Effect of egg yolk on the detection of Mycobacterium avium subsp. paratuberculosis using the ESP II liquid culture system.

Rapid diagnosis of paratuberculosis in infected cattle is important for the successful control of Johne disease within herds. Thus, improving culture methods for Mycobacterium avium subsp. paratuberculosis (M. paratuberculosis) will aid in the identification of asymptomatic animals. Egg yolk is a component of the media used for growing M. paratuberculosis, but its requirement as a supplement has not been reported. Using the ESP II liquid culture system, 2 different sources and 5 concentrations (3.3%, 1.6%, 0.8%, 0.4%, and 0%) of egg yolk were analyzed. Egg yolk source did not affect either recovery rate or time to detection, but both parameters were significantly improved when the 3.3% egg yolk concentrations (final volume) were used over media containing no egg yolk. This study also assessed the recovery of M. paratuberculosis from fecal samples that were cultured multiple times using Herrold egg yolk agar (HEY). Specimens containing greater than 70 cfu/g feces could routinely be identified as positive for M. paratuberculosis after only 1 culture attempt, whereas specimens with fewer bacteria were only intermittently positive, even after 5 replicate cultures. Therefore, this study indicates that the sensitivity of the Trek Diagnostic ESP II liquid culture system for M. paratuberculosis is affected by egg yolk concentration and that single culture attempts using HEY solid media may not identify specimens containing low numbers of bacteria.