STAPHLOCOCCI AUREUS

Question 1

CLASSFICATION OF STAPHLOCOCCI AUREUS

Answer 1

gram + cocci, grape like structure, golden yellow, non-sporing, non-motile, non-capsulated

Question 2

BIOCHEM/PHYSIO OF STAPHLOCOCCI AUREUS

Answer 2

Catalase +, blood agar 10% nacl, coagulase +, oxidase -, facultative anerobe.

Question 3

PATHOGENESIS OF STAPHLOCOCCI AUREUS

Answer 3

-most virulent type
- infection caused by significant host compromise:
via:
1. break in skin or insertion of foreign body
2. wound, surgical infection or central venous catheter
3. obstruction of hair follicle
4. immune system compromised

Staphlococci aureus may be:

• infection
• intoxication
• both

Staphlococci aureus virulent factors are:

1) cell wall
- capsule - polypeptide microcapsule with increased resistance to phagocytosis
- protein A - binds to Fc region of IgG and has an antiopsonin effect.
- Fibronectin Binding Protein - promotes binding to mucosal cells and matrices
- clumping - FnBp enhances clumping in plasma.

2) Cytolytic exotoxins - a, b, y, d referred to as hemolysins polymerize into tubes and pierce the membrane.
3) Panton-valentine leukocidin - lyses PMNs

4) Super antigen exotoxins - have an increased affinity for T cell receptor MHCII antigen complexes causing increase T lymph response.
- enterotoxin - ABCDEG, produced by 1/2 of staph aureus isolates. ingestion can lead to food poisoning. stimulates vomitting center in brain. more heat stable
- Toxic SS toxin - causes TSS
- Exfolatin - causes scaled skin in children, cleaves desmoglein 1.

Question 4

PATHOGENICITY OF STAPHLOCOCCI AUREUS

Answer 4

1) Localized skin infection
- small superficial abcess involving sweat, sebcaceous glands and hair follicles.
- subcutaneous abcess - faruncles or boils forming around foreign bodies.
- larger deeper infections carbuncles which lead to bacteremia.

2) Diffuse skin infection
- impetigo - pyoderma which is superficial spreading crusty skin lesion seen in children

3) deeper localized infection such as
- acute or chronic infection of the bone – osteomyelitis
- acute infection of the joint – septic joint

4) other infections are acute endocarditis, septicemia and necrotizing pneumonia.
5) Toxinoses - toxic shock syndrome - characterized by high fever, rash, vomiting, diarrhea, hypotension & multiorgan involvement

staphlococcal gastroenteritis - caused by ingestion of contaminated food.

Scaled skin syndrome - superficial bullae appears due to toxin which causes marked epithelial desquamation.

Question 5

IMMUNITY OF STAPHLOCOCCI AUREUS

Answer 5

does not exibit longlasting immunity as demonstrated by continous susceptibility

Question 6

EPIDEMIOLOGY OF STAPHLOCOCCI AURUES

Answer 6

• carried by healthy individuals on skin & mucous membranes
• carriers can also be a source of infection to themselves and others by contamination of formites such as doorknobs or food.

Question 7

TREATMENT OF STAPHLOCOCCI AUREUS

Answer 7

methicillin susceptible : oxacillin, niacillin

methicillin resistant: vancomycin, trimethoprim, daptomycin

Question 8

PREVENTION OF STAPHLOCOCCI AUREUS

Answer 8

No vaccine or preventable drug available

Question 9

LABORATORY IDENTIFICATION

Answer 9

gram + cocci
grape like clusters
coagulase +
catalase +
mannitol +
oxidase -
yellow - hemolytic
BA 10% Nacl
pus from abcess
sputum from people with pneumonia
faeces/ vomit from people with food poisoning
blood speticemia