Laboratory Diagnosis of Gonorrhoea

Antibiotic Susceptibility Testing of N. Gonorrhoeae

These discs are available commercially from Oxoid. If obtaining discs is difficult, contact your Regional Reference Centre for further information.

Reading zones (annular radius)

The annular radius (mm) is the shortest distance from the edge of the disc to the edge of confluent growth.

The edge of confluent growth usually corresponds to the sharpest edge of the zone. Because chocolate agar plates are used, zone sizes must be measured from the inoculated surface.

Interpreting zone sizes into category of sensitivity

The size of the zones of inhibition is determined by measuring the annular radius (mm). The category to which the isolate is allocated is decided by referring to Table 2 and the accompanying notes. The organism may then be placed into an appropriate category of sensitivity for each antibiotic tested. Comparisons should always be made with reference cultures supplied. Ideally, these should be tested simultaneously and regularly.

Interpreting zone sizes for individual patient management

The above descriptions apply to interpretative criteria for epidemiological purposes. When antibiotic susceptibility testing results are required for individual patient management rather than for epidemiology, refer to Table 3.

Table 2. Interpretative criteria for epidemiological purposes

Measurements are the annular radii in mm and not zone diameters

*   Quinolone testing is performed using a combination of both nalidixic acid (30 mg) and ciprofloxacin (1 mg) discs. The category of 'susceptible', 'less sensitive' or 'resistant' for quinolones is derived by considering the annular radius measurements obtained with both antibiotic discs.

** No zone is usually obtained with the nalidixic acid disc with strains showing any degree of altered quinolone susceptibility (less sensitive or resistant). A small percentage of less sensitive strains may produce a reduced zone to nalidixic acid, all still <6 mm. Further discrimination (quinolone less sensitive or quinolone resistant) is then provided with the ciprofloxacin 1 mg disc.

*** A small percentage of strains with a ciprofloxacin MIC = 0.5 mg/L, i.e. upper limit of 'less sensitive', may produce an annular radius <6 mm and thus fall into the resistant category by the disc screening kit. This is acceptable.

Table 3. Interpretative criteria for patient management

*  no resistance reported

Quality control

The following quality control conditions should be adhered to, as per the checklist (see page 34).

Media (especially important for commercially prepared plates)

Check the approximate depth of agar and weight of agar plates. Test the control strains and check that the zone sizes are within the acceptable range and make sure that a good lawn growth is obtained.

Check with your source of pre-poured agar plates to ensure that the correct medium is being used, e.g. Oxoid, BBL or LABM Columbia agar base for chocolate agar.

Inoculum

Bacterial material must be seen on the tip of the straight wire. After overnight incubation there must be confluent growth on the sensitivity plate. A very light suspension will alter the results more than a slightly heavier inoculum.

Potency of antibiotic discs

Check that the correct disc potency is used. Some reference strains are of limited use since the zone sizes will fall outside the acceptable range only when the potency of the disc has dropped by more than half. For this reason it is imperative that the antibiotic discs are handled as follows:

1.      Disc stocks not in immediate use must be stored at or below –20 ºC.

2.      Discs in use must be stored in an air-tight container (dispenser) with dessicant at 4 ºC.

3.      Discs must be allowed to reach room temperature before opening the container to avoid condensation on the discs as this will inactivate some antibiotics, e.g. benzyl penicillin.

4.      Do not use discs past their date of expiry.