Aims:To compare the detection capabilities of the non-radiometric MGIT TM (Mycobacteria Growth Indicator Tubes) and radiometric BACTEC 460TB culture systems (Becton Dickinson, Cowley, Oxford, UK) for recovering Mycobacterium avium subsp. paratuberculosis from milk. Methods and Results: Ultra heat treated (UHT) milk samples spiked with different levels of M. paratuberculosis (10-10 7 cells ml )1 ) were inoculated into MGIT and BACTEC media (containing recommended supplements) with and without prior chemical decontamination of the milk samples with 0AE75% (w/v) cetylpyridinium chloride for 5 h. Time for the detection of growth in days was recorded for each culture system, and a M. paratuberculosis count for each milk sample was calculated from BACTEC readings using a published formula. Correlation between MGIT and BACTEC detection times was 0AE6983. Both culture systems were capable of detecting 10-100 M. paratuberculosis cells ml )1 in milk within 30-40 days when no decontamination treatment was applied, but only 10 2 -10 3 cells ml )1 or greater when chemical decontamination was applied before culture.Conclusions, Significance and Impact of Study: The non-radiometric MGIT system could be substituted for the radiometric BACTEC system for the culture of M. paratuberculosis from milk without loss of detection sensitivity. Chemical decontamination before culture caused a significant reduction in numbers of viable M. paratuberculosis in all spiked milk samples resulting in decreased detection capability for both culture systems.
A PCR assay has been developed for the detection of Campylobacter jejuni and Camp. coli in water samples. The sample is filtered through a membrane which is subjected to sonication to release the impacted cells. After removal of the filter from the cell suspension and a freeze/thaw cell lysis step, a semi-nested PCR is carried out on the filtrate using the primers CF02, CF03 and CF04 (Camp. jejuni flaA and flaB gene sequences). Incorporation of a sonication stage allows removal of the filter membrane since they have been shown to inhibit the PCR. In experiments with spike water samples (20 ml) a theoretical sensitivity of 10-20 campylobacter cells ml-1 was achieved. Using a sample volume of 100 ml this sensitivity can be increased to approximately 2 campylobacter cells ml-1.
The resistance of three strains of Escherichia coli O157:H7 in their stationary growth phase to starvation (24 h in water at 37 degrees C) followed by a heat treatment (56 degrees C for up to 90 min) was determined. Starvation was found to increase significantly the resistance of two strains (NCTC 12079; eae+, VT1+, VT2+, and ATCC 43889 eae+, VT2+) but not the remaining strain (ATCC 43890 eae+, VT1+). Strain NCTC 12079 (only one tested) was shown to retain all of the three virulence factors after the two stresses. De novo protein synthesis was shown to be required for heat resistance. Evidence using an rpoS mutant indicated a central role for this gene in inducing heat resistance after a starvation stress. It is hoped that this work will contribute to more accurate risk assessments in certain food processing operations.
M.T. ROWE AND R.B. KIRK. 2001. Micro‐organisms which are subjected to non‐lethal stress can exhibit significantly greater resistance when both the same or an unrelated stress is subsequently reapplied. This latter phenomenon is termed ‘cross‐protection’. In experiments using three strains of Escherichia coli harbouring cytotoxic necrotizing factor 1 and/or cytolethal distending toxin all three exhibited significantly greater (P < 0·05) resistance to salt (20% w/v) or heat (56 °C for up to 75 min) when prestressed with lactic acid (pH 4). This work indicates that the cross‐protection phenomenon should be taken into account when devising food process operations designed to minimize the risk posed by these pathogens.
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