Physical and metabolic causes of sub-lethal damage in Listeria monocytogenes after long-term chilled storage at 4 degrees C

Sublethally injured microorganisms in food processing and preservation: Quantification, formation, detection, resuscitation and adaption

Sublethally injured state has been recognized as a survival strategy for microorganisms suffering from stressful environments. Injured cells fail to grow on selective media but can normally grow on nonselective media. Numerous microorganism species can form sublethal injury in various food matrices during processing and preservation with different techniques. Injury rate was commonly used to evaluate sublethal injury, but mathematical models for the quantification and interpretation of sublethally injured microbial cells still require further study. Injured cells can repair themselves and regain viability on selective media under favorable conditions when stress is removed. Conventional culture methods might underestimate microbial counts or present a false negative result due to the presence of injured cells. Although the structural and functional components may be affected, the injured cells pose a great threat to food safety. This work comprehensively reviewed the quantification, formation, detection, resuscitation and adaption of sublethally injured microbial cells. Food processing techniques, microbial species, strains and food matrix all significantly affect the formation of sublethally injured cells. Culture-based methods, molecular biological methods, fluorescent staining and infrared spectroscopy have been developed to detect the injured cells. Cell membrane is often repaired first during resuscitation of injured cells, meanwhile, temperature, pH, media and additives remarkably influence the resuscitation. The adaption of injured cells negatively affects the microbial inactivation during food processing.

A Population Balance Model to Describe the Evolution of Sublethal Injury

The detection and quantification of sublethal injury (SI) of pathogenic microorganisms has become a common procedure when assessing the efficiency of microbial inactivation treatments. However, while a plethora of studies investigates SI in function of time, no suitable modelling procedure for SI data has been proposed thus far. In this study, a new SI model structure was developed that relies on existing microbial inactivation models. This model is based on the description of inactivation kinetics between the subpopulations of healthy, sublethally injured and dead cells. The model was validated by means of case studies on previously published results, modelled by different inactivation models, i.e., (i) log-linear inactivation; (ii) biphasic inactivation; and (iii) log-linear inactivation with tailing. Results were compared to those obtained by the traditional method that relies on calculating SI from independent inactivation models on non-selective and selective media. The log-linear inactivation case study demonstrated that the SI model is equivalent to the use of independent models when there can be no mistake in calculating SI. The biphasic inactivation case study illustrated how the SI model avoids unrealistic calculations of SI that would otherwise occur. The final case study on log-linear inactivation with tailing clarified that the SI model provides a more mechanistic description than the independent models, in this case allowing the reduction of the number of model parameters. As such, this paper provides a comprehensive overview of the potential and applications for the newly presented SI model.

Sublethal injury and virulence changes in Listeria monocytogenes and Listeria innocua treated with antimicrobials carvacrol and citral

The aim of this study was to evaluate the effect of two antimicrobial substances, carvacrol and citral, on Listeria monocytogenes and Listeria innocua cells, as well as possible virulence changes in injured cells, using Caenorhabditis elegans as a model test. The results indicated that the percentage of sublethal damage was higher in L. monocytogenes than in L. innocua. The results of the study carried out by using C. elegans indicated that C. elegans fed in a lawn of L. monocytogenes previously treated with carvacrol showed a loss in life span (p ≤ 0.05) as compared with L. monocytogenes treated with citral, Escherichia coli OP50 as a negative control, and treated and untreated L. innocua. Egg laying was also affected: worms fed in a lawn of treated and untreated L. monocytogenes laid fewer eggs than those fed in a lawn of treated and untreated L. innocua or fed with OP50 as a negative control. Worms fed in a lawn of treated and untreated L. innocua also laid fewer eggs than those fed with OP50 as a negative control. A phenotype named bag of worms and an undescribed new one, "vulva inflammation", were also observed.

Acetic acid induces pH-independent cellular energy depletion in Salmonella enterica

Weak organic acids are widely used as preservatives and disinfectants in the food industry. Despite their widespread use, the antimicrobial mode of action of organic acids is still not fully understood. This study investigated the effect of acetic acid on the cell membranes and cellular energy generation of four Salmonella strains. Using a nucleic acid/protein assay, it was established that acetic acid did not cause leakage of intracellular components from the strains. A scanning electron microscopy study further confirmed that membrane disruption was not the antimicrobial mode of action of acetic acid. Some elongated Salmonella cells observed in the micrographs indicated a possibility that acetic acid may inhibit DNA synthesis in the bacterial cells. Using an ATP assay, it was found that at a neutral pH, acetic acid caused cellular energy depletion with an ADP/ATP ratio in the range between 0.48 and 2.63 (p<0.05) that was apparent for the four Salmonella strains. We suggest that this effect was probably due solely to the action of undissociated acid molecules. The antimicrobial effect of acetic acid was better under acidic conditions (ADP/ATP ratio of 5.56 ± 1.27; p<0.05), where the role of both pH and undissociated acid molecules can act together. We concluded that the inhibitory effect of acetic acid is not solely attributable to acidic pH but also to undissociated acid molecules. This finding has implication for the use of acetic acid as an antimicrobial against Salmonella on food products, such as chicken meat, which can buffer its pH.

Influence of stress on individual lag time distributions of Listeria monocytogenes

The effects of nine common food industry stresses on the times to the turbidity (T(d)) distribution of Listeria monocytogenes were determined. It was established that the main source of the variability of T(d) for stressed cells was the variability of individual lag times. The distributions of T(d) revealed that there was a noticeable difference in response to the stresses encountered by the L. monocytogenes cells. The applied stresses led to significant changes of the shape, the mean, and the variance of the distributions. The variance of T(d) of wells inoculated with single cells issued from a culture in the exponential growth phase was multiplied by at least 6 and up to 355 for wells inoculated with stressed cells. These results suggest stress-induced variability may be important in determining the reliability of predictive microbiological models.

Survival of Listeria monocytogenes strain H7762 and resistance to simulated gastric fluid following exposure to frankfurter exudate

Listeria monocytogenes strain H7762, a frankfurter isolate, was tested to determine whether it was able to survive at 4 degrees C in frankfurter pack fluid (exudate) and to determine whether food exposure affects its acid sensitivity. Cultures were sampled and tested for acid sensitivity by challenge with simulated gastric fluid (SGF). SGF challenges performed immediately after inoculation revealed that between 20 and 26% of the cells survived the full 30 min of SGF challenge regardless of whether the cells were inoculated into brain heart infusion broth (BHI) or exudate. After 2 days of incubation, cells exposed to both exudate and BHI had significantly decreased SGF resistance; however, the cells exposed to exudate were significantly more SGF resistant than cells exposed to BHI (after 15 min of SGF treatment, 33% of the exudate-exposed cells survived and 12% of the BHI-exposed cells survived). L. monocytogenes exposed to exudate had greater SGF resistance at all challenge times compared with BHI-exposed cells from day 2 through day 4. From days 8 to 15, exudate-exposed cells continued to have greater SGF resistance than BHI-exposed cells up to 10 min of SGF challenge but were as sensitive as the BHI-exposed cells at 20 to 30 min of challenge. By day 25, cells exposed to exudate were significantly more sensitive to SGF challenge than BHI-exposed cells. The survivor data generated from SGF challenges were modeled by a nonlinear regression analysis to calculate the underlying distribution of SGF resistance found in the challenged populations. These analyses indicated that L. monocytogenes exposed to exudate at 4 degrees C had a broader distribution of resistance to SGF compared with cells exposed to BHI at 4 degrees C. In addition, the mean time of death during SGF treatment was greater after exposure to exudate, indicating that cells exposed to exudate were more resistant to killing by SGF These data suggest that exposure to frankfurter exudate might render L. monocytogenes more able to survive the stomach environment during the initial stages of infection.

Growth kinetics and cell morphology of Listeria monocytogenes Scott A as affected by temperature, NaCl, and EDTA

Growth kinetics and morphological characteristics of Listeria monocytogenes Scott A grown under stress conditions induced by increasing levels of NaCl and EDTA were studied as a function of temperature. L. monocytogenes Scott A was inoculated into brain heart infusion broth (pH 6) at 19, 28, 37, and 42 degrees C. Test cultures contained NaCl (at concentrations of 4.5, 6.0, and 7.5%) or EDTA (at concentrations of 0.1, 0.2, and 0.3 mM); control cultures contained 0.5% NaCl. Growth curves were fitted from plate count data by the Gompertz equation, and growth kinetics parameters were derived. Stationary-phase cells were examined by scanning and transmission electron microscopy. Generation times (GTs) and lag phase duration times (LPDs) increased as additive levels were increased. The bacterium grew at all NaCl levels. At 37 and 42 degrees C, growth was slow in media containing 7.5% NaCl, and no growth occurred in media containing 0.3 mM EDTA. Temperature was a major factor in certain stress conditions that led to cell elongation and loss of flagella. Cells in control media at 28 degrees C grew as short rods (0.5 by 1.0 to 2.0 microm), while at 42 degrees C most cells were 4 to 10 times as long. Higher levels of NaCl at higher temperatures resulted in longer and thicker cells. At 28 degrees C, 0.1 mM EDTA had little effect on growth kinetics and morphology; however, 0.3 mM EDTA caused a sixfold increase in GT and LPD and loss of flagellae, with most cells being two to six times as long as normal. Cell length did not correlate with growth kinetics. The results of this study suggest that the effect of altered morphological characteristics of L. monocytogenes cells grown under stress on the virulence and subsequent survival of these cells should be investigated.

Combined antimicrobial effect of nisin and a listeriophage against Listeria monocytogenes in broth but not in buffer or on raw beef

The effect of nisin and listeriophage LH7, alone and in combination, on the growth and survival of two strains of Listeria monocytogenes in broth and two model food systems, with appropriate controls, was determined. Growth curves for both bacterial strains in tryptic soy broth incubated at 7 or 30 degrees C, and with the addition of nisin and/or listeriophage at lag, mid-exponential or early stationary phase, were obtained by measuring absorbance at 550 nm. Numbers of mixed populations of both L. monocytogenes strains in phosphate buffered saline (pH 5.5) and on vacuum-packaged fresh beef, both stored for 4 weeks at 4 degrees C, and with the addition of nisin and/or listeriophage, were determined. This was achieved by plating appropriately diluted samples on both Tryptic Soy Agar and Modified Oxford Agar to determine both L. monocytogenes numbers and the presence of sub-lethal injury. In broth nisin alone, reduced levels or prevented growth of the two strains under the conditions studied, but regrowth to levels equivalent to those of untreated cells, occurred. Listeriophage LH7 alone, on the other hand, had no effect in broth under the conditions studied. Notably, however, a mixture of nisin and listeriophage displayed a combined effect in broth and reduced levels of cells substantially without regrowth under the conditions studied. In both model food systems only nisin appeared to be active, in a manner consistent with existing literature, and no combined action was apparent. The use of nisin and listeriophage has potential to control L. monocytogenes in foods but a further understanding of the interactions in this complex system needs to be achieved before it could be applied practically.

Detection of heat injury in Listeria monocytogenes Scott A

Methods of detecting live pathogens in foods that may be growth inhibited following heat treatment are essential to food safety. Among the techniques available, reverse transcription polymerase chain reaction (RT-PCR) amplification of messenger RNA from heat-injured Listeria monocytogenes Scott A is preferable to direct PCR in an attempt to avoid false positives from dead cells. The RT-PCR has a detection limit of 3 x 10(6) CFU/g, compared to 3 CFU/g for untreated controls, but may not be suitable for the identification of all viable cells. Physically apparent changes in cellular structures from heat injury in L. monocytogenes are expected to result. Ultrastructural analyses did depict notable heat damage as cytoplasmic clearing after 5 min at 60 degrees C. The heat-injured survivors can be readily distinguished from total viable cells using selective media. As a result, combinations of molecular and visual methods including selective media improve detectability of heat-injured, viable L. monocytogenes Scott A.

The role of L-carnitine and glycine betaine in the survival and sub-lethal injury of non-growing Listeria monocytogenes cells during chilled storage

AIMS

To determine the role played by previous growth in the presence of osmolytes on the subsequent survival and sub-lethal injury of L. monocytogenes during long-term chilled storage in a model buffer system.

METHODS AND RESULTS

Four Listeria monocytogenes strains were grown separately to stationary phase in Listeria minimal medium (DM) alone or in DM with 4% NaCl alone, or both these media supplemented with 1 mM L-carnitine and/or 1 mM glycine betaine. Cells were resuspended in phosphate buffered saline (pH 5.5) and stored for four weeks at 4 degrees C. Initially, and at weekly intervals, samples were plated on both Tryptic Soy Agar and Tryptic Soy Agar with 4% NaCl to determine total numbers and degree of sub-lethal injury in the populations. The numbers of cells within all strains after growth to stationary phase, except one which increased ( approximately 2 log cfu ml-1, P < 0.05) in the presence of NaCl, were not influenced significantly by previous growth conditions (P > 0.05). During subsequent chilled storage, however, numbers of all strains grown in the presence of NaCl remained constant while those grown in its absence decreased. The rate and magnitude of the decrease in cell numbers was strain dependent. The initial percentage of sub-lethal injury increased significantly in all strains when grown previously in the presence of L-carnitine (P < 0.05). During subsequent chilled storage sub-lethal injury increased for all strains in a manner that was strain dependent, but not related to the previous growth conditions.

CONCLUSION

Previous growth in the presence of osmolytes of NaCl, but not osmolytes alone, increases the subsequent survival, but not percentage sub-lethal injury, of L. monocytogenes during subsequent chilled storage in buffer.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows that risks associated with L. monocytogenes in chilled food may be influenced by the individual life histories of the cells.

Survival of osmotic and acid stress by Listeria monocytogenes strains of clinical or meat origin

The ability of 30 Listeria monocytogenes strains, 15 of meat origin and 15 of clinical origin, to use carnitine as an osmoprotectant and to resist acid stress was determined. All strains examined were able to use carnitine as an osmoprotectant, indicating the importance of this characteristic to the survival of L. monocytogenes in natural environments. Clinical and meat strains, however, differed with respect to this characteristic. Specifically, 73% of meat strains reached a lower maximum cell density in the presence of carnitine with osmotic stress than in its absence with no stress. Only 33% of clinical strains displayed the same feature whereas the remaining clinical strains reached a higher maximum cell density in the presence of carnitine with osmotic stress than in its absence with no stress. The physiological reasons and advantage of this difference are unclear. When exposed to conditions of severe acid stress (pH 2.5) for 2 h, only two L. monocytogenes strains (L66 and L78), both of meat origin, displayed significant reductions (P < 0.05) in number (3.51 and 2.79 log cfu, respectively). Acid-sensitive strains were not found among the clinical isolates examined, highlighting the importance of acid stress resistance in the infection process.