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STAPHYLOCOCCUS AUREUS
Morphology :
Gram positive cocci about 1µm in diameter, arranged in irregular clusters. They are non-motile, non-spore forming occasionally capsulate.
Cultural
Characters :
Staphylococci are facultative anaerobes, grow readily on most bacteriologic media. Optimum temperature for growth is 37°C, but form pigment best at room temperature (20-25°C). Staphylococcus aureus forms grey to golden yellow colonies and produces various degree of haemolysis. On mannitol salt agar; the selective medium for Staphylococci, yellow haloes will surround colonies of Staphylococcus aureus due to acid formation. Non-pathogenic species don't ferment mannitol and the colonies will be pink in colour.
Resistance To
Physical And Chemical Agents :
Staphylococci are relatively resistant to dryness, heat (they withstand50°C for 30 minutes) and 10% sodium chloride. They are variably sensitive to many antimicrobial drugs, but are readily killed by certain antiseptics and disinfectants at their correct dilution. 
Biochemical
Reactions :
Staphylococci produce catalase which differentiates them from streptococci. They ferment many carbohydrates with acid production. Staphylococcus aureus is characterized by the production of coagulase which coagulates plasma, while other staphylococci are coagulase negative. Staphylococcus aureus ferments mannitol with acid production as well as several other sugers and liquefy gelatin.
Antigenic
Characters :
Staphylococci contain antigenic polysaccharides (peptidoglycan) and proteins as well as other substances important in cell wall structure. Protein A is a cell wall component of many Staphylococcus aureus strains, it is antiphagocytic and binds non- specifically to Fc portion of IgG molecule, leaving specific Fab sites free. The subsequent reaction of Fab with homologus (test bacteria) antigen is visualized by clumping of staphylococci. This is the basis of coagglutination method. Most Staphylococcus aureus strains have coagulase or clumping factor on the cell wall surface, which binds to fibrinogen yielding aggregation of the bacteria. Some Staphylococcus aureus strains have capsules, which inhibit phagocytosis.
Pathogenesis :
The pathogenic capacity of a given strain of Staphylococcus aureus is the combined effect of its (A) Structural virulence factors (peptidoglycan layer, protein A and capsule, if present). (B) Production of extracellular factors (toxins and enzymes).
I. Toxins :
1. Haemolysins : An alpha toxin (haemolysin), lyses erythrocytes and damages platelets and vascular smooth muscles. It is dermonecrotic and lethal. Beta, gamma and delta haemolysins, all lyse human (and other species) erythrocytes, but their role in pathogenicity is less well defined. 2. Leucocidins : Cause loss of motility and destruction of leucocytes. 3. Enterotoxins : (A-F) Are important causes of food poisoning, produced when Staphylococcus aureus grows in carbohydrate and protein foods. They are heat stable (resist boiling for 30 minutes) and are resistant to the action of gut enzymes. 4. Exfoliative Toxin : It causes generalized desquamation of the skin. 5. Toxic Shock Syndrome Toxin-1 : (TSST-1) Produced by certain strains. It is the same as enterotoxin F.
II. Enzymes :
1. Coagulase : It is an exo-enzyme, which may be cell bound acting directly on fibrinogen or may be free requiring an accessory factor present in plasma. It coagulates plasma, it deposits fibrin on the surface of staphylococci alerting their ingestion by phagocytic cells or their destruction within such cells. 2. Staphylokinase : It causes fibrinolysis and clot dissociation and the formation of suppurative micro-emboli responsible for septic metastasis. 3. Hyaluronidase : Spreading factor It causes hyaluronic acid hydrolysis, which is the fundamental substance forming the connective tissue. 4. β-lactamase : Penicillinase It inactivates penicillin resulting in resistance to this antibiotic. 5. Deoxyribonuclease : It hydrolyses DNA.
Clinical
Findings :
Staphylococcus aureus is the most important human staphylococcal pathogen. Some of the infections caused by Staphylococcus aureus include : (I) Pyogenic infections : (characterized by production of suppurative and necrotic lesions). 1. Focal : suppuration: folliculitis, impetigo, furuncles, carbuncles, breast abcess, cellulitis, post operative wound infection. 2. Disseminated : with visceral localization and suppuration. From any focus the organism may spread via the lymphatics and blood stream to other parts of the body leading to osteomyelitis, lung abscess, brain abscess, endocarditis, pneumonia and meningitis. (II) Toxin-mediated illness : 1. Food poisoning : due to ingestion of meal containing the enterotoxin previously secreted by Staphylococcus. It is characterized by short incubation period (1-8 hours), nausea, violent vomiting and diarrhoea, followed by rapid convalescence. There is no fever. 2. Toxic shock syndrome : due to TSST-1, there is an abrupt onset of fever, vomiting, diarrhoea, scarletiniform rash, hypotension with cardiac and renal failure in severe cases, usually there is no bacteraemia. 3. Scalded skin syndrome : due to exfoliative toxin causing generalized skin desquamation.  Type 2 diabetes millitus patient has furuncles
Epidemiology :
The nasal carriage of Staphylococcus aureus occurs in 40-50% of humans. Human skin is densely colonized with coagulase-negative species and to a lesser extent with Staphylococcus aureus. The chief sources of infection are the shedding human lesions, contaminated fomites, human respiratory tract and skin. Contact spread is important in hospitals, where a large proportion of staff and patients carry antibiotic-resistant staphylococci in the nose or on the skin and this may lead to serious epidemics in newborn nurseries, intensive care units and operating rooms.
Bacteriophage
Typing :
Bacteriophage typing is the method of choice adopted by many countries for the epidemiological study of the isolated strains and tracing the source of infection. Strains of Staphylococcus aureus are generally distinguished from one another by their patterns of susceptibility to lysis by an internationally recognized set of 23 standard typing phages.
Detection Of
Carriers :
In tracing the source of infection, the suspected patients, hospital staff or food handlers are tested for the possibility of being nasal or skin carriers of strain causing either the hospital epidemic or food poisoning. The isolated strains are then subjected to phage typing.
Laboratory
Diagnosis :
Specimens : Pus swab or other material as indicated by type of infection. Smears : Gram-strained smears often reveal Gram-positive cocci arranged singly, in pairs or clusters among pus cells. Culture : Specimens are plated on blood agar and MacConkey's agar. In case of staphylococcal food poisoning the specimens (food, vomitus, stools) are plated on mannitol salt agar. Colonies are identified by their morphology, colour (pigment formation) and presence of haemolysis. The colonies are further identified by their biochemical activities and coagulase production (slide or tube coagulase test).
Treatment :
Because of the frequancy of drug-resistant strains, staphylococcal isolates should be tested for antimicrobial susceptibility to help in the choice of the drug. Methicillin-resistant Staphylococcus aureus (MRSA) : These strains are considered resistant to all beta lactam antibiotics including penicillins, cephalosporins and also non-classic beta lactams such as imipenem. Some strains are also resistant to other drugs including aminoglycosides. The great majority of these strains are sensitive to vancomycin. |
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STAPHYLOCOCCUS AUREUS
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