Staphylococci

Question 1

What is the classification of the staphylococci?

Answer 1

Gram-positive, non–spore-forming cocci

Question 2

What are the clinically significant staphylococci?

Answer 2

• S. aureus
• S. epidermidis
• S. saprophyticus

Question 3

What is the appearance of staphylococci, microscopically and macroscopically (including the results of different biochemical tests)?

Answer 3

• 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

Question 4

Why are S. aureus colonies yellow–gold?

Answer 4

They produce a carotenoid endopigment

Question 5

What are the characteristics of the staphylococcal cell wall?

Answer 5

• 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

Question 6

What is the role of peptidoglycan in staphylococcal cell walls?

Answer 6

Has endotoxin-like activity, stimulating:

• production of endogenous pyrogens,
• activation of complement,
• production of IL-1, and
• aggregation of neutrophils

Question 7

What is the significance of teichoic acids in staphylococcal cell walls?

Answer 7

They are normally poorly immunogenic but stimulate antibody responses when bound to peptidoglycan

Question 8

How do adhesion proteins in staphylococcal cell walls function?

Answer 8

Clumping factor proteins A and B bind fibrinogen and convert it to insoluble fibrin, causing the staphylococci to clump/aggregate (they have fibrinogen activity)

Question 9

What are the adhesion proteins of staphylococci called?

Answer 9

Microbial surface components recognizing adhesive matrix molecules (MSCRAMM)

Question 10

What is the role of the extracellular capsule in staphylococci?

Answer 10

It protects bacteria by inhibiting phagocytosis of the organisms by neutrophils

Question 11

Where is S. aureus, and other coagulase-negative staphylococci, found as part of the normal flora?

Answer 11

• Skin
• Nares (nostrils)
• Oropharynx
• Gastrointestinal tract
• Genitourinary tract

Question 12

Where in the airways is transient or persistent S. aureus carriage more common?

Answer 12

Anterior nasopharynx (in 20–50% of humans)

Question 13

How are staphylococci transmitted?

Answer 13

• Direct contact
• Contact with fomites
• S. aureus also infects cattle and can be transmitted that way

Question 14

Which groups have a higher prevalence of S. aureus in the nasopharynx?

Answer 14

• Hospitalized patients
• Medical personnel
• Persons with eczematous skin diseases
• Those who regularly use needles (illicitly or for medical reasons)

Question 15

Which staphylococci were considered an epidemiological threat?

Answer 15

Strains of methicillin-resistant S. aureus (MRSA) and VRSA (vancomycin-resistant)

Question 16

What is the history of MRSA?

Answer 16

• 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

Question 17

What are the toxin-mediated diseases caused by S. aureus?

Answer 17

• Scalded skin syndrome
• Food poisoning
• Toxic shock

Question 18

What are the suppurative infections caused by S. aureus?

Answer 18

• Impetigo
• Folliculitis
• Furuncles (boils)
• Carbuncles
• Abscesses formed anywhere in the body

Question 19

What is scalded skin syndrome?

Answer 19

Disseminated desquamation of epithelium in infants; blisters with no organisms or leukocytes, mediated by S. aureus toxins

Question 20

What is impetigo?

Answer 20

Localized cutaneous infection characterized by pus-filled vesicles on an erythematous (red) base

Question 21

What are the toxins produced by staphylococci?

Answer 21

• Enterotoxins
• TSST-1
• α-Toxin

Question 22

When in the growth cycle are staphylococcal surface proteins expressed?

Answer 22

The exponential-growth phase

Question 23

When in the growth cycle are staphylococcal toxins expressed?

Answer 23

The stationary phase

Question 24

What is folliculitis?

Answer 24

Impetigo involving hair follicles

Question 25

What are furuncles (boils)

Answer 25

Large, painful, pus-filled cutaneous nodules

Question 26

What are carbuncles?

Answer 26

The product of the coalescence of furuncles with extension into subcutaneous tissues and evidence of systemic disease (fever, chills, bacteremia)

Question 27

What are the features of staphylococcal food poisoning?

Answer 27

• 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

Question 28

How is staphylococcal food poisoning transmitted?

Answer 28

By contamination of the food by a human carrier

Question 29

Can staphylococcal food poisoning be prevented by heating contaminated food?

Answer 29

No, as the toxins are heat stable

Question 30

What are the diseases caused by coagulase-negative staphylococci?

Answer 30

• 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

Question 31

Why are coagulase-negative staphylococci particularly amenable to causing catheter and shunt infections?

Answer 31

They have a polysaccharide slime layer that bonds them to catheters and shunts and protects them from antibiotics and inflammatory cells