Lab-12-
StaphylococciStaphylococci are gram positive cocci , arranged in irregular clusters like grapes, they are facultative anaerobic organisms that grow well on most nutrient media. Non motile and non sporulating bacteria . There are three practical clinically important species :
Staph.aureus
Staph. epidermidis
Staph. saprophyticus
Staph. epidermidis : Is the most frequent inhabitant of human surface tissues, including skin mucous membranes, it is not usually pathogenic, but it may cause serious infections, If it has unusual opportunity to enter past suffice barriers for example cardiac surgery patients or those with indwelling intra venous catheters.
Staph. saprophyticus : Has been implicated in a cute urinary tract infections in young women 16 25 years. It has not been found among the normal flora and is not yet known to cause other types of infection.
Staph. aureus : Strains produce a number of toxins and enzyme that can exert harmful effects on the cells of the infected host.
Hemolysin : can destroy red blood cells.
Catalase : Decompose H2O2
Coagulase : Coagulate plasma .
Leukocidin : destroy leucocytes.
Hyaluronidase : act on substracte that is structural component of connecvtive tissue ( break down the tissue & permit staphylococci to penetrate more deeply, hens it's called spreading factor, some strept also produce it.
Cytotoxins : Cell lysis .
Penicillinase : break down blactam drugs .
Protein A : Anti phagocytic .
Staphylokinase : can dissolve fibrin clots.
Lipase : breaks lipds of cell membrane .
DNase : DNA hydrolysis .
Enterotoxin : Elaborated by some strains of Staph. aureus [ can responsible of gastro-enteritis or staphylococcal food poisoning ]
Pathogenicity :
Common skin infections caused by Staph. aureus include : pimples, furncles ( boils ) and impetigo.
Systemic infections ( deep tissue ) infections that result from Staph. aureus also produce infections of the sinuses ( sinusitis ) and middle ear ( otitis media ) .
Rarely causes oral infection, may cause angular chelitis together with Candida at the angles of the mouth.
Isolation and Identification of Staphylococci :
The laboratory diagnosis of Staphylococci is made by identifying the organism ( usually Staph. aureus ) in clinical specimen representing the site of infection :Pus from skin lesion, sputum from pneumonia ( when pneumonia is suspected ) urine, spinal fluid or blood. Special care must be taken, not to contaminate the specimen with normal flora.
The laboratory identification of staphylococci include :
Microscopic morphology.Colonial appearance on blood agar .
For demonstrating hemolytic activity since some streptococci as well as many strains of Staph. aureus are beta hemolytic ( usually but not always ) . While Staph. epidermidis are almost _ always non hemolytic .
On blood agar Staph. aureus usually golden to yellow pigment, Where Staph. epidermidis has a white pigment and Staph. saprophyticus either yellow or white pigment
3- Catalase tests
4-Coagulase test
5-Protein A latex agglutination test
Protein A, synthesized by almost all strains of Staph. aureus , has a special affinity to fragment of immunoglobulin G ( IgG ). Hence when latex practicles coated with IgG are mixed with a suspension of organism visible agglutination of latex particles occurs .6- Reaction to carbohydrate mannitol.
7-Dnase test : Most coagulase positive staphylococcus are produced this enzyme.
Procedure :Inoculate DNase medium with staphylococcus , after incubation at 37Co for 24hrs, flood the plate with 1 NHCL .Result : A clear zone around the positive DNase strain, the remain plate will be opaque because of the precipitation of DNA by HCL.
8-Novobiocin sensitivity test:
Staph. Saprophyticus is distinguished by its resistance to tower concentration of Novo biocin disc test, this test is used to differentiating . two coagulase negative species of staphylococci, the zone of inhibition around Staph. saprophyticus is less than 16 mm.Not : Staph. aurens is a major cause of nosocomial infections, transmited from hospital or the environment.
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