VOL: 96, ISSUE: 38, PAGE NO: 8
Rose Cooper, PhD, BSc, PGCE, is a microbiologist, School of Applied Sciences, University of Wales Institute, Cardiff
Professor Keith Harding, MB, MRCGP, FRCS, Director, Wound Healing Research Institute, CardiffWhen it was first developed, penicillin effectively treated Staphylococcus aureus infections by inhibiting bacterial cell-wall biosynthesis. Within 10 years, however, more than 80% of S. aureus strains were resistant to it.
When it was first developed, penicillin effectively treated Staphylococcus aureus infections by inhibiting bacterial cell-wall biosynthesis. Within 10 years, however, more than 80% of S. aureus strains were resistant to it.
Penicillin contains what is known as a β-lactam ring, which targets and binds with a group of enzymes that are vital for the synthesis and maintenance of the bacterial cell wall, resulting in rapid cell death. Most of the resistant strains were able to synthesise an enzyme, called penicillinase or β-lactamase, which deactivated the antibiotic through cleavage of the β-lactam ring.
Penicillinase-stable antibiotics such as methicillin - and later cloxacillin and flucloxacillin - were developed to counter this resistance, but methicillin-resistant strains of S. aureus (MRSA) were detected almost immediately. Because these bacteria also produce an enzyme for cell-wall production that did not bind readily and was therefore not easily inhibited by methicillin, they were resistant to all β-lactam antibiotics.
Methicillin is highly toxic to patients so it is no longer used in clinical practice, but some strains of MRSA have acquired resistance to a further range of antibiotics. The extent of multiple resistance varies from strain to strain and must be established in a laboratory.
The clinical significance of MRSA is often misunderstood. It is a potential pathogen that is capable of colonising healthy people without causing harm, but can initiate infections in compromised patients. Effective control is vital because of its ability to spread and colonise debilitated patients and the difficulties involved in treating systemic MRSA infection.
The emergence of epidemic strains of MRSA, especially EMRSA-15 and EMRSA-16, together with the escalating cost of controlling the spread of MRSA, have prompted a re-examination of the strategies available to minimise both its spread and infection rates. Revised guidelines for the control of MRSA in hospitals have been formulated (Ayliffe et al, 1998).
For nurses, the main points to remember about MRSA infection are:
- MRSA is commonly found in granulating wounds, but its true prevalence in patients in hospitals and the community is not known;
- Screening policies may vary between hospital and community settings, but it is the responsibility of clinicians to comply with local as well as national guidelines;
- The estimation of MRSA carriage is influenced by microbiological methodology, frequency of sampling and choice of site sampled;
- All MRSA infections must be treated with appropriate systemic therapy after laboratory evaluation of antibiotic sensitivity;
- To reduce cross-infection rates, policies should aim to eradicate MRSA carriage by health care workers and hospital patients. Antimicrobial strategies should also minimise the risk of developing new resistant strains by adopting suitable therapy regimens. The topical application of mupirocin is recommended three times a day for 7-10 days. It is therefore inappropriate to limit the application of mupirocin to wounds to once a day with routine dressing changes;
- The management of MRSA in wounds will be influenced by whether it is causing infection and whether it is likely to transfer to debilitated patients;
- The impact of discharging MRSA-colonised patients to the community must be evaluated in terms of the risks of transferring MRSA to other susceptible people;
- MRSA may not delay healing, so where there is no risk of cross-infection there may be no need to treat a colonised wound;
- It may not be possible to eradicate MRSA from chronic wounds because undetectable levels of bacteria may persist;
- Innovative approaches to clearing MRSA from wounds need urgent investigation.