Staphylococci

Staphylococcus

Micrococcus

Stomatococcus

• GPC - grape-like clusters

• Part of normal flora of skin & mucous membranes of humans/other warm blooded animals

• Facultative anaerobes

• Beta-hemo / CAT+ / DNAse+

Staphylococcus aureus

• GPC - grape-like clusters

• Part of normal flora of skin & mucous membranes of humans/other warm blooded animals

• Facultative anaerobes

• Non-hemo, white / CAT+ / DNAse+

Coagulase neg staphyloccoci

Catalase test differentiates:

Staph (+) from Strep (-)

Enzyme that breaks down hydrogen peroxide (H₂O₂) into H₂O and O₂

Catalase

False (+) risks of Catalase test (2)

1) Reverse order of steps, using an iron loop

2) Blood from BAP taken up with colony (RBCs contain small amount of catalase)

Coagulase test divides:

Coag ±Staph

Coag (+) - S. aureus

Coag (-) - S. epidermidis, S. saprophyticus, S. lugdunensis

What is the slide coag test?

Mix isolate with saline on slide

Add drop of plasma, mix, read immediately: clumps → positive

What is the free tube coag test?

Emulsify a loopful of isolate in small amount of plasma in a tube

Incubate 35C for 4 hours

Gently tilt tube: clot formation → positive

False positives for slide coag test

Organism autoagglutinates (clumps with saline)

Test not read within 10s (use BAP)

False negatives for tube coag test

S. aureus strains that lyse clot after prolonged incubation

Latex agglutination kit detects what?

Detect clumping factor and protein A

What can give false negative results in latex agglutination?

MRSA

What are the coag negative Staphs?

S. epidermidis

S. saprophyticus

S. lugdunensis

Most commonly isolated CNS?

S. epidermidis

What CNS is associated with UTIs (young women/older men) and Novobiocin resistant?

S. saprophyticus

What CNS is the frequent cause of endocarditis, ferments mannitol, PYR+, and frequently slide coag+?

S. lugdunensis

Colony: Opaque, smooth, beta-hemo

CAT+ / COAG+ / PYR -

Can tolerate high salt concentrations (MSA) → ferments mannitol (yellow colonies)

S. aureus

Most CNS do not ferment mannitol, so the colony colors will be:

Red/pink

S. aureus infections (7)

Skin

Gastrointestinal (via enterotoxin)

Pneumonia

Osteomyelitis

Endocarditis

Toxic Shock Syndrome

Scalded Skin Syndrome (caused by exofoliatin)

What infection is associated with toxin TSST-1 and high absorbency tampon use?

Toxic shock syndrome

What is the most important mechanism for MRSA resistance? What does it code for?

mecA gene - This gene codes for a new penicillin binding protein (PBP2a), with a low affinity for beta-lactam antimicrobics

What test can detect mecA gene?

PCRs

How does VRSA work?

Due to VanA operon that alters vancomycin’s cell wall target

Commensal, free-living GPC

Often arranged in regular packets of 4 or 8 (tetrads)

CAT+ / COAG -

Appear yellow and non-hemo on BAP

Generally contaminants

Micrococcus

What are the differential tests for Micrococcus?

Bacitracin and furazolidone susceptibility (lawn + disc method)

Modified oxidase test (detect cytochrome oxidase, blue = positive)

GPC in clumps or tetrads

CAT - (pseudocatalase) / Capsule +

“Stick Staph” bc it’s gray-white colonies strongly stick to agar

Part of normal flora

Rothia mucilaginosus

CAT +

Modified Oxidase -

Ferments glucose anaerobically +

Bacitracin R

Furazolidone S

Staphylococcus

CAT +

Modified Oxidase +

Ferments glucose anaerobically -

Bacitracin S

Furazolidone R

Micrococcus

CAT -

Modified Oxidase -

Ferments glucose anaerobically +

Bacitracin R

Furazolidone S

Stomatococcus