Liquid Culture Contamination and sources of contamination

Liquid Culture Contamination and Sources of Contamination

Liquid media are more prone to contamination than solid media. Therefore, it is essential to process specimens with extreme care, adhering very closely to procedures and recommendations.

Sources of Contamination:

1.       Improper or under-decontamination of the specimen
2.       Thick mucoid specimens that are hard to liquefy 
3.       Prolonged storage and transportation time of the specimen after collection. In such situations, especially in hot weather, bacteria tend to overgrow and are hard to kill by routine decontamination procedures.

4.       Use of non-sterile materials such as pipettes, tubes, etc.

         The incidence of contamination with bacteria (other than mycobacteria) varies from laboratory to laboratory, depending upon several factors. According to the NTEP guidelines, up to a 5% contamination rate is acceptable in cultures of clinical specimens on solid media. However, for liquid media, slightly higher contamination may be accepted (up to 7-8%). A very low contamination rate (less than 3%) may indicate too harsh a decontamination process, which would also affect the growth of mycobacteria and may reduce the positivity rate and increase the time-to-detection of positive mycobacterial culture.

   Resource

   Mycobacteriology Laboratory Manual 

5. Assessment