Gram Positive: Staphylococci

Question 1

Major identifying features of Staphylococci.

Answer 1

• Gram positive cocci that are arranged in clusters (as opposed to strips of bundles as in Streptococci)
• catalase positive (Streptococci are negative)
• facultative anaerobes
• most strains are penicillin resistant because of penicillinase
• these are largely normal flora and are essentially found everywhere

Question 2

What are the three major pathogenic species of Staphylococci?

Answer 2

• Staph aureus, Staph epidermidis, and Staph saprophyticus

- (Staph aureus is the only coagulase positive species of the three)

Question 3

Staphylococcus aureus

Answer 3

• beta-hemolytic, coagulase positive, gives off a golden pigment on sheep blood agar
• exotoxin release causes: gastroenteritis (food poisoning, enterotoxin), toxic shock syndrome (TSST-1), scalded skin syndrome (exfoliatin)
• direct organ invasion causes: acute bacterial endocarditis, osteomyelitis, septic arthritis, pneumonia (rare but very destructive), meningitis, sepsis, UTI
• virulence factors: protein A, coagulase, hemolysins, penicillinase, hyaluronidase, staphylokinase, exotoxins (exfoliatin, enterotoxin, TSST-1)

Question 4

What is food poisoning?

Answer 4

• the ingestion of pre-formed exotoxins (enterotoxins) resulting in diarrhea
• episodes last less than 24 hours
• (vs. an actual infection with a pathogen)
• main culprits are Staph aureus and Bacillus cereus

Question 5

What is toxic shock syndrome?

Answer 5

• caused by the TSST-1 (toxic shock syndrome toxin-1) exotoxin of Staph aureus
• the toxin stimulates TNF and IL-1, inducing a large systemic cytokine response: acute onset of fever, nausea, vomiting, diarrhea, diffuse rash
• associated with septic shock

Question 6

What is acute endocarditis?

Answer 6

• the rapid destruction of HEALTHY heart valves via invasion with Staph aureus
• associated with IV drug use; the vegetations on the valves grow rapidly and are prone to embolization
• (compare to subacute bacterial endocarditis that involves damaged valves via Strep viridans and group D strep)

Question 7

What is MRSA?

Answer 7

• Methicillin-resistant Staph aureus
• these strains are resistant to the penicillinase-resistant penicillins
• treat with vancomycin (but we now have VRSA thanks to VRE)
• associated with hospitals
• CA-MRSA is community acquired MRSA and is becoming more and more common

Question 8

Staphylococcus epidermidis

Answer 8

• (coagulase negative)
• a frequent, harmless skin contaminant of blood cultures
• pathogenisis associated with long term catheters and IVs; the most common organism related to infections of prosthetic devices (because they form biofilms)

Question 9

Staphylococcus saprophyticus

Answer 9

• (coagulase negative)
• the 2nd most common cause of UTIs in the community (especially in sexually active females); the number 1 most common cause is E. coli