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​​Microbiological Samples

Samples for bacterial culture should preferably be taken before instituting antibiotic therapy as one or two doses of an antibiotic may prevent the isolation of an infecting pathogen.  Situations where antibiotics should be given immediately are:
  • Life threatening infections e.g. meningococcal meningitis.
  • Situati​ons where investigations cannot be done quickly e.g. tuberculosis.
If a patient is already receiving antimicrobial therapy and bacteriological investigation is warranted, consider suspending therapy (the patient's clinical condition permitting) for 48 - 72 hours before taking samples and restarting on a different regimen. If the patient is already receiving antibiotics, please indicate this on the request form.

Samples should be taken from, or as close as possible to, the anatomical focus of the infection. Some organisms will not survive in or on adjacent tissues (e.g. gonococcus is rarely isolated from high vaginal swabs even when the infection is obvious on a cervical swab).

In addition, colonising bacterial flora may be construed as having a pathogenic role if the infected site is not sampled directly (e.g. overgrowth of potential pathogens often occurs on the surface of infected wounds, ulcers or discharging sinuses but they are seldom responsible for underlying infection).

Examine the sample before forwarding it to the laboratory to check if it is a good one or not (e.g. a rectal swab without faecal staining, or a `sputum' sample that is simply frothy saliva are considered worthless).

Preserve the quality of the sample by putting it in the correct leak proof container or transport medium and ensuring that it reaches the laboratory within a reasonable time.

Please explain to patients what the microbiological specimen sample is for and how it might affect their management.  Clear and simple instructions and explanations may encourage their active participation, which may be crucial to obtaining a good quality sample. This applies particularly to urine collection but also to faeces and sputum.