Evaluation of denaturing gradient gel electrophoresis for the detection of mycobacterial species and their potential association with waterborne pathogens

The versatility of denaturing gradient gel electrophoresis (DGGE) protocol provides enough grounds for its wide application over an array of microorganisms. This work was designed to evaluate DGGE for the detection and confirmation of mycobacteria and their association, if any, with waterborne pathogens. A total of 76 samples comprising raw untreated water, schmutzdecke, floccules and final treated water obtained from a common water source, and two water treatment works (WTW1 and WTW2), were analysed. Thirty-five species were identified from the overall samples, with 7% (5/76), 13% (10/76) and 26% (20/76) from the common raw water source, WTW1 and WTW2 respectively. The majority of the species were Cyanobacteria, with high dominance in the raw water entering WTW2. In the final treated water of WTW1 Eutreptiella braarudii was found, and that of WTW2 contained Anabaena nereformis, Anabaena torulosa and Podocarpus nerrifolius. Furthermore, one Mycobacterium species was found in the raw water of WTW1 aside from the detection of Mycobacterium avium ssp. paratuberculosis by the technique. No association between mycobacteria and the other species was observed. This implies DGGE may be employed to study the diversity of other akin mycobacterial species from various sources, and not as a direct means of elucidating microbial associations.

Optimisation of decontamination method and influence of culture media on the recovery of Mycobacterium avium subspecies paratuberculosis from spiked water sediments

The recovery of Mycobacterium avium subspecies paratuberculosis (Map) from the environment can be a laborious process - owing to Map being fastidious, its low number, and also high numbers of other microbial populations in such settings. Protocols i.e. filtration, decontamination and modified elution were devised to recover Map from spiked water sediments. Three culture media: Herrold's Egg Yolk Media (HEYM), Middlebrook 7H10 (M-7H10) and Bactec 12B were then employed to grow the organism following its elution. In the sterile sediment samples the recovery of Map was significant between the time of exposure for each of HEYM and M-7H10, and insignificant between both media (P < 0.05). However, in the non-sterile sediment samples, the HEYM grew other background microflora including moulds at all the times of exposure whilst 4 h followed by M-7H10 culture yielded Map colonies without any background microflora. Using sterile samples only for the Bactec 12B, the recovery of Map decreased as time of exposure increased. Based on these findings, M-7H10 should be considered for the recovery of Map from the natural environment including water sediments where the recovery of diverse microbial species remains a challenge.

SIGNIFICANCE OF THE STUDY

Map is a robust pathogen that abides in the environment. In water treatment operations, Map associates with floccules and other particulate matter including sediments. It is also a fastidious organism, and its detection and recovery from the water environment is a laborious process and can be misleading within the abundance of other mycobacterial species owing to their close resemblance in phylogenetic traits. In the absence of a reliable recovery method, Map continues to pose public health risks through biofilm in household water tanks, hence the need for the development of a reliable recovery protocol to monitor the presence of Map in water systems in order to curtail its public health risks.

Occurrence of Mycobacterium avium subsp. paratuberculosis in raw water and water treatment operations for the production of potable water

Mycobacterium avium subsp. paratuberculosis (Map) causes Johne's disease of cattle and is implicated as a cause of Crohn's disease in humans. The organism is excreted in animal faeces and can contaminate water catchment areas. This coupled with Map's survival in the environment means that water destined for domestic use may be a source of exposure. This work was designed to determine the occurrence of Map in Lough Neagh (the largest freshwater lake in the British Isles), used as a reservoir, and in two water treatment works (WTW1 and WTW2) which abstract from the lough and which have slow sand filtration (SSF) and dissolved air flotation respectively as their principal treatment regimes. The organism was not detected in lough water samples by culture (n=70) but 29% (20/70) were positive by PCR. In the raw water to WTW1 and WTW2 no culture positives were detected but 54% (13/24) and 58% (14/24) respectively were PCR positive. In WTW1 there were no culture positives at the SSF or final water but 31% (8/26) and 45% (9/20) respectively were PCR positive. In WTW2 similar results were obtained with 26% (6/23) and 48% (11/23) in the floccules and final water respectively. At WTW2 however one culture positive was detected in the final water. This latter finding is of concern. The inability to reach definitive conclusions indicates the need for further research, particularly in the detection methods for viable Map.

Detection of Mycobacterium avium ssp. paratuberculosis in cheese, milk powder and milk using IS900 and f57-based qPCR assays

AIMS

To develop a quantitative PCR assay for sensitive and specific detection of Mycobacterium avium ssp. paratuberculosis (Map) in a range of dairy products.

METHODS AND RESULTS

TaqMan(®) assays were designed to target the IS900 and f57 genetic elements of Map. Both real-time PCR assays were integrated with the Adiapure(®) Map DNA extraction kit and assessed separately for the detection/quantification of Map in spiked milk, Cheddar cheese and milk powder. Assays were validated against Cheddar cheese samples containing known concentrations of Map. The IS900 qPCR assay was significantly more sensitive than the assay based on the f57 primer/probe. At a threshold cycle value of 38, limits of detection (LOD) for the IS900 qPCR assay were 0·6 CFU ml(-1), 2·8 CFU g(-1) and 30 CFU g(-1) for artificially contaminated pasteurized milk, whole milk powder and Cheddar cheese, respectively. The respective LOD's for the f57 assay were 6·2 CFU ml(-1), 26·7 CFU g(-1) and 316 CFU g(-1).

CONCLUSION

The integrated Adiapure(®) extraction - IS900 real time assay described is a sensitive, quantitative method for the detection of Map in dairy products. This is the first study to consider qPCR as a quantitative estimation of Map-DNA in cheese and whole milk powder.

SIGNIFICANCE AND IMPACT OF THE STUDY

The assay developed allows sensitive detection and quantification of Map DNA in a range of dairy products which is valuable for the screening and surveillance of this potential zoonotic organism.

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 degrees 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.

Mycobacterium avium ssp. paratuberculosis and its potential survival tactics

Mycobacterium avium ssp. paratuberculosis (Map) is an important animal pathogen with a potential, but as yet unproven, role in human disease. This review briefly describes the characteristics of Map that distinguish it from other Mycobacterium spp., presenting new information arising from completion of the sequencing of the Map genome. It then focuses on the potential mechanisms Map might employ to survive and disseminate in the environment, including interaction with protozoa and insects, dormancy, biofilm formation and aerosolization.

Characteristics of cytotoxic necrotizing factor and cytolethal distending toxin producing Escherichia coli strains isolated from meat samples in Northern Ireland

Swabs collected from pig, lamb and beef carcasses and samples of pork, lamb and beef mince were cultured for Escherichia coli strains. Strains harbouring cytotoxic necrotizing factors (CNF1 and 2) and cytolethal distending toxins (CDT-I,-II,-III and -IV) were identified in plate cultures of the isolates by colony hybridization with labelled probes and multiplex PCR assays. Simplex and multiplex PCR assays were used to further characterize the isolates to determine the presence of P, S and F17 fimbriae as well as afimbrial adhesins and haemolysin. The serotype was also determined where possible. Thirty strains with the capacity to code for CNF (4), CDT (24) or both (2) were isolated and characterized, and a wide range of associated factor patterns was observed. The methods utilized were successful in demonstrating the detection of viable strains with potentially significant pathogenic factors from human food sources.

Development of an IMS-PCR assay for the detection of Mycobacterium avium ssp. paratuberculosis in water

AIMS

To develop a sensitive detection method for Mycobacterium avium ssp. paratuberculosis (Map) in water by modifying and optimizing an existing immunomagnetic separation polymerase chain reaction (IMS-PCR) technique.

METHODS AND RESULTS

Sterile distilled water (50 ml) spiked with 10(6) Map ml(-1) was subjected to either filtration (0.45 microm pore size) followed directly by IS900 PCR (method 1) or centrifugation (2500 g for 20 min) followed by IMS and IS900 PCR (method 2). Method 2 permitted the detection of Map, whereas method 1 did not. Method 2 was then optimized by adding different concentrations of Tween 80 (0.05, 0.1, 0.2, 0.4 and 0.6% v/v) to water samples spiked with Map (10(6)-1 CFU ml(-1)) prior to centrifugation, and assessing the impact of this action on the detection sensitivity of subsequent IMS-PCR. The optimum Tween 80 concentration was found to be 0.4%, which permitted the detection of 10 Map CFU ml(-1) in spiked water samples by IMS-PCR.

CONCLUSIONS

This method will be used to determine the incidence of Map in water destined for domestic use in future studies.

SIGNIFICANCE AND IMPACT OF THE STUDY

A sensitive method for the detection of Map in water involving addition of 0.4% Tween 80, centrifugation and IMS-PCR was developed.

Persistence of Mycobacterium paratuberculosis during manufacture and ripening of cheddar cheese

Model Cheddar cheeses were prepared from pasteurized milk artificially contaminated with high 10(4) to 10(5) CFU/ml) and low (10(1) to 10(2) CFU/ml) inocula of three different Mycobacterium paratuberculosis strains. A reference strain, NCTC 8578, and two strains (806PSS and 796PSS) previously isolated from pasteurized milk for retail sale were investigated in this study. The manufactured Cheddar cheeses were similar in pH, salt, moisture, and fat composition to commercial Cheddar. The survival of M. paratuberculosis cells was monitored over a 27-week ripening period by plating homogenized cheese samples onto HEYM agar medium supplemented with the antibiotics vancomycin, amphotericin B, and nalidixic acid without a decontamination step. A concentration effect was observed in M. paratuberculosis numbers between the inoculated milk and the 1-day old cheeses for each strain. For all manufactured cheeses, a slow gradual decrease in M. paratuberculosis CFU in cheese was observed over the ripening period. In all cases where high levels (>3.6 log(10)) of M. paratuberculosis were present in 1-day cheeses, the organism was culturable after the 27-week ripening period. The D values calculated for strains 806PSS, 796PSS, and NCTC 8578 were 107, 96, and 90 days, respectively. At low levels of contamination, M. paratuberculosis was only culturable from 27-week-old cheese spiked with strain 806PSS. M. paratuberculosis was recovered from the whey fraction in 10 of the 12 manufactured cheeses. Up to 4% of the initial M. paratuberculosis load was recovered in the culture-positive whey fractions at either the high or low initial inoculum.

Effect of chemical decontamination and refrigerated storage on the isolation of Mycobacterium avium subsp. paratuberculosis from heat-treated milk

AIMS

To assess the impact of chemical decontamination and refrigerated storage before culture on the recovery of Mycobacterium avium subsp. paratuberculosis from heat-treated milk.

METHODS AND RESULTS

Five-millilitre samples of ultra heat-treated (UHT) milk spiked with Myco. paratuberculosis NCTC 8578, B4 or 806R (ca 10(6) CFU ml(-1)) were heated at 63 degrees C for 20 or 30 min by submersion in a water bath. Heat-treated milk (0.5 ml) was cultured immediately into BACTEC 12B medium or refrigerated at 4 degrees C for 48 h before culture. Milk samples that received a 20-min heat treatment were also subjected to decontamination with 0.75% cetylpyridinium chloride (CPC) for 5 h at room temperature before inoculation into BACTEC 12B medium when tested immediately and after 48 h at 4 degrees C. BACTEC vials were monitored for evidence of growth over an 18-week incubation period at 37 degrees C. CPC decontamination resulted in a significant reduction in the number of culture-positive milk samples recovered immediately after heating (P < 0.05) and after refrigerated storage for 48 h (P < 0.01). Refrigerated storage for 48 h before testing did not have any significant effect, beneficial or detrimental, on Myco. paratuberculosis recovery rates.

CONCLUSIONS

CPC decontamination applied to milk immediately or 48 h after heating will adversely affect the recovery of viable Myco. paratuberculosis, possibly leading to nonrecovery of the organism although viable cells are present in the original milk sample.

SIGNIFICANCE AND IMPACT OF THE STUDY

Published pasteurization studies in which milk samples were decontaminated before culture will have underestimated the survival capability of Myco. paratuberculosis after high-temperature, short-time pasteurization. CPC decontamination should not be applied to pasteurized milk in future studies.

Evaluation of culture media for the recovery of Mycobacterium avium subsp. paratuberculosis from Cheddar cheese

AIMS

The study evaluated the efficacy of four Mycobacterium avium subsp. paratuberculosis (MAP) culture media in suppressing commonly used starter cultures and typical nonstarter microflora present during the manufacture and ripening of Cheddar cheese, with a view to identify a suitable medium for the enumeration of MAP during laboratory-scale Cheddar production.

METHODS AND RESULTS

Four Cheddar starter cultures and Cheddar cheese manufactured with these starters were inoculated onto Herrold's egg yolk medium (HEYM); HEYM supplemented with vancomycin, amphotericin B and nalidixic acid (HEYM/VAN); Middlebrook 7H10 agar containing polymyxin, amphotericin B, nalidixic acid, trimethoprim and azlocillin (PANTA) antibiotic supplement; and BACTEC 12B radiometric medium with and without a preliminary decontamination step (0.75% w/v hexadecylpyridinium chloride (HPC), 5 h). The inclusion of a decontamination step inhibited all Cheddar cheese starter and nonstarter micro-organisms. The medium 7H10/PANTA and to a lesser extent HEYM/VAN were effective inhibitors of cheese microflora when no decontamination step was employed.

CONCLUSIONS

Middlebrook 7H10 medium, supplemented with PANTA antibiotics, suppressed all micro-organisms associated with ripening Cheddar cheese manufactured with pasteurized milk.

SIGNIFICANCE AND IMPACT OF THE STUDY

A MAP culture medium has been identified, which may be used to enumerate this bacterium during the laboratory manufacture and ripening of Cheddar cheese and hence facilitate further research into the persistence of this pathogen in the product.

Iodixanol development of a laboratory-scale technique to monitor the persistence of Mycobacterium avium subsp. paratuberculosis in Cheddar cheese

Mycobacterium avium subsp. paratuberculosis (Map) is a potential human pathogen known to be present in raw milk from infected dairy herds. Current pasteurisation regimes do not totally inactivate Map resulting in the possibility of viable cells being present in pasteurised milk used for Cheddar cheese production. A laboratory-based method, ensuring strict safety precautions, was developed to manufacture 800-g Cheddar blocks, experimentally contaminated (postpasteurisation) with two different strains of Map. The composition of the model Cheddar produced was consistent with commercial product. Syneresis of the cheese curd caused a 1 log10 concentration of Map numbers from milk to cheese for a strain isolated from pasteurised milk. The type strain NCTC 8578 did not show a similar concentration effect, but did however survive the Cheddar manufacturing process. A small percentage (<5%) of the Map load for each strain was recovered in the whey fraction during the process.

Comparative evaluation of the MGIT and BACTEC culture systems for the recovery of Mycobacterium avium subsp. paratuberculosis from milk

AIMS

To compare the detection capabilities of the non-radiometric MGIT (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 (10-107 cells ml-1) were inoculated into MGIT and BACTEC media (containing recommended supplements) with and without prior chemical decontamination of the milk samples with 0.75% (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 0.6983. 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 102-103 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.

Comparative evaluation of four decontamination protocols for the isolation of Mycobacterium avium subsp. paratuberculosis from milk

AIMS

Four chemical decontamination protocols for milk were compared with respect to mean percentage recovery of spiked Mycobacterium avium subsp. paratuberculosis, minimum detection limit and ease of application.

METHODS AND RESULTS

Raw milk spiked with 106 cfu M.a. paratuberculosis was decontaminated prior to culture by: (1) treatment with 0.75% (w/v) hexadecylpyridinium chloride (HPC) for 5 h; (2) and (3) Cornell methods employing brain heart infusion broth containing 0.75% (w/v) and 0.9% (w/v) HPC, respectively; and (4) a C18-carboxypropylbetaine (CB-18)

METHOD

The 0.75% HPC method yielded the highest mean percentage recovery of M.a. paratuberculosis (28.7%) and was capable of detecting the lowest number of cells (30 cfu/40 ml).

CONCLUSION

Treatment of milk with 0.75% HPC for 5 h was shown to be superior to the other methods for decontaminating milk prior to culture for M.a. paratuberculosis.

SIGNIFICANCE AND IMPACT OF STUDY

Certain chemical decontamination protocols are too harsh for application to milk. The "best" decontamination protocol only recovered a fraction of the M.a. paratuberculosis cells present in a milk sample.

Bactericidal effect of chlorine on Mycobacterium paratuberculosis in drinking water

AIMS

One possible route of transmission of Mycobacterium paratuberculosis from cattle to humans is via contaminated water supplies. The aim of this work was to determine whether this organism can survive standard water treatment processes.

METHODS AND RESULTS

Two strains of M. paratuberculosis (bovine strain, NCTC 8578 and human strain Linda, ATCC 43015) were subjected to various chlorine concentrations (0.5, 1.0 and 2.0 microg ml(-1)) for 15 and 30 min. Chlorine test solutions were made up in two types of water, sterile water that had been deionized and subjected to reverse osmosis (DRO) and DRO water containing MgCl(2), CaCl(2), NaHCO(3) and bovine serum albumin (0.3% w/v), the latter to mimic conditions the organism would experience in commercial water treatment operations.

CONCLUSION

The data showed that when initial inoculum levels were high (10(6) cfu ml(-1)) neither M. paratuberculosis strain was completely killed at the free chlorine concentrations and contact times applied. Log10 reductions in the range 1.32-2.82 were observed. The greatest log(10) reduction in cell numbers (2.82 and 2.35 for the bovine and human strains, respectively) was observed at the highest chlorine concentration (2 microg ml(-1)) and longest contact time (30 min).

SIGNIFICANCE AND IMPACT OF THE STUDY

This work highlights the need for further research into the survival of M. paratuberculosis during water treatment.

Incidence of Mycobacterium paratuberculosis in raw sheep and goats' milk in England, Wales and Northern Ireland

A blind survey of 104 raw sheep and goats' milk samples (90 goat, 14 sheeps) from bulk tanks on farms throughout England, Wales and Northern Ireland was carried out over a 5-month period (January-May 1998) in order to determine the incidence of Mycobacterium paratuberculosis. Each milk sample (100 ml) was divided into two 50ml portions. One portion was decontaminated with 0.75% hexadecylpyridinium chloride for 5h before culture on slopes of Herrold's egg yolk medium and in BACTEC radiometric medium. The second portion was subjected to immunomagnetic separation followed by IS900 PCR (IMS-PCR). The IMS-PCR assay was employed in order to provide a more rapid indication of the presence of M. paratuberculosis in each milk sample than is possible by culture. Information on the Johne's disease status of the sheep and goat herds that took part in the survey was not sought at the time of milk sampling. However, it subsequently emerged that at least some of the herds whose bulk milk was tested during this study were previously or currently infected with Johne's disease. Overall, during this survey one raw goats' milk sample tested positive for the presence of M. paratuberculosis by IMS-PCR (<1% of milk samples tested) but no viable M. paratuberculosis were isolated by culture. The results of this study suggest that bulk raw sheep and goats' milk from these regions of the UK may not represent significant vehicles of transmission of M. paratuberculosis to humans.

Cross-protection phenomenon in Escherichia coli strains harbouring cytotoxic necrotizing factors and cytolethal distending toxins

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 degrees 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.

Improved detection of Mycobacterium avium subsp. paratuberculosis In milk by immunomagnetic PCR

The potential use of a novel immunomagnetic PCR (IMS-PCR) technique as a rapid method to screen milk samples for the presence of Mycobacterium avium subsp. paratuberculosis (M. ptb) was assessed. Immunomagnetic separation (IMS) for M. ptb, developed at Queen's University, Belfast, was applied to milk samples prior to IS900 PCR in order to selectively concentrate any M. ptb cells present and, at the same time, separate the cells from constituents of milk likely to inhibit subsequent PCR. This increased the sensitivity of IS900 PCR. IMS-PCR sensitivity could be further increased by initial centrifugation (2500 g for 20 min) of larger volumes of milk (10 and 50 ml), and resuspension of the sediment into a 1 ml volume appropriate for IMS treatment. Following IMS, template DNA for IS900 PCR was obtained by heating the bead-cell suspension in a thermal cycler at 100 degrees C for 15 min. It was estimated that the IMS-PCR assay could detect approximately 10(3)CFU of M. ptb per 50 ml of milk (equivalent to 20 CFU/ml), whereas the minimum detection limit of direct IS900 PCR was estimated at 10(5)CFU of M. ptb per 50 ml (equivalent to 2000 CFU/ml). A blind trial was carried out in which a total of 40 spiked (10(6)CFU M. ptb) and unspiked, raw and laboratory-pasteurised milk samples were independently tested by IMS-PCR and conventional IS900 PCR. IMS-PCR correctly identified 97. 5% of milk samples (sensitivity 100%, specificity 95%), including spiked milk samples before and after laboratory-pasteurisation. One false positive result was obtained which may have resulted from carryover between samples during the IMS procedure. Conventional IS900 PCR correctly identified only 72.5% of the same 40 milk samples (sensitivity 23%, specificity 100%). IMS-PCR was also shown to be capable of detecting natural M. ptb infection in raw sheep's milk, and raw and commercially pasteurised cows' milk.

Effect of nutrient starvation on the resistance of Escherichia coli O157:H7 to subsequent heat stress

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.

Effect of higher pasteurization temperatures, and longer holding times at 72 degrees C, on the inactivation of Mycobacterium paratuberculosis in milk

Raw cow's milk spiked with 10(6) cfu ml-1 of Mycobacterium paratuberculosis was subjected to heat treatments of 72, 75, 78, 80, 85 or 90 degrees C for 15 s, and 72 degrees C for 20 and 25 s, using laboratory pasteurizing units. Three bovine strains of Myco. paratuberculosis were studied (NCTC 8578, B2 and DVL 943). Each strain was subjected to all the heat treatments indicated on three separate occasions. Although each of the heat treatments achieved a substantial (5-6 log10) reduction in numbers of viable Myco. paratuberculosis, small numbers of the organism (4-16 cfu 10 ml-1) survived in a proportion of the milk samples at each of the higher temperatures investigated, right up to 90 degrees C for 15 s. A longer holding time of 25 s at 72 degrees C was found to be more effective at inactivating Myco. paratuberculosis. Only one of the three strains studied, B2, yielded small numbers of survivors after heating at 72 degrees C for 20 s, but it was completely inactivated by extending the holding time at 72 degrees C by a further 5 s to 25 s. It was concluded that a longer holding time is more likely to achieve the complete inactivation of Myco. paratuberculosis in milk than a higher pasteurization temperature.

Isolation of Mycobacterium paratuberculosis from milk by immunomagnetic separation

An immunomagnetic separation (IMS) technique was developed to facilitate selective isolation of Mycobacterium paratuberculosis cells from milk. Rabbit polyclonal antibodies against radiation-killed intact M. paratuberculosis cells were produced and used to coat sheep anti-rabbit immunoglobulin G (IgG) type M-280 Dynabeads. The rabbit anti-M. paratuberculosis IgG-coated beads (IMB) reacted strongly with laboratory strains of M. paratuberculosis as determined by slide agglutination, and microscopic examination confirmed that M. paratuberculosis cells attached to the IMB. The IMB were found to have a maximum binding capacity of 10(4) to 10(5) CFU of M. paratuberculosis. Studies showed that IMS selectively recovered M. paratuberculosis from inoculated milk containing as few as 10 CFU of M. paratuberculosis per ml when 10 microliter IMB (ca. 10(6) beads) was added to 1 ml of milk and the preparation was incubated for 30 min at room temperature with gentle agitation. Larger volumes of milk (10 and 50 ml) were centrifuged and resuspended in 1 ml of phosphate-buffered saline-0.05% Tween 20 prior to IMS in order to increase the sensitivity of the method. Currently, primary isolation of M. paratuberculosis from a milk sample relies on chemical decontamination, followed by culturing on Herrold's egg yolk medium, which must be incubated at 37 degreesC for up to 18 weeks. The potential value of our IMS method is as an aid for rapid detection of M. paratuberculosis in milk when it is used in conjunction with end point detection methods, such as IS900 PCR or an enzyme-linked immunosorbent assay.

Effect of high-temperature, short-time (HTST) pasteurization on milk containing low numbers of Mycobacterium paratuberculosis

The efficacy of high-temperature, short-time (HTST) pasteurization (72 degrees C/15 s) when low numbers (< or = 10(3) cfu ml-1) of Mycobacterium paratuberculosis are present in milk was investigated. Raw cows' milk spiked with Myco. paratuberculosis (10(3) cfu ml-1, 10(2) cfu ml-1, 10 cfu ml-1, and 10 cfu 50 ml-1) was subjected to HTST pasteurization using laboratory pasteurizing units. Ten bovine strains of Myco. paratuberculosis were tested in triplicate. Culture in BACTEC Middlebrook 12B radiometric medium detected acid-fast survivors in 14.8% and 10% of HTST-pasteurized milk samples at the 10(3) and 10(2) cfu ml-1 inoculum levels, respectively, whereas conventional culture on Herrold's egg yolk medium containing mycobactin J detected acid-fast survivors in only 3.7% and 6.7% of the same milk samples. IS900-based PCR confirmed that these acid-fast survivors were Myco. paratuberculosis. No viable Myco. paratuberculosis were isolated from HTST-pasteurized milk initially containing either 10 cfu ml-1 or 10 cfu 50 ml-1.

Effect of non-target cells on the sensitivity of the PCR for Escherichia coli O157:H7

The sensitivity of a multiplex PCR for the virulence factors VT1, VT2 and eaeA specific for enterohaemorrhagic Escherichia coli (target cells, T) was adversely affected when non-pathogenic E. coli (non-target, NT) cells were present. In the absence of NT cells the sensitivity, obtained by decimally diluting the T cell culture, was T-10 (< 10 cfu ml-1) for eaeA and VT2, and T-5 (ca 10(4) cfu ml-1 T) for VT1 virulence factors. When approximately 10(9) cfu ml-1 NT cells (NT0 dilution) were present, the sensitivity dropped to T-1, T-3 and T-1 (ca 10(7), 10(6) and 10(8) cfu ml-1 T) for eaeA, VT2 and VT1, respectively. At NT-1 (ca 10(8) cfu ml-1 NT) or higher dilutions the sensitivity of eaeA and VT2 was the same as when no NT cells were present. In respect of VT1 the sensitivity gradually increased until at NT-4 the sensitivity was the same as when NT cells were completely absent. This work indicates that caution should be exercised when interpreting PCR results particularly when substantial non-target cell populations are suspected.

Thermal inactivation of several Mycobacterium spp. in milk by pasteurization

The thermal inactivation of Mycobacterium avium, Myco. bovis, Myco. fortuitum, Myco. intracellulare and Myco. kansaii in milk at 63.5 degrees C was investigated. Survivors were enumerated after heating for 0, 5, 10, 15, 20 and 30 min and thermal death curves were constructed for each species. Mycobacterium bovis and Myco. fortuitum were found to exhibit linear thermal death curves and neither species demonstrated any survival after heating at 63.5 degrees C for 30 min (equivalent to holder pasteurization). In contrast, Myco. avium, Myco. intracellulare and Myco. kansasii yielded thermal death curves which exhibited significant 'tailing' and all three strains survived holder pasteurization.

Inactivation of Mycobacterium paratuberculosis in cows' milk at pasteurization temperatures

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.

Development of a selective plating technique for the recovery of Escherichia coli O157:H7 after heat stress

The use of Sorbitol MacConkey Agar supplemented with 4-methylumbelliferyl beta-D-glucuronide (MSMA), which is commonly used in the isolation of Escherichia coli O157:H7, has been shown to perform poorly when stressed cells of the pathogen are present. The incorporation of a resuscitation period (2 h at 25 degrees C) on Trypticase Soy Agar (TSA) before overlay with MSMA was found to significantly (P < or = 0.01) improve recovery of heat-stressed (52 degrees C/60 min) cells. Maximal recovery was, however, obtained by adding catalase (1000 U) to the TSA before overlaying with MSMA. This recovery protocol was shown not to result in the loss of the major known virulence factors of E. coli O157:H7 (genes encoding eae, VT1 and VT2).

A PCR assay for the detection of Campylobacter jejuni and Campylobacter coli in water

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.

Effect of carbon dioxide on growth and extracellular enzyme production by Pseudomonas fluorescens B52

The effect of carbon dioxide (30 mM.l-1) on growth and extracellular protease and lipase production by Pseudomonas fluorescens B52 growing in a simulated milk medium at 7 degrees C in fermenter was investigated. The addition of carbon dioxide was shown to have a differential effect with extracellular enzyme production, particularly lipase, being inhibited to a greater extent than bacterial growth and final cell numbers.