Staphylococcus: Morphology, Cultural Characteristics, Pathogenicity, Antibiotic Sensitivity

Staphylococci are gram positive cocci. These are common organism found in the environment. They are present on the skin and in the anterior nostrils as commensals. Important pathogenic species are:

• Staphylococcus aureus
• Staphylococcus epidermidis
• Staphylococcus saprophyticus

MORPHOLOGY

They are Gram-positive cocci, 0.5-1 µm in diameter, arranged in irregular clusters, singly or in pairs.

CULTURAL CHARACTERISTICS

They are facultative anaerobes but grow best in aerobic environment at 35ᵒC – 37ᵒC on blood agar and mannitol salt agar as a selective medium. This selective medium is specially used in cases of food poisoning by staphylococcus. S. aureus colonies are about 1-2 mm in size and yellow to golden in color. A zone of complete hemolysis can usually be seen when cultured on blood agar.  S. epidermidis colonies are white and usually do not produce hemolysis.

ENZYMES OF STAPHYLOCOCCUS AUREUS

1. Catalase: converts H₂O₂ to H₂O and O₂.
2. Coagulase: converts fibrinogen to fibrin.
3. DNAse: Splits deoxyribonucleic acid (DNA).
4. Phosphatase: breaks phosphates.
5. Lipase: breaks fats.
6. Hyaluronidase: splits hyaluronic acid.
7. Staphylokinase: causes fibtrinolysis.
8. β-lactamase: breaks down the penicillin by attacking its structural ring.

TOXINS OF STAPHYLOCOCCUS AUREUS

1. Haemolysins, α, β, γ, θ.
2. Toxic shock syndrome toxin (TSS).
3. Exfoliative toxin causes peeling of skin and scalded skin syndrome.
4. Leucocidin (Panton-Valentine [P-V] substance) kills white blood cells.
5. Enterotoxin (A-F) causes food Poisoning.

PATHOGENICITY

The pathogenic species is S. aureus. It caused:

1. Boils, abscesses furuncles and carbuncles
2. Wound infection
3. Hospital Infections
4. Conjunctivitis
5. Pneumonia, osteomyelitis, meningitis, endocarditis
6. Food Poisoning
7. Scalded Skin Syndrome in children
8. Toxic Shock Syndrome

S. epidermidis is a normal commensal may cause endocarditis especially in prosthetic valves, ventricular shunts and in drug addicts. This organism is also an important cause for intravascular catheter associated blood stream and other infections particularly in immunocompromised patients. S. saprophyticus causes urinary tract infections in females. Both are coagulase and DNAse negative and can be differentiated by putting up the antimicrobial sensitivity disk of Novobiocin or colistin (Polymyxin). S. saprophyticus is resistant to novobiocin and susceptible to Novobiocin and resistant to colistin.

Biochemical Reactions of Staphylococcus aureus:

ANTIBIOTIC SENSITIVITY

Antibiotic disks employed in sensitivity testing of staphylocacci are penicillin (>90% of Staphylococci are penicillin resistant). Oxacillin, Erythromycin, Tetracycline, Ceephalosporins (1st generation), Linomycin, Clindamycin, Fusidic acid, Vancomycin, Teicoplanin, Gentamicin, Amikacin, Quinolones, Rifampicin, The susceptibility aganist   colxacillin, instead the disk of oxacillin (1μg) is used. If Staphylococcus aureus is resistant to oxacillin it is labelled as methicilin resistant and known as MRSA (Methicillin resistant Staphylococcus aureus). MRSA shows multi-resistant to antibiotics and is invariably resistant to other B-lactam antibiotics (cephalosporins, Imipenem etc.). They are very important hospital pathogens and are extremely difficult to eradicate.