Staph aureus

Question 1

Habitat

Answer 1

anterior nares

Question 2

Colonization

Answer 2

axilla, vagina, pharynx

Question 3

Mode of transmission

Answer 3

Traumatic introduction, direct contact with infected person, or inanimate objects

Question 4

Clinical significance

Answer 4

Chronic infections, indwelling devices, skin injuries, immune response defects

Question 5

Virulence factors

Answer 5

• Enterotoxins that are heat stable @ 100C for 30 mins
• A,B,D cause food poisoning
• F causes TSS (Toxic Shock Syndrome Toxin 1)
• Exfoliatin (Epidermolytic toxin) causes Scalded Skin Syndrome or Ritter’s Disease
• Cytolytic Toxins
• Alpha-hemolysin: lyses rbcs, damages plts, causes severe tissue damage
• B-hemolysin: acts on sphingomyelin in the plasma membrane of rbcs (CAMP Test)
• Gamma: only found in Panton-Valentine Leukocidin, PVL is lethal to PMN’s
• Hyaluronidase: hydrolyzes hyaluronic acid in connective tissue allowing spread of infection throughout the body
• Coagulase: virulence marker (converts fibrinogen to fibrin), two types, bound and free
• Lipase: allows colonization of the skin
• Penicillinase: confers resistance
• Protein A: in cell wall, ability to bind to the Fc part of IgG, this neutralizes IgG and will block phagocytosis

Question 6

Clinical Infections

Answer 6

Impetigo, surgical wound infections, carbuncles, furuncles/boils, food poisoning (symptoms occur 2-8 hrs after eating), Scalded Skin Syndrome/Ritter’s Disease, TTS-1, respiratory (uncommon), pneumonia, bacteremia, osteomyelitis/arthritis, endocarditis, pseudomembranous enterocolitis

Question 7

Specimen collection and handling

Answer 7

Samples must be taken from the actual site of collection, prevent delay, prevent drying and minimize growth of contaminating organisms

Question 8

Colony morph

Answer 8

smooth, butyrous, white-yellow, creamy, can have B-hemolysis

Question 9

Biochem results

Answer 9

Catalase: +
Citrate: +
Coagulase: +
Gelatin Hydrolysis: +
Methyl Red: +
Nitrate Reduction: +
OF: +
Urease: +
VP: +

Ferments: DNase, fructose, galactose, glucose, lactose, maltose, mannitol, mannose, ribose, sucrose, trehalose

Question 10

Why is Staph aureus resistant to penicillin?

Answer 10

Beta-lactamase producers breakdown the beta-lactam ring of penicillin, so the antibiotics is deactivated before it acts on bacterial cells.

Question 11

How do you test for methicillin resistance for Staph aureus and Staph epidermidis?

Answer 11

Test for sensitivity to cefoxitin, isolates resistant to this are considered methicillin resistant.

Question 12

What causes methicillin resistance?

Answer 12

the mecA gene

Question 13

What is the gold standard for MRSA testing?

Answer 13

Nucleic acid probe or PCR

Question 14

Antimicrobial Susceptibility - VRSA

Answer 14

Vancomycin combined with rifampin or gentamicin is commonly used to treat staphylococcal infections, there has been an increase in vancomycin resistance