Methicillin Resistant Staphylococcus Aureus
MRSA is the term used for bacteria of the
Staphylococcus aureus group (S. aureus) that are resistant to the usual
antibiotics used in the treatment of infections with such organisms.
Traditionally MRSA stood for methicillin resistance but the term increasingly
refers to a multi-drug resistant group. Such bacteria often have resistance to
many antibiotics traditionally used against S.aureus.
This resistance to methicillin is due to the
presence of the mec gene in the bacteria. This alters the site at which
methicillin binds to kill the organism. Hence, methicillin is not able to
effectively bind to the bacteria.
Infections caused by MRSA are the same as
other staphylococcal infections because the organism itself is not any more
virulent (or infectious) than usual type S.aureus.
Like other S.aureus, MRSA can colonise the
skin and body of an individual without causing sickness, and in this way it can
be passed on to other individuals unknowingly. Problems arise in the treatment
of overt infections with MRSA because antibiotic choice is very limited.
Staphylococcal skin infections
Staphylococci (‘staph’) are a common type
of bacteria that live on the skin and mucous membranes (eg. in nose) of humans.
Staphylococcus aureus (S. aureus) is the most important of these bacteria in
human diseases. Other staphylococci, including S. epidermidis, are considered
commensals, or normal inhabitants of the skin surface.
About 15-40 per cent of healthy humans are
carriers of S. aureus, that is, they have the bacteria on their skin without any
active infection or disease (colonisation). The carrier sites are usually the
nostrils and fexures, where the bacteria may be found intermittently or every
time they are looked for.
Bacteriology
Staphylococcal aureus bacteria are classified
as Gram-positive cocci based on their appearance under a microscope. They may
occur singly or grouped in pairs, short chains or grape-like clusters. They are
usually facultative anaerobes, that is, they are capable of surviving at various
levels of oxygenation, and are generally very hardy organisms.
They are only able to invade via broken skin
or mucous membranes, hence intact skin is an excellent human defence. Once they
have invaded they have various ways to avoid host defences. They:
- Hide their antigens to avoid an immune
response
- Kill infection-fighting cells (phagocytes)
- Survive within host infection-fighting
cells.
- Develop resistance to antibiotics
- Release toxins (intoxication) - these do
not require the presence of live bacteria to have an effect.
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