Heat inactivation data for Mycobacterium avium subsp. paratuberculosis: implications for interpretation

Inactivation kinetics of 280 nm UV-LEDs against Mycobacterium abscessus in water

Nontuberculous mycobacteria (NTM) are opportunistic premise plumbing pathogens (OPPPs) that cause a burdensome waterborne respiratory disease. Due to their resistance to chemical disinfectants and regrowth in biofilms in drinking water distribution systems, treatment can be better performed using small ultraviolet disinfection units at the point-of-use (POU), such as at a tap or showerhead. Ultraviolet light-emitting diodes (UV-LEDs) are well suited for such applications, but fluence-response data are not available for one of the most important NTM, Mycobacterium abscessus. In this study, a bench-scale 280 nm UV-LED apparatus was used to irradiate M. abscessus in a water matrix. The fluence-response profile was sigmoidal, exhibiting both shoulder and tailing phenomena. Simple linear regression and the Geeraerd's inactivation kinetics model yielded k values of 0.36 and 0.37 cm²/mJ, respectively, revealing that M. abscessus is more resistant to UV than Pseudomonas aeruginosa and Legionella pneumophila, which suggests that NTM are among the most UV-resistant OPPPs. Results of this study suggest that 280 nm UV-LED irradiation can be an effective and practical option to inactivate M. abscessus at the POU. Disinfection units that can deliver a fluence of 10 mJ/cm² are expected to achieve nearly 2 log (99%) inactivation of M. abscessus.

Efficacy of dairy on-farm high-temperature, short-time pasteurization of milk on the viability of Mycobacterium avium ssp. paratuberculosis

Feeding pasteurized milk to suckling calves is a popular practice used increasingly on dairy farms. Waste milk is frequently fed to calves because of its high nutritional value and economic benefits compared to milk replacement products. However, one of the disadvantages of feeding waste milk is the potential for exposure to a high number of bacterial contaminants, which may lead to serious illnesses or infections in calves. One of these contaminants is Mycobacterium avium ssp. paratuberculosis (MAP), the causative agent of Johne's disease (paratuberculosis). The transmission and distribution of paratuberculosis in dairy herds occurs mostly through the feeding newborn calves with contaminated colostrum or milk, because this age group is believed to be most susceptible to infection. To reduce the risk of transmission of pathogens, on-farm pasteurization of milk has become increasingly popular. In this study, we analyzed the efficacy of a new commercial high-temperature, short-time pasteurizer (73.5°C for 20 to 25 s) in terms of MAP inactivation under experimental on-farm conditions. The pasteurizer uses a newly developed steam-heating technique, allowing for the pasteurization of the transition milk without clumping. In 3 independent trials, we spiked fresh raw milk samples to a level of 10⁷ or 10⁴ viable MAP cells/mL before pasteurization. We examined the thermal inactivation and viability of MAP using culture and a D29 bacteriophage-based assay. To verify the identity and number of MAP cells, we also performed PCR assays. Pasteurization of the inoculated milk (10⁷ and 10⁴ MAP cells/mL) resulted in a remarkable reduction in viable MAP cells. The mean inactivation rate of MAP ranged from 0.82 to 2.65 log₁₀ plaque-forming units/mL, depending on the initial MAP amount inoculated and the addition of conservative agents to the pasteurized milk. Nevertheless, approximately 10³ MAP cells/mL remained viable and could be transferred to calves after high-temperature, short-time pasteurization of milk.

Dry-heat inactivation of "Mycobacterium canettii"

Objective

“Mycobacterium canettii” is responsible for non-transmissible lymph node and pulmonary tuberculosis in persons exposed in the Horn of Africa. In the absence of direct human transmission, contaminated water and foodstuffs could be sources of contamination. We investigated the dry-heat inactivation of “M. canettii” alone and mixed into mock-infected foodstuffs by inoculating agar cylinders and milk with 10⁴ colony-forming units of “M. canettii” CIPT140010059 and two “M. canettii” clinical strains with Mycobacterium tuberculosis H37Rv as a control.

Results

Exposed to 35 °C, M. tuberculosis H37Rv, “M canettii” CIPT140010059 and “M. canettii” 157 exhibited a survival rate of 108, 95 and 81%, which is significantly higher than that of “M. canettii” 173. However, all tested mycobacteria tolerated a 90-min exposure at 45 °C. In the foodstuff models set at 70 °C, no growing mycobacteria were visualized. This study supports the premise that “M. canettii” may survive up to 45 °C; and suggests that contaminated raw drinks and foodstuffs but not cooked ones may be sources of infection for populations.

Thermal Inactivation of Mycobacterium avium subsp. paratuberculosis in Artificially Contaminated Milk by Direct Steam Injection

The efficiency of direct steam injection (DSI) at 105 °C for 3 s to inactivate Mycobacterium avium subsp. paratuberculosis in milk at a pilot-plant scale was investigated. Milk samples were artificially contaminated with M. avium subsp. paratuberculosis and also with cow fecal material naturally infected with M. avium subsp. paratuberculosis. We also tested milk artificially contaminated with Mycobacterium smegmatis as a candidate surrogate to compare thermal inactivation between M. smegmatis and M. avium subsp. paratuberculosis. Following the DSI process, no viable M. avium subsp. paratuberculosis or M. smegmatis was recovered using culture methods for both strains. For pure M. avium subsp. paratuberculosis cultures, a minimum reduction of 5.6 log10 was achieved with DSI, and a minimum reduction of 5.7 log10 was found with M. smegmatis. The minimum log10 reduction for wild-type M. avium subsp. paratuberculosis naturally present in feces was 3.3. In addition, 44 dairy and nondairy powdered infant formula (PIF) ingredients used during the manufacturing process of PIF were tested for an alternate source for M. avium subsp. paratuberculosis and were found to be negative by quantitative PCR (qPCR). In conclusion, the results obtained from this study indicate that a >7-fold-log10 reduction of M. avium subsp. paratuberculosis in milk can be achieved with the applied DSI process.

IMPORTANCE

M. avium subsp. paratuberculosis is widespread in dairy herds in many countries. M. avium subsp. paratuberculosis is the causative agent of Johne's disease in cattle, and infected animals can directly or indirectly (i.e., fecal contamination) contaminate milk. Despite much research and debate, there is no conclusive evidence that M. avium subsp. paratuberculosis is a zoonotic bacterium, i.e., one that causes disease in humans. The presence of M. avium subsp. paratuberculosis or its DNA has been reported in dairy products, including pasteurized milk, cheese, and infant formula. In light of this, it is appropriate to evaluate existing mitigation measures to inactivate M. avium subsp. paratuberculosis in dairy products. The work conducted in this study describes the efficacy of direct steam injection, a thermal process commonly used in the dairy industry, to eliminate M. avium subsp. paratuberculosis and a surrogate bacterium in milk, thus ensuring the absence of M. avium subsp. paratuberculosis in dairy products subject to these process conditions.

Estimation of Mycobacterium avium subsp. paratuberculosis growth parameters: strain characterization and comparison of methods

The growth rate of Mycobacterium avium subsp. paratuberculosis was assessed by different methods in 7H9 medium supplemented with OADC (oleic acid, albumin, dextrose, catalase), Tween 80, and mycobactin J. Generation times and maximum specific growth rates were determined by wet weight, turbidometric measurement, viable count, and quantitative PCR (ParaTB-Kuanti; F57 gene) for 8 M. avium subsp. paratuberculosis strains (K10, 2E, 316F, 81, 445, 764, 22G, and OVICAP 49). Strain-to-strain differences were observed in growth curves and calculated parameters. The quantification methods gave different results for each strain at specific time points. Generation times ranged from an average of 1.4 days for viable count and qPCR to approximately 10 days for wet weight and turbidometry. The wet-weight, turbidometry, and ParaTB-Kuanti qPCR methods correlated best with each other. Generally, viability has been assessed by viable count as a reference method; however, due to M. avium subsp. paratuberculosis clumping problems and the presence of noncultivable M. avium subsp. paratuberculosis cells, we conclude that qPCR of a single-copy gene may be used reliably for rapid estimation of M. avium subsp. paratuberculosis bacterial numbers in a sample.

Mycobacterium avium subsp. paratuberculosis in dairy products, meat, and drinking water

Mycobacterium avium subsp. paratuberculosis (Map) is the cause of Johne's disease, a chronic infection of the gut, in ruminant animals that provide milk and/or meat for human consumption. Map also may be involved in Crohn's disease and type 1 diabetes in humans. Although the role of Map in human diseases has not been established, minimizing the exposure of humans to the organism is considered desirable as a precautionary measure. Infected animals can shed Map in feces and milk, and the organism can become disseminated in tissues remote from the gut and its associated lymph nodes. The presence of at least some Map in raw milk and meat and in natural waters is likely, but the numbers of Map in those foods and waters should be reduced through cooking or purification. The available information relating to Map in milk and dairy products, meats, and drinking water is reviewed here for assessment of the risks of exposure to Map from consumption of such foods and water.

Assessment of food as a source of exposure to Mycobacterium avium subspecies paratuberculosis (MAP)

The National Advisory Committee on Microbiological Criteria for Foods assessed the importance of food as a source of exposure to Mycobacterium avium subspecies paratuberculosis (MAP). MAP is the causative agent of Johne's disease, which affects primarily the small intestine of all ruminants. The significance of MAP as a human pathogen is unknown and is being investigated by several research groups. This document also reviews the efficacy of current detection methods, processing interventions, and MAP inactivation. Research needs related to MAP are provided. The Committee reached the following conclusions: current methods for detection of MAP have significant limitations, and a standard method for the detection of viable MAP cells is needed. Aside from MAP-infected domestic ruminant animals, the organism is found infrequently. If MAP in cattle is controlled, the source of MAP in other animals, food, and water may largely be eliminated. Milk, particularly raw milk, may be a likely food source for human exposure to MAP. Given the prevalence of MAP in U.S. cattle herds, ground beef may be a potential source of MAP. Although humans may be exposed to MAP through a variety of routes, including food and the environment, the frequency and amount of exposure will require additional research.

A robust method for bacterial lysis and DNA purification to be used with real-time PCR for detection of Mycobacterium avium subsp. paratuberculosis in milk

A possible mode of transmission for the ruminant pathogen Mycobacterium avium subsp. paratuberculosis (MAP) from cattle to humans is via milk and dairy products. Although controversially, MAP has been suggested as the causative agent of Crohn's disease and its presence in consumers' milk might be of concern. A method to detect MAP in milk with real-time PCR was developed for screening of bulk tank milk. Pellet and cream fractions of milk were pooled and subjected to enzymatic digestion and mechanical disruption and the DNA was extracted by automated magnetic bead separation. The analytical sensitivity was assessed to 100 organisms per ml milk (corresponding to 1-10 CFU per ml) for samples of 10 ml. The method was applied in a study of 56 dairy herds to compare PCR of farm bulk tank milk to culture of environmental faecal samples for detection of MAP in the herds. In this study, 68% of the herds were positive by environmental culture, while 30% were positive by milk PCR. Results indicate that although MAP may be shed into milk or transferred to milk by faecal contamination, it will probably occur in low numbers in the bulk tank milk due to dilution as well as general milking hygiene measures. The concentration of MAP can therefore be assumed to often fall below the detection limit. Thus, PCR detection of MAP in milk would be more useful for control of MAP presence in milk, in order to avoid transfer to humans, than for herd prevalence testing. It could also be of value in assessing human exposure to MAP via milk consumption. Quantification results also suggest that the level of MAP in the bulk tank milk of the studied Danish dairy herds was low, despite environmental isolation of MAP from the herds.

Pulsed electric field treatment as a potential method for microbial inactivation in scaffold materials for tissue engineering: the inactivation of bacteria in collagen gel

AIMS

To investigate the effectiveness of pulsed electric field (PEF) treatment as a new method for inactivation of micro-organisms in complex biomatrices and to assess this by quantifying the inactivation of Escherichia coli seeded in collagen gels.

METHODS AND RESULTS

PEF was applied to E. coli seeded collagen gels in static (nonflowing) chambers. The influence of electric field strength, pulse number and seeded cell densities were investigated. The highest level of inactivation was obtained at the maximum field strength of 45 kV cm(-1). For low levels of E. coli contamination (10(3) CFU ml(-1)), PEF treatment resulted in no viable E. coli being recovered from the gels. However, PEF treatment of gels containing higher cell densities (>or=10(4) CFU ml(-1)) did not achieve complete inactivation of E. coli.

CONCLUSIONS

PEF treatment successfully inactivated E. coli seeded in collagen gels by 3 log(10) CFU ml(-1). Complete inactivation was hindered at high cell densities by the tailing effect observed.

SIGNIFICANCE AND IMPACT OF THE STUDY

PEF shows potential as a novel, nondestructive method for decontamination of collagen-based matrices. Further investigation is required to ensure its compatibility with other proteins and therapeutic drugs for tissue engineering and drug delivery applications.

A radial mode ultrasonic horn for the inactivation of Escherichia coli K12

Tuned cylindrical radial mode ultrasonic horns offer advantages over ultrasonic probes in the design of flow-through devices for bacterial inactivation. This study presents a comparison of the effectiveness of a radial horn and probe in the inactivation of Escherichia coli K12. The radial horn is designed using finite element analysis and the predicted modal parameters are validated using experimental modal analysis. A validated finite element model of the probe is also presented. Visual studies of the cavitation fields produced by the radial horn and probe are carried out using luminol and also backlighting to demonstrate the advantages of radial horns in producing a more focused cavitation field with widely dispersed streamers. Microbiological studies show that, for the same power density, better inactivation of E. coli K12 is achieved using the radial horn and, also, the radial horn offers greater achievable power density resulting in further improvements in bacterial inactivation. The radial horn is shown to be more effective than the probe device and offers opportunities to design in-line flow-through devices for processing applications.

Pooled faecal culture for the detection of Mycobacterium avium subsp paratuberculosis in goats

OBJECTIVE

To evaluate pooled faecal culture for herd diagnosis of caprine Johne's disease and relate these findings to faecal shedding rates of Mycobacterium avium subsp paratuberculosis (Map).

DESIGN

Radiometric broth culture was applied to several pooling dilutions, and shedding rates were estimated from a regression equation based on bacterial growth rates and known processing losses during radiometric culture.

PROCEDURE

Sixteen faecal samples from goats naturally infected with sheep (n = 3) or cattle (n = 13) strains of Map, were diluted in normal goat faeces from 1 in 5 to 1 in 50. Cultures were confirmed by IS900 polymerase chain reaction and restriction endonuclease analysis, and mycobactin dependency. The numbers of viable Map in the culture inocula were determined by endpoint titration (most probable number) of nine samples and related to a cumulative growth index.

RESULTS

A pooling dilution of 1 in 25 with an incubation period of 10 weeks detected 13 of 16 culture positive goats, all shedding > or = 2 x 10(4) Map per gram of faeces. Two samples containing very low numbers of Map (< 2 x 10(3)/g) were only culture positive from undiluted faeces. Thirteen of 16 goats were considered to be shedding low to moderate concentrations of Map (< 2 x 10(5)/g faeces).

CONCLUSIONS

These data support a pooling dilution of 1 in 25 for application of pooled faecal culture as a diagnostic tool in caprine Johne's disease control. A test based on this dilution would reduce laboratory costs of whole herd testing in goats by approximately 40% relative to serology and 75 to 90% relative to individual faecal culture.

Effective heat inactivation of Mycobacterium avium subsp. paratuberculosis in raw milk contaminated with naturally infected feces

The effectiveness of high-temperature, short holding time (HTST) pasteurization and homogenization with respect to inactivation of Mycobacterium avium subsp. paratuberculosis was evaluated quantitatively. This allowed a detailed determination of inactivation kinetics. High concentrations of feces from cows with clinical symptoms of Johne's disease were used to contaminate raw milk in order to realistically mimic possible incidents most closely. Final M. avium subsp. paratuberculosis concentrations varying from 10(2) to 3.5 x 10(5) cells per ml raw milk were used. Heat treatments including industrial HTST were simulated on a pilot scale with 22 different time-temperature combinations, including 60 to 90 degrees C at holding (mean residence) times of 6 to 15 s. Following 72 degrees C and a holding time of 6 s, 70 degrees C for 10 and 15 s, or under more stringent conditions, no viable M. avium subsp. paratuberculosis cells were recovered, resulting in >4.2- to >7.1-fold reductions, depending on the original inoculum concentrations. Inactivation kinetic modeling of 69 quantitative data points yielded an E(a) of 305,635 J/mol and an lnk(0) of 107.2, corresponding to a D value of 1.2 s at 72 degrees C and a Z value of 7.7 degrees C. Homogenization did not significantly affect the inactivation. The conclusion can be drawn that HTST pasteurization conditions equal to 15 s at > or =72 degrees C result in a more-than-sevenfold reduction of M. avium subsp. paratuberculosis.

Detection of Mycobacterium avium subspecies paratuberculosis genetic components in retail cheese curds purchased in Wisconsin and Minnesota by PCR

Research has been focused on the detection of Mycobacterium avium subspecies paratuberculosis (MAP) in pasteurized milk; however, pasteurized milk is a key ingredient in a variety of food products. Therefore, MAP contamination in milk-derived products must be investigated. We undertook a six-month study to investigate the presence of viable MAP and MAP genetic components in cheese curds purchased from retail outlets in the northern and southern regions of Wisconsin and Minnesota. A total of 98 retail cheese curd samples were tested for MAP by PCR prescreen, culture on Herrold's egg yolk agar slants with mycobactin J and amphoteracin B, naladixic acid, and vancomycin, and slant rinse PCR using IS900 and hspX primer sets. Although no viable MAP were able to be cultured, 5% of the samples were PCR positive with both the IS900 and hspX primer sets (MAP-specific DNA) when prescreened and 1% of the samples were PCR positive with both the IS900 and hspX primer sets when culture slants were rinsed and tested.

Heat inactivation of Mycobacterium avium subsp. paratuberculosis in milk

The effectiveness of pasteurization and the concentration of Mycobacterium avium subsp. paratuberculosis in raw milk have been identified in quantitative risk analysis as the most critical factors influencing the potential presence of viable Mycobacterium paratuberculosis in dairy products. A quantitative assessment of the lethality of pasteurization was undertaken using an industrial pasteurizer designed for research purposes with a validated Reynolds number of 62,112 and flow rates of 3,000 liters/h. M. paratuberculosis was artificially added to raw whole milk, which was then homogenized, pasteurized, and cultured, using a sensitive technique capable of detecting one organism per 10 ml of milk. Twenty batches of milk containing 10(3) to 10(4) organisms/ml were processed with combinations of three temperatures of 72, 75, and 78 degrees C and three time intervals of 15, 20, and 25 s. Thirty 50-ml milk samples from each processed batch were cultured, and the logarithmic reduction in M. paratuberculosis organisms was determined. In 17 of the 20 runs, no viable M. paratuberculosis organisms were detected, which represented > 6-log10 reductions during pasteurization. These experiments were conducted with very heavily artificially contaminated milk to facilitate the measurement of the logarithmic reduction. In three of the 20 runs of milk, pasteurized at 72 degrees C for 15 s, 75 degrees C for 25 s, and 78 degrees C for 15 s, a few viable organisms (0.002 to 0.004 CFU/ml) were detected. Pasteurization at all temperatures and holding times was found to be very effective in killing M. paratuberculosis, resulting in a reduction of > 6 log10 in 85% of runs and > 4 log10 in 14% of runs.

Attachment of Mycobacterium avium subspecies paratuberculosis to bovine intestinal organ cultures: Method development and strain differences

Mycobacterium avium spp. paratuberculosis (MAP) causes chronic granulomatous inflammation of the intestinal tract in many species of animals, but the mechanisms of disease are poorly understood. Attachment of bacteria to epithelial cells is a critical step in pathogenesis of many mucosal diseases. The goal of these studies was to develop an in vitro method to study attachment of MAP to bovine intestinal epithelial cells. Short-term, bovine intestinal organ cultures were used to show a significant difference in the ability of radiolabelled MAP strains to attach to intestinal epithelium. We found significant differences in the ability of different strains of MAP to attach, but there were no differences in attachment among different regions of the intestinal tract. Examination of acid fast stained tissue sections of organ cultures demonstrated that organisms were located adjacent to mucosal epithelium or within goblet cells. Coating of the organisms with fibronectin, which has been shown to be involved in attachment of many mycobacteria, including MAP, affected the attachment of the MAP strains in different ways, but did not affect the overall attachment of the organisms to different regions of the gastrointestinal tract. This organ culture method should also prove useful for defining the molecular mechanisms of attachment and interactions of MAP with intestinal epithelium.

Detection of viable Mycobacterium avium subsp. paratuberculosis in retail pasteurized whole milk by two culture methods and PCR

Cattle with Johne's disease can shed live Mycobacterium avium subsp. paratuberculosis (MAP) in their milk, and MAP can survive under simulated commercial pasteurization conditions. In several studies conducted in the United Kingdom and Canada, MAP DNA has been detected in retail pasteurized milk samples; however, in one study in the United Kingdom viable MAP was identified in commercially pasteurized milk. A double-blind study involving two laboratories was undertaken to evaluate retail pasteurized whole milk in the United States. Marshfield Clinic Laboratories used solid culture medium (Herrold's egg yolk agar slants with mycobactin J and amphotericin B, nalidixic acid, and vancomycin), and TREK Diagnostic Systems, Research and Development used liquid culture medium (ESP culture system). Cultures at both laboratories were confirmed by PCR. A total of 702 pints of retail whole milk were purchased in three of the top five milk-producing states (233 from California, 234 from Minnesota, and 235 from Wisconsin) over a 12-month period and were tested for the presence of viable MAP. The criteria used for identifying samples as positive for viable MAP were similar to those followed by most laboratories (positive culture with PCR confirmation). The combined data from the two laboratories revealed the presence of viable MAP in 2.8% of the retail whole milk pints tested. Although the number of samples containing viable MAP was similar among states (P > 0.05), there was a seasonal effect on the presence of viable MAP in retail milk (P = 0.05). More MAP-positive samples were identified during the third quarter of the year (July through September). Of the 22 brands of retail milk tested, 12 (55%) yielded at least one sample positive for viable MAP.

Possible association of GroES and antigen 85 proteins with heat resistance of Mycobacterium paratuberculosis

Conflicting reports on the heat resistance of Mycobacterium paratuberculosis prompted an examination of the effect of culture medium on this property of the organism. M. paratuberculosis was cultured in three types of media (fatty acid-containing medium 7H9-OADC (oleic acid-albumin-dextrose-catalase supplement) and glycerol-containing media WR-GD and 7H9-GD [glycerol-dextrose supplement]) at pH 6.0. M. paratuberculosis grown under these three culture conditions was then tested for heat resistance in distilled water at 65 degrees C. Soluble proteins and mycolic acids of M. paratuberculosis were evaluated by two-dimensional electrophoresis (2-DE) and thin-layer chromatography (TLC), respectively. The type of culture medium used significantly affected the heat resistance of M. paratuberculosis. The decimal reduction times at 65 degrees C (D(65 degrees C) values; times required to reduce the concentration of bacteria by a factor of 10 at 65 degrees C) for M. paratuberculosis strains grown in 7H9-OADC were significantly higher than those for the organisms grown in WR-GD medium (P < 0.01). When the glycerol-dextrose supplement of WR was substituted for the fatty acid supplement (OADC) in 7H9 medium (resulting in 7H9-GD), the D(65 degrees C) value was significantly lower than that for the organism grown in 7H9-OADC medium (P = 0.022) but higher than that when it was cultured in WR-GD medium (P = 0.005). Proteomic analysis by 2-DE of soluble proteins extracted from M. paratuberculosis grown without heat stress in the three media (7H9-OADC, 7H9-GD, and WR-GD) revealed that seven proteins were more highly expressed in 7H9-OADC medium than in the other two media. When the seven proteins were subjected to matrix-assisted laser desorption ionization-mass spectrometric analysis, four of the seven protein spots were unidentifiable. The other three proteins were identified as GroES heat shock protein, alpha antigen, and antigen 85 complex B (Ag85B; fibronectin-binding protein). These proteins may be associated with the heat resistance of M. paratuberculosis. Alpha antigen and Ag85B are both trehalose mycolyltransferases involved in mycobacterial cell wall assembly. TLC revealed that 7H9-OADC medium supported production of more trehalose dimycolates and cell wall-bound mycolic acids than did WR-GD medium. The present study shows that in vitro culture conditions significantly affect heat resistance, cell wall synthesis, and protein expression of M. paratuberculosis and emphasize the importance of culture conditions on in vitro and ex vivo studies to estimate heat resistance.

Variation in resistance of Mycobacterium paratuberculosis to acid environments as a function of culture medium

Acid resistance of Mycobacterium paratuberculosis was examined as a function of growth conditions (i.e., in vitro growth medium and pH). M. paratuberculosis was cultured in either fatty acid-containing medium (7H9-OADC) or glycerol-containing medium (WR-GD or 7H9-GD) at two culture pHs (pHs 6.0 and 6.8). Organisms produced in these six medium and pH conditions were then tested for resistance to acetate buffer at pHs 3, 4, 5, and 6 at 20 degrees C. A radiometric culture method (BACTEC) was used to quantify viable M. paratuberculosis cell data at various acid exposure times, and D values (decimal reduction times, or the times required to kill a 1-log(10) concentration of bacteria) were determined. Soluble proteins of M. paratuberculosis grown under all six conditions were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) to identify proteins that may be associated with acid resistance or susceptibility. The culture medium affected growth rate and morphology: thin floating sheets of cells were observed in 7H9-OADC versus confluent, thick, waxy, and wrinkled pellicles in WR-GD. Culture medium pH affected growth rate (which was highest at pH 6.0), but it had little or no effect on D values for M. paratuberculosis at any test pH. When grown in 7H9-OADC, M. paratuberculosis was more acid resistant at all test pHs (higher D values) than when grown in WR-GD. Glycerol appeared to be the culture medium component most responsible for lower levels of M. paratuberculosis acid resistance. When glycerol was substituted for OADC in the 7H9 medium, D values were significantly lower than those of 7H9-OADC-grown M. paratuberculosis and were approximately the same as those for M. paratuberculosis grown in WR-GD medium. Comparison of the SDS-PAGE protein profiles for M. paratuberculosis cultures grown in 7H9-OADC, WR-GD, or 7H9-GD medium revealed that increased expression of 34.2- and 14.0-kDa proteins was associated with higher levels of acid resistance of M. paratuberculosis grown in 7H9-OADC medium and that 56.6- and 41.3-kDa proteins were associated with lower levels of acid resistance. This is the first report showing that in vitro culture conditions significantly affect growth characteristics, acid resistance, and protein expression of M. paratuberculosis, and the results emphasize the importance of culture conditions for in vitro susceptibility studies.

Pasteurization of milk and the heat resistance of Mycobacterium avium subsp. paratuberculosis: a critical review of the data

Mycobacterium avium subsp. paratuberculosis (M. paratuberculosis) causes Johne's disease in ruminants (including cattle, sheep and goats) and other animals, and may contribute to Crohn's disease in humans. This possibility, and the fact that M. paratuberculosis may be present in raw milk, make it important to ensure that the heat treatment specified for pasteurization of milk will give acceptable inactivation of this bacterium, with an adequate margin of safety. Published studies of the heat resistance of this bacterium in milk have given widely differing results. Possible reasons for these differences, and the technical problems involved in the work, are reviewed. It is concluded that there is a need (i) for the adoption of an agreed Performance Criterion for pasteurization of milk in relation to this bacterium, (ii) a need for definitive laboratory experiments to understand and determine the heat resistance of M. paratuberculosis, and (iii) a need for an assessment of whether the minimum heat treatments specified at present for pasteurization of milk (Process Criteria) will meet the Performance Criterion for M. paratuberculosis. Measures are also required to ensure that commercial processes deliver continually the specified heat treatment, and to ensure that post-pasteurization contamination is avoided.