Does Mycobacterium paratuberculosis Survive Current Pasteurization Conditions?

In a recent article, Stabel, Steadham, and Bolin reported that Mycobacterium paratuberculosis did not survive high-temperature, short-time (HTST) pasteurization simulated with an Armfield HTST laboratory pasteurizer (7). The authors effectively dismissed the considerable data generated by previous studies (1–5) which indicated that current HTST pasteurization conditions may not be effective in killing M. paratuberculosis and chose to conclude that turbulent flow of milk during pasteurization was essential for complete killing of contaminating M. paratuberculosis. However, Hope et al. (4) also employed small-scale continuous-flow pasteurizing equipment and reported that viable M. paratuberculosis cells were isolated from HTST-pasteurized milk initially containing 10⁴ M. paratuberculosis CFU/ml. These findings would tend to invalidate the argument regarding turbulent versus static heating put forward by Stabel et al. to explain the results of their study. In my opinion there are a number of other factors which may explain their inability to isolate viable M. paratuberculosis during their recent study.

My primary concern is that on the one hand Stabel et al., keen to simulate commercial HTST pasteurization conditions as closely as possible, used an Armfield HTST laboratory pasteurizer but on the other hand proceeded to use an inoculum which had been both frozen and sonicated before addition to the raw milk. M. paratuberculosis cells naturally present in infected milk would be subject to neither of these treatments prior to commercial pasteurization, and none of the previous studies of M. paratuberculosis and pasteurization have taken this approach. The detrimental effect of freezing at −80°C on the viability of M. paratuberculosis has been reported (6), and sonication applied to cells already injured by freeze-thawing may also have affected cell viability. The reason given by Stabel et al. for the use of sonication is fully appreciated. However, recent studies by Sung et al. (8) and ourselves (unpublished data) have shown that declumped M. paratuberculosis cells are much less heat resistant than clumped cells. Consequently, by incorporating a sonication step before heating, Stabel et al. were effectively increasing the chances of inactivating M. paratuberculosis during pasteurization. Further sonication of M. paratuberculosis cells in the pasteurized milk before enumeration may also have contributed to their inability to detect viable but sublethally injured M. paratuberculosis, if any existed, in the pasteurized product. In my experience, heat-treated M. paratuberculosis cells are not recovered by BACTEC culture if PANTA antibiotic supplement is added to the BACTEC medium, whereas they are recoverable if the PANTA is omitted. This observation clearly illustrates the sublethally injured status of M. paratuberculosis cells after heating and the adverse effect that additional stress can have on the viability of the organism.

In my opinion, Stabel et al. studied the effect of HTST pasteurization on potentially injured M. paratuberculosis cells which were likely to be more heat sensitive from the outset. I would contend that the methodology employed by these authors less accurately reflected the condition of M. paratuberculosis cells occurring in naturally infected milk than that used in previous studies, and consequently, the results of this study must be considered in light of this fact.