CH. 14: Staphylococci and other GP Bacteria

Disk diffusion assays

Sensitive: zone of inhibition, size determines how sensitive

Resistant: not killed by antibiotic

Staphylococci

• GPC

• catalase - positive

• modified oxidase - negative

• mostly skin flora, few pathogens

• opaque colonies - creamish colors

Micrococci

• G. stain similar to Staphylococci

• catalase - positive

• modified oxidase - positive

• mostly skin flora, not usually pathogenic

• often produce bright yellow colonies, BUT, not always

Coagulase positive Staph species(s)

S. aureus

Coagulase negative Staph species(s)

S. epidermidis

S. saprophyticus

S. lugdunensis

Coagulase testing for enzyme “clumping factor”

coagulase tube testing:

• most accurate, bacteria + plasma in tube, detects bound AND free coagulase

coagulase slide testing:

• detects BOUND only, bacteria + plasma on slide, 10-15 sec results, some false negs

staph latex testing:

• most common, detects bound coagulase AND protein A (clumping protein), 10-15 sec results, very sensitive and specific

Pyrrolidonyl arylamidase (PYR) test

• RAPID test to detect presence of PYR enzyme

  • red color change if present

Clinically revelant Staphylococci

S. aureus

• most clinically significant species

• produce several enterotoxins and exotoxins

• colony morphology: beta-hem. on BAP, buttery to creamy color, convex round smooth colonies

S. aureus skin and wound infections

• folliculitis, furuncles, and carbuncles

  • opp. skin infections caused by injury

  • carb. are multiple furun. that fuse

• abscess

  • necrosis

  • pus-filled

  • can be caused by carb., insect bites, needles, other penetration

• treatment: antibiotics, surgery

S. aureus - Impetigo

• superficial skin infection

  • large pustules

• highly contagious

  • most common in children

  • direct contact spread

• treatment: topical anti. or goes away on own

S. aureus - Scalded Skin Syndrome

• exfoliative dermatitis

  • most common in newborns or immunocompromised

• causes

  • exotoxins that can travel all over body

• blisters to whole body skin loss

• treatment: IV anti.

S. aureus - Toxic Shock Syndrome

• multisystem disease caused by systemic production of TSST-1 toxin

• most common long-term tampon use

  • toxins absorbed thru vaginal walls

• hypotension, fever, chills, vomiting, diarrhea, shock

• treatment: IV anti. and supportive therapy, can be fatal

S. aureus - food poisoning

• GI disturbance

• toxins produced in improperly stored food

  • ingested

  • most common in meats, dairy, eggs, mayo at room temp for long time

• rapid onset (2-8 hrs), but resolves by itself in 24-48 hrs

Staphylococcus epidermidis

• usually normal skin flora, but can cause hospital-acquired infections

• most common in foreign-surface in body

  • catheters, prosthetic valve endocarditis, CSF shunts

• colony morphology: gamma hem. on BAP, light gray, convex smooth colonies

Staphylococcus saprophyticus

• most associated w/ UTIs in sexually active young females

• adheres to vaginal/vulval mucosa

  • rarely causes infection in other mucous membranes

• colony morphology: gamma hem. on BAP, bright white

Methicillin-resistant S. aureus (MRSA)

• resistant to ALL beta-lactam anti.

• produces PBP2a - penicillin binding protein

  • deactivates beta-lactam to make ineffective

• vancomycin thru IV only treatment

Vancomycin-resistant S. aureus (VRSA)

very rare, but dangerous emerging pathogen