Effect of high pressure and pasteurization on Mycobacterium avium ssp. paratuberculosis in milk

Aims:  To determine the effect of high pressures alone and in conjunction with pasteurization on the viability of two strains of Mycobacterium avium ssp. paratuberculosis (Map). Methods and Results:  Map in a milk matrix was subjected to 400, 500 and 600 MPa with and without pasteurization (72°C for 15 s) and plated onto Herrold’s egg yolk medium (HEYM) and Middlebrook 7H10 (7H10) agar, both containing antibiotic supplements. Medium 7H10 was found to give a significantly (P < 0·001) better recovery than HEYM. A significantly greater (P < 0·001) reduction in viable numbers was observed using 500 MPa (mean log reduction of 6·52) compared with 400 MPa (mean log reduction of 2·56) and between 400 MPa and control (no applied pressure) for 10 min treatments. A treatment time of 10 min resulted in significantly (P < 0·001) fewer survivors than 5 min. Low numbers of survivors were still detected when pressure treatment at 400 and 600 MPa was combined with subsequent pasteurization. Conclusions:  The use of high-pressure was effective in reducing viable numbers of Map but even when combined with pasteurization there were still survivors, albeit when high inoculum levels of Map were used. Significance and Impact of the Study:  To the authors’ knowledge the work reported here represents the first study of the efficacy of high-pressure treatments alone and in combination with pasteurization to kill Map. The results indicate that further research is warranted before more commercial-scale studies are commissioned.