Monitoring Liquid Culture Contamination

The growth of contaminant bacteria results in positive fluorescence. Therefore, it is important to observe all fluorescent positive MGIT tubes visually for turbidity and to make an AFB smear. If an MGIT tube broth is heavily turbid, contamination is suspected even if the AFB smear is positive. 

Usually, contaminating bacteria causes heavy turbidity, although M. tuberculosis growth appears as particles without significant turbidity, while some of the NTM may produce light turbidity. 

Contamination may be confirmed by the following method: 

1. Make a smear and stain with Ziehl-Neelsen stain. The presence of non-acid-fast contaminant bacteria on the smear confirms contamination.

2. Sub-culture a loopful on blood agar. If blood agar is not available, use chocolate agar or a brain heart infusion (BHI) agar plate. Several specimens (up to 4) may be carefully inoculated on a plate (small streak for each specimen, properly labelled). Divide the plate and identify the specimen number with a marker. Incubate these subcultures at 35ºC ±1ºC and observe after 24-48 hours. If contaminating growth appears, confirm again by gram and ZN stain.

3. If contamination is confirmed with a negative AFB smear from the broth, discard the specimen and report it as contaminated. The isolation procedure can be repeated if contamination is confirmed with a positive AFB smear from the broth.

Resource

Mycobacteriology Laboratory Manual

Assessment