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Culture and Sensitivity

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Julian Ketel,

BSc (Hons), DipN, RGN, RMN, RNMH, is specialist nurse - infection prevention and control,Walsall Primary Care Trust

Microscopy, culture and sensitivity (MC&S) is the term often used when sending microbiological samples to the laboratory.

Microscopy, culture and sensitivity (MC&S) is the term often used when sending microbiological samples to the laboratory.

Microscopy

The sample can be viewed under a microscope within minutes of arriving at the laboratory. This enables a quick initial report, such as ‘Gram positive cocci seen’, to be telephoned to the clinician if necessary. Combined with an accurate clinical picture this may be enough to initiate targeted treatment.

Culture

A sample from the original culture is grown to create a pure sample to enable further identification of an organism. This is of particular value if the organism is unusual, and may confirm or indicate a change to treatment. Culturing can also enable a bacterial count to be made, which can assist in deciding whether a wound is colonised or infected. In colonised wounds bacteria multiply but do not elicit a host response; infected wounds contain multiplying bacteria which induce a pathological response (Flanagan, 2000; Box 1). Numbers of bacteria are expressed as colony-forming units (cfu) e.g. 104cfu/ml for a urine sample. A cfu is a colony arising from a single viable bacterium.

Sensitivity

A pure culture is seeded onto an agar plate containing discs saturated with various antibiotics. If the organism grows up to a disc it is resistant to that antibiotic; if there is a clear zone around the disc it is susceptible. This can confirm a patient is on the correct treatment or not and can further help with identification e.g. methicillin-resistant Staphylococcus aureus (MRSA).

Sampling

Reliable MC&S results are only possible if a good sample is obtained. Samples should be taken aseptically. If pus is present it should be sampled, as it is the best indicator of pathogens (Health Protection Agency 2003a; 2003b). Specimens should be sent as soon as possible, as some organisms will not survive and others will be overgrown by more vigorous strains (Wilson, 2001; Greenwood et al, 1997). If samples are not sent immediately they must be stored appropriately - some require storage at room temperature and others refrigeration. For storage methods see local procedures, infection control teams, textbooks such as Lawrence and May (2003) or laboratory staff.

Documentation and results

Clear documentation on the laboratory form is essential and should include patient details, suspected illness/initial diagnosis, site/type of specimen, condition of site if appropriate and any relevant treatment such as antibiotics. The result should be assessed within the context of the clinical picture: ϠIf a urine sample shows a high bacterial count and the patient has signs of clinical infection, the two may be linked. However, a high count alone may not always indicate infection. ϠIf a wound shows no sign of infection and only its surface is sampled, the result will show only what is colonising or contaminating its surface. It is questionable whether such wounds should be sampled (Wilson, 2001), as contamination and colonisation do not usually compromise healing. In fact benign bacteria may even help to avoid infection by preventing the ingress of pathological strains (Selwyn, 1975). ϊSamples can be taken for screening e.g. nasal swab for MRSA. This should be stated on the form so only these bacteria are tested for and therefore the result will only indicate their presence or absence and sensitivities.

Common clinical signs of wound infection

Cardinal signs of infection:Redness;Heat;Swelling;Pain or change in pain;Impairment of movement.

Other signs of infection:Presence of pus;Increased wetness;Change in appearance;Bridging;Odour (offensive);Cellulitis;Delayed healing;Discoloration;Pocketing;Wound breakdown;Abscess formation.

REFERENCES

Flanagan M. (2000) Essential Wound Healing Part 4: Wound Infection. London: Emap Healthcare. Greenwood, D. et al (1997) Medical Microbiology (15th edn). Edinburgh: Churchill Livingstone. Health Protection Agency (2003a) Standard Operating Procedure: Investigation of skin and superficial wound swabs. London: HPA. Health Protection Agency (2003b) Standard Operating Procedure: Investigation of abscesses and of postoperative wound and deep seated infections. London: HPA. Lawrence, J., May, D. (2003) Infection Control in the Community. Edinburgh: Churchill Livingstone. Selwyn, S. (1975) Natural antibiosis among skin bacteria as a primary defence against infection. British Journal of Dermatology; 93: 487-493. Wilson, J. (2001) Infection Control in Clinical Practice (2nd edn). London: Bailli貥 Tindall. .

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