Staphylococci

They are virulent organisms associated with various human infections ranging from boils to sepsis.

Morphology

Staphylococci are gram positive cocci arranged in grape-like clusters. Typically non capsulated with the exception of a few strains which have a microcapsule which is a thin layer of polysaccharide.

Numerous, spherical (cocci), Gram-positive, Staphylococcus aureus bacteria.

Classification

Depending on the coagulase test Staphylococci can be differentiated as coagulase positive which is Staphylococcus aureus or coagulase negative which are Staphylococcus epidermidis and Staphylococcus saprophyticus. All staphylococci are catalase positive.

Coagulase test

The enzyme coagulase converts soluble fibrinogen in plasma to insoluble fibrin. The test can be done on a slide or a tube. A smooth suspension of staphylococci is mixed with plasma. Visible clumping on a slide test or clot formation on a tube test is a positive result.

Catalase test

The enzyme catalase breaks down hydrogen peroxide to water and oxygen and helps the bacteria to survive human macrophages. The test is done by mixing 3% H2O2 with a bacterial inoculum (growth from culture). Brisk bubbling is a positive test and lack of bubbling is a negative test.

Human Pathology

S.aureus causes many pyogenic infections like boils, folliculitis, abscess, cellulitis, septic arthritis, osteomyelitis, impetigo,sepsis etc. It also causes toxin mediated diseases like food poisoning (enterotoxin), Toxic shock syndrome (TSST toxin) and scalded skin syndrome (exfoliatin).

Enterotoxin mediated food poisoning

Symptoms such as nausea, vomiting, abdominal cramps and diarrhea start within 30 mins to 8 hours of ingestion of contaminated food. Commonly implicated food items are sliced meats, pastries, sandwiches and puddings. It is due to preformed enterotoxin in contaminated food. Antibiotics do not help in treatment.

Toxic shock syndrome

Presents like a septicemia with fever, shock, hypotension, a desquamative, reddish macular rash and end organ failure. Due to TSST toxin which is a superantigen. Patients have a history of tampon use, recent childbirth or staphylococcal infections like infected burns or wounds.

Scalded skin syndrome

Mostly seen in young children. Characterised by fever, blisters, skin peeling with a positive Nikolsky sign (skin peels off with gentle pressure). Due to toxin exfoliatin.

S.epidermidis causes device or implant associated infections like intravenous catheter associated sepsis, prosthetic heart valve endocarditis, intraperitoneal catheter associated peritonitis, prosthetic joint associated arthritis etc. It is attributed to production of glycocalyx by S. epidermidis.

S.saprophyticus causes urinary tract infections in young sexually active women.

Antibiotics for staphylococci

Beta lactamase producing S.aureus can be treated with oxacillin, nafcillin, cephalosporins, amoxicillin plus clavulanic acid and vancomycin. MRSA are treated with vancomycin, daptomycin and for minor or skin MRSA infections treat with trimethoprim plus sulfamethoxazole or clindamycin. VRSA strains are treated with daptomycin or quinupristin dalfopristin.

S.epidermidis infections are treated with vancomycin.

S.saprophyticus infections are treated with quinolones or trimethoprim-sulfamethoxazole.

Laboratory diagnosis of staphylococcal infections

Important points

1. Always do gram stain and culture of pus from infected sites

2. In toxic shock syndrome blood cultures may be negative but if you culture from the source of infection it will be positive.

3. Bacteriophage typing can be done for epidemiological purposes to trace the source of an outbreak.