Comparative evaluation of the MGIT (TM) and BACTEC culture systems for the recovery of Mycobacterium avium subsp paratuberculosis from milk

Aims: To compare the detection capabilities of the non-radiometric MGIT(TM) (Mycobacteria Growth Indicator Tubes) and radiometric BACTEC 460TB culture systems (Becton Dickinson, Cowley, Oxford, UK) for recovering Mycobacterium avium subsp. paratuberculosis from milk. Methods and Results: Ultra heat treated (UHT) milk samples spiked with different levels of M. paratuberculosis (10-10(7) cells ml(-1)) were inoculated into MGIT and BACTEC media (containing recommended supplements) with and without prior chemical decontamination of the milk samples with 0.75% (w/v) cetylpyridinium chloride for 5 h. Time for the detection of growth in days was recorded for each culture system, and a M. paratuberculosis count for each milk sample was calculated from BACTEC readings using a published formula. Correlation between MGIT and BACTEC detection times was 0.6983. Both culture systems were capable of detecting 10-100 M. paratuberculosis cells ml(-1) in milk within 30-40 days when no decontamination treatment was applied, but only 10(2)-10(3) cells ml(-1) or greater when chemical decontamination was applied before culture. Conclusions, Significance and Impact of Study: The non-radiometric MGIT system could be substituted for the radiometric BACTEC system for the culture of M. paratuberculosis from milk without loss of detection sensitivity. Chemical decontamination before culture caused a significant reduction in numbers of viable M. paratuberculosis in all spiked milk samples resulting in decreased detection capability for both culture systems.