When experimental models of MAP infection are employed to investigate the efficacy of vaccination or other therapeutic strategies, quantification of tissue concentrations of MAP is desirable. This has most commonly been achieved through processing tissue samples for culture, and plate counts of MAP colonies grown on solid media such as Herrold's Egg Yolk Media (HEYM). This procedure requires up to 16 weeks of incubation, whereas growth in liquid media can be achieved in a much shorter time period. The time required for the mycobacterial growth to trigger detection in an automated liquid culture system (Time To Positive, TTP) is correlated to the amount of MAP in the original inoculum. The purpose of this study was to compare the use of automated liquid culture of MAP, using the MGIT system (Becton-Dickinson) to conventional quantification of MAP using HEYM, using tissues obtained from calves experimentally infected with MAP.

Twelve male Holstein calves were given an oral challenge of 10⁹ CFU of MAP on days 21 and 22 of life. Calves were euthanized at 100 days of age, and 33 tissue samples (intestine, mesenteric lymph nodes) were processed for MAP culture. Tissue samples were disrupted with a stomacher, decontaminated with HPC, and plated on 4 tubes of HEYM (200 ul inoculum per tube) or in one tube of liquid MGIT media (100 ul/tube). The number of colonies per tube of HEYM were counted after 16 weeks of incubation. The MGIT tubes were incubated until signalled positive by the Bactec unit. Time to positive was recorded, and converted to a CFU value by use of a standard curve constructed by plotting time to positive vs. CFU/ml for inocula created from known concentration MAP suspension.

Of 396 samples, 3 were lost to contamination and excluded from analysis. Of 393 remaining samples, 269 (68%) were positive on both culture systems, 83 were negative on both systems (21%), for a total of 89% agreement between methods. Of the discordant samples, 36 were positive on HEYM only, and 5 were positive on MGIT only. The CFU as determined by HEYM was significantly (but inversely) correlated with the TTP in MGIT (r = -0.96).

On the basis of these results, automated liquid culture detection of MAP in tissue samples provides a more rapid method to quantify tissue concentration of MAP, compared with HEYM. Comparable results are obtained with both methods. In this experiment, in which half of the calves received MAP vaccination prior to challenge, differences in MAP tissue colonization of experimental vs. control groups was detected at a similar level of statistical significance for the two culture methods.