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Inactivation of Mycobacterium paratuberculosis in cows' milk at pasteurization temperatures
I R Grant1, H J Ball, S D Neill
1Department of Food Science (Food Microbiology), Queen's University of Belfast, North Ireland, United Kingdom.
Abstract:
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.
Insights
Pasteurization may not fully inactivate Mycobacterium paratuberculosis in milk, especially at high bacterial concentrations. Further research is needed to ensure complete elimination of this pathogen during milk processing.
Area of Science:
- Food Microbiology
- Veterinary Microbiology
- Public Health
Background:
- Mycobacterium paratuberculosis is a causative agent of Johne's disease in cattle.
- The presence of M. paratuberculosis in milk poses a potential public health risk.
- Standard pasteurization methods aim to eliminate harmful bacteria in milk.
Purpose of the Study:
- To investigate the thermal inactivation of M. paratuberculosis in cow's milk using laboratory pasteurization.
- To determine the efficacy of holder and high-temperature, short-time (HTST) pasteurization against M. paratuberculosis.
- To construct a thermal death curve for M. paratuberculosis.
Main Methods:
- Inoculation of cow's milk with M. paratuberculosis at varying concentrations (10^7 and 10^4 CFU/ml).
- Application of standard holder (63.5°C for 30 min) and HTST (71.7°C for 15 s) pasteurization methods.
- Viability assessment using microbial culture (HEYM, BACTEC) and PCR confirmation.
Main Results:
- M. paratuberculosis was isolated from 96% (holder) and 85% (HTST) of samples with high initial contamination (10^7 CFU/ml).
- Isolation rates decreased to 50% (holder) and 58% (HTST) with lower initial contamination (10^4 CFU/ml).
- The thermal death curve exhibited significant "tailing," indicating slow inactivation of some cells.
Conclusions:
- Standard and HTST pasteurization may not guarantee complete inactivation of M. paratuberculosis in milk, particularly with high bacterial loads.
- The "tailing" phenomenon in the thermal death curve suggests a need for optimized or alternative inactivation strategies.
- Further studies are warranted to ensure the microbiological safety of milk concerning M. paratuberculosis contamination.