Staphylococcus aureus (which is occasionally given the nickname golden staph) is a bacterium, frequently living on the skin or in the nose of a healthy person, that can cause illnesses ranging from minor skin infections (such as pimples, boils, and cellulitis) and abscesses, to life-threatening diseases such as pneumonia, meningitis, endocarditis and septicemia. Each year some 500,000 patients in American hospitals contract a staphylococcal infection. It is a spherical bacterium.
Microbiology
Staphylococcus aureus appears as a Gram-positive coccus, in grape-like clusters when viewed through a microscope and as large, round, golden-yellow colonies, often with β-hemolysis, when grown on blood agar plates. Hence, the word "aureus" which means gold in Latin. S. aureus can be differentiated from most other staphylococci by the coagulase test. S. aureus is coagulase-positive, while most other staphylococci are coagulase-negative. S. aureus is also catalase positive and thus able to convert hydrogen peroxide (H2O2) to water and oxygen, which makes the Catalase test useful to distinguish staphylococci from enterococci and streptococci.
The species has been subdivided into two subspecies: S. aureus aureus and S. aureus anaerobius. The latter requires anaerobic conditions for growth, is an infrequent cause of infection, and is rarely encountered in the laboratory.
Antibiotic sensitivity
S. aureus has become resistant to many commonly used antibiotics. Up to 90% of all Staphylococcus isolates are resistant to penicillin, which has led to the introduction of flucloxacillin and cloxacillin as first-line antistaphylococcal antibiotics.
An increasing problem since the 1950s has been resistance of S. aureus to flucloxacillin, oxacillin, and similar β-lactam antibiotics that are deactivated by β-lactamase. As methicillin is used in laboratories to assess for this type of resistance, the term Methicillin-resistant Staphylococcus aureus (MRSA) is in use to denote these strains. MRSA is generally sensitive to the glycopeptide antibiotics vancomycin and teicoplanin.
In 1997, physicians were alarmed to encounter staph strains that resist even vancomycin, to which it had previously always been sensitive. Due to this resistance, S. aureus is sometimes referred to as a superbug.
Staphylococcal resistance to penicillins and cephalosporins is expressed as beta-lactamase production: enzymes which break down the beta-lactam ring of the penicillin molecule. Other resistance-conferring mutations include altered penicillin-binding proteins to which penicillins bind poorly.
With the increased incidence of MRSA infections, vancomycin or teicoplanin (glycopeptide antibiotics) are often a treatment of choice in infections with methicillin-resistant Staphylococcus aureus (MRSA). Vancomycin-resistant Staphylococcus aureus (VRSA) is a strain of Staphylococcus aureus that has become resistant to the glycopeptides. Three cases of VRSA infection have been reported in the United States[1].
Role of pigment in resistance
The vivid yellow pigmentation of S. aureus may be a factor in its virulence. When comparing a normal strain of S. aureus with a strain modified to lack the yellow coloration, the pigmented strain was more likely to survive dousing with an oxidizing chemical such as hydrogen peroxide than the mutant strain was.
Colonies of the two strains were also exposed to human neutrophils. The mutant colonies quickly succumbed while many of the pigmented colonies survived. Wounds on mice were swiped with the two strains. The pigmented strains created lingering abscesses. Wounds with the unpigmented strains healed quickly.
These tests suggest that the yellow pigment may be key to S. aureus's ability survive immune attacks. Drugs that inhibit the bacterium's production of the carotenoids responsible for the yellow coloration may weaken it and renew its susceptibility to antibiotics. [2]
Role in disease
Staphylococcus lives as a commensal on the skin and in the nose of humans and other animals, as well as in the environment. It can infect other tissues when normal barriers have broken down (e.g. skin or mucosal lining). This leads to furuncles (boils) and carbuncles (a collection of furuncles). In infants Staphylococcus aureus infection can cause a severe disease SSSS (staphylococcal scalded skin syndrome).[3]
Staphylococcal infections can be spread through contact with pus from an infected wound, skin to skin contact with an infected person, and contact with objects such as towels, sheets, clothing, or athletic equipment used by an infected person.
Deep Staphylococcus infections can be very severe. Prosthetic joints are particularly at risk, and staphylococcal endocarditis (infection of the heart valves) and pneumonia may be rapidly fatal.
Basic handwashing techniques are generally effective in preventing the transmission of Staphylococcus aureus. By the use of "standard precautions", and where necessary additional precautions, the risk of transmission can be further reduced.
Note
- ^ Menichetti F (2005). "Current and emerging serious Gram-positive infections". Clinical Microbiology and Infection 11 (Suppl 3): 22-8. PMID 15811021.
- ^ Liu GY, Essex A, Buchanan JT, Datta V, Hoffman HM, Bastian JF, Fierer J, Nizet V (2005). "Staphylococcus aureus golden pigment impairs neutrophil killing and promotes virulence through its antioxidant activity". Journal of Experimental Medicine 202 (2): 209-15. PMID 16009720.
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