Efficiency of high pressure treatment for destruction of Listeria monocytogenes in fruit juices

The objective of this study was to compare high pressure resistance of Listeria monocytogenes strains at 25 degrees C and 50 degrees C at 350 MPa and to use high pressure (250 MPa and 350 MPa) at 30 degrees C and 40 degrees C for the inactivation of the relatively most pressure resistant strain inoculated in pasteurized apple, apricot, cherry and orange juices. L. monocytogenes CA was found to be the relatively most pressure resistant strain and increasing pressurization from 250 MPa to 350 MPa at 30 degrees C had an additional three to four log cycle reduction in viability, still leaving viable cells after 5 min. When 350 MPa at 40 degrees C for 5 min was applied more than eight log cycle reduction in cell population of all fruit juices was achieved. This study demonstrated that low temperature (40 degrees C) high pressure (350 MPa) treatment has the potential to inactivate relatively pressure resistant L. monocytogenes strains inoculated in different fruit juices within 5 min.     

Thermal inactivation of Enterococcus faecium: effect of growth temperature and physiological state of microbial cells

AIMS: To provide data on the effects on culture temperature and physiological state of cells on heat resistance of Enterococcus faecium, which may be useful in establishing pasteurization procedures. METHODS AND RESULTS: The heat resistance of this Ent. faecium (ATCC 49624 strain) grown at different temperatures was monitored at various stages of growth. In all cases, the bacterial cells in the logarithmic phase of growth were more heat sensitive. For cells which had entered in the stationary phase, D70 values of 0.53 min at 5 degrees C, 0.74 min at 10 degrees C, 0.83 min at 20 degrees C, 0.79 min at 30 degrees C, 0.63 min at 37 degrees C, 0.48 min at 40 degrees C and 0.41 min at 45 degrees C were found. By extending the incubation times cells were more heat resistant as stationary phase progressed, although a different pattern was observed for cells grown at different temperatures. At the lower temperatures heat resistance increased progressively, reaching D70 values of 1.73 min for cells incubated at 5 degrees C for 50 days and 1.04 min for those grown at 10 degrees C for 16 days. At other temperatures assayed heat resistance became stable for late stationary phase cells, reaching D70 values of 1.05, 1.08 and 1.01 min for cultures incubated at 20, 30 and 37 degrees C. Heat resistance of cells obtained at higher temperatures, 40 and 45 degrees C, was significantly lower, with D70 values of 0.76 and 0.67 min, respectively. Neither the growth temperature nor the growth phase modified the z-values significantly. CONCLUSIONS: D70 values obtained for Ent. faecium (ATCC 49624) varies from 0.33 to 1.73 min as a function of culture temperature and physiological state of cells. However, z values calculated were not significantly influenced by these factors. A mean value of 4.50 +/- 0.39 degrees C was found. SIGNIFICANCE AND IMPACT OF THE STUDY: Overall results strongly suggest that, to establish heat processing conditions of pasteurized foods ensuring elimination of Ent. faecium, it is advisable to take into account the complex interaction of growth temperature and growth phase of cells acting on bacterial thermal resistance.     

Detection and characterization of enterocins from Enterococcus sp

Examination of 90 isolates of Enterococcus sp. revealed production of enterocin by two isolates of E. faecalis which was inhibitory to Listeria monocytogenes. Two isolates of E. gallinarum produced enterocin active against Staphylococcus aureus. None of the isolates antagonized Salmonella enteritidis. The enterocins of E. faecalis isolates were inactivated by alpha-chymotrypsin but not by trypsin and papain, while those of E. gallinarum were resistant to all the three enzymes. Enterocins produced by all 4 strains were resistant to heating at 60 degrees C for 30 min and 80 degrees C for 10 min, but sensitive to 121 degrees C for 15 min. At 100 degrees C for 10 min, two enterocins, one each of E. faecalis and E. gallinarum were inactivated, while the remaining two retained the bactericidal activity.     

Saccharomyces cerevisiae strains from traditional fermentations of Brazilian cachaça: trehalose metabolism, heat and ethanol resistance

Nine indigenous cachaça Saccharomyces cerevisiae strains and one wine strain were compared for their trehalose metabolism characteristics under non-lethal (40°C) and lethal (52°C) heat shock, ethanol shock and combined heat and ethanol stresses. The yeast protection mechanism was studied through trehalose concentration, neutral trehalase activity and expression of heat shock proteins Hsp70 and Hsp104. All isolates were able to accumulate trehalose and activate neutral trehalase under stress conditions. No correlation was found between trehalose levels and neutral trehalase activity under heat or ethanol shock. However, when these stresses were combined, a positive relationship was found. After pre-treatment at 40°C for 60 min, and heat shock at 52°C for 8 min, eight strains maintained their trehalose levels and nine strains improved their resistance against lethal heat shock. Among the investigated stresses, heat treatment induced the highest level of trehalose and combined heat and ethanol stresses activated the neutral trehalase most effectively. Hsp70 and Hsp104 were expressed by all strains at 40°C and all of them survived this temperature although a decrease in cell viability was observed at 52°C. The stress imposed by more than 5% ethanol (v/v) represented the best condition to differentiate strains based on trehalose levels and neutral trehalase activity. The investigated S. cerevisiae strains exhibited different characteristics of trehalose metabolism, which could be an important tool to select strains for the cachaça fermentation process.     

A survey for serotyping, antibiotic resistance profiling and PFGE characterization of and the potential multiplication of restaurant Salmonella isolates

AIMS: The aims of this research were (1) to determine the occurrence of Salmonella in Irish restaurant kitchens and (2) to investigate the serovar, genotype, antibiotic resistance profile and survival/growth profile of the Salmonella under catering chilled storage and temperature abuse conditions. METHODS: Five sites/tools in each of 200 randomly selected restaurant kitchens were examined for the presence of presumptive Salmonella spp. by enrichment. Serotyping, antibiotic resistance studies and genotyping were performed using the Kauffmann-White, CLSI and PulseNet methods, respectively. Survival/growth was investigated in milk, meat and vegetable products. RESULTS: Presumptive isolates from 15 of the 200 restaurant kitchens were recovered and confirmed as Salmonella positive. Seven different serovars showing a variety of antibiotic resistance profiles were detected. PFGE profiles suggested that isolates from geographically adjacent restaurants were related. Salmonella survived in foods stored at typical catering refrigeration temperatures and increased by approximately 0.8 log(10) CFU ml(-1) per day in food products stored under conditions of thermal abuse (20 degrees C). CONCLUSIONS: Inadequate hygiene has resulted in contamination of restaurant kitchens with Salmonella, which may persist/multiply in cross-contaminated foods. SIGNIFICANCE AND IMPACT OF THE STUDY: This study highlights the need for greater hygiene in restaurant kitchens coupled with rapid chilling of food not for immediate consumption and reheating before subsequent serving.     

Induction of viable but nonculturable state in Vibrio parahaemolyticus and its susceptibility to environmental stresses

AIMS: This work analysed factors that influence the induction of viable but nonculturable (VBNC) state in the common enteric pathogen, Vibrio parahaemolyticus. The susceptibility of the VBNC cells to environmental stresses was investigated. METHODS AND RESULTS: Bacterium was cultured in tryptic soy broth-3% NaCl medium, shifted to a nutrient-free Morita mineral salt-0.5% NaCl medium (pH 7.8) and further incubated at 4 degrees C in a static state to induce the VBNC state in 28-35 days. The culturability and viability of the cells were monitored by the plate count method and the Bac Light viable count method, respectively. Cells grown at the optimum growth temperature and in the exponential phase better induced the VBNC state than those grown at low temperature and in the stationary phase. Low salinity of the medium crucially and markedly shortened the induction period. The VBNC cells were highly resistant to thermal (42, 47 degrees C), low salinity (0% NaCl), or acid (pH 4.0) inactivation. CONCLUSIONS: Optimal conditions for inducing VBNC V. parahaemolyticus were reported. The increase in resistance of VBNC V. parahaemolyticus to thermal, low salinity and acidic inactivation verified that this state is entered as part of a survival strategy in an adverse environment. SIGNIFICANCE AND IMPACT OF THE STUDY: The methods for inducing VBNC V. parahaemolyticus in a markedly short time will facilitate further physiological and pathological study. The enhanced stress resistance of the VBNC cells should attract attention to the increased risk presented by this pathogen in food.     

Identification of plant-associated enterococci

AIMS: To employ an in vitro screening regime to select a probiotic Bifidobacterium strain to complement resistant starch (Hi-maizetrade mark) in a synbiotic yoghurt. METHODS AND RESULTS: Of 40 Bifidobacterium isolates examined, only B. lactis Laftitrade mark B94 possessed all of the required characteristics. This isolate hydrolysed Hi-maizetrade mark, survived well in conditions simulating passage through the gastrointestinal tract and possessed technological properties suitable for yoghurt manufacture. It grew well at temperatures up to 45 degrees C, and grew to a high cell yield in an industrial growth medium. In addition to resistant starch, the organism was able to utilize a range of prebiotics including inulin, and fructo-, galacto-, soybean- and xylo-oligosaccharides. Pulse field gel electrophoresis of restriction enzyme cut chromosomal DNA revealed that B. lactis Laftitrade mark B94 was very closely related to the B. lactis Type Strain (DSM 10140), and to the commercial strains B. lactis Bb-12 and B. lactis DS 920. However, B. lactis Laftitrade mark B94 was the only one of these isolates that could hydrolyse Hi-maizetrade mark. This phenotypic difference did not appear to be due to the presence of plasmid encoded amylase. Bifidobacterium lactis Laftitrade mark B94 survived without substantial loss of viability in synbiotic yoghurt containing Hi-maizetrade mark during storage at 4 degrees C for six weeks. CONCLUSION: Bifidobacterium lactis Laftitrade mark B94 is a promising new yoghurt culture that warrants further investigation to assess its probiotic potential. SIGNIFICANCE AND IMPACT OF THE STUDY: In vitro screening procedures can be used to integrate complementary probiotic and prebiotic ingredients for new synbiotic functional food products.     

Bacteriocin-like substance production by Bacillus licheniformis strain P40

AIMS: To investigate the production of bacteriocin-like compounds by Bacillus spp. isolated from the Amazon basin. METHODS AND RESULTS: An antimicrobial substance produced by Bacillus licheniformis strain P40 was inhibitory to a broad range of indicator strains, such as Listeria monocytogenes, Bacillus cereus and clinical isolates of Streptococcus spp. The compound was stable at 100 degrees C, but lost its activity when treated at 121 degrees C/103.5 kPa for 15 min. It was resistant to the proteolytic action of trypsin and papain but sensitive to pronase E and was stable within a wide range of pH (3-11). The substance was bactericidal and bacteriolytic to L. monocytogenes. CONCLUSIONS: An antibacterial peptide produced by Bacillus licheniformis was characterized, presenting a broad spectrum of activity against pathogenic and spoilage organisms. SIGNIFICANCE AND IMPACT OF THE STUDY: The identification of a substance active against important pathogens addresses an important aspect of food safety.     

Relationship between thermotolerance and hydrophobin-like proteins in aerial conidia of Beauveria bassiana and Paecilomyces fumosoroseus as fungal biocontrol agents

AIMS: This study was to illustrate the relationship between the thermotolerance and the contents of hydrophobin-like or formic-acid-extractable (FAE) proteins in aerial conidia of Beauveria bassiana and Paecilomyces fumosoroseus produced on rice-based substrate. METHODS AND RESULTS: Survival indices of 11 isolates were separately assessed as a ratio of the viability of conidia after 3-150 min thermal stress at 48 degrees C over that of unstressed conidia and fitted well to a survival model (r(2) >/= 0.97). For a given isolate, the fitted model generated an LT(50), the time for 50% viability loss under the stress. The LT(50)s of six B. bassiana isolates (10.1-61.9 min) and five P. fumosoroseus isolates (2.8-6.2 min) were correlated (r(2) = 0.81) with FAE protein contents (6.9-23.4 microg mg(-1)). The survival indices of a fixed B. bassiana isolate after 45-min thermal stress at 48 degrees C were also correlated to the FAE protein contents from conidia produced on glucose-, sucrose-, or starch-based substrate (0.79 相似文献   

The use of ribotyping and antibiotic resistance patterns for identification of host sources of Escherichia coli strains

AIMS: To compare antibiotic resistance and ribotyping patterns ability to identify triplicate isolates sent from a group of 40 Escherichia coli taken from seven host sources. METHODS AND RESULTS: Of the 120 isolates, 22 isolates were resistant to ampicillin, streptomycin, tetracycline and trimethoprim and 98 isolates were susceptible. Antibiotic patterns identified 33 of the triplicates and three of the six groups had isolates from multiple hosts. Ribotyping divided the isolates into 27 ribotype groups with all triplicates grouped into the same ribotype group with one host per group. CONCLUSIONS: Antibiotic susceptibility pattern placed 98 of the isolates in a single group with 50% of the antibiotic susceptibility pattern groups containing multiple host species. Ribotyping groups were host specific with each host having one to seven ribotype groups. SIGNIFICANCE AND IMPACT OF THE STUDY: Antibiotic susceptibility pattern groups have been used for environmental source identification and faecal pollution tracking, however these groups do not always distinguish between host species. Stability of the markers is a potential concern and this system can only be used if antibiotic resistance levels are high in the isolates studied. All isolates have a ribotype group which was stable and like other molecular methods has advantages over antibiotic susceptibility pattern groups which uses a phenotypic method.     

Comparison of methods for the detection of methicillin resistance in Staphylococcus aureus isolates from food products

AIMS: To compare several methods for detection of methicillin resistance in Staphylococcus aureus isolates from food. METHODS AND RESULTS: Two hundred S. aureus isolates from food of animal origin were screened for methicillin resistance by a PCR assay specific for the mecA gene, an oxacillin agar screen test and a cefoxitin disk diffusion test. Six out of 200 strains (3%) were found to be methicillin-resistant Staphylococcus aureus (MRSA) by PCR. The oxacillin agar screen test detected only one of the MRSA isolates (sensitivity of 16.7%) and mischaracterized three additional strains as MRSA (specificity of 98.45%). None of the MRSA strains was detected by the cefoxitin test (sensitivity of 0%), while 15 methicillin-susceptible S. aureus (MSSA) strains were misclassified as resistant (specificity of 92.3%). Fifteen MSSA strains displayed a beta-lactamase hyperproducer-like phenotype. The six MRSA (mecA-positive) strains resembled the characteristics of heteroresistant strains. CONCLUSIONS: As MRSA of animal origin may display atypical phenotypes, PCR appears to be more reliable for detection of methicillin resistance in animal strains. SIGNIFICANCE AND IMPACT OF THE STUDY: The study stresses the need for implementing the methods of screening S. aureus from food of animal origin for methicillin resistance.     

Physiological diversity and trehalose accumulation in Schizosaccharomyces pombe strains isolated from spontaneous fermentations during the production of the artisanal Brazilian cachaça

Twenty-seven Schizosaccharomyces pombe isolates from seven cacha?a distilleries were tested for maximum temperature of growth and fermentation, osmotolerance, ethanol resistance, invertase production, and trehalose accumulation. Two isolates were selected for studies of trehalose accumulation under heat shock and ethanol stress. The S. pombe isolates were also characterized by RAPD-PCR. The isolates were able to grow and ferment at 41 degrees C, resisted concentrations of 10% ethanol, and grew on 50% glucose medium. Four isolates yielded invertase activity of more than 100 micromol of reducing sugar x mg(-1) x min(-1). The S. pombe isolates were able to accumulate trehalose during stationary phase. Two isolates, strains UFMG-A533 and UFMG-A1000, submitted to a 15 min heat shock, were able to accumulate high trehalose levels. Strain UFMG-A533 had a marked reduction in viability during heat shock, but strain UFMG-A1000 preserved a viability rate of almost 20% after 15 min at 48 degrees C. No clear correlation was observed between trehalose accumulation and cell survival during ethanol stress. Strain UFMG-A1000 had higher trehalose accumulation levels than strain UFMG-A533 under conditions of combined heat treatment and ethanol stress. Molecular analysis showed that some strains are maintained during the whole cacha?a production period; using the RAPD-PCR profiles, it was possible to group the isolates according to their isolation sites.     

Heat and salt stress in the food pathogen Bacillus cereus

AIMS: The effects of stresses imposed on bacterial contaminants during food processing and treatment of packaging material were evaluated on the food pathogen Bacillus cereus. METHODS AND RESULTS: Conditions were established which allowed the cells to adapt to heat, ethanol and hydrogen peroxide stresses, but not to osmotic shock. Cross protection between stresses indicated a clear hierarchy of resistance with salt protecting against hydrogen peroxide, which protected against ethanol, which protected against heat shock. The cultures were shown to be most sensitive to heat, ethanol and oxidative stress at mid-exponential phase and to become resistant at stationary phase. Adaptive levels of stressor were found to induce synthesis of general stress and stress-specific proteins and differential accumulation of proteins was demonstrated between heat- or salt-stressed and unstressed cells. CONCLUSIONS: Sequencing revealed that a number of glycolytic enzymes were regulated by heat and osmotic shocks and that the chaperone GroEL was induced by heat shock. SIGNIFICANCE AND IMPACT OF THE STUDY: The implications of the physiological data in designing storage and processing conditions for food are discussed. The identification of stress-regulated proteins reveals a clear role for glycolysis in adaptation to heat shock and osmotic stress.     

Characterization of Salmonella enterica serovar Enteritidis isolates recovered from blood and stool specimens in Thailand

ABSTRACT: BACKGROUND: Bacteremia due to Salmonella spp. is a life-threatening condition and is commonly associated with immune compromise. A 2009 observational study estimated risk factors for the ten most common non-typhoidal Salmonella (NTS) serovars isolated from Thai patients between 2002-2007. In this study, 60.8% of Salmonella enterica serovar Enteritidis isolates (n = 1517) were recovered from blood specimens and infection with Salmonella serovar Enteritidis was a statistically significant risk factor for bacteremia when compared to other NTS serovars. Based on this information, we characterized a subset of isolates collected in 2008 to determine if specific clones were recovered from blood or stool specimens at a higher rate. Twenty blood isolates and 20 stool isolates were selected for antimicrobial resistance testing (MIC), phage typing, PFGE, and MLVA. Result Eight antibiogrammes, seven MLVA types, 14 XbaI/BlnI PFGE pattern combinations, and 11 phage types were observed indicating considerable diversity among the 40 isolates characterized. Composite analysis based on PFGE and MLVA data revealed 22 genotypes. Seven of the genotypes containing two or more isolates were from both stool and blood specimens originating from various months and zones. Additionally, those genotypes were all further discriminated by phage type and/or antibiogramme. Ninety percent of the isolates were ciprofloxacin resistant. CONCLUSIONS: The increased percentage of bloodstream infections as described in the 2009 observational study could not be attributed to a single clone. Future efforts should focus on assessing the immune status of bacteriaemic patients and identifying prevention and control measures, including attribution studies characterizing non-clinical (animal, food, and environmental) isolates.     

Storage of pathogenic leptospires in liquid nitrogen

The virulence and viability of various serovars of Leptospira interrogans were successfully preserved by storage in liquid nitrogen. Dimethyl sulphoxide at a final concentration of 2.5% (v/v) was added as cryoprotectant to a culture of leptospires grown in Ellinghausen-McCullough-Johnson-Harris medium. Ampoules were cooled at a controlled rate of 1 degree-3 degrees C/min to -70 degrees C, then transferred to the liquid phase of a liquid nitrogen storage unit. Glycerol was discounted as a cryoprotectant as it was found to be approximately 10 times more toxic than dimethyl sulphoxide to four of five serovars used in this study. The viability of nine strains has so far been observed over a period of 8-22 months storage in liquid nitrogen and full viability of all strains has been preserved over this period. Virulence of strains of serovars pomona and hardjo was well preserved, as demonstrated by challenge tests in guinea pigs and domestic pigs.     

Factors affecting the pressure resistance of some Campylobacter species

AIMS: To compare pressure resistance between strains of Campylobacter jejuni, Campylobacter coli, Campylobacter lari and Campylobacter fetus, and to investigate the effect of suspending medium on pressure resistance of sensitive and more resistant strains. METHODS AND RESULTS: Six strains of C. jejuni and four each of C. coli, C. lari and C. fetus were pressure treated for 10 min at 200 and 300 MPa. Individual strains varied widely in pressure resistance but there were no significant differences between the species C. jejuni, C. coli and C. lari. Campylobacter fetus was significantly more pressure sensitive than the other three species. The pressure resistance of C. jejuni cultures reached a maximum at 16-18 h on entry into stationary phase then declined to a minimum at 75 h before increasing once more. Milk was more baroprotective than water, broth or chicken slurry but did not prevent inactivation even of a resistant strain at 400 MPa. CONCLUSIONS: Pressure resistance varies considerably between species of Campylobacter and among strains within a species, and survival after a pressure challenge will be markedly influenced by culture age and food matrix. SIGNIFICANCE AND IMPACT OF THE STUDY: Despite the strain variation in pressure resistance and protective effects of food, Campylobacter sp. do not present a particular problem for pressure processing.     

Antibiotic resistant Escherichia coli and Salmonella in Russian rooks (Corvus frugilegus) wintering in the Czech Republic

AIMS: To characterize antibiotic resistant Escherichia coli and Salmonella isolates in rooks wintering in the Czech Republic. METHODS AND RESULTS: Three hundred and sixty-three faeces samples from rooks were examined for antibiotic resistant Escherichia coli and Salmonella. Altogether 13.7%E. coli isolates were resistant to antimicrobial agents tested. The dominant type of resistance was to tetracycline. Resistant E. coli isolates were examined for antibiotic resistance genes and class 1 integrons. Five of 29 antibiotic resistant isolates possessed the int1 gene. Nine Salmonella isolates (2.5%) were found in rook faeces. All the isolates belonged to serotype Salmonella enterica serovar Enteritidis phage type PT8 and PT23. CONCLUSIONS: The study suggests that rooks can be infected by antibiotic resistant E. coli and Salmonella isolates, probably reflecting the presence of such isolates in their sources of food and/or water in the environment. SIGNIFICANCE AND IMPACT OF THE STUDY: Rooks can serve as reservoirs and vectors of antibiotic resistant E. coli and Salmonella isolates and potentially transmit these isolates over long distances.     

Further comparison of temperature effects on growth and survival of Clostridium perfringens type A isolates carrying a chromosomal or plasmid-borne enterotoxin gene

Clostridium perfringens type A isolates can carry the enterotoxin gene (cpe) on either their chromosome or a plasmid, but food poisoning isolates usually have a chromosomal cpe gene. This linkage between chromosomal cpe isolates and food poisoning has previously been attributed, at least in part, to better high-temperature survival of chromosomal cpe isolates than of plasmid cpe isolates. In the current study we assessed whether vegetative cells and spores of chromosomal cpe isolates also survive better than vegetative cells and spores of plasmid cpe isolates survive when the vegetative cells and spores are subjected to low temperatures. Vegetative cells of chromosomal cpe isolates exhibited about eightfold-higher decimal reduction values (D values) at 4 degrees C and threefold-higher D values at -20 degrees C than vegetative cells of plasmid cpe isolates exhibited. After 6 months of incubation at 4 degrees C and -20 degrees C, the average log reductions in viability for spores of plasmid cpe isolates were about fourfold and about threefold greater, respectively, than the average log reductions in viability for spores from chromosomal cpe isolates. C. perfringens type A isolates carrying a chromosomal cpe gene also grew significantly faster than plasmid cpe isolates grew at 25 degrees C, 37 degrees C, or 43 degrees C. In addition, chromosomal cpe isolates grew at higher maximum and lower minimum temperatures than plasmid cpe isolates grew. Collectively, these results suggest that chromosomal cpe isolates are commonly involved in food poisoning because of their greater resistance to low (as well as high) temperatures for both survival and growth. They also indicate the importance of proper low-temperature storage conditions, as well as heating, for prevention of C. perfringens type A food poisoning.     

Variation in resistance of natural isolates of Staphylococcus aureus to heat, pulsed electric field and ultrasound under pressure

AIMS: To study and compare the resistance of 15 Staphylococcus aureus isolates to heat, pulsed electric field (PEF) and ultrasound (UW) under pressure (manosonication, MS). METHODS AND RESULTS: Survival curves to heat (58 degrees C), to PEF (22 kV cm(-1), 2 micros square wave pulses) and to UW under pressure (117 microm, 20 kHz, 200 kPa) were obtained and inactivation parameters (decimal reduction times for heat and UW under pressure, and b-values for PEF) were calculated. A wide resistance variation to heat treatment, but not to PEF and MS, was observed amongst the 15 strains. CONCLUSIONS: There was no relationship between the resistances to the three physical agents studied. Staphylococcus aureus was relatively resistant to MS but sensitive to PEF. Heat resistance varied with strain and was positively correlated to carotenoid pigment content. SIGNIFICANCE AND IMPACT OF THE STUDY: Results would help in defining safe food preservation processes. Care should be taken to choose the most adequate strain of S. aureus to model food preservation processing.     

Inactivation of Bacillus spores in reconstituted skim milk by combined high pressure and heat treatment

AIMS: To determine the resistance of a variety of Bacillus species spores to a combined high pressure and heat treatment; and to determine the affect of varying sporulation and treatment conditions on the level of inactivation achieved. METHODS AND RESULTS: Spores from eight Bacillus species (40 isolates) were high pressure-heat treated at 600 MPa, 1 min, initial temperature 72 degrees C. The level of inactivation was broad (no inactivation to 6 log10 spores ml(-1) reduction) and it varied within species. Different sporulation agar, high pressure equipment and pressure-transmitting fluid significantly affected the response of some isolates. Varying the initial treatment temperature (75, 85 or 95 degrees C) shifted the relative order of isolate high pressure-heat resistance. CONCLUSIONS: The response of Bacillus spores to combined high pressure-heat treatment is variable and can be attributed to both intrinsic and extrinsic factors. The combined process resulted in a high level of spore inactivation for several Bacillus species and is a potential alternative treatment to traditional heat-only processes. SIGNIFICANCE AND IMPACT OF THE STUDY: Sporulation conditions, processing conditions and treatment temperature all affect the response of Bacillus spores to the combined treatment of high pressure and heat. High levels of spore inactivation can be achieved but the response is variable both within and between species.