Staphylococcus aureus

Staphylococcus aureus

Staphylococcus: are cluster forming Gram positive cocci.

The main species of medical importance is: Staphylococcus aureus. S.epidermidis and S.saprophyicus.

Habitat: Environment. Part normal microbial flora of the skin, upper respiratory tract and intestinal tract. S. aureus is carried in the nose of 40% or more of healthy people.

Pathogenicity

S. aureus causes boils, styes, pustules, impetigo, infections of wounds (cross-infections), ulcers and burns, osteomyelitis, mastitis, septicaemia, meningitis, pneumonia and pleural empyema.

Also, toxic food-poisoning (rapid onset, no fever), toxic shock syndrome and toxic skin exfoliation.

Extracellular enzymes and toxins produced by strains of S. aureus that contribute to its invasiveness and pathogenicity

● Coagulase: Clots plasma, interferes with phagocytosis, facilitates spread in the tissues.

● Haemolysins: Lyze red cells.

● Leukocidin: Kills leucocytes.

● Fibrinolysin: Digests fibrin.

● Lipase: Breaks down fat.

● Hyaluronidase: Facilitates spread in tissues by destroying hyaluronic acid (component of connective tissue).

● Protein A: Antiphagocytic (prevents complement activa-tion).

● Enterotoxins (heat stable): Food-poisoning(particularly vomiting).

● Toxic shock syndrome toxin-1: Shock, rash.

● Epidermolytic toxins A and B: Generalized peeling of the skin.

● Chemotaxis inhibitory protein: Inhibits migration and activation of neutrophils.

LABORATORY FEATURES

Specimens: Pus and swabs from infected sites, sputum, CSF, blood. Faeces, vomit and the remains of food when (foodpoisoning).

Morphology

Staphylococci are Gram positive cocci of uniform size, occurring c in groups but also singly and in pairs. Non-motile and non capsulate.

Culture

Grow well aerobically and in a carbon dioxide (CO₂)enriched atmosphere. Most strains also grow anaerobically, but less well. Can grow at 10–42 ^oC, with an optimum of 35–37 ^oC.

Blood agar, chocolate (heated blood) agar: Produces yellow (golden yellow) to cream or occasionally white 1–2 mm in diameter colonies. Some strains are  betahaemolytic when grown aerobically. Colonies are slightly raised and easily emulsified.

MacConkey agar: Smaller (0.1–0.5 mm) colonies. Most strains are lactose fermenting.

Mannitol salt agar: On Manitol salt agar selective and differential media for S.aureus isolation from faecal and nasal carrier. S.aureus ferment manitol and the colonies surrounded by yellow zones due to acid production.

 S. aureus is able to grow on agar containing 70–100 g/l sodium chloride.

Biochemical tests: S. aureus is:

● Coagulase positive.

● DNA-ase positive.

● Catalase positive.

Several latex agglutination test kits are available to identify S. aureus based on the detection of clumping factor and, or, protein A (latex particles are sensitized with fibrinogen and immunoglobulin G).

Pastorex Staph Plus test

It detects all strains of S. aureus, including up to 95% MRSA strains (reagent contains antibodies to the capsular polysaccharides found in MRSA as well as fibrinogen and protein A).

Dryspot Staphytect Plus

Colonies of S. aureus are emulsified in saline and mixed with the dry reagent. Agglutination of the blue latex particles indicates a positive test.

Commercially available test kits to confirm MRSA

Test kits have become available to detect penicillin binding protein 2 (PBP2) for the rapid detection of MRSA. An example of a PBP2 latex agglutination test is Mastalex MRSA. The test has been shown to be 97% specific and  sensitive for the detection of MRSA. PBP2-based tests are expensive.

Antimicrobial susceptibility

Sensitive to Penicillins* Vancomycin, Macrolides, Cephalosporins and Fusidic acid *Most strains of S. aureus (particularly hospital strains) are resistant to penicillin due to the production of plasmid-coded beta-lactamase.

MRSA (methicillin resistant S. aureus): These strains are resistant to methicillin and related penicillins and are particularly difficult to treat because they are also resistant to most other common antibiotics. Vancomycin is often needed to treat MRSA infections.

Other pathogenic Staphylococcus species

_ Staphylococcus saprophyticus: Causes urinary tract infections in sexually active women.

_ Staphylococcus epidermidis: May cause endocarditis and bacteraemia following infection of cannulae, indwelling catheters, shunts or other appliances positioned in the body. Infections are difficult to treat due to the resistance of S. epidermidis to many antimicrobials.

Microscopically: As. aureus.

Culturally the colonies of S. epidermidis are white and usually non-haemolytic. The colonies of S. saprophyticus may be white or yellow. They are non-haemolytic. Growth may not occur on MacConkey agar. S. saprophyticus and S. epidermidis are coagulase negative.

Biochemical reactions that differentiate S. epidermidis and S. saprophyticus from S. aureus

Test                      S. aureus              S. epidermidis       S. saprophyticus

Coagulase             +                 ––                         –       

DNA-ase              +                 +Weak                  –

Mannitol*             +                 -                            +

Trehalose*           +                 -                            +      

Sucrose*               +                 +                           +

Novobiocin (5µg)S                  S                           R

Notes: *Fermentation tests: __Sugar fermented with acid production (sugar tablet tests are available from Rosco Diagnostics) S _ Susceptible, R _ Resistant