Author Topic: STAPHYLOCOCCI  (Read 3629 times)

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Offline Seeraar

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STAPHYLOCOCCI
« on: November 06, 2009, 12:37:58 AM »
   STAPHYLOCOCCI:-
•   Gram +ve spherical cells arranged in grape-like clusters.
•   Ferment carbohydrates &produce pigments.
•   Some are members of m\m &skin flora of humans.
•   Produce PYOGENIC infections.
•   Pathogenic Staph.haemolizes blood-coagulate plasma &produce extra cellular enzymes &toxins.
•   Enterotoxin produces food poisoning.
•   Develop resistance to antibiotics .
•   30 sps. are known &the 3 of medical importance are Staph. aureus.- Staph.epidermidis &Staph.saprophiticus.
•   Non-motile.
CULTURE:   
•    Best growth on bacteriologic media under aerobic conditions at 37C.Form pigment at room temperature.
•   Produces smooth round colonies.
•   Produces CATALANE which differentiate it from STREP.
•   Ferment carbohydrates &produce lactic acid but no gas.
•   Sesitive to antibiotics- some strains are resistant.



ANTIGENIC STRUCTURE
o   Contain antigenic polysaccharides &proteins.
o   Peptidoglycan-(cell wall) a poly saccharide polymer-is destroyed by strong acids &lysozyme &it is important in pathogensis of infection &production of Interlukin-1 &opsonic anyibodies.
o   Teichoic acid-polymers of glycerol or ribitol phosphate linked to peptidoglycan &can be antigenic.Antiteichoic antibodies could be detected in parients with endocarditis due to Staph.aureus.
o   Protein A- a cell wall component that binds to IgG.

TOXINS &ENZYMES:-
•   Catalane convertsHO2 into H2O &O2 .test differenciate Staph.(+ve) from Strep.(- ve).-
•   COAGULASEP:-enzyme-like protein that clot oxalated or citrated plasma.Coagulase deposit fibrin on the surface of Staph.altering phagocytosis.
•   Hyaluronidase:- spreading factor.
Exotoxins:-
Several lethal toxins to animals on injection cause necrosis of skin(alpha-beta-gama &delta toxins).
-Leukocidin:   can kill white blood cells of many animals.
-Exfoliative toxin: includes 2 proteins that lead to the generalized desquamation of the Staph.scalded skin syndrome.
-Toxic shock syndrome toxin:  patients with toxic shock syndrome produce(TSST-1) which resembles enterotoxinF &pyrogenic exotoxin-C.In humans produces feverand shock.

Enterotoxins:-
Six(A-B-C-D-E&f) soluble toxins:   produced by 50% of Staph.aureus strains.They are heat stable and resist boiling for 30 minutes.Important cause of food poisoning.



PATHOGENISIS:-
   Staph. are members of normal flora of human skin- occurs in nasal passage-found in clothing-bed linens &other fomites.
   Pathogenic capacity depends on extracellular factors &toxins and invasive properties of the strain.
   Food poisoning depends on injestion of enerotoxins.
   Bacteremia & abcesses in all organs.
   Pathogenic Staph.aureus produces coagulase-yellow pigment and is hemolytic.Staph. epidermidis is non-pathogenic andStaph.saprophiticus causes urinary tract infection in young women.

CLINICAL FINDINGS:-
       *Pimple—hair follicle infection or abscess.
        *Direct contamination of wounds-post operative surgery or trauma.
       *Bacteremia-endocarditis-osteomyelitis-meningitis &pulmonary infection.
        *Food poisoning with short incubation period(1-8 hr.)-violent nausea-vomiting-
&diarrhea and rapid convalescence –no fever.


  Toxic Shock Syndrome:
high fever – vomiting – diarrhea – mualgias - scarlatiniform rash - hypotention with cardiac &renal failure in severe cases
 It occurs in young women 5days after menses who use tampons- but also in children or men with Staph. infection.The syndrome is found in vagina-on tampons-in wounds-other localized infection &throat BUT never in blood stream.
DIAGNOSTIC LABORATORY TESTS:-
o   Specimens:-surface swab-pus-blood-tracheal aspirate-spinal fluid for culture-Antibody demonstration in serum is of no value.
o   Smears:-Staph. are seen in stained smears of pus &sputum.
o   CULTURE:-Blood agar –typical colonies in 18hrs.at 37 incubation.Pigments are produced on optimal room temperature.Contaminants are inhibited by 7.5% salt on media.
o   CATALASE TEST:-drop of H2O2+small amount of bacterial growth is placed in solution-O2 is released in bubbles.
o   COAGULATE TEST:-Citrated rabbit or human plasma diluted 1:5 is mixed with equal volume of broth culture or colonies incubated at37C-clot is formed in 1-4hrs if test is +ve.
o   Susceptibility testing.
Serological typing tests:- no practical value.
Epidomiology :-
Staph. are ubiquitous human parasites &human infection chiefly shed infection &also fomites-respiratory tract &skin.
Carriers of antibiotic resistant strains constitute a hazard in hospitals