Gram Positive Cocci: Staphylococci and Streptococci (trans 6) Flashcards
PYOGENIC COCCI
Pus-producing cocci
Types:
o Staphylococcus
o Streptococcus
o Gram-negative Neisseria
GENUS STAPHYLOCOCCUS
Gram-positive, spherical cells, usually arranged in grapelike irregular clusters (division occurs simultaneously in several planes)
Natural habitat: mammalian body surfaces
Pathogenic when surface barrier is breached and organisms gain access to tissues
ALL are CATALASE (+) (review: catalase reaction – the enzyme catalase catalyzes the conversion of H2O2 to H2O and O2. When H2O2 is added to a bacterial colony, liberation of O2 as gas bubbles signifies a positive catalase reaction)
Only Staphylococcus aureus is COAGULASE (+) (review: coagulase reaction – citrated plasma is mixed with broth culture or bacterial growth on agar, formation of clot after 1-4 hours signifies a positive coagulase reaction)
Genera Staphylococcus
o Staphylococcus aureus (golden)
o Staphylococcus epidermidis (over the skin)
o Staphylococcus saprophyticus (rotten plants)
Staphylococcus aureus
Gram-positive, round, cluster-forming coccus
Non-motile, non-spore forming, facultative anaerobe
o In the presence of oxygen: uses TCA for energy
o In the absence of oxygen: uses fermentative mechanisms for energy
Ferments MANNITOL (as distinguished from S. epidermidis and S. saprophyticus)
CATALASE (+)
COAGULASE (+)
- Coagulase: an enzyme that causes citrated plasma to clot. It makes this organism more virulent.
Forms gray to golden yellow colony on blood agar, hence called “aureus” meaning golden.
Location: nasal passages (most numerous), skin, inguinal area, and mucous membranes
Resistant to drying, can withstand 50oC for 30 min and thus can persist for long periods, as long as 2 weeks on fomites (inanimate objects, e.g. gauze), which can then serve as sources of infection.
Resistant to 9% NaCl but inhibited by certain chemicals like 3% Hexachlorophene found on antibacterial soaps.
o Hence they are halophilic or salt-loving.
β-Lactamase production is common: resistance to penicillin or other cephalosporin (plasmid controlled)
Resistant to Nafcillin, Methicillin, Oxacillin: in genes independent of plasmids
o Attributed in genes of the bacteria (SCCmec)
May be susceptible to Vancomycin
o That’s why it’s the drug of choice for Methicillin Resistant Staphylococcus aureus (MRSA)
Staphylococcus aureus - Culture Characteristics
Grows well on routine media like Nutrient Agar or Blood Agar Plate
Readily grows under aerobic or microaerophilic conditions and most rapidly at 37°C
Smooth, opaque, round, convex golden yellow colonies surrounded by β-hemolysis or complete hemolysis (complete lysis of RBCs around the colony)
Antigenic Structure
- Peptidoglycan
- Teichoic Acids
- Protein A
- Fibronectin-binding protein: (FnBP)
- Clumping Factor
- Capsular polysaccharide - Anti-phagocytic
Staphylococcus aureus - Antigenic Structure
Peptidoglycan
Very thick peptidoglycan (found in cell wall)
Destroyed by acids or lysozyme (found in tears and saliva). So when you put saliva in your wound, it will not heal but would be more infected because of the other anaerobic organisms resting in the saliva.
Elicits production of pyrogens (IL1) and opsonic antibodies by monocytes
Chemoattractant for polymorphonuclear cells (PMNs)
Staphylococcus aureus - Antigenic Structure
Teichoic Acids
Stimulates production of antibodies
Binds the peptidoglycan
Extracellular teichoic acid can consume early reacting complement components in the serum which protects the organism from complement mediated destruction
Staphylococcus aureus - Antigenic Structure
Protein A
Antigenic structure found in the bacterial cell wall
Called Microbial Surface Component Recognizin Adhesive Matrix Molecules (MSCRAMM)
Antiphagocytic: Protein A attaches to the Fc part of IgG (except IgG3) instead of Fab => making the bacteria unrecognizable to phagocytes (Normally: the Fab portion of the Ab is the one that attaches to the bacteria, and the macrophage attaches itself to the Fc portion of the Ab prior to phagocytosis)
Only S. aureus has protein A
Staphylococcus aureus - Antigenic Structure
Fibronectin-binding protein: (FnBP)
Promote binding to fibronectin in mucosal cells \and tissue matrices
Adds to the bacteria’s virulence
Staphylococcus aureus - Antigenic Structure
Clumping Factor
Causes clumping of S. aureus when mixed with plasma (aggregated bacteria is anti-phagocytic => adhere to fibrinogen and fibrin
Staphylococcus aureus - Determinants of Pathogenicity 1. Extracellular Enzymes COAGULASE LIPASE CATALASE HYALURONIDASE /SPREADING FACTOR STAPHYLOKINASE (FIBRINOLYSIN) NUCLEASE
2. Toxins ALPHA TOXIN BETA TOXIN DELTA TOXIN GAMMA TOXIN LEUKOCIDIN ENTEROTOXIN EXFOLIATIVE TOXIN TOXIC SHOCK SYNDROME TOXIN (TSST-1) **Cytolytic Exotoxins: α, β, γ, and δ toxins attack mammalian cell (including red blood cell) membranes (Hemolysins)
Staphylococcus aureus - Determinants of Pathogenicity
Extracellular Enzymes: COAGULASE
o When present in a culture, automatically, it’s S.aureus!
o Causes clotting of plasma
o Deposits fibrin on the surface of staphylococcus, perhaps altering their ingestion by phagocytic cells (Antiphagocytic, acts like a protective shield just like an armor)
o Synonymous with invasive potential
Staphylococcus aureus - Determinants of Pathogenicity
Extracellular Enzymes: LIPASE
o Hydrolyzes lipids including the oil on body surfaces (ex. skin and subcutaneous tissues)
o For invasion of cutaneous and subcutaneous tissues
o Correlated with ability to produce BOILS (“pigsa”)
Staphylococcus aureus - Determinants of Pathogenicity
Extracellular Enzymes: CATALASE
o Converts H2O2 => O2 + H2O
o Normally, hydrogen peroxide is toxic to cells, but with catalase, it is neutralized or deactivated
Staphylococcus aureus - Determinants of Pathogenicity
Extracellular Enzymes: HYALURONIDASE/SPREADING FACTOR
o Hydrolyzes hyaluronic acid in the ground substance of connective tissue facilitating spread of organism
o Also called “spreading factor”
o Correlated to the ability to invade
Staphylococcus aureus - Determinants of Pathogenicity
Extracellular Enzymes: STAPHYLOKINASE (FIBRINOLYSIN)
o Converts plasminogen => plasmin
o Produces dissolution of clots and aids in spread of the organism
o The normal body response is that clots wall off the infection containing it in one area but this enzyme digests that wall helping the organism to spread
o Streptococcus also possess this
Staphylococcus aureus - Determinants of Pathogenicity
Extracellular Enzymes: NUCLEASE
o Can cleave either DNA or RNA (to liquefy the secretions more so that it would be easier for the staph to spread)
o Viscosity of pus is due to Deoxyribonucleoprotein
o Nuclease lowers the viscosity, facilitating the spread of the organism
Staphylococcus aureus - Determinants of Pathogenicity
Toxins: ALPHA TOXIN
o Hemolytic and dermonecrotic (produce necrosis of tissues)
o Injures the circulatory system, muscles, renal cortex tissues, damages macrophages and platelets
o Polymerizes into tubes that pierce membranes, resulting in the loss of important molecules and osmotic lysis
Staphylococcus aureus - Determinants of Pathogenicity
Toxins: BETA TOXIN
o Damages sphingomyelin on RBC membrane producing hemolysis
Staphylococcus aureus - Determinants of Pathogenicity
Toxins: DELTA TOXIN
o Damages RBC, Macrophage, Lymphocytes, Neutrophils, Platelets
Staphylococcus aureus - Determinants of Pathogenicity
Toxins: GAMMA TOXIN
o Hemolytic activity
Staphylococcus aureus - Determinants of Pathogenicity
Toxins: LEUKOCIDIN
o Panton Valentine Leuokocidin (PVL): 2 proteins (interacts with Gamma Toxin to form 6 potential 2-component toxins)
o Pore-forming toxin: toxic to PMN (polymorphonuclear neutrophils => lysis)
o Production of this toxin makes strains more virulent (PVL is encoded on a mobile phage unlike the hemolysins)
Superantigen Exotoxin (e.g., Toxic Shock Syndrome Toxin-1)
Superantigen: only a small amount is needed to produce a profound effect
Affinity for T cell receptor–MHC Class II antigen complex
Stimulate enhanced T-lymphocyte response
This major T-cell activation can causeToxic Shock Syndrome, by releasing large amounts of T-cell cytokines, such as IL-2, IFN-γ, &TNF-α into the circulation
IL-2 effect: flu-like symptoms, vomiting/nausea, rash, weakness or shortness of breath, diarrhea, low BP, drowsiness or confusion, loss of appetite
TNF effect: systemic inflammation; able to induce fever and apoptotic cell death
Important virulence factor in CA-MRSA infections
Staphylococcus aureus - Determinants of Pathogenicity
Toxins: ENTEROTOXIN
o Heat stable exotoxin resistant to gut enzymes
o Stimulates the vomiting center and increases fluid transudation into the intestine (causing diarrhea)
Staphylococcus aureus - Determinants of Pathogenicity
Toxins: EXFOLIATIVE TOXIN
o Epidermolytic toxin which dissolves the matrix of the epidermis producing generalized desquamation with pus on the surface
o Causes Staphylococcal Scalded Skin Syndrome by dissolving the mucopolysaccharide matrix of the epidermis
Staphylococcus aureus - Determinants of Pathogenicity
Toxins: TOXIC SHOCK SYNDROME TOXIN (TSST-1)
o Binds to MHC Class II on T-cells causing release of cytokines that mediate shock like IL-2, TNF, and INF => shock of vascular systems
o Associated with fever, shock, and multisystem involvement, including a desquamative skin rash
Staphylococcus aureus - Pathogenesis
Anterior nares: major reservoir of infection
The perineum is another carriage site
Lesions with draining pus on the hands of the patients: disseminates the organism via touching others
Abscess: characteristic of a Staph infection. Organisms penetrate a sebaceous gland or a hair shaft, multiply and produce the infection.
Skin/Mucous membrane is an excellent protective barrier. Destruction of the integrity of these surfaces predispose to staph infection. (ex. Burns)
Staphylococcus aureus - Clinical Infections
FOLLICULITIS FURUNCLE CARBUNCLE IMPETIGO STAPHYLOCOCCAL SCALDED SKIN SYNDROME STAPHYLOCOCCAL FOOD POISONING OSTEOMYELITIS SEPTICEMIA ACUTE ENDOCARDITIS PNEUMONIA SEPTIC ARTHRITIS TOXIC SHOCK SYNDROME
Staphylococcus aureus - Clinical Infections
FOLLICULITIS
o Infection of hair follicles
o Manifestations: yellow discoloration of skin near hair follicle, the base of the hair shaft is surrounded by redness and a greenish tinge
o Treatment: during the old days - clamp and pull out the hair, the pus comes with it, no antibiotic needed
Staphylococcus aureus - Clinical Infections
FURUNCLE
o When folliculitis extends to subcutaneous tissues
o Focal suppurative lesion
o Treatment: do incision and drainage
Staphylococcus aureus - Clinical Infections
CARBUNCLE
o Furuncle extending to the deeper fibrous tissues
o Multiple foci and multiple draining sinuses
Staphylococcus aureus - Clinical Infections
IMPETIGO
o Encrusted pustules on the superficial skin of children and infants
o Starts as vesicles/vesicular lesions then becomes pustular
o When crusts are removed, a red denuded surface is exposed, most of the time with pus
o Highly contagious
o 2 causes: Staphylococcus aureus and Streptococcus pyogenes (classical impetigo — produced more by S. pyogenes)
Staphylococcus aureus - Clinical Infections
STAPHYLOCOCCAL SCALDED SKIN SYNDROME
STAPHYLOCOCCAL SCALDED SKIN SYNDROME
o Due to Exfoliative Toxin
o Generalized painful erythema and bulbous desquamation of large areas of the skin
Staphylococcus aureus - Clinical Infections
STAPHYLOCOCCAL FOOD POISONING
o Most common form of bacterial food poisoning
o Preformed toxin in food contaminated by the hands of the food workers
o The problem is food with enterotoxin are normal in odor and taste, you don’t know that it’s contaminated
o Rapid onset of symptoms like abdominal pain, vomiting & diarrhea 2-6 days after infection
o Immediate effect: 2-6 (1-8) hours for sign and symptoms to appear
Staphylococcus aureus - Clinical Infections
OSTEOMYELITIS
o Follows a hematogenous spread usually from a furuncle or carbuncle
o Or a contiguous spread because of the close proximity especially if it’s on the shin bone because it’ll just erode into the bone
o Localizes at the diaphysis of long bones with accumulation of pus
o Fever, chills, pain over the bone and on movement
Staphylococcus aureus - Clinical Infections
SEPTICEMIA
o A generalized infection with sepsis or bacteremia that may be associated with a known focus like a septic joint or even an ordinary carbuncle
o Pwedeng sa boils (pigsa) lang magsimula ang septicemia
Staphylococcus aureus - Clinical Infections
ACUTE ENDOCARDITIS
o Acute = immediate
o Associated with intravenous drug abuse caused by injection of contaminated preparations or by needles contaminated with S. aureus
o S. aureus also colonizes the skin around the injection site, and if the skin is not sterilized before injection, the bacteria can be introduced into soft tissues and the bloodstream
o Can kill you in a week or so
o Differentiate this from subacute endocarditis caused by Streptococcus viridans (takes a long time, around 1 yr)
Staphylococcus aureus - Clinical Infections
PNEUMONIA
o Common in hospitalized patients; follows a viral influenza infection
o Rapid destruction of lung parenchyma resulting in cavitations
o The most common type of hospital acquired pneumonia
o Common in malnourished post-measles children
Staphylococcus aureus - Clinical Infections
SEPTIC ARTHRITIS
o Invasion of synovial membrane resulting in a closed infection of the joint cavity
o Staphylococcal lesion on a joint
o Painful, red, swollen joint with a limited range of motion
Staphylococcus aureus - Clinical Infections
TOXIC SHOCK SYNDROME
o Common in menstruating women who use tampons or anybody who has a Staph wound infection
o Women brought to the ER during that time (1970’s) forgot that they still have a tampon inside => cause of infection
o TSS results in high fever, rash with diffuse erythema followed by desquamation, vomiting, diarrhea, hypotension and multi-organ involvement (especially GI, renal and/or hepatic damage)
Staphylococcus aureus - Laboratory Diagnosis
Gram Stain of Pus/Sputum
o Gram-positive cocci in clusters is a presumptive diagnosis of Staphylococcus
Culture
o Specimen: Pus, blood, tracheal fluid, aspirate or CSF
o Gold standard for definitive diagnosis
o Blood Agar Plate: (+) for S. aureus when there are yellow colonies with β-hemolysis
o Mannitol Salt Agar Plate: (+) for S. aureus when the agar plate color changes from red to yellow due to the fermentation of mannitol
*note: only Staph aureus can ferment mannitol
Catalase Test
o Use 3% H2O2on specimen
o Appearance of bubbles indicate that the test is (+)
Coagulase Test
o Citrated plasma plus equal volume of broth. Incubated
o Formation of a clot in 1-4 hours means the test is (+)
Remember: “All Staph are Catalase (+), Strep is Catalase (-), only S. aureus is Coagulase (+), other staph are Coagulase (-)”
Staphylococcus aureus - Treatment
Basic Principles of Treatment:
o Adequate drainage (IND – Incision and Drainage)
o Removal of all foreign bodies
“Antimicrobials are less effective for Staph when there is an abscess formation so the abscess must be removed first.”
Oral Cloxacillin, IV Nafcillin, Vancomycin
New drug: Flucloxacillin
Cloxacillin Capsule 500mg (every 6 hours for 10 days)
For those allergic to penicillin: Clindamycin capsule 300mg every 6 hours
Pediatrics: Cloxacillin at 50mg/kg/day divided into 4 doses
Methicillin Resistant Staphylococcus aureus (MRSA)
o Resistant to all β-lactam antibiotics (Penicillins and Cephalosporins)
o Vancomycin - choice for empiric treatment of life-threatening MRSA, 1g every 12 hours
o Unfortunately, there’s already resistance to this drug so alternative drugs are used such as Quinupristin Dalfopristin, Linezolid, and Daptomycin
Staphylococcus aureus - Prevention
Proper disposal of discharge of contaminated fomites
o Wound-infected gauze – survive for 2 weeks
Washing of hands before and after the patient contact
Mupirocin to nasal and perineal carriage sites (Bactroban/Bactifree in the Phils)
Anti Staph Drug + Rifampicin – provides long term suppression (for long term suppression or abscess that do not respond to Cloxacillin)
2015 literatures: use Clindamycin + Ciprofloxacin
Staphylococcus epidermidis
Identification
o CATALASE (+), COAGULASE (-)
o White colonies on blood agar plate
o Does not ferment mannitol
Epidemiology
o Member of the normal skin flora as well as the respiratory and gastrointestinal tracts
o Usual target: immunocompromised patients
o Infection is usually the contamination of a surgical site from patients or others skin or nasopharynx.
Pathogenesis
o Low virulence for the normal host
o Life threatening infection when host defenses are breached
o Predilection for prosthetic devices (ex. total hip replacement)
Treatment
o Vancomycin 1g every 12 hours
o Vancomycin sensitivity remains the rule, but resistant isolates have been reported
Staphylococcus saprophyticus
Identification
o CATALASE (+), COAGULASE (-)
Epidemiology
o Occurs on the skin and periurethral area and as a transient urethral flora
Pathogenesis
o Common cause of symptomatic UTI in sexually active young women
o Novobiocin resistant (distinguishing characteristic from the other two Staph species)
REMEMBER All Staph are CATALASE (+) Strep are CATALASE (-) Only Staphylococcus aureus is COAGULASE (+) Other Staph are COAGULASE (-)
GENUS STREPTOCOCCUS
27 recognized species
Streptococci are gram positive, non-motile, and catalase negative.
They are ovoid to spherical in shape and occur as pairs or chains
Most are facultative anaerobes because they grow fermentatively even in the presence of oxygen because of their complex nutritional requirements.
Blood enriched medium is generally used for their isolation
Does NOT produce gas
Classification of Streptococci
- Hemolysis Patterns on BAP
- Lancefield Classification
- Capsular Polysaccharides - Antigenic specificity is used to classify S. pneumoniae into 90 types, and to type Group B streptococci
- Biochemical Reactions
GENUS STREPTOCOCCUS - Classification of Streptococci
Hemolysis Patterns on BAP
β-Hemolysis: clear zone of hemolysis due to complete lysis of RBC
α-Hemolysis: greenish discoloration (biliverdin) due to incomplete hemolysis
γ-Hemolysis: non-hemolytic
GENUS STREPTOCOCCUS - Classification of Streptococci
Lancefield Classification
System based on the antigenic composition of cell wall carbohydrates in β- hemolytic strep
Groups A,B,C,D,G: Associated with human infection (according to Jawetz, groups A, B, C, F and G are said to cause disease in humans)
GENUS STREPTOCOCCUS - Classification of Streptococci
Biochemical Reactions
Streptococci ferment carbohydrates
CATALASE (-)
Optochin Sensitivity
o Optochin is an antibiotic disk (derivative of quinine)
o S. pneumoniae is sensitive, other Strep species are resistant
PYR Disk
o Ability to hydrolyze pyrrolidonyl
o (+) – red color (S. pyogenes and Group D Enterococci)
o (-) – yellow (No Change)–Strep viridans
Oxidase Test (Doc said, “nevermind the oxidase test”)
o Presence of cytochrome oxidase in certain bacteria. Organism will react to it and will change the color of the paper into:
o (+) – violet blue (Neisseriae)
o (-) – S. pyogenes
Group A β-Hemolytic Streptococci (Streptococcus pyogenes) - Typical Organism
o Lancefield Group A, Gram-positive, spherical and in chains
o Facultative anaerobe
o CATALASE (-), OXIDASE (-), PYR (+)
o Killed in 30 min at 60°C
o Bacitracin sensitive – 95% accurate
Bacitracin test is performed to determine whether the organism is Streptococcus pyogenes or not
S. pyogenes is the only bacteria sensitive to bacitracin.
In Bacitracin test, a bacitracin disk is placed in a plate agar culture with S. pyogenes. After 18-24 hours of incubation, a zone of inhibition is seen around the disk.
Group A β-Hemolytic Streptococci (Streptococcus pyogenes) - Culture
o Blood Agar Plate (BAP): Primary isolation of S. pyogenes
o Clinical specimens can be processed by POUR or STREAK PLATE methods
o Optimal pH 7.4- 7.6 at 37°C
o Since it is facultative, culture growth is enhanced by a low O2 tension or by increased CO2
o Discoid colonies, domed, grayish to opalescent with a zone of β-hemolysis
o Utilizes glucose with lactic acid as product
Group A β-Hemolytic Streptococci (Streptococcus pyogenes) - Antigenic Structure and Determinants of Pathogenicity
- M PROTEIN
- LIPOTEICHOIC ACID
- PROTEIN F (FIBRONECTIN-BINDING PROTEIN)
- CAPSULAR POLYSACCHARIDE
- PROTEIN F (FIBRONECTIN-BINDING PROTEIN)
Group A β-Hemolytic Streptococci (Streptococcus pyogenes)
Antigenic Structure and Determinants of Pathogenicity - M PROTEIN
Hair-like projections of the streptococcal cell wall
Major virulence factor of S. pyogenes
Antiphagocytic: inhibits opsonization and the complement system
Immunity to Group A strep. is due to antibodies against M Protein (but there are 150 types of M Protein so a patient can have repeated infections with group A S. pyogenes of different M types)
A person may be immune to 1 M protein but may not be immune to another M proteins)
Type I-M induces antibodies that react with human cardiac muscle
There is a cross reaction with human cardiac muscle and heart bound antigens => destruction of heart valves => Rheumatic Fever and Rheumatic Heart Disease
Type II-M: no consequence
Group A β-Hemolytic Streptococci (Streptococcus pyogenes)
Antigenic Structure and Determinants of Pathogenicity - LIPOTEICHOIC ACID
Attached to peptidoglycans and covers the pili which consists partly of M proteins
It mediates buccal epithelial cell adherence (adheres to fibronectin on epithelial cells)
Cytotoxic to host cells
Group A β-Hemolytic Streptococci (Streptococcus pyogenes)
Antigenic Structure and Determinants of Pathogenicity - CAPSULAR POLYSACCHARIDE
Made up of hyaluronic acid capsules in some Group A streptococci
Hyaluronic acid mimics the ground substance of connective tissue, so the body will not act against it, and protects the organism from being phagocytosed (anti-phagocytic)
Group A β-Hemolytic Streptococci (Streptococcus pyogenes)
Antigenic Structure and Determinants of Pathogenicity - PROTEIN F (FIBRONECTIN-BINDING PROTEIN)
Mediates attachment to fibronectin in the pharyngeal epithelium
M proteins and lipoteichoic acids also bind to fibronectin
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Toxins and Enzymes
- HEMOLYSINS
- PYROGENIC EXOTOXIN (ERYTHROGENIC TOXIN)
- STREPTOKINASE
- STREPTODORNASE
- HYALURONIDASE
- DIPHOSPHOPYRIDINE NUCLEOTIDASE - Ability to kill leukocytes
Group A β-Hemolytic Streptococci (Streptococcus pyogenes):
Toxins and Enzymes - HEMOLYSINS
Streptolysin O (SLO)
o Oxygen labile
o Cytolytic activity against RBC, PMN, platelets, and the reason for β-hemolysis
o Antigenic and stimulates production of the antibody, antistreptolysin O (ASO) in 10-14 days
o ASO Titer: indicator of recent infection with S. pyogenes (>200iu = positive for S. pyogenes)
Streptolysin S
o Oxygen stable
o Lytic for red and white blood cells
o Non-antigenic
o Responsible for the surface hemolysis seen on BAP
Group A β-Hemolytic Streptococci (Streptococcus pyogenes):
Toxins and Enzymes - PYROGENIC EXOTOXIN (ERYTHROGENIC TOXIN)
90% of S. pyogenes: Exotoxin A, B, C
For S. pyogenes to produce this, it has to be infected by a virus, bacteriophage confers a gene to S. pyogenes that allows it to make the toxin
Heat labile but stable to acid, alkali, and pepsin
Exotoxin A
o Associated with Toxic Shock Syndrome and Scarlet fever
o Superantigen producing release of cytokines that mediate shock/tissue injury
Exotoxin B
o Necrosis of tissues (Necrotizing Fasciitis)
Exotoxin C
o Causes increased permeability of the blood-brain barrier to toxins and bacteria
o Has a pyretic effect on the hypothalamus
Group A β-Hemolytic Streptococci (Streptococcus pyogenes):
Toxins and Enzymes - STREPTOKINASE
Transform plasminogen into plasmin, a proteolytic enzyme to digest fibrin and other proteins facilitating the rapid spread of the organism
Group C Streptokinase: Used as IV for treatment of pulmonary emboli, coronary artery and venous thrombosis
Group A β-Hemolytic Streptococci (Streptococcus pyogenes):
Toxins and Enzymes - STREPTODORNASE
Helps to liquefy exudates
DNAses that degrade the viscous DNA in necrotizing tissue or exudates, aiding the spread of infection
Group A β-Hemolytic Streptococci (Streptococcus pyogenes):
Toxins and Enzymes - HYALURONIDASE
Splits hyaluronic acid in the ground substance of connective tissue thus aiding in the spread of the microorganism
Spreading factor
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Pathogenesis and Clinical Findings
A. Diseases due to invasion by S. pyogenes
1. ERYSIPELAS
2. STREPTOCOCCAL CELLULITIS
3. NECROTIZING FASCIITIS (STREP. GANGRENE)
4. PUERPERAL FEVER/ CHILDBED FEVER
5. SCARLET FEVER
6. BACTEREMIA/SEPSIS/STREPTOCOCCAL TOXIC SHOCK SYNDROME
B. Local infection with S. pyogenes
1. STREPTOCOCCAL SORE THROAT/PHARYNGITIS/TONSILLITIS
2. STREPTOCCOCUS PYODERMA / IMPETIGO
C. Post-Streptococcal Diseases
1. ACUTE GLOMERULONEPHRITIS (AGN
2. RHEUMATIC FEVER
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Pathogenesis and Clinical Findings
Diseases due to invasion by S. pyogenes - ERYSIPELAS
Skin and subcutaneous tissue infection usually on the face or the lower extremities
With massive brawny/elevated edema and rapidly advancing margin of infection, which is well-delineated
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Pathogenesis and Clinical Findings
Diseases due to invasion by S. pyogenes - STREPTOCOCCAL CELLULITIS
Rapidly spreading infection of the skin and subcutaneous tissues associated with trauma, burns, and wounds
Erythema, pain, tenderness, and swelling occur
Compared to erysipelas, the affected region in cellulitis is not raised/elevated; the line between involved and uninvolved tissue is indistinct
Not well demarcated
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Pathogenesis and Clinical Findings
Diseases due to invasion by S. pyogenes - NECROTIZING FASCIITIS (STREP. GANGRENE)
Rapidly spreading necrosis of the skin, subcutaneous tissue, and fascia (usu within hours) due to M protein that blocks phagocytosis and Exotoxin B that causes the necrosis of the tissues, allowing bacteria to move rapidly through tissues
Symptoms may include a toxic shock-like syndrome, fever, hypotension, multi-organ involvement, as a sunburn-like rash, or a combination of these symptoms.
Must be recognized early. Surgically remove the fascia (Surgical debridement)
Called “Flesh-eating bacteria” due to necrotizing Exotoxin B
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Pathogenesis and Clinical Findings
Diseases due to invasion by S. pyogenes - PUERPERAL FEVER/ CHILDBED FEVER
Septicemia (blood poisoning) secondary to an endometritis occurring after birth
May be introduced by the healthcare workers (unhygienic practices) to the mother giving birth
Chills, fever, abdominal distention, and tenderness, serosanguinous vaginal discharge
High mortality rate before the advent of antibiotics
Ignatius Semmelweis – obstetrician in charge of 2 OB wards in Austria. One ward was run by midwives and it has a zero death rate, the other ward is run by medical students and the death rate was hitting the ceiling. The medical students during that time went straight from anatomy lab dissection and deliver babies.
Semmelweis instituted strict hand washing, and the death rate dropped to zero
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Pathogenesis and Clinical Findings
Diseases due to invasion by S. pyogenes - SCARLET FEVER
Associated with severe, purulent inflammation of the posterior oropharynx and tonsillar areas with a sunburn-like rash on the neck, trunk, and extremities, armpits and inguinal areas (called Pastia’s lines), in response to the release of pyrogenic/erythrogenic exotoxin to which the patient does not have antibodies
“Strawberry tongue” due to an erythrogenic toxin
Transmitted by droplets and fomites
Patient dies of toxic shock syndrome
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Pathogenesis and Clinical Findings
Diseases due to invasion by S. pyogenes - BACTEREMIA/SEPSIS/STREPTOCOCCAL TOXIC SHOCK SYNDROME
Isolation of Group A β-hemolytic streptococci from blood or another normally sterile body site in the presence of shock and multi-organ failure.
Mediated by the production of streptococcal pyrogenic exotoxins causing massive, nonspecific, T-cell activation and cytokine release
Flu-like symptoms, followed shortly by necrotizing soft tissue infection, shock, acute respiratory distress syndrome, and renal failure, followed by death
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Pathogenesis and Clinical Findings
Local infection with S. pyogenes - STREPTOCOCCAL SORE THROAT/PHARYNGITIS/TONSILLITIS
Occurs as a subacute nasopharyngitis and sore throat with a thin serous discharge and little fever, and may extend to the middle ear and mastoid (otitis media)
In adults, there is intense nasopharyngitis, tonsillitis, intense redness, edema of the mucous membranes, with purulent exudate
Enlarged, tender cervical lymph nodes around the submandibular area and fever are also present
The pharynx may appear beefy red with exudates
Rheumatic Fever may be a sequelae
o Caused by cross reactions between antigens of the heart and joint tissues, and the streptococcal antigen (M Protein)
o Characterized by fever, rash, carditis, and arthritis
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Pathogenesis and Clinical Findings
Local infection with S. pyogenes - STREPTOCCOCUS PYODERMA/IMPETIGO
Small vesicles progressing to weeping pustular lesions; denuded surfaces with pus
Group A Strep skin infection may precede glomerulonephritis (post-streptococcal/post-infectious glomerulonephritis)
Although S. aureus is recovered from most contemporary cases of impetigo, S. pyogenes is the classic cause of this syndrome
The disease begins on any exposed surface, most commonly the legs
Typically affecting children, common in tropical climates, people living in poor areas, conditions of poor hygiene
It can cause severe and extensive lesions on the face and limbs
May start as just a scratch in the skin or an insect bite => inoculated the bacteria
Treatment
o Topical agent such as mupirocin ointment
o Systemically treated with penicillin or a first generation cephalosporin
o Doc: “Don’t use Silver sulfadiazine cream to your patients with impetigo because when it heals it will turn into black. Your patient will look like dalmatian. So use mupirocin instead.”
Impetigo is best prevented by attention to adequate hygiene!
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Pathogenesis and Clinical Findings
Post-Streptococcal Diseases
1-4 weeks after acute infection
Not directly related to the bacteria but due to a hypersensitivity response
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Pathogenesis and Clinical Findings Post-Streptococcal Diseases - ACUTE GLOMERULONEPHRITIS (AGN) **REMEMBER! “CHEAP BUN” C – Elevated Creatinine H – Hematuria/ Hypertension E – Edema A – Azotemia P – Proteinuria BUN – Elevated BUN
Due to nephritogenic strains
Preceded more by skin infection rather than respiratory infection
Due to Ag-Ab complexes on the basement membrane, producing inflammation
Edema, hypertension, azotemia (elevated BUN and serum creatinine levels), hematuria, and proteinuria
Some go into chronicity leading to Renal Failure
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Pathogenesis and Clinical Findings
Post-Streptococcal Diseases - RHEUMATIC FEVER
Certain Group A strepcocci contain membrane antigens that causes a cross-reaction with human heart and joint tissue antigens
The patient produces antibodies that damage heart muscles and valves
Preceded by a respiratory infection
Typical symptoms: fever, malaise, arthritis, carditis
If you have another Strep infection, the effect is recurrent and cumulative => give the patient prophylaxis for Strep
Tendency to be reactivated by recurrent strep infection producing cumulative heart damage (valvular damage)
o If a patient is getting 6 attacks of tonsillitis a year, for the last 3 or 4 years, it is best recommended that the tonsils be already taken out.
o Prophylaxis is given: Benzathine Penicillin injection every 28 days, or Erythromycin tablets 250mg BID for life.
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Diagnosis
Specimen
o Definitive diagnosis: direct culture of posterior pharynx and tonsils. Swabs are inoculated on broth or blood agar.
o Vesicular or pustular fluid
o Cellulitis and Erysipelas material: Aspiration of tissue fluids from the advancing border of erysipelas or by subcutaneous injection of sterile saline followed by reaspiration.
o Serum for antibody determination (ASO)
Smears from Pus
o Gram (+), usually seen as single cocci or pairs rather than definite chains
Smears from Throat swab
o Rarely contributory
o S. viridans has the same appearance as S. pyogenes so check the hemolysis!
Culture
o Streptococcus pyogenes: β-hemolysis on BAP; PYR(+)(red color), Inhibited by Bacitracin
o Streptococcus viridans: α-hemolytic on BAP; PYR(- (yellow color); Bacitracin (-)
Serologic Test
o Estimates rise in antibody titer (ASO Titer)
Antigen Detection Test
o Rapid detection of Group A antigen from a throat swab using agglutination
o In a (+) test, the latex particles clump together, whereas in a (-) test, they stay separate, giving the suspension a milky appearance
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Treatment
Penicillin, Erythromycin
Antimicrobials – no effect on established glomerulonephritis or rheumatic fever
Give antimicrobials to prevent post-streptococcal diseases and recurrence
Group A β-Hemolytic Streptococci (Streptococcus pyogenes): Prevention and Control
Nasal discharges are the most dangerous source for spread
Prophylactic antibiotics for surgery patients with known heart valve deformity or prosthetic heart valves
Eradicate Group A Strep in patient with respiratory or skin infections
Prophylaxis for those who have suffered an attack of rheumatic fever
Group B Streptococci (Streptococcus agalactiae)
Grow as diplococci or in short chains, encapsulated
Normal flora of the pharynx, GIT, and vagina
β-hemolytic and forms large, mucoid colonies
Leading cause of Neonatal Septicemia and Meningitis
o Acquired from the mother during delivery
o Incidence is higher when there is prolonged labor, premature rupture of membranes (PROM), or obstetric manipulation
o REMEMBER! “B” IS FOR BABY (Group B strep)
Can also cause puerperal infection
Immunocompromised/Elderly: can cause cellulitis, skin and soft tissue infections in diabetics, UTI, pneumonia, endocarditis and septic arthritis
Infection during first month of life may also present as Respiratory Distress Syndrome (RDS)
o Neonates with meningitis do not present with a stiff neck, which is the classic sign seen in adults. They display nonspecific signs such as fever, vomiting, poor feeding, and irritability.
Penicillin G is the drug of choice: 12-24 million units/day
Other drugs: Erythromycin, Chloramphenicol, Cephalosporins, Vancomycin, Imipenem, Clindamycin
Group C Streptococci (Streptococcus equisimilis)
Representative Organisms
1. Streptococcus equisimilis
a. May cause pharyngitis, puerperal sepsis, endocarditis, bacteremia, osteomyelitis, brain abscess, post-op wound infection, and pneumonia
b. Can be pathogenic
c. Also the source of streptokinase used for people with stroke/heart attack
2. Streptococcus zooepidemicus
3. Streptococcus dysgalactiae
Characteristics
o ALL are β-hemolytic EXCEPT S. dysgalactiae
o Identified by reactions with specific antisera
Group D Streptococci Representative Organisms o Enterococcal 1. Enterococcus faecalis 2. Enterococcus faecium 3. Enterococcus durans o Non-enterococcal 1. Streptococcus equinus (only seen in horses) 2. Streptococcus bovis - most important to human disease
Characteristics
Enterococcus
o Can grow as diplococci or short chains
o Part of the normal enteric flora, inhabits the skin & the upper repiratory, & the gut
o Gamma to alpha hemolytic, PYR (+),
o Grows in 40% bile
o (+) Esculin hydrolysis (black)
o (+) growth in 6.5% NaCl (distinguishing feature), halophilic
Streptococcus bovis
o Lyse by 6.5% NaCl
o The rest are same as enterococcus (grows in 40% biles, can hydrolyze esculin
Group D Streptococci
Clinical Infection
o E. faecalis: UTI, biliary infection, septicemia, endocarditis, wound infection, intra-abdominal abscess
o S. bovis: endocarditis or bacteremia, associated with GI malignancy (colonic CA)
o REMEMBER!The presence of S. bovis in the blood – alert the clinician to a possible occult malignancy, almost always associated with colonic cancer.
Treatment Enterococcus o Penicillin + Gentamycin/Streptomycin o Vancomycin + Erythromycin o E. faecium is more likely to be vancomycin or multiply resistant than E. faecalis S. bovis o Penicillin G
**S. anginosus-milleri Group
- S. constellatus,
- S. intermedius,
- S. anginosus,
- S. milleri
Normal flora of oral cavity & gingival crevices
May be classified as Viridans strep because they are also α-hemolytic
Dental, brain, lung, and intra-abdominal abscess
Viridans Group Streptococci Group of Streptococci o S. mitis, o S. mutans, o S. salivarius, o S. sanguis α-hemolytic but may also be non-hemolytic (-) PYR, (-) Bacitracin NOT inhibited by optochin NOT soluble in bile Most prevalent member of the normal flora of the mouth and the upper respiratory tract Also found in the GIT & female genital tract
Pathogenesis
o May reach the bloodstream because of dental manipulation, trauma, or GI or GU instrumentation and cause subacute endocarditis. *(Subacute because it takes about a year before it is fatal. An underlying heart valve structural defect must be present)
o Wound infection, meningitis, biliary and intra abdominal infections may occur
o S. sanguis: most frequent single species causing Subacute bacterial endocarditis
o S. mutans: dental caries & dental plaque
Streptococcus pneumoniae
General Characteristics
Most common cause of community acquired pneumonia and meningitis in adults
Causes otitis media, septicemia, sinusitis, bronchitis, and bacteremia
Normal inhabitants of the upper respiratory tract of 5-40% of humans
Gram (+) lancet-shaped diplococci, encapsulated, non motile, in chains or pairs
Most energy comes from the fermentation of glucose which is accompanied by the rapid production of lactic acid (which limits its growth).
Morphology and Identification
Complex nutritional requirements
Has absolute nutritional requirement for choline
α-hemolytic on BAP
Facultative anaerobe
For primary isolation:
o Trypticase soy agar or Tryptone soya agar (TSA)
o Brain Heart Infusion broth enriched with 5% defibrinated blood
- Young cultures of encapsulated pneumococci produce circular, glistening, dome shaped colonies 1mm in diameter; later center of colonies collapse
- Unencapsulated strain produce rough colonies
Streptococcus pneumoniae: Laboratory Identification
OPTOCHIN SENSITIVITY
BILE SOLUBILITY
QUELLUNG REACTION
Animal inoculation (not being done any more)
Streptococcus pneumoniae: Laboratory Identification
OPTOCHIN SENSITIVITY
o Disc with a quinine derivative – inhibits the growth of pneumococci but NOT Viridans strep
o Used to distinguish the two organisms because both are α-hemolytic
Streptococcus pneumoniae: Laboratory Identification
BILE SOLUBILITY
o Autolytic amidase or autolysin that cleaves the peptidoglycan present in pneumococci but NOT in viridans strep
o The amidase is activated by bile and bile salts, β-lactam antibiotics and a stationary phase resulting in LYSIS of the organism
o Amidase cleaves the peptidoglycan which results in the lysis of the organism. This lysis is responsible for the clear finding observed when bile salt is added
Two Methods:
1) Agar plate culture
2) Broth culture method
Streptococcus pneumoniae: Laboratory Identification
BILE SOLUBILITY - Two Methods:
1) Agar plate culture
Requirements:
o Young α-hemolytic colonies on blood agar
o 2% sodium deoxycholate solution
A loopful or a small drop 2% bile solution is placed over selected α-hemolytic colonies. The plate is incubated at 37˚C for 30 mins.
Result: A positive test is indicated by the disappearance of the colonies treated with 2% bile solution leaving behind areas of α-hemolysis
2) Broth culture method
Requirements:
o Young α-hemolytic colonies
o Serum/digest/infusion broth
o 10% sodium deoxycholate solution
Few identical alpha hemolytic colonies are picked up and inoculated into suitable broth. Two sets of broth are taken; one marked test and the other marked control. The tubes are incubated at 37˚C for a duration of18-24hrs.
0.5mL of 10% bile solution is added to the “test” tube and incubated at 37˚C for 15 minutes.
Result: A positive test is indicated by the clearance in the “test” tube but not in the “control” tube
Streptococcus pneumoniae: Laboratory Identification
QUELLUNG REACTION
**pneumonia quellung reaction. A positive reaction shows clumping because of cross-linking of the antibodies and pneumococci. The halo effect is apparent capsular swelling. A negative control would show no clumping or capsular swelling
o Pneumococci + Polyvalent antiserum = capsule swelling
o Most useful and rapid method for identification of pneumococci in sputum, CSF, exudates
o Polyvalent antiserum of “omniserum” contains antibodies for all types of Pneumococcus
Streptococcus pneumoniae: Pathogenesis
Types of Pneumococci:
o Adults: Types 1-8 –75% of Pneumonia cases
o Children: Types 6, 14, 19, 23
Streptococcus pneumoniae: Determinants of pathogenicity
o POLYSACCHARIDE CAPSULE
91 types; Pneumococcus damages host tissue as long as it is outside the phagocytic cell
Capsule is antiphagocytic
Antigenic
o PILI
Enable the attachment of encapsulated pneumococci to the epithelial cells of the upper respiratory tract
o NEURAMINIDASE
Disrupts cell membranes; contributes to invasiveness
o PROTEASES
Immunoglobulin degrading extracellular proteases
Eliminate IgA, IgG, IgM
o CHOLINE BINDING PROTEIN A
Major adhesin allowing the pneumococcus to attach to carbohydrates on epithelial cells of the human nasopharynx
o PNEUMOLYSIN O TOXIN (more important)
Hemolysin
Inhibits chemotaxis of PMN
Toxic effect on respiratory epithelium producing ciliary slowing and epithelial disruption.
Produces paralysis of mucociliary tract that’s why its hard to expectorate
This toxin stimulates production of pro-inflammatory cytokines
Streptococcus pneumoniae:
Predisposing Factors
o Viral/other respiratory infections: damages respiratory epithelium
o Accumulated secretions: protection from phagocytosis
o Bronchial obstruction
o Irritants that disturb mucociliary action (ex: smoking)
o Alcohol, drugs, anesthesia, morphine: depresses phagocytosis and the cough reflex, and facilitates aspiration of the bacteria
o Pulmonary congestion (the fluid in the lungs is a good medium for pneumococci), CHF, prolonged bed rest (you cannot expectorate)
o Malnutrition and immunosuppression
Pathology
o Outpouring of edema fluid in the alveoli which facilitates microbial multiplication & spread to other alveoli
o PMN’s & RBC’s accumulate in the alveoli => consolidation
o Pneumococci reaches the bloodstream via the lymphatics, if it reaches the brain => meningitis
o Later: phagocytes take up & digest the pneumococci
Streptococcus pneumoniae: Clinical Findings
- PNEUMOCOCCAL PNEUMONIA/ACUTE BACTERIAL PNEUMONIA
- OTITIS MEDIA
- MENINGITIS
- BACTEREMIA/SEPSIS
Streptococcus pneumoniae: Clinical Findings
PNEUMOCOCCAL PNEUMONIA/ACUTE BACTERIAL PNEUMONIA
Rarely a primary infection, usually there is a prior infection by a virus
Results only when the normal defense barriers of the respiratory tract are breached
Pneumonia is frequently preceded by an upper or middle respiratory viral infection, which predisposes to S. pneumoniae infection of the pulmonary parenchyma
A leading cause of death, especially in older adults and those whose resistance is impaired
Acute fever, chills, severe pleuritic pain
Cough with “rusty” mucopurulent sputum – (compared to “currant jelly sputum,” which is Klebsiella pneumonia)
Complications:
Pleural effusion that can lead to empyema (pus in the pleural space)
Meningitis, pericarditis, endocarditis
Streptococcus pneumoniae: Clinical Findings
OTITIS MEDIA
The most common bacterial infection of children
Streptococcus pneumoniae: Clinical Findings
MENINGITIS
Pneumococcus is the most common cause of meningitis in adults and of recurrent meningitis in all age groups
Usually preceded by pulmonary infection, URTI, sinusitis, or otitis media
Streptococcus pneumoniae: Clinical Findings
BACTEREMIA/SEPSIS
BACTEREMIA/SEPSIS
Common in individuals who are functionally or anatomically asplenic (sickle cell disease) or immunocompromised
Streptococcus pneumoniae: Diagnostic Laboratory Test
Specimens o Nasopharyngeal swab o Pus o Spinal fluid o Blood - Blood should be drawn for culture before antibiotics are given o Sputum From the lungs, not saliva Gram Stain: (+) lancet-shaped diplococci – a presumptive diagnosis of pneumococcal pneumonia can be made Culture: brain heart infusion (BHI), Trypticase Soy agar and broth with 5% blood, BAP α-hemolytic colonies on BAP Bile-soluble Optochin sensitive (+) Quellung reaction
Streptococcus pneumoniae: Diagnostic Treatment
o Penicillin G
o Cephalosporin (Cefotaxime; Ceftriaxone)
o Erythromycin
o Chloramphenicol for meningitis (not used in the US because it can cross BBB even without inflammation)
* Note that penicillin can penetrate the blood brain barrier only when there is an inflammation
Streptococcus pneumoniae: Diagnostic Control
o Vaccines provide 90% protection
o Pneumococcal polysaccharide vaccine: PPV immunizes against 23 serotypes (adults); does not work for children, you have to conjugate it to a protein
o Pneumococcal conjugate vaccine 13: PCV 13 – effective in infants and toddlers between 6 weeks to 5 years
Staphylococci are relatively resistant to drying, heat (they withstand 50°C for 30 minutes), and 9% sodium chloride but are readily inhibited by certain chemicals (eg, 3% hexachlorophene).
Resistance to nafcillin (and to methicillin and oxacillin)
is independent of β-lactamase production. Resistance to nafcillin is encoded and regulated by a sequence of genes found in a region of the chromosome called the staphylococcal cassette chromosome mec (SCCmec). Specifically, the mecA gene on this locus encodes a low-affinity penicillin binding protein (PBP2a) that is responsible for the resistance. There are 12 different SCCmec types. Types I, II, and III are associated with hospital-acquired infections and may contain genes that encode resistance to other antimicrobials as well. SCCmec type IV has principally been found in community-acquired methicillin-resistant S aureus (CA-MRSA)
α-Hemolysin is a heterogeneous protein that acts on a broad spectrum of eukaryotic cell membranes. The β-toxin degrades sphingomyelin and therefore is toxic for many kinds of cells, including human red blood cells. The δ-toxin is heterogeneous and dissociates into subunits in nonionic detergents. It disrupts biologic membranes and may have a role in S aureus diarrheal diseases. The γ hemolysin is a leukocidin that lyses white blood cells and is composed of two proteins designated S and F.
Panton-Valentine Leukocidin has two components, and unlike the chromosomally encoded hemolysins, PVL is encoded on a mobile phage. It can kill white blood cells of humans and rabbits. The two components designated as S and F act synergistically on the white blood cell membrane as described for γ toxin. This toxin is an important virulence factor in CA-MRSA infections. Both groups of hemolysins are regulated by agr
Staphylococci exfoliative toxin
These epidermolytic toxins of S aureus are two distinct proteins of the same molecular weight. Exfoliative toxin A is encoded by eta located on a phage and is heat stable (resists boiling for 20 minutes).
Exfoliative toxin B is plasmid mediated and heat labile. These epidermolytic toxins yield the generalized desquamation of the staphylococcal scalded skin syndrome by dissolving the mucopolysaccharide matrix of the epidermis. The toxins are superantigens.
Staphylococci Toxic Shock Syndrome Toxin
Most S aureus strains isolated from patients with toxic shock syndrome produce a toxin called toxic shock syndrome toxin-1 (TSST-1), which is the same as enterotoxin F. TSST-1 is the prototypical superantigen
TSST-1 binds to major histocompatibility class (MHC) class II molecules, yielding T-cell stimulation, which promotes the protean manifestations of the toxic shock syndrome. The toxin is associated with fever, shock, and multisystem involvement, including a desquamative skin rash. The gene for TSST-1 is found in about 20% of S aureus isolates, including MRSA.
Staphylococci - Regulation of Virulence Determinants
The expression of staphylococcal virulence determinants is regulated by several systems that sense and respond to environmental signals. The first of these systems consists of two proteins (two-component systems), an example of which is accessory gene regulator (agr). The other two systems consist of DNA-binding proteins (eg, Sar proteins) and small regulatory RNAs, respectively (eg, RNAIII).
Binding of sensors to specific extracellular ligands or to a receptor results in a phosphorylation cascade that leads to binding of the regulator to specific DNA sequences. This ultimately leads to activation of transcription-regulating functions. There are several well-described two-component regulatory systems in S aureus. These include agr, the best described, sae RS, srrAB, arlSR, and lytRS.
Staphylococci - Regulation of Virulence Determinants
The accessory gene regulator (agr) is essential in quorum-sensing control of gene expression. It controls the preferential expression of surface adhesins (protein A, coagulase, and fibronectin binding protein) and production of exoproteins (toxins such as TSST-1) depending upon the growth phase (and hence bacterial density).
At low cell density, the promoter P2 is off, and transcriptions of transmembrane protein, AgrB, peptide precursor, AgrD, transmembrane sensor, AgrC, and transcription regulator, Agr A, are at low levels. As cell density increases during stationary growth phase, the AgrC sensor activates the regulator AgrA. AgrA is a DNA-binding protein that activates promoter P2 and promoter P3. Promoter P3 initiates transcription of δ-hemolysin and an effector called RNAIII, which downregulates the expression of surface adhesins and activates secretion of exoproteins at both the transcriptional and translational levels. Agr is also positively controlled by a DNA-binding protein called SarA (encoded by sar)
Staphylococci - Regulation of Virulence Determinants
At least four additional two-component regulatory systems have been shown to affect virulence gene expression. These are called sae, S aureus exoproteins; srrAB, staphylococcal respiratory response; arlS, autolysis-related locus sensor; and lytRS
Sae regulates gene expression at the transcriptional level and is essential for production of α-toxin, β-hemolysins, and coagulase. Its activity is independent from that of agr. SsrAB is important for regulation of virulence factor expression that is influenced by environmental oxygen. The arlSR locus is important to the control of autolysis and decreases the activation of the agr locus. The lytRS locus is also involved in autolysis.
Streptococci - Group-Specific Substance (Lancefield Classification)
This carbohydrate is contained in the cell wall of many streptococci and forms the basis of serologic grouping into Lancefield groups A – H and K – U. The serologic specificity of the group-specific carbohydrate is determined by an amino sugar.
For group A streptococci, this is rhamnose-N-acetylglucosamine; for group B, it is rhamnose-glucosamine polysaccharide; for group C, it is rhamnose- N -acetylgalactosamine; for group D, it is glycerol teichoic acid containing d-alanine and glucose; and for group F, it is glucopyranosyl-N-acetylgalactosamine.
