Staphylococci Flashcards
What is the classification of the staphylococci?
Gram-positive, non–spore-forming cocci
What are the clinically significant staphylococci?
- S. aureus
- S. epidermidis
- S. saprophyticus
What is the appearance of staphylococci, microscopically and macroscopically (including the results of different biochemical tests)?
- Resemble a cluster of grapes
- S. aureus colonies have a yellow or gold color
- S. epidermidis colonies have a gray–white appearance on first isolation
- S. aureus colonies are coagulase-positive
- Are all catalase-positive
- S. aureus produces various amonts of hemolysis
Why are S. aureus colonies yellow–gold?
They produce a carotenoid endopigment
What are the characteristics of the staphylococcal cell wall?
- Gram-positive
- Contains peptidoglycan
- Contains teichoic acids
- Contains adhesion proteins (MSCRAMM), e.g. staphylococcal protein A, clumping factor proteins A and B
- Is covered with a polysaccharide capsule
What is the role of peptidoglycan in staphylococcal cell walls?
Has endotoxin-like activity, stimulating:
- production of endogenous pyrogens,
- activation of complement,
- production of IL-1, and
- aggregation of neutrophils
What is the significance of teichoic acids in staphylococcal cell walls?
They are normally poorly immunogenic but stimulate antibody responses when bound to peptidoglycan
How do adhesion proteins in staphylococcal cell walls function?
Clumping factor proteins A and B bind fibrinogen and convert it to insoluble fibrin, causing the staphylococci to clump/aggregate (they have fibrinogen activity)
What are the adhesion proteins of staphylococci called?
Microbial surface components recognizing adhesive matrix molecules (MSCRAMM)
What is the role of the extracellular capsule in staphylococci?
It protects bacteria by inhibiting phagocytosis of the organisms by neutrophils
Where is S. aureus, and other coagulase-negative staphylococci, found as part of the normal flora?
- Skin
- Nares (nostrils)
- Oropharynx
- Gastrointestinal tract
- Genitourinary tract
Where in the airways is transient or persistent S. aureus carriage more common?
Anterior nasopharynx (in 20–50% of humans)
How are staphylococci transmitted?
- Direct contact
- Contact with fomites
- S. aureus also infects cattle and can be transmitted that way
Which groups have a higher prevalence of S. aureus in the nasopharynx?
- Hospitalized patients
- Medical personnel
- Persons with eczematous skin diseases
- Those who regularly use needles (illicitly or for medical reasons)
Which staphylococci were considered an epidemiological threat?
Strains of methicillin-resistant S. aureus (MRSA) and VRSA (vancomycin-resistant)
What is the history of MRSA?
- Strains began spreading in the 1980s, mostly in susceptible hospitalized patients
- In 2003, new strains began spreading outside hospitals (community acquired), causing cutaneous infections and pneumonia
What are the toxin-mediated diseases caused by S. aureus?
- Scalded skin syndrome
- Food poisoning
- Toxic shock
What are the suppurative infections caused by S. aureus?
- Impetigo
- Folliculitis
- Furuncles (boils)
- Carbuncles
- Abscesses formed anywhere in the body
What is scalded skin syndrome?
Disseminated desquamation of epithelium in infants; blisters with no organisms or leukocytes, mediated by S. aureus toxins
What is impetigo?
Localized cutaneous infection characterized by pus-filled vesicles on an erythematous (red) base
What are the toxins produced by staphylococci?
- Enterotoxins
- TSST-1
- α-Toxin
When in the growth cycle are staphylococcal surface proteins expressed?
The exponential-growth phase
When in the growth cycle are staphylococcal toxins expressed?
The stationary phase
What is folliculitis?
Impetigo involving hair follicles
What are furuncles (boils)
Large, painful, pus-filled cutaneous nodules
What are carbuncles?
The product of the coalescence of furuncles with extension into subcutaneous tissues and evidence of systemic disease (fever, chills, bacteremia)
What are the features of staphylococcal food poisoning?
- It is an intoxication, not an infection, as it is mediated by toxins
- Onset of the disease is abrupt and rapid, with an incubation period of about 4 hours
- Symptoms last less than 24 hours
- Symptoms include severe vomiting, diarrhea, and abdominal pain or nausea
How is staphylococcal food poisoning transmitted?
By contamination of the food by a human carrier
Can staphylococcal food poisoning be prevented by heating contaminated food?
No, as the toxins are heat stable
What are the diseases caused by coagulase-negative staphylococci?
- Wound infections, characterized by erythema and pus at the site of a traumatic or surgical wound
- UTIs, especially associated with urinary catheters, causing dysuria and pyuria
- Catheter and shunt infections
- Prosthetic device infections, e.g. endocarditis of artificial valves
Why are coagulase-negative staphylococci particularly amenable to causing catheter and shunt infections?
They have a polysaccharide slime layer that bonds them to catheters and shunts and protects them from antibiotics and inflammatory cells
