Gram-positive Cocci Flashcards
the reservoirs of Staph aureus
• Normal flora of nares, skin, various mucosal surfaces
the reservoirs of Staph epidermidis
• Normal inhabitant of skin and mucous membranes
the transmission of Staph aureus
- From endogenous flora to normally sterile site
- Person-to-person via fomites, direct contact, respiratory droplets
- Ingestion of food containing toxin
the transmission of Staph epidermidis
- From normal flora to normally sterile site
* Spread person-to-person
Major diseases caused by Staph aureus
Cutaneous infections
Impetigo
o Primarily affects young children
o Flattened red spot → pus filled vesicle that crusts
Cellulitis
o Infection of skin and soft tissues (dermis and subcutaneous fat layers)
o Treat with systemic antibiotics
Folliculitis
o Infection of hair follicles
o Often resolves on own; may need systemic therapy
Furuncle (boil)
o Small, painful infection of skin and soft tissues
o Infection resolves with drainage or incision
Carbuncle
o When furuncles coalesce and extend to deeper tissue
o Usually on back of neck
o May result in bacteremia
Wound infections
o Staph colonizing skin is introduced during surgery or trauma
o Manage infection by draining pus and removing foreign object
o May require antibiotic therapy
Mastitis
o Colonized infant transfers Staph to mother during breastfeeding
o Pain, swelling, redness of breast tissue
Food poisoning
• Toxin-mediated = toxin grows in food
• Heat-stable, not affect appearance or taste of food
• Causes abdominal pain, severe vomiting, diarrhea
• Occurs 2-4 hrs after ingestion, lasts about 24 hrs
Toxic shock syndrome (TSS)
• Mediated by sugerantigen (TSST-1)
• Associated with tampon use or surgical packing
• Abrupt onset: fever, hypotension, diffuse erythematous rash, shock, desquamation of skin
• Multiple organs may be involved
• Fluid resuscitation is critical!
Scalded skin syndrome
• Mediated by toxin
• Abrupt onset of erythema and blisters over entire body, desquamation of top layer of epidermis
• Most common in neonates and young children
• Mortality rate is low
Bacteremia and endocarditis
• 50% of cases = following a surgical procedure or use of intravascular catheter
• With prolonged cases = dissemination to other body sites
• If go to heart → endocarditis
• Mortality rate approaches 50% (80% in MRSA endocarditis)
Pneumonia
• Associated with ventilator use
• Also from aspiration of oral secretions
• Seen in very young, elderly, CF patients COPD, post-influenza
Osteomyelitis
• From contiguous (adjoining) soft tissue infection or hematogenous dissemination to bone
• Staph aureus is main cause
Septic arthritis
• Staph aureus is primary cause
Major diseases caused by Staph epidermidis
leading nosocomial pathogen
Hospital acquired bacteremia
• #1 cause
Endocarditis
• Primarily involves prosthetic valves
• Indolent course of disease
• Need to replace heart valves
Catheter and shunt infection
• Access to blood stream → develop persistent bacteremia → endocarditis may result
Implanted medical device infection
• Prosthetic joints, breast implants, artificial lens
• Biofilms form
• Need to remove device
Surgical site infection
Virulence factors from Staph aureus
- Techoic acid = binds fibronectin and aids in attaching to mucosal surface
- Protein A = binds to Fc portion of IgG → inhibits Ab-mediated clearance by phagocytes
- Capsule = anti-phagocytic
Toxins:
• Cytotoxins: α, β, Δ, γ, and Panton-Valentine (PV) leukocidin
o Damage cell membranes
o Mediate destruction of many cell types
o PV leukocidin seen in most community-acquired MRSA = forms pores = allows Staph aureus to penetrate tissues
• Exfoliative toxins
o Split intracellular bridges in stratum granulosum epidermis → desquamation = scalded skin syndrome
• Enterotoxins
o Heat stable
o Stimulate GI symptoms (vomiting)
o Produced by up to 50% of all Staph aureus
• Toxic shock syndrome toxin-1 (TSST-1)
o Nonspecific activation of 30-40% of T cells and cytokine release
o Superantigen = lots of cytokine release
o Produces leakage or destruction of endothelial cells
Enzymes • Coagulase o Converts fibrinogen to fibrin o Fibrin protects organism from phagocytosis o Best marker of acute virulence • Catalase o Catalyzes breakdown of toxic hydrogen peroxide (accumulates during bacterial metabolism) • Antibiotic resistance genes
Virulence factors from Staph epidermidis
- Hydrophobic synthetic polymers = initial adherence to medical devices
- Some strains produce viscous polysaccharide slime/biofilm
- Provides additional adhesion
- Acts as barrier to antimicrobials and host defense
- Often resistant to beta-lactam antibiotics
Groups at risk for Staph aureus
o Menstruating women = toxic shock syndrome
o Neonates = scalded skin syndrome
o Patients with intravenous catheters or implanted medical devices = bacteremia and endocarditis
o Patients with immunocompromised pulmonary function or preceeding viral infection = pneumonia
o IV drug users = bacteremia and endocarditis
o Patients with neutrophil dysfunction or granulocytopenia = primarily skin and soft tissue infections
o Burn patients or patients with compromised skin integrity (including surgical wounds) = primarily soft tissue infections and bacteremia
o Healthy adults = cryptogenic bacteremia
Groups at risk for Staph epidermidis
o Patients with intravenous catheters
o Patients with prosthetic materials
Gram Stain reactions and lab tests for Staph aureus
Gram-positive cocci
o Clusters
o Catalase-positive
o Facultative
Culture o Coagulase-positive o Facultative anaerobe but prefers O2 Sheep blood agar: • Grows as 3-5 mm smooth white colonies • Non-mucoid (no capsule) • Commonly has golden color • Most colonies are hemolytic Mannitol salt agar • 7.5% salt selects for staphylococci • Staph aureus ferments mannitol → turns media yellow
Gram Stain reactions and lab tests for Staph epidermis
Gram-positive cocci
o Clusters
o Catalase-positive
o Facultative
Culture: o Coagulase-negative Sheep blood agar: • Small, grey to white colonies • Non-hemolytic Mannitol salt agar • Grows but does not ferment mannitol • Facultative anaerobe but prefers O2
Staph aureus prevention techniques
- Hand hygiene
- Chemoprophylaxis to prevent post-op surgical site infections (especially with cardiac and hip replacements)
- Antiseptics applied to skin prior to surgical procedures
- Proper cleaning of wounds
- Women who have had TSS should not use tampons unless have detectable antibodies to TSST-1
- Properly refrigerate foods
- Food handlers with furuncles should not work
- No anti-staphylococcal vaccine available
Staph epidermidis prevention techniques
- Limit long-term catheter use
- Prompt wound treatment
- Chemoprophylaxis to prevent post-op surgical site infections (especially with cardiac and hip replacements)
- Antiseptics applied to skin prior to surgical procedures
- No vaccine yet
Treatment of Staph infections
o Penicillin-sensitive strains (rare) = penicillin
o Oral treatment of penicillin-resistant infections:
1) Methicillin (Nafcillin or Oxycillin) IV or Dicloxacillin
2) Other choices: cephalosporins, carbapenems, combination beta-lactam/beta-lactamase inhibitor durgs
3) Clindamycin if penicillin allergy
o For MRSA strains (Methicillin-resistant) = Vancomycin
• Other choices:
1) For MRSA pneumonia and soft tissue infections (but not bacteremias) = Linezolid
2) For severe infections (but not pneumonia) = Daptomycin
3) For soft tissue infections = clindamycin, trimethoprim-sulfamethoxazole, minocycline
o Vancomycin-resistant strains (VRS) = Linezolid or Daptomycin
Specific treatments:
o Abscesses = incision and drainage; may use antibiotics
o Bacteremia and endocarditis = prolonged IV therapy
o TSS = reapid resuscitation with massive amounts of IV fluids, source control, antibiotic therapy
Staph saprophyticus
o Coagulase negative staph
o Colonizes skin and urethral area
o May result in UTI
Group A Strep reservoirs
skin, upper respiratory tract, may be carried in nasal and pharyngeal mucosa
Streptococci transmission
o Person-to-person via direct contact with secretions or respiratory droplets via coughing or sneezing
Major diseases caused by Group A Strep (Strep pyogenies)
Pharyngitis
• Acute inflammation of pharynx and tonsils
• Result: fever, painful swallowing
• Incubation period of 2-4 days
• Group A strep = most common bacterial cause of pharyngitis in school age children
• Most sore throats NOT due to Strep but from viral infection or mycoplasma
• Clinical features alone only 50-75% accurate in Strep pharyngitis diagnosis
• Must culture throat or do rapid antigen test
• Complication: Scarlet Fever
• Occurs when infecting strain is lysogenized by bacteriophage → production of exotoxin
• Develop diffuse erythematous rash 1-2 days after pharyngitis symptoms
• Strawberry tongue common
Impetigo
• Skin colonization plus trauma (insect bites)
• Pus-filled vesicles rupture and crust over
• Also from Staph aureus alone or in combination with Strep pyogenes
Erysipelas
• Skin infection with spreading of erythema and edema with well-demarcated edges
• Usually in children and older adults
• Commonly seen in legs rather than face
Cellulitis
• Acute inflammation of skin and deeper subcutaneous CT
• Not clearly defined edges of infection
• Caused: Group A Strep, Staph aureus, others
Necrotizing fasciitis
• Infection deep in subcutaneous tissues
• Extensive tissue destruction
• Requires debridement of non-viable tissue
Streptococcal Toxic shock syndrome
• Begins with soft tissue inflammation at site of infection
• Pain fever, chills, non-specific symptoms
• Progresses to shock and multi-organ failure
• Patients are bacteremic; most have necrotizing fasciitis
• Need debridement or amputation
• Associated with streptococcal pyrogenic exotoxin (acts as superantigen)
• High mortality: 30-40%
• Other form = S. aureus
• Most are tampon-related
• Mortality low (<1%)
