A competitive microflora increases the resistance of Salmonella typhimurium to inimical processes: evidence for a suicide response

Oxidative damage and delayed replication allow viable Mycobacterium tuberculosis to go undetected

[Figure: see text].

Salmonella Cold Stress Response: Mechanisms and Occurrence in Foods

Since bacteria in foods often encounter various cold environments during food processing, such as chilling, cold chain distribution, and cold storage, lower temperatures can become a major stress environment for foodborne pathogens. Bacterial responses in stressful environments have been considered in the past, but now the importance of stress responses at the molecular level is becoming recognized. Documenting how bacterial changes occur at the molecular level may help to achieve the in-depth understanding of stress responses, to predict microbial fate when they encounter cold temperatures, and to design and develop more effective strategies to control pathogens in food for ensuring food safety. Microorganisms differ in responding to a sudden downshift in temperature and this, in turn, impacts their metabolic processes and can cause various structural modifications. In this review, the fundamental aspects of bacterial cold stress responses focused on cell membrane modification, DNA supercoiling modification, transcriptional and translational responses, cold-induced protein synthesis including CspA, CsdA, NusA, DnaA, RecA, RbfA, PNPase, KsgA, SrmB, trigger factors, and initiation factors are discussed. In this context, specific Salmonella responses to cold temperature including growth, injury, and survival and their physiological and genetic responses to cold environments with a focus on cross-protection, different gene expression levels, and virulence factors will be discussed.

Survival of Campylobacter jejuni in chicken meat at frozen storage temperatures

The aim of this study was to determine the survival of Campylobacter jejuni in chicken meat samples at frozen temperatures and given length of incubation and to determine the impact of aerobic bacteria on the survival of C. jejuni. The chicken meat samples were inoculated with C. jejuni NCTC 11351 suspensions and stored in bags at temperatures of -20°C and -70°C. The mean value of C. jejuni from meat samples decreased from 7.52 log10 CFU/g after 30 minutes of incubation at ambient temperature, to 3.87 log10 CFU/g on the eighth week of incubation at -20°C, and to 3.78 log10 CFU/g at incubation at -70°C after the same incubation period. Both freezing temperatures, -20°C and -70°C, decreased the number of campylobacters. The presence of aerobic mesophilic bacteria did not influence the survival of C. jejuni in chicken meet samples. Keeping poultry meat at freezing temperatures is important for the reduction of C. jejuni, which has a strong influence on the prevention of occurrence of campylobacteriosis in humans.

Survival of Campylobacter jejuni on beef and pork under vacuum packaged and retail storage conditions: examination of the role of natural meat microflora on C. jejuni survival

The ability of Campylobacter jejuni ATCC 11168 to survive on beef and pork stored under chilled, vacuum packaged and retail display conditions were examined. In addition, the effect of natural microflora on commercial beef and pork on the survival of C. jejuni under these storage conditions was examined. When sterile cores of beef and pork were inoculated with ∼ 10(5) to 10(6) cfu cm(-2)C. jejuni, and were stored under aerobic or vacuum packaged conditions at -1.5 or 4 °C, its numbers dropped significantly and C. jejuni could not be enumerated by direct plating after 21 d of the 6 wks study. In contrast, survival of C. jejuni on commercial vacuum packaged beef and pork was significantly enhanced, resulting in only 1 log cfu cm(-2) reduction at the end of 6 wks. During 7 d of display in a retail case, numbers of C. jejuni dropped quickly, but could be enumerated by direct plating even after the 7 d. The presence of high numbers of inoculated C. jejuni on beef and pork had no significant effect on the natural microflora numbers compared to uninoculated controls when the meat was stored either in vacuum or in a retail display case. These results show that natural microflora on vacuum packaged meat afford enhanced survival of C. jejuni present on the surfaces of both beef and pork when stored at refrigeration temperatures. Hence, strict hygienic practices or the implementation of decontamination technologies are recommended to ensure safety of meat with respect to this pathogen.

Biological approach to modeling of Staphylococcus aureus high-hydrostatic-pressure inactivation kinetics

Graphs for survival under high hydrostatic pressure (450 MPa; 25°C; citrate-phosphate buffer, pH 7.0) of stationary-growth-phase cells of eight Staphylococcus aureus strains were found to be nonlinear. The strains could be classified into two groups on the basis of the shoulder length. Some of them showed long shoulders of up to 20 min at 450 MPa, while others had shoulders of <3.5 min. All strains showed tails. No significant differences in the inactivation rate were found during the log-linear death phase among the eight strains. The entry into stationary growth phase resulted both in an increase in shoulder length and in a decrease in the inactivation rate. However, whereas shoulder length proved to depend on sigma B factor activity, the inactivation rate did not. Recovery in anaerobiosis decreased the inactivation rate but did not affect the shoulder length. Addition of the minimum noninhibitory concentration of sodium chloride to the recovery medium resulted in a decrease in shoulder length and in an increase in the inactivation rate for stationary-growth-phase cells. In the tail region, up to 90% of the population remained sensitive to sodium chloride.

Defining established and emerging microbial risks in the aquatic environment: current knowledge, implications, and outlooks

This timely review primarily addresses important but presently undefined microbial risks to public health and to the natural environment. It specifically focuses on current knowledge, future outlooks and offers some potential alleviation strategies that may reduce or eliminate the risk of problematic microbes in their viable but nonculturable (VBNC) state and Cryptosporidium oocysts in the aquatic environment. As emphasis is placed on water quality, particularly surrounding efficacy of decontamination at the wastewater treatment plant level, this review also touches upon other related emerging issues, namely, the fate and potential ecotoxicological impact of untreated antibiotics and other pharmaceutically active compounds in water. Deciphering best published data has elucidated gaps between science and policy that will help stakeholders work towards the European Union's Water Framework Directive (2000/60/EC), which provides an ambitious legislative framework for water quality improvements within its region and seeks to restore all water bodies to "good ecological status" by 2015. Future effective risk-based assessment and management, post definition of the plethora of dynamic inter-related factors governing the occurrence, persistence and/or control of these presently undefined hazards in water will also demand exploiting and harnessing tangential advances in allied disciplines such as mathematical and computer modeling that will permit efficient data generation and transparent reporting to be undertaken by well-balanced consortia of stakeholders.

Heat and acid tolerance responses of Listeria monocytogenes as affected by sequential exposure to hurdles during growth

This study aimed to evaluate the effects of the level and sequence of hurdles, applied during growth, on the subsequent heat and acid tolerances of a 10-strain composite of Listeria monocytogenes. Individual strains were grown in glucose-free tryptic soy broth with 0.6% yeast extract (TSBYE-G). Then cultures were mixed and inoculated in fresh TSBYE-G (0.5% NaCl, pH 7.42; control), TSBYE-G that was supplemented with 3% NaCl (3.5% NaCl in total), or TSBYE-G with pH adjusted to 6.01 or 5.04 with lactic acid and incubated at 30 degrees C for 24 h. Furthermore, the culture composite was exposed to the following five combinations of double sequential hurdles (12 h in each at 30 degrees C): NaCl then pH 6.01, NaCl then pH 5.04, pH 7.42 then NaCl, pH 5.04 then NaCl, and pH 6.01 then NaCl. The heat and acid tolerances of the culture were assessed at 57 degrees C (for 2 h) and at pH 3.5 (for 7 h), respectively, in TSBYE-G. No significant (P > or = 0.05) differences in thermotolerance were observed among cultures exposed to various stresses. In contrast, the acid resistance followed the order: pH 6.01 = NaCl > NaCl then pH 5.04 > pH 6.01 then NaCl = pH 5.04 > pH 5.04 then NaCl > pH 7.42 then NaCl > control. The results suggest that exposure of L. monocytogenes to NaCl and low pH during growth may not affect its heat (57 degrees C) tolerance, but it may increase its acid (pH 3.5) resistance, depending on the sequence and intensity of the applied stresses.

Stress, sublethal injury, resuscitation, and virulence of bacterial foodborne pathogens

Environmental stress and food preservation methods (e.g., heating, chilling, acidity, and alkalinity) are known to induce adaptive responses within the bacterial cell. Microorganisms that survive a given stress often gain resistance to that stress or other stresses via cross-protection. The physiological state of a bacterium is an important consideration when studying its response to food preservation techniques. This article reviews the various definitions of injury and stress, sublethal injury of bacteria, stresses that cause this injury, stress adaptation, cellular repair and response mechanisms, the role of reactive oxygen species in bacterial injury and resuscitation, and the potential for cross-protection and enhanced virulence as a result of various stress conditions.

Survival at refrigeration and freezing temperatures of Campylobacter coli and Campylobacter jejuni on chicken skin applied as axenic and mixed inoculums

Campylobacter is considered to be the most common cause of bacterial diarrhoeal illness in the developed world. Many cases are thought to be acquired from consumption of undercooked poultry. The aim of this study was to compare the effect of the rate of cooling on the survival, at 4 degrees C and -20 degrees C, of Campylobacter coli and Campylobacter jejuni strains, inoculated on chicken skin from axenic culture or as mixed inoculums. Strains chilled in a domestic refrigerator varied in their tolerance to storage at 4 degrees C. Statistically significant differences between strains applied as axenic or mixed inoculums were observed for specific strain combinations using two-way ANOVA, including the enhanced survival of antibiotic resistant C. coli 99/367 at 4 degrees C. The use of rapid cooling (at -20 degrees C/min) enhanced the survival of all the Campylobacter strains chilled to 4 degrees C compared to standard refrigeration. Freezing to -20 degrees C reduced viable counts by 2.2-2.6 log10 CFU/cm(2) in 24 h. Rapid cooling to -20 degrees C (at -30 degrees C/min) enhanced the survival of C. coli 99/367 compared to freezing in a domestic freezer. Statistically significant interaction terms between specific strains were observed in mixed inoculums chilled to -20 degrees C by freezing in a domestic freezer and by rapid chilling to -20 degrees C. Rapid chilling of poultry, particularly for 4 degrees C storage may enhance survival of Campylobacter and although this is an issue that affects meat quality, it should be considered by poultry processors.

Stress and How Bacteria Cope with Death and Survival

Bacterial populations that are exposed to rapidly changing and sometimes hostile environments constantly switch between growth, survival, and death. Understanding bacterial survival and death are therefore cornerstones in a full comprehension of microbial life. During the last few years, new insights have emerged regarding the mechanisms of bacterial inactivation under stressful conditions. Particularly under mildly lethal stress, the ultimate cause of inactivation often seems mediated by the cell itself and is subject to additional regulation that integrates information about the global state of the cell and its environmental and social surrounding. This article explores the thin line between bacterial growth and inactivation and focuses on some emerging bacterial survival strategies, both from an individual cell and from a population perspective.

The pattern of pleiomorphism in stressed Salmonella Virchow populations is nutrient and growth phase dependent

AIMS

To describe the interactions of imposed osmotic and nutritional stress on the morphology of stationary and exponential phase S. Virchow cells.

METHODS AND RESULTS

This study examined the morphology and viability of osmotically stressed exponential and stationary phase cultures of Salmonella Virchow under nutritionally deficient and competent conditions. In addition to normal morphology, salt-stressed cultures exhibited filamentous and spherical morphotypes, which were capable of reversion to normal morphology on stress removal. Proportions of atypical morphotypes were influenced by the phase of growth when the stress was applied. Salt-stressed exponential phase populations contained 54% filamentous, 30% spherical forms, salt-stressed stationary phase populations contained 16% filamentous, 79% spherical forms. Proportions of morphotypes were also influenced by the nutrient status of the medium, but not by metabolic by-products.

CONCLUSIONS

Development of a range of morphotypes in response to stress (osmotic/nutritional), may offer population level advantages, increasing the survival potential of the population.

SIGNIFICANCE AND IMPACT OF STUDY

The application of sublethal concentrations of salt may stimulate S. Virchow morphotype diversity, improving survival and rates of poststress recovery.

Suicide through stress: A bacterial response to sub-lethal injury in the food environment

The response of bacteria to sub-lethal injury is an important aspect of food microbiology as many inimical processes to which bacteria are subjected during processing are non-lethal. For pathogens like Salmonella and Escherichia coli, the difference in injury levels of exponential phase cells compared to their stationary phase counterparts in this regard is well recognised and evident for a variety of inimical processes. The expression of a range of stress resistance genes under the control of the sigma factor RpoS provides some explanation for the greater resistance of stationary phase cells. However in 1997 the suicide response hypothesis was put forward as an explanation for the observed response of Salmonella and E. coli to sub-lethal stresses. This hypothesis arose as an explanation for the observed protection of Salmonella and E. coli strains to heat and freeze-thaw injury by the presence of a high level of competitor organisms, a protection that had been shown to be RpoS independent. The central tenet of this theory was that under sub-lethal stress bacteria produce a burst of intracellular free radicals and it is these that lead to sub-lethal injury and/or death. Exponential phase cells because of their more active metabolism are more susceptible to this effect and suffer greater damage. This paper reviews the origins of this theory, the evidence for a free radical response and explores the potential mechanisms by which competitor cells produce a protective effect.

Effects of storage and the presence of a beef microflora on the thermal resistance of Salmonella Typhimurium DT104 in beef and broth systems

AIMS

To investigate the effects of storage and the presence of a beef microflora on the thermal resistance of Salmonella serotype Typhimurium DT104 on beef surfaces and in a broth system during subsequent heat treatments after extended low-temperature storage (4 degrees C for 14 days) or mild temperature abuse (10 degrees C for 7 days).

METHODS AND RESULTS

Surviving Salm. Typhimurium DT104 cells were estimated after heating in a water bath (55 degrees C) by plating beef and broth samples on tryptone soya agar and overlaying with xylose-lysine-deoxycholate agar. In beef and broth systems, D(55) values for Salm. Typhimurium DT104 stored at 4 degrees C or 10 degrees C in the presence or absence of a beef microflora were significantly lower (P < 0.01) than the D values for this organism heat-treated immediately after inoculation. In beef systems, the D(55) values were significantly lower (P < 0.05) in the presence of a beef microflora than the D(55) values obtained in 'pure' culture under all temperature/storage combinations. However, in broth systems, there was no significant difference between the D(55) values obtained in 'pure' culture and the D(55) values obtained from systems containing beef microflora.

CONCLUSIONS

Storage of Salm. Typhimurium DT104 significantly reduced the thermal resistance of the pathogen in beef and broth systems. In the presence of high numbers of a Gram-negative beef microflora, the heat sensitivity of the pathogen was further increased on beef surfaces but not in broth.

SIGNIFICANCE AND IMPACT OF THE STUDY

Studies investigating the survival of Salm. Typhimurium DT104 in different food systems will help define safe food preservation processes and will aid in the elimination this pathogen from the food production environments.

Mode of Salmonella and Escherichia coli O157:H7 inactivation by a stabilized oxychloro-based sanitizer

AIM

To determine the mechanisms by which a stabilized oxychloro (SOC)-based sanitizer, applied to decontaminate seeds destined for sprout production, inactivates Escherichia coli O157:H7 ph1 and Salmonella serotype Meleagridis.

MATERIALS AND RESULTS

The action of SOC on the metabolism, membrane and DNA integrity of Salmonella and E. coli O157:H7 was studied. In both pathogens, there was an oxidative burst and depletion of intracellular glutathione (GSH) upon initial exposure to 200 ppm SOC. Metabolic activity, measured via bioluminescence, decreased over a 4-h period in E. coli O157:H7 ph1 cells exposed to SOC. Membrane integrity, assessed through viability staining, decreased progressively over 23 h when exposed to SOC. The appearance of auxotrophic mutants suggested that DNA damage had also occurred. Enzymes rich in disulfide bonds (alkaline phosphatase and protease) were sensitive to the chlorite-based sanitizer. Through challenging other microbial types, it was found that Gram positive had higher tolerance to SOC than Gram negatives with the exception of Salmonella. MS2 bacteriophage was highly sensitive; however, Bacillus endospores were not inactivated by SOC.

CONCLUSIONS

SOC inactivates E. coli O157:H7 and Salmonella through GSH oxidation and disruption of disulfide bonds. Ultimately, membrane damage resulting from prolonged exposure to SOC leads to the loss of cell viability.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results provide a basis for understanding why extended treatment times are required to inactivate bacteria using SOC.

Impact of inoculum preparation and storage conditions on the response of Escherichia coli O157:H7 populations to undercooking and simulated exposure to gastric fluid

This study evaluated the impact of inoculum preparation and storage conditions on the response of Escherichia coli O157:H7 exposed to consumer-induced stresses simulating undercooking and digestion. Lean beef tissue samples were inoculated with E. coli O157:H7 cultures prepared in tryptic soy broth or meat decontamination runoff fluids (WASH) or detached from moist biofilms or dried biofilms formed on stainless steel coupons immersed in inoculated WASH. After inoculation, the samples were left untreated or dipped for 30 s each in hot (75 degrees C) water followed by lactic acid (2%, 55 degrees C), vacuum packaged, stored at 4 (28 days) or 12 degrees C (16 days), and periodically transferred to aerobic storage (7 degrees C for 5 days). During storage, samples were exposed to sequential heat (55 degrees C; 20 min) and simulated gastric fluid (adjusted to pH 1.0 with HCl; 90 min) stresses simulating consumption of undercooked beef. Under the conditions of this study, cells originating from inocula of planktonic cells were, in general, more resistant to heat and acid than cells from cultures grown as biofilms and detached prior to meat inoculation. Heat and acid tolerance of cells on meat stored at 4 degrees C was lower than that of cells on nondecontaminated meat stored at 12 degrees C, where growth occurred during storage. Decontamination of fresh beef resulted in injury that inhibited subsequent growth of surviving cells at 12 degrees C, as well as in decreases in resistance to subsequent heat and acid stresses. The shift of pathogen cells on beef stored under vacuum at 4 degrees C to aerobic storage did not affect cell populations or subsequent survival after sequential exposure to heat and simulated gastric fluid. However, the transfer of meat stored under vacuum at 12 degrees C to aerobic storage resulted in reduction in pathogen counts during aerobic storage and sensitization of survivors to the effects of sequential heat and acid exposure.

Induction of oxidative stress by high hydrostatic pressure in Escherichia coli

Using leaderless alkaline phosphatase as a probe, it was demonstrated that pressure treatment induces endogenous intracellular oxidative stress in Escherichia coli MG1655. In stationary-phase cells, this oxidative stress increased with the applied pressure at least up to 400 MPa, which is well beyond the pressure at which the cells started to become inactivated (200 MPa). In exponential-phase cells, in contrast, oxidative stress increased with pressure treatment up to 150 MPa and then decreased again, together with the cell counts. Anaerobic incubation after pressure treatment significantly supported the recovery of MG1655, while mutants with increased intrinsic sensitivity toward oxidative stress (katE, katF, oxyR, sodAB, and soxS) were found to be more pressure sensitive than wild-type MG1655. Furthermore, mild pressure treatment strongly sensitized E. coli toward t-butylhydroperoxide and the superoxide generator plumbagin. Finally, previously described pressure-resistant mutants of E. coli MG1655 displayed enhanced resistance toward plumbagin. In one of these mutants, the induction of endogenous oxidative stress upon high hydrostatic pressure treatment was also investigated and found to be much lower than in MG1655. These results suggest that, at least under some conditions, the inactivation of E. coli by high hydrostatic pressure treatment is the consequence of a suicide mechanism involving the induction of an endogenous oxidative burst.

Premature Salmonella Typhimurium growth inhibition in competition with other Gram-negative organisms is redox potential regulated via RpoS induction

AIMS

To identify the role of oxidation-reduction (redox) potential in the premature growth inhibition and RpoS induction in Salmonella serotype Typhimurium in competitive growth experiments.

METHODS AND RESULTS

Oxidation-reduction potential was measured throughout the growth of a minority population of Salm. Typhimurium in mixed cultures with other Gram-negative and Gram-positive organisms. A lux-based reporter was also used to evaluate RpoS activity in Salm. Typhimurium in competitor studies. In a mixed culture, the multiplication of a minority population of Salm. Typhimurium was inhibited when competing Gram-negative organisms entered the stationary phase. This was not seen when the competing flora was Gram-positive. The change in redox potential during growth in mixed cultures was closely linked to the inhibition of Salm. Typhimurium growth by Gram-negative competitors. An artificially induced drop in redox potential earlier during growth in mixed cultures with Gram-negative organisms reduced the time to RpoS induction in Salm. Typhimurium and thus inhibited its multiplication prematurely. In contrast, RpoS induction and growth inhibition were prevented under high redox potential conditions.

CONCLUSIONS

This work shows that the inhibitory activity of competitive organisms can be mediated through their effect on redox potential-regulated RpoS induction.

SIGNIFICANCE AND IMPACT OF THE STUDY

Redox potential is shown to be an important determinant of Salm. Typhimurium growth, an observation with practical implications both for its control and detection.

Source of tryptone in growth medium affects oxidative stress resistance in Escherichia coli

AIMS

To investigate the influence of the source of tryptone in the growth medium on the resistance of Escherichia coli to various types of oxidative stress.

METHODS AND RESULTS

Cultures of Escherichia coli MG1655 were grown in Luria-Bertani (LB) medium at 37 degrees C to stationary phase, harvested, and subsequently subjected to various types of oxidative stress. A marked difference in oxidative stress sensitivity was observed depending on the origin of the tryptone in the LB medium used to grow the cultures. Cells harvested from LB containing tryptone from source x (LBx) were more sensitive to inactivation by the superoxide generating compound plumbagin and by t-butyl peroxide, and to growth inhibition by the lactoperoxidase enzyme system, than cells harvested from LB containing tryptone from source y (LBy). By monitoring expression of a panel of stress gene promotors linked to the gfp (green fluorescent protein) gene, and using Delta2-22 alkaline phosphatase as a probe for disulphide bridge formation from protein sulphydryl groups, it was demonstrated that a greater cytoplasmic oxidative stress existed in cells during growth in LBy than in LBx.

CONCLUSIONS

Depending on the source of tryptone, bacteria may experience different levels of oxidative stress in tryptone-containing nonselective growth media. Although these levels of oxidative stress are subinhibitory, they may trigger a stress response that makes the bacteria more resistant to a subsequent exposure to a lethal or inhibitory level of oxidative stress.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work highlights the importance of controlling very subtle differences in composition of nonselective growth media in studies on bacterial physiology.

Oxidation-reduction potential regulates RpoS levels in Salmonella Typhimurium

AIMS

The aim of this work was to investigate the connection between oxidation-reduction (redox) potential and stationary phase induction of RpoS in Salmonella Typhimurium.

METHODS AND RESULTS

A lux-based reporter was used to evaluate RpoS activity in S. Typhimurium pure cultures. During growth of S. Typhimurium, a drop in the redox potential of the growth medium occurred at the same time as RpoS induction and entry into stationary phase. An artificially induced decrease in redox potential earlier during growth reduced the time to RpoS induction and Salmonella entered the stationary phase prematurely. In contrast, under high redox conditions, Salmonella grew unaffected and entered the stationary growth phase as normal, although RpoS induction did not occur. As a consequence, stationary phase cells grown in the high redox environment were significantly more heat sensitive (P < 0.05) than those grown under normal conditions.

CONCLUSIONS

This work suggests that redox potential can regulate RpoS levels in S. Typhimurium and can thus, control the expression of genes responsible for thermal resistance.

SIGNIFICANCE AND IMPACT OF THE STUDY

The ability to manipulate RpoS induction and control stationary phase gene expression can have important implications in food safety. Early RpoS induction under low redox potential conditions can lead to enhanced resistance in low cell concentrations to inimical processes such as heat stress. Inhibition of RpoS induction would abolish stationary phase protective properties making cells more sensitive to common food control measures.

Influence of extended acid stressing in fresh beef decontamination runoff fluids on sanitizer resistance of acid-adapted Escherichia coli O157:H7 in biofilms

This study evaluated resistance to sanitizing solutions of Escherichia coli O157:H7 cells forming biofilms on stainless steel coupons exposed to inoculated meat decontamination runoff fluids (washings). A previously acid-adapted culture of a rifampicin-resistant derivative of E. coli O157:H7 strain ATCC 43895 was inoculated in unsterilized or sterilized combined hot-water (85 degrees C) and cold-water (10 degrees C) (50/50 [vol/vol]) composite water (W) washings (pH 6.29 to 6.47) and in W washings mixed with 2% acetic acid (pH 4.60 to 4.71) or in 2% lactic acid W washings (pH 4.33 to 4.48) at a ratio of 1/99 (vol/vol). Stainless steel coupons (2 by 5 by 0.08 cm) were submerged in the inoculated washings and stored for up to 14 days at 15 degrees C. Survival of E. coli O157:H7 was determined after exposure (0 to 60 s for cells in suspension and 0 to 300 s for attached cells) to two commercial sanitizers (150 ppm peroxyacetic acid and 200 ppm quaternary ammonium compound) at 2, 7, and 14 days. E. coli O157:H7 attached more rapidly to coupons submerged in washings containing the natural flora than to those without. The attached cells were more resistant to the effects of the sanitizers than were the cells in suspension, and survival was highest in the presence of the natural flora. Attached cells in the presence of dilute acid washings were more sensitive to subsequent sanitizer treatments than were cells generated in the presence of W washings. Under the conditions of this study, cells of E. coli O157:H7 in W washings were more sensitive to acidic (peroxyacetic acid) than to alkaline (quaternary ammonium) sanitizers during storage. These results suggest that meat processing plants that apply no decontamination or that use only water washings of meat should consider using acidic sanitizers to enhance biofilm removal. Plants that apply both water and acidic washings may create a sublethal acid-stressing environment in the runoff fluids, sensitizing biofilm cells to subsequent sanitizing treatments.

Laboratory heating studies with Salmonella spp. and Escherichia coli in organic matter, with a view to decontamination of poultry houses

AIMS

To determine a temperature-humidity-time treatment that eliminates Salmonella and Escherichia coli in substrates representing organic matter in poorly cleaned poultry houses, i.e. worst case scenario laboratory tests.

METHODS AND RESULTS

Organic matter (poultry faeces and feed) in a 2.5-cm layer was inoculated with 2 x 10(5)-3 x 10(6) Salmonella g(-1), left undried or dried at ca. 30% relative humidity (RH) during a 10-day period, and temperature increased at 1 degrees C h(-)1 to the final heating temperature of 50, 55, 60, 65 or 70 degrees C and held at 16-30 or 100% RH. All samples were tested for Salmonella according to predetermined sampling time schedules and faecal samples were also tested for naturally occurring E. coli. Overall, humidity was an important factor in the elimination of Salmonella and E. coli. Results for recovery of Salmonella and E. coli were highly associated.

CONCLUSIONS

The application of >/=60 degrees C and 100% RH during a 24-h period eliminated Salmonella and E. coli in all samples. Escherichia coli could be used as an indicator bacterium for the elimination of Salmonella.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results from worst case scenario laboratory tests could be applied in steam heating of persistently Salmonella-infected poultry houses. The use of E. coli as an indicator bacterium for the validation of Salmonella results should be considered.

Cell-cell interactions influence resistance and survival of Salmonella serotype Typhimurium to environmental stress

AIMS

The aim of this work was to study the effects of prolonged nutrient stress on survival, cell interactions and resistance to inimical processes in Salmonella serotype Typhimurium.

METHODS AND RESULTS

Salmonella Typhimurium cells were subjected to prolonged incubation in the stationary phase of growth and the properties of starved cells (old) were investigated with reference to those of exponentially-growing cells (young). Competition experiments between old and young cells revealed cell-cell interactions that influenced stationary phase survival and response of the bacterium to heat stress. During prolonged incubation of cells, cycles of resistance and sensitivity to heat stress were identified. Competition experiments between old and young cells revealed that the resistance of young cells to heat increased to levels more like those of stationary phase cells than growing cells. The presence of old cells influenced the phenotype of young cells, possibly by means of cell-cell interactions. There was no evidence for the involvement of any extracellularly-produced factors in this phenomenon, but a requirement that the old competitor cells be viable could be demonstrated.

CONCLUSIONS

It is proposed that the complex interactions within stationary phase cultures of Salm. Typhimurium may be due to cycles of mutation in concert with an as yet undefined interaction between old cells and growing ones.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides evidence for active and diverse responses to nutrient stress within populations of Salm. Typhimurium that promote survival and that may be important for the success of this bacterium as a pathogen.

Photodynamic effects of antioxidant substituted porphyrin photosensitizers on gram-positive and -negative bacterial

Photodynamic treatment of the gram-negative bacteria Escherichia coli B and Acinetobacter baumannii and the gram-positive bacterium Staphylococcus aureus was performed using two newly devised and synthesized antioxidant carrier photosensitizers (antioxidant carrier sensitizers-2 [ACS-2] and antioxidant carrier sensitizers-3 [ACS-3]), which are butyl hydroxy toluene and propyl gallate substituted haematoporphyrins, respectively. It was found that ACS-2 is less reactive than other photosensitizers previously used for the same purpose, whereas ACS-3 is very effective against the multidrug-resistant bacterium A. baumannii, causing its complete eradication at a low fluence (approximately 7.5 J/cm2) of blue light (407-420 nm) and a low concentration (10 microM). At a higher fluence (approximately 37.5 J/cm2) complete eradication of E. coli B can be obtained under the same conditions. Furthermore, X-ray microanalysis and ultrastructural changes indicate that ACS-3, especially in the case of photodynamic treatment of A. baumannii, interferes with membrane functions and causes the inactivation of the bacterium. ACS-3 may be suggested as a specific photosensitization agent for photoinactivation of gram-negative bacteria.

Ecology of mixed biofilms subjected daily to a chlorinated alkaline solution: spatial distribution of bacterial species suggests a protective effect of one species to another

Three bacterial strains (Kocuria sp. C714.1, Brevibacterium linens B337.1 and Staphylococcus sciuri CCL101) were grown together on stainless steel and were subjected daily to a commercial alkaline chlorine solution (22 mg l-1 of free chlorine, pH 11) over a period of 4 weeks. After the daily chemical shock, culture madia [1:20 dilution of tryptic soy broth (TSB-YE/20) or diluted whey] was deposited on the biofilms. The chemical shocks led first to a drop in the culturable population, followed by an increase and finally stabilization at around 106-107 CFU cm-2 by day 11 of the experiment. These changes in the microbial population can be attributed to a decreasing susceptibility to the antimicrobial agent with biofilm age, and to the consumption of free chlorine by biofilm exoproteins. The microbial composition appeared to be linked to the free chlorine concentration that depended on exoprotein production. At the end of the experiment, exoprotein production was greater for biofilms grown in TSBYE/20 than in whey. As a consequence, biofilms grown in whey did not neutralize the chlorine and the dominant strain was the one having the highest resistance to chlorine: K. varians. When biofilm were grown in TSBYE/20, chlorine was neutralized and the dominant strain was the one having the highest growth rate: S. sciuri. The presence of chlorine may also explain the distribution of S. sciuri cells as a ring around Kocuria sp. microcolonies. When chlorine was totally consumed by the biofilm during the chemical shock, S. sciuri was no longer grouped around Kocuria sp. microcolonies but was evenly scattered over the substratum as single cells or in small clusters, as it was before any chemical treatment. These findings strongly suggest protection of S. sciuri by Kocuria sp. microcolonies against the chlorinated solution. This phenomenon, added to the low susceptibility phenotype of the biofilm cells, could at least partly explain the survival of microbial cells in an adverse environment.

The importance of RpoS in the survival of bacteria through food processing

The resistance of bacteria to environmental stresses is recognised as an increasingly important area of microbiology. In particular, the alternative sigma factor RpoS has been shown to produce greater stress resistance in stationary phase cells of Salmonella and Escherichia coli compared with those in exponential phase. Our work has shown that RpoS can be induced in exponential phase in response to a number of inimical processes used in the food industry, including changes in water activity produced using a range of humectants and preservatives. The presence of high levels of competitor cells will also lead to early induction of RpoS in Salmonella by an as yet unknown mechanism. High levels of competitor cells also provide Salmonella with an increased resistance to heat and freeze-thaw injury; the mechanism for this, however, is rpoS independent and has lead to the theory of a holistic mechanism for sub-lethal injury in respiring bacteria--the bacterial suicide response. This hypothesis predicts that sub-lethal injury occurs through the production of free radical species and not by the action of the applied inimical process per se. The demonstration of the production of a free radical burst when cells are subjected to differing types of stresses has been shown by a number of methods.

Influence of the natural microbial flora on the acid tolerance response of Listeria monocytogenes in a model system of fresh meat decontamination fluids

Depending on its composition and metabolic activity, the natural flora that may be established in a meat plant environment can affect the survival, growth, and acid tolerance response (ATR) of bacterial pathogens present in the same niche. To investigate this hypothesis, changes in populations and ATR of inoculated (10(5) CFU/ml) Listeria monocytogenes were evaluated at 35 degrees C in water (10 or 85 degrees C) or acidic (2% lactic or acetic acid) washings of beef with or without prior filter sterilization. The model experiments were performed at 35 degrees C rather than lower (8.0 log CFU/ml) by day 1. The pH of inoculated water washings decreased or increased depending on absence or presence of natural flora, respectively. These microbial and pH changes modulated the ATR of L. monocytogenes at 35 degrees C. In filter-sterilized water washings, inoculated L. monocytogenes increased its ATR by at least 1.0 log CFU/ml from days 1 to 8, while in unfiltered water washings the pathogen was acid tolerant at day 1 (0.3 to 1.4 log CFU/ml reduction) and became acid sensitive (3.0 to >5.0 log CFU/ml reduction) at day 8. These results suggest that the predominant gram-negative flora of an aerobic fresh meat plant environment may sensitize bacterial pathogens to acid.

Development and demonstration of RNA isolation and RT-PCR procedures to detect Escherichia coli O157:H7 gene expression on beef carcass surfaces

Preventing the development of pathogen resistance to processing and preservation techniques will require an understanding of the genetic mechanisms that pathogens use in situ to adapt and develop tolerance to stresses they encounter in the food environment. RNA isolation and reverse-transcription (RT)-PCR protocols were developed as tools to detect gene expression in bacteria on beef carcass surfaces. The utility of these procedures was demonstrated by detecting the expression of a selectively-inducible green fluorescent protein (GFP) gene in a plasmid-transformed strain of Escherichia coli O157:H7 inoculated onto beef carcass surface tissue. These procedures should serve as useful tools for studying the genetic responses of bacteria when exposed to antimicrobial interventions applied to food animal carcasses.

Review of studies on the thermal resistance of Salmonellae

Heat resistance data for different serotypes of Salmonella enterica in different food products and laboratory media are reviewed. From all D-values reported, the highest heat resistance of Salmonella was in liquid eggs and liquid egg yolks. The equation from a line drawn through the highest D-values, and above all values reported, was log D-value = 11.7 - 0.188T degrees C. From this equation, the calculated z-value was 5.3 degrees C (9.5 degrees F), and a process at 71degrees C (160 degrees F) will require 1.2 s to inactivate 1 log of Salmonella cells. This calculation did not include data that evaluated the heat resistance after stress conditions or data for Salmonella Senftenberg. The heat resistance of Salmonella is highly influenced by the strain tested, the type of experiment (log reduction versus end-point), culture conditions prior to the experiment, heating menstruum, and recovery conditions. Heat resistance data for Salmonella are still nonexistent or scarce in chicken meat, fruit juices, and aquacultured fish.

How novel methods can help discover more information about foodborne pathogens

Considerable emphasis is being placed on quantitative risk assessment modelling as a basis for regulation of trade in food products. However, for models to be accurate, information about the behaviour of potential pathogens in foods needs to be available. The question is how to obtain this knowledge in a simple and cost effective way. One technique that has great potential is the use of reporter bacteria which have been genetically modified to express a phenotype that can be easily monitored, such as light production in luminescent organisms. Bacteria carrying these (lux) genes can easily be detected using simple luminometers or more sophisticated low light imaging equipment.By monitoring light output from these bacteria over time, it can easily be determined if the organism is growing (resulting in an increase in light emission), is dead (causing a decrease in light production) or is injured (light output remains constant). The use of imaging systems allows the response of bioluminescent bacteria to be studied directly on the food, making the technique even more useful. Applications of bioluminescence are discussed below and include use as reporters of gene expression; biocide efficacy and antibiotic susceptibility; sub-lethal injury; adhesion and biofilm formation; the microbial ecology of foods; pathogenesis; and as biosensors.

Bacterial viability and culturability

Renewed interest in the relationships between viability and culturability in bacteria stems from three sources: (1) the recognition that there are many bacteria in the biosphere that have never been propagated or characterized in laboratory culture; (2) the proposal that some readily culturable bacteria may respond to certain stimuli by entering a temporarily non-culturable state termed 'viable but non-culturable' (VBNC) by some authors; and (3) the development of new techniques that facilitate demonstration of activity, integrity and composition of non-culturable bacterial cells. We review the background to these areas of interest emphasizing the view that, in an operational context, the term VBNC is self-contradictory (Kell et al., 1998) and the likely distinctions between temporarily non-culturable bacteria and those that have never been cultured. We consider developments in our knowledge of physiological processes in bacteria that may influence the outcome of a culturability test (injury and recovery, ageing, adaptation and differentiation, substrate-accelerated death and other forms of metabolic self-destruction, prophages, toxin-antitoxin systems and cell-to-cell communication). Finally, we discuss whether it is appropriate to consider the viability of individual bacteria or whether, in some circumstances, it may be more appropriate to consider viability as a property of a community of bacteria.

Induction of rpoS in Salmonella typhimurium by nutrient-poor and depleted media is slower than that achieved by a competitive microflora

The presence of a viable competitive microflora, at greater than 10(8) cfu ml-1, is known to advance the induction of RpoS-mediated gene expression in a sub-population of Salmonella typhimurium. As starvation is known to induce RpoS, one action of the competitive microflora could be to cause depletion of essential nutrients. The aim of the current experiments was to determine whether this was the case by examining RpoS induction in Salm. typhimurium in reduced nutrient media. RpoS-mediated gene expression in Salm. typhimurium was not advanced so significantly in 'conditioned' or diluted medium as it was in the presence of competitors, which indicates that nutrient depletion was not the responsible mechanism. The effect of a competitive microflora has implications for models of bacterial survival during food processing, as RpoS ultimately regulates both stress resistance and virulence.