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Appl. Environ. Microbiol., 02 1996, 631-636, Vol 62, No. 2
IR Grant, HJ Ball, SD Neill and MT Rowe
The thermal inactivation of 11 strains of Mycobacterium paratuberculosis at
pasteurization temperatures was investigated. Cows' milk inoculated with M.
paratuberculosis at two levels (10(7) and 10(4) CFU/ml) was pasteurized in
the laboratory by (i) a standard holder method (63.5 degrees C for 30 min)
and (ii) a high-temperature, short- time (HTST) method (71.7 degrees C for
15 s). Additional heating times of 5, 10, 15, 20, and 40 min at 63.5
degrees C were included to enable the construction of a thermal death curve
for the organism. Viability after pasteurization was assessed by culture on
Herrold's egg yolk medium containing mycobactin J (HEYM) and in BACTEC
Middlebrook 12B radiometric medium supplemented with mycobactin J and
sterile egg yolk emulsion. Confirmation of acid-fast survivors of
pasteurization as viable M. paratuberculosis cells was achieved by
subculture on HEYM to indicate viability coupled with PCR using M.
paratuberculosis-specific 1S900 primers. When milk was initially inoculated
with 10(6) to 10(7) CFU of M. paratuberculosis per ml, M. paratuberculosis
cells were isolated from 27 of 28 (96%) and 29 of 34 (85%) pasteurized milk
samples heat treated by the holder and HTST methods, respectively.
Correspondingly, when 10(3) to 10(4) CFU of M. paratuberculosis per ml of
milk were present before heat treatment, M. paratuberculosis cells were
isolated from 14 of 28 (50%) and 19 of 33 (58%) pasteurized milk samples
heat treated by the holder and HTST methods, respectively. The thermal
death curve for M. paratuberculosis was concave in shape, exhibiting a
rapid initial death rate followed by significant "tailing." Results
indicate that when large numbers of M. paratuberculosis cells are present
in milk, the organism may not be completely inactivated by heat treatments
simulating holder and HTST pasteurization under laboratory conditions.
Copyright © 1996, American Society for Microbiology
Inactivation of Mycobacterium paratuberculosis in cows' milk at pasteurization temperatures
Department of Food Science (Food Microbiology), Queen's University of Belfast, North Ireland, United Kingdom.
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