Performing an antibiotic sensitivity test

The organism is suspended into sterile saline (at a concentration of approximately 106 colony-forming units/mL) and spread across the surface of an appropriate culture medium and a selection of antibiotics (at specific concentrations impregnated into small paper discs) are added to the surface. The plate is incubated for 18-24 hours. The diameter of the zone of inhibition is accurately measured and the results are interpreted as susceptible or resistant

An antibiotic sensitivity plate showing zones of inhibition around discs

Figure 3.8 An antibiotic sensitivity plate showing zones of inhibition around discs.

Reproduced by kind permission of Don whitley scientific Ltd. Copyright © 2015 Don whitley scientific ltd., uK.

based on the size of the zone, guided by standard methods. This is shown in Figure 3.8.

A range of antibiotic sensitivities will be reported and this will offer the clinician some choice in the selection of appropriate therapy. Factors other than organism sensitivity alone may be considered by the clinician, such as patient intolerance of an antibiotic, the cost of treatment, and so on.

The final microbiological report will collate together all the results obtained in the laboratory. These may include the following:

  • ? The result of direct microscopic examination of the sample (Gram stain). This will indicate the groups of bacteria (if present) such as ‘Gram-positive cocci’, ‘Gram-negative bacilli’, and so on. It may also indicate the presence or absence of pus seen in the stained film.
  • ? A list of relevant or significant organisms (which may or may not be recognized pathogens) with antibiotic sensitivities if appropriate.
  • ? The presence of any normal or other flora present in the specimen, such as mixed faecal organisms, skin flora, and so on.
  • ? Any comments appended to the microbiological results by the Medical Microbiologist. These may include recommended antibiotic treatment, or the need for further investigations

Traditionally, the final report described above would be a printed hard copy and this would be appended to the patient’s notes. More commonly, the report will now be generated electronically.

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