Gram+ and Gram- Cocci (Kaul) - 4/26/16 Flashcards
Discuss 4 overall characteristics of Gram+ cocci.
- Important human pathogens
- Adapted to living on/in humans
- Three genera comprise majority: Staphylococcus, Streptococcus, Enterococcus
- Virulence factors (factors that enable an organism to produce disease)
- Adhesins/cell surface factors
- Secreted enzymes/toxins
Describe characteristics of Staphylococci:
- Gram Stain?
- Skin Flora?
- Anaerobe type?
- Heat resistant?
- Growth and appearance on blood agar?
- Catalase positive or negative?
Describe catalase test.
- Gram Stain? Cells in cluster (bunch of grapes)
- Skin Flora? Normal - most abundant on our skin
- Anaerobe type? Facultative (can grow both aerobically and anaerobically)
- Heat resistant? Yes, resistant to heat and drying- persist on fomites; hardy bacteria - can exist on metal instruments, door knobs, beds, etc…
- Grows well on blood agar; golden-yellow colonies
- Catalase positive or negative? Positive (unlike streptococci)
Describe the catalase test:
- Drop H2O2 onto microscope slide. Mix in unknown bacteria (blue circle)… gram+. Mix in with H2O2. If bubbles –> staph cocci… if no bubbles –> strep cocci
- Most common infection in cuts and wounds
Discuss the pathogenic strains of Staphylococci.
S. aureus = major pathogen (coagulase positive)
All other pathogenic strains = coagulase negative Staph (abb. CoNS or CNS); e.g. S. epidermidis, S. saprophyticus
Describe GENERAL CHARACTERISTICS of Staphylococcus aureus.
- “Golden” colonies on agar
- Coagulase positive (coagulase clots plasma)
- Normal flora of anterior nares in 1/3 of people
- Most common human pathogen
- Ferments mannitol (if ferments mantel, turns agar yellow)
Virulence factors:
- Protein A (binds to antibodies on Fc portion) - evades immune system
- Microcapsule
- Adhesins
Describe VIRULENCE FACTORS of Staphylococcus aureus.
S. aureus produces many exotoxins that damage membranes:
S. aureus superantigen toxins stimulate T lymphocytes
Virulence factors:
- Protein A (anti-opsonin effect by binding to antibodies on Fc portion) - evades immune system
- Microcapsule - antiphagocytic polysaccharide “microcapsule”
- Adhesins - facilitate attachment to host cells/connective tissue
Exotoxins:
- alpha-toxin: Hemolysins (punches holes in RBCs)
- beta-toxin: lipase - damages membranes (basis of CAMP test used to ID Strep)
- PVL (leukocidin): (punches holes in WBCs); produced predominantly by CA-MRSA; destroys neutrophils
Invasins (burrow into cell)
- Staphylokinase
- Hyaluronidase
- Lipase
Describe SUPERANTIGEN TOXINS of Staphylococcus aureus.
- Non-specifically crosslink MHC to TCR
- Activates T-cells with differing specificities
- Up to 20% of all T-cells activated!
- Overproduction of cytokines (IL-1, IL-2, TNF)
- Toxic shock syndrome toxin (TSST-1) - IL-1, IL-2, TNF
- Enterotoxins (“food poisoning”) - nausea, vomiting, no fever
- Exfoliatin (“scalded skin syndrome”) - skin peels off
Describe EPIDEMIOLOGY of Staphylococcus aureus.
- Carriage rate for healthy adults = 20-30%
- Very common hospital-associated infection (facilitated by ability to persist on fomites)
- Predisposition to infection include: tissue injury (surgical/battle wounds), pre-existing primary infection, diabetes, immunodeficiency, poor hygiene, nutrition
- Infections can be localized or systemic
Describe CLINICAL MANIFESTATIONS of Staphylococcus aureus.
SSTIs, Infections, Toxinoses
Most common clinical presentation is SSTIs (skin, soft tissue infections):
- Furuncles: small PUS-filled local infections
- Carbuncles: larger skin abscesses
- Impetigo: spreading, crusted skin infection
- Cellulitis: deep skin infection
Infections of other tissues, potentially from metastasis of superficial infections:
- Osteomyelitis: S. aureus = most common cause of bone infections in children
- Septic joint/septic arthritis: especially in children
- Pneumonia: often follows viral flu infections, particularly in hospitalized patients
- Acute endocarditis: frequently associated with IV drug abuse (rapid onset of vegetation on valve –> can embolize to other tissue including lungs or brain)
- Bloodstream infections: Bacteremia and Septicemia
Toxinoses:
- TSS from TSST-1 exotin results in high fever, sunburn-like rash and multi-organ failure
- “Food poisoning” or gastroenteritis from enterotoxins - ingestion of heat stable enterotoxins formed riectly from contaminated food; acute onset of GI distress –> projectile vomiting
- Scalded Skin Syndrome - exfoliatin toxin induced bright red flush, blisters (bullae) –> bullous impetigo –> desquamation of epidermis
Describe ANTIBIOTIC RESISTANCE to Staphylococcus aureus.
1945: Penicillin
1955: Almost all S. aureus resistant to penicillin (due to penicillinase - cleaved beta-lactam ring of penicillin, inactivating it)
Anti-staphylococcal “penicillinase-resistant” beta-lactams developed - methicillin, oxacillin, nafcillin
MRSA (methicillin resistant S. aureus) emerges! SJDFKLSDF :(
Vancomycin - glycopeptide
VISA (Vancomycin Intermediate S. aureus) and VRSA (Vancomycin Resistant S. aureus) - relatively uncommon but growing in importance
Describe ANTIMICROBIAL TREATMENT for Staphylococcus aureus.
Can’t use penicillin - completely resistant
Can use:
- Beta lactamase resistant penicillins (Nafcillin)
- Vancomycin - glycopeptide
- Daptomycin - lipopeptide
- Linezolid - oxazolidone
- Ceftaroline - cephaolosporin with affinity for PBP2a
Describe GENERAL CHARACTERISTICS, VIRULENCE, COMMON CLINICAL MANIFESTATIONS, TREATMENT OPTIONS, and DIAGNOSTICS of Staphylococcus epidermidis.
GENERAL:
- Major component of normal skin flora
- Cause wound infections through broken skin
DIAGNOSTICS:
- Gram stain: Gram positive cocci in clusters
- Catalase positive
- Coagulase negative
VIRULENCE:
- Relatively less virulent
- Produces cell surface polysaccharide “slime” - adheres to bioprosthetic* materials and acts as barrier to antibiotics
COMMON CLINICAL MANIFESTATIONS:
- Frequently involved in nosocomial and opportunistic infections - catheters or medical devices, IV lines
TREATMENT OPTIONS:
- Vancomycin
- Most are HIGHLY RESISTANT to penicillins and oxacillins
Describe GENERAL CHARACTERISTICS, VIRULENCE, COMMON CLINICAL MANIFESTATIONS, TREATMENT OPTIONS, and DIAGNOSTICS of Staphylococcus saprophyticus.
GENERAL:
- Normal vaginal flora (infrequently found on skin)
DIAGNOSTICS:
- Gram stain: Gram positive cocci in clusters
- Catalase positive
- Coagulase negative
- Novobiocin resistant
COMMON CLINICAL MANIFESTATIONS:
- UTI, cystitis in women
TREATMENT OPTIONS:
- Pencillin G
- Distinguished from other CoNS and S. aureus by its natural resistance to novobiocin
Describe characteristics of Streptococcus.
- Gram-positive spherical/ovoid cocci arranged in long chains; commonly in pairs
- ~25 species
- CATALASE NEGATIVE (distinguishing factor from Staph)
- Most parasitic forms are fastidious and require enriched media (not hardy)
- Sensitive to drying and heat (not hardy like staph)
- Aerotolerant anaerobes (facultative anaerobes)
- Natural habitat = mucous membranes (abundant in normal flora of oral cavity, GI, respiratory, and GU tracts)
Classification based on (1) hemolysis pattern on blood agar and (2) cell wall antigen
Characterization technique 1: Compare the three different types of hemolysis.
- Beta-hemolysis = complete erythryocyte destruction (bright yellow halo)
- Alpha-hemolysis = Erythrocytes damaged by peroxide, hemoglobin turns green/brown
- Gamma-hemolysis = No hemolysis
Characterization technique 2: What are Streptococcus Lancefield Groups?
- Serological classification (dependent on antigen)
- Based on antigenic cell wall polysaccharide called C-substance
- Groups A-U: Reaction w/ specific antisera tested in a slide agglutination assay; common human pathogens: Groups A, B, D, and “none” (no Lancefield Group)
*REVIEW
Describe VIRULENCE FACTORS, COMMON CLINICAL MANIFESTATIONS, and EPIDEMIOLOGY of Group A Streptococci (GAS): S. pyogenes
VIRULENCE FACTORS:
- M-protein (~80 types!): essential for infection; highly variable antigenic –> prevents activation of complement by alternate pathway; anti-phagocytic
- Hemolysins: Oxygen-sensitive [Streptolysin O = SLO]; Oxygen-stable [Streptolysin S]
- Streptococcal pyrogenic exotoxins (SPE) A, B, C (some strains)
- Erhtyrogenic toxin: SPE = streptococcal pyrogenic exotoxin, a superantigen toxin –> causes rash of Scarlet fever
- Exoenzymes: Streptokinase - dissolves clots by activating plasmin
COMMON CLINICAL MANIFESTATIONS:
- Streptococcal pharyngitis “strep throat”: purulent inflammation in pharynx; can be associated with scarlet fever (toxin mediated skin rash)
- Streptococcal skin infections: impetigo (crusted skin infection) or erysypelas (redness) –> cellulitis or more severe necrotizing fasciitis
- Streptococcal toxic shock syndrome: superantigen pyrogenic toxin mediated shock and multi-organ failure
Post-infection sequelas of untreated infections - antibody-mediated:
- Acute Rheumatic Fever: antibodies to M protein are thought to cross-react with heart tissue and damage heart valves; (2-3 weeks after pharyngitis)
- Acute glomerulonephritis: injury to kidney (1 week after pharyngitis or skin infection)
EPIDEMIOLOGY:
- Inhabits throat, nasopharynx, occasionally skin in humans
- Transmission - contact, droplets, food
- Spreads easily in crowded environments
- Portal of entry generally skin or pharynx
- Children predominant group affected for cutaneous/throat infections (~30% all bacterial pharyngitis in children due to GAS)
- Systemic infections/progressive sequelae possible if untreated
Describe characteristics of Group B Streptococci: S. agalactiae
- Normal flora of female repro tract
- Leading cause of neonatal sepsis
- Women routinely screened and treated for GBS colonization prior to delivery