Risk factors for suicide in the Israeli army between the years 1992-2012: A case-control study.

OBJECTIVE: Young age, availability of weapons, and stressful life events, increase the risk of suicide. The aim of the present study was to assess additional risk factors for suicide in the Israeli army. METHODS: We conducted a case-control study, to assess risk factors for suicide. The cases comprised soldiers who died by suicide during their military service (n=462; 0.039% of all soldiers in the cohort). The control group consisted of soldiers who did not commit suicide but were in active service during the investigated period (n=1,170,895; 99.96%). Predictor variables, including socio-demographic and psychiatric diagnoses, were considered. RESULTS: Using a Generalized Linear Model with a Binary Logistic dependent variable to predict suicide, while controlling the effect of intervening variables, we found the following variables enhanced the risk for committing suicide: male (RR=6.703; P<0.001), country of origin: Ethiopia (RR=4.555; P=0.014), low socioeconomic status (RR=1.448; P=0.016) and low adjustment difficulties (RR=2.324; P<0.001). In addition, we found that in males only, Cluster B Personality Disorder (RR=2.548; P=0.027), low (RR=1.657; P=0.002), to average motivation to serve in a combat unit (RR=1.322; P=0.046) increased the risk for suicide. CONCLUSIONS: IDF Soldiers bearing a psychiatric diagnosis or severe adjustment difficulties remained tightly monitored through their military service, and were found to be at a lower risk for suicide. However, those enlisted with mild (low) difficulties, were found to be at greater risk for suicide, as well as soldiers whose country of origin is Ethiopia. Suicide prevention program should focus on monitoring soldiers with these risk factors, together with soldiers' guidance regarding help seeking and de-stigmatizing suicide.

An effective suicide prevention program in the Israeli Defense Forces: A cohort study.

OBJECTIVE: To evaluate the effectiveness of the IDF Suicide Prevention Program, implemented since 2006. DESIGN: Quasi-experimental (before and after) cohort study. PARTICIPANTS: Two cohorts of IDF mandatory service soldiers: the first inducted prior to (1992-2005, n=766,107) and the second subsequent to (2006-2012, n=405,252) the launching of the intervention program. EXPOSURE: The IDF Suicide Prevention Program is a population-based program, incorporating: reducing weapon availability, de-stigmatizing help-seeking behavior, integrating mental health officers into service units, and training commanders and soldiers to recognize suicide risk factors and warning signs. MAIN OUTCOME MEASURE: Suicide rate and time to suicide in cohorts before and after exposure to the Suicide Prevention Program. RESULTS: Trend analysis showed lower suicide rates in the cohort after intervention. The hazard ratio for the intervention effect on time to suicide was 0.44 (95% CI=0.34-0.56, P<.001) among males. Lower risk was associated with: male gender; born in Israel; higher socio-economic status; higher intelligence score; and serving in a combat unit (HR=0.43: 95% CI=0.33-0.55). CONCLUSIONS: There was a 57% decrease in the suicide rate following the administration of the IDF Suicide Prevention Program. The effect of the intervention appears to be related to use of a weapon, and being able to benefit from improved help-seeking and de-stigmatization. Future efforts should seek to extend the program's prevention reach to other demographic groups of soldiers. The success of the IDF program may inform suicide prevention in other military organizations and in the civilian sector.

Emotional regulation of mental pain as moderator of suicidal ideation in military settings.

BACKGROUND: In the Israel Defense Forces (IDF) as well as in most armies throughout the world, the leading cause of death during peace-time is suicide. This study examines emotional regulation of mental pain as a contributor to suicidal ideation in soldiers. METHODS: One hundred sixty-eight IDF soldiers (aged 18-21years, 59% males) completed the following self-report questionnaires: Scale for Suicide Ideation (SSI); Orbach & Mikulincer Mental Pain Scale (OMMP); and Emotional Regulation of Mental Pain questionnaire. Participants were divided into 3 groups: soldiers who attempted suicide (AS group, n=58), soldiers under treatment by a mental health professional and reporting no suicidal behavior (PT group, n=58), and controls (C group, n=50). RESULTS: Suicide ideation, mental pain, and low emotional regulation were significantly higher in the suicidal group as compared to the two other groups (P<0.001). Mental pain was significantly related to more suicide ideation in soldiers with low ability to regulate mental pain (P<.001 for the interaction). CONCLUSION: Emotional regulation of mental pain moderates the link between mental pain and suicide ideation. Soldiers with high mental pain and low regulation of mental pain exhibited higher suicidal ideation. These findings may assist in planning prevention programs in the army and similar settings.

Correlations between interpersonal and cognitive difficulties: relationship to suicidal ideation in military suicide attempters.

BACKGROUND: Understanding suicidal ideation may help develop more effective suicide screening and intervention programs. The interpersonal and the cognitive-deficit theories seek to describe the factors leading to suicidal behavior. In the military setting it is common to find over- and under-reporting of suicidal ideation. This study sought to determine the relationship between these two models and determine to what degree their components can indirectly predict suicidal ideation. METHODS: Suicide attempters (n=32) were compared with non-suicidal psychologically treated peers (n=38) and controls (n=33), matched for sex and age (mean 19.7years). Pearson's analysis was used to quantify the relationship between the variables from the two models and hierarchal regression analysis was used to determine the explanation of suicidal ideation variance by these variables. RESULTS: Suicide attempters have more difficulties in problem-solving, negative emotion regulation and burdensomeness compared with their peers (P<.001). These variables are all closely correlated with each other and to suicide ideation (r>±0.5; P<.001). Prior suicide attempt, loneliness and burdensomeness together explain 65% (P<.001) of the variance in suicidal ideation. CONCLUSIONS: Suicidal ideation is strongly correlated with components of interpersonal and cognitive difficulties. In addition to assessing current suicidal ideation, clinicians should assess past suicide attempt, loneliness and burdensomeness.

Differentiating Army suicide attempters from psychologically treated and untreated soldiers: a demographic, psychological and stress-reaction characterization.

BACKGROUND: Suicide is the leading cause of death in most armies during peace-time. The recent dramatic rise in suicides in the US Army further focuses attention on the causes of suicidal behavior in the military. METHODS: This study investigated demographic characteristics, psychological profile and stress-related risk factors associated with suicide attempts in Israelis aged 18-21 years, who served in the Army in 2009. Soldiers who attempted suicide (N=60) were compared to soldiers treated by a mental health professional, but reported no suicidal behavior (N=58), and to controls (N=50). RESULTS: Suicide attempters had lower socioeconomic status and less cognitive ability compared with treated soldiers and untreated control soldiers. Only 25% of the suicide attempters had received mental healthcare prior to the attempt. The majority of the attempts were non-lethal (86.2%), and only 5.2% used firearms. Attempters had more previous suicide attempts (37.9%) and deliberate selfharm incidents (19.3%), compared to almost no such behaviors in the other two groups. Following the suicide attempt, 77% were diagnosed with moderate to severe mental disorders, 44.8% personality disorders and 8.6% mood disorders. Attempters reported higher levels of general stress compared to their peers in the other two groups. Being away from home and obeying authority were especially more stressful in attempters. CONCLUSIONS: Young soldiers are less prone to seek mental health assistance, despite suffering from higher levels of stress. Screening is required to detect soldiers at risk for suicidal behavior and preventive intervention will require active outreach.

Immobilization and detection of Listeria monocytogenes.

A procedure was developed for immobilization of Listeria monocytogenes cells on metal hydroxides coupled with detection and enumeration using an automated optical system. The results of the immobilization procedure (<1 h) and detection during overnight incubation agreed with calculated plate counts, and this technique is simple and rapid and provides samples that are ready for confirmation of the presence of the pathogen by rapid methods.

Enhanced antimicrobial effects of combination of lactate and diacetate on Listeria monocytogenes and Salmonella spp. in beef bologna.

The antimicrobial activities of salts of organic acids such as lactate and acetate are well documented, but there is limited information on their effect when used in combination. We previously reported enhanced inhibition of Listeria monocvtogenes and Salmonella enterica serovar Enteritidis in sterile comminuted beef at 5 and 10 degrees C by combinations of sodium lactate (SL) (2.5%) and sodium diacetate (SDA) (0.2%). The present study was undertaken to evaluate the inhibitory effect of these salts, alone and in combination, in ready-to-eat (RTE) meat. Single strains and six-strain mixtures of each of the pathogens ( approximately 3 log CFU/g) were tested in beef bologna during aerobic storage at 5 and 10 degrees C for up to 60 days. The growth rate of the six-strain mixture of Listeria was faster than that of the single strain (Scott A) in the lactate/diacetate-free product. While each of the salts delayed growth of the listeriae at 5 degrees C, the effect of their combination was listericidal for the single strain and listeriostatic for the six-strain mixture. Enhanced inhibition by the salt combination was also observed at 10 degrees C. Salmonella numbers declined to undetectable levels in the untreated meat product and in each of the treatments after 20-30 days. However, the decline was more rapid in meat with the combination of the salts during storage at both 5 and 10 degrees C. Each of the salts further delayed the growth of the background microflora during storage at 5 degrees C, with their combinations showing the most effect.

Effects of sodium chloride and lactates on chemical and microbiological changes in refrigerated and frozen fresh ground pork.

Changes in fat oxidation and color of freshly ground pork, 14% fat, during storage at 2 °C for 15 days and at -20 °C for up to 70 days were determined. The fresh meat was further treated with NaCl (1 and 2%, by weight), sodium or potassium lactate (2%), and combinations of NaCl and lactates. The microbial stability of the refrigerated meat was enhanced by the lactates, and their combinations with NaCl were more effective than lactates alone in delaying the onset of spoilage. Changes in TBARS were seen in the untreated meat only after it was judged spoiled when stored at 2 °C (day 7), and after 69 days at -20 °C. Among treatments, fat stability was highest in lactate-treated meat, and TBARS values were generally similar to those in untreated meat. NaCl enhanced fat oxidation, and the effects of 2% salt were significantly higher than those of 1% (P<0.05). Combinations with lactates reduced the prooxidant effects of NaCl at both storage temperatures, and the effects were more pronounced in meat with 2% NaCl stored at -20 °C. The red color (a* value) was enhanced by NaCl and lactates immediately after their addition to the meat. However, values for all treatments, including the untreated meat, declined rapidly after 4 days at 2 °C, and were 50-70% lower than the initial values after 8 days. Color stability in the frozen meat was highest in control and lactate-treated samples throughout the storage period. It was lowest in samples with 2% NaCl, whether alone or in combination with lactates. Sodium or potassium lactate (2%) enhanced the microbial stability of refrigerated pork without deleterious effects on its color or fat stability. Combinations of lactates with NaCl improved the fat stability, particularly during storage at -20 °C, by reducing the prooxidant activity of NaCl.

Enhanced inhibition of Listeria monocytogenes and salmonella enteritidis in meat by combinations of sodium lactate and diacetate.

The antimicrobial activities of sodium lactate (SL) and sodium acetate (SA) are well documented, but there is limited information on the effect of their combination or of the combination of SL and sodium diacetate (SDA) on survival and growth of Listeria monocytogenes and salmonellae in meat. Effects of SL (1.8 and 2.5%), SDA (0.1 and 0.2%), or SA (0.2%) and their combinations on the behavior of L monocytogenes and Salmonella enterica serovar Enteritidis were investigated in sterile comminuted beef (pH 6.3, 79% moisture) during storage at 5 and 10 degrees C. Although L. monocytogenes grew faster than Salmonella Enteritidis in control samples at 10 degrees C, numbers of both pathogens increased from 3.5 to approximately 8.0 log CFU/g after 20 days. SL (1.8%) decreased the growth rate of both L. monocytogenes and Salmonella Enteritidis. SDA (0.2%) was more effective than SL in decreasing the growth rate of L monocytogenes, and it caused a more than 1 log CFU/g decline in initial numbers of Salmonella Enteritidis during storage for 25 days at 10 degrees C. Synergy was observed by combinations of SL and SDA. Combinations of 2.5% SL and 0.2% SDA were bacteriostatic to L. monocytogenes and bactericidal to Salmonella Enteritidis after 20 days at 10 degrees C. At 5 degrees C, a listeriostatic effect was produced by 1.8% SL + 0.1% SDA, whereas numbers of Salmonella Enteritidis were less than 10 cells/g after refrigeration for 30 days. Although SA was consistently and significantly less inhibitory than SDA, its mixtures with SL also demonstrated synergistic activity against both pathogens. Combinations of 2.5% SL and 0.2% SDA can be expected to greatly enhance the safety of refrigerated and temperature-abused ready-to-eat meats.

Automated simultaneous detection of low levels of listeriae and salmonellae in foods.

Salmonella spp. and Listeria monocytogenes continue to be major pathogens of concern to food processors. However, routine screening of food samples to detect these pathogens is generally labor intensive and costly. Automated optical procedures for the detection of salmonellae and listeriae in foods were developed in our laboratory. In the present study we report their adaptation to a simultaneous recovery and detection procedure. Milk, shell eggs, fresh and ready-to-eat (RTE) meats or raw chicken contaminated with a combination of sub-lethally injured salmonellae and listeriae (10-50 cells each) were incubated for 6 h at 35 degrees C in modified universal pre-enrichment broth (MUPB). Volumes (4 ml) were then transferred to vials containing selective liquid media for these pathogens (4 ml), and incubated overnight at 35 degrees C in a BioSys instrument. The presence of the pathogens was identified by a black coloration of the media and a sharp drop in light transmittance caused by hydrogen sulfide production (Salmonella organisms), or esculin hydrolysis (Listeria organisms). There was no difference in the detection time of salmonellae when incubated alone or with listeriae, but listeriae grew at a slower rate in the presence of salmonellae, resulting in a delay of < or = 1 h in their detection. Overall, the detection of 10-50 salmonellae and 10-50 listeriae in 25 g of the tested foods required a total of 24 h. Confirmation of the pathogens by PCR-based assay (6 h) was completed the following day directly from positive vials, requiring a total of < or = 30 h for detection and confirmation. Negative samples required no confirmation. The testing system was confirmed in 70 naturally contaminated foods.

Rapid detection of low levels of Listeria in foods and next-day confirmation of L. monocytogenes.

Outbreaks of foodborne listeriosis caused by Listeria monocytogenes in recent years, and the high mortality rate associated with listeriosis, have raised the need for reliable and rapid detection of the pathogen. A simple, automated method was developed for the detection of Listeria organisms in foods. It consists of a 6-h pre-enrichment step followed by overnight incubation in selective broth at 35 degrees C. Changes in light transmittance in the selective broth are registered continuously by an optical sensor of the BioSys instrument (MicroSys, Ann Arbor, MI), and recorded in the computer. Esculin hydrolysis by listeriae results in black coloration of the media that causes a sharp drop in light transmittance, whereas negative samples remain colorless. Confirmation of L. monocytogenes is carried out only on esculin-positive samples and is completed within 6 h. Detection of 10-50 cells of Listeria inoculated into 25 g of food was confirmed in shell eggs, milk and ground beef. Naturally contaminated raw and ready-to-eat foods were further screened to validate the procedure.

A new rapid automated method for the detection of Listeria from environmental swabs and sponges.

Many food and meat processors test environmental swabs and sponges to confirm the absence of Listeria spp. Spectral pattern changes in a liquid growth medium, resulting from esculin hydrolysis by Listeria in contaminated swabs and sponges, were automatically monitored by the BioSys instrument in a semifluid layer (SFL). The blackening of SFL in modified MOX broth resulted in sharply declining curves, which were easily detected by the instrument. The instrument detected all nine strains of Listeria monocytogenes tested. None of the gram negative organisms (Proteus, Escherichia coli, Pseudomonas, Citrobacter and Yersinia) were detected by the system, nor were most gram positive organisms, including Bacillus, Streptococcus, and Lactobacillus strains, Staphylococcus aureus, Enterococcus faecium and E. faecalis, which hydrolyze esculin, produced black colonies on PALCAM and Oxford media and were also detected in the system. A total of 122 sponges and swabs collected at food processing plants were evaluated by this method. Of these, 99 were negative, and 11 were positive. L. innocua was the dominant Listeria species in these environmental samples. Good correlation was obtained between numbers of Listeria and detection times of esculin hydrolysis: 1000 CFU/swab were detected in 10-13 h, whereas 1-10 CFU/swab were detected in less than 22 h. The total assay time was 26 h.

Simple sequence repeats in Escherichia coli: abundance, distribution, composition, and polymorphism.

Computer-based genome-wide screening of the DNA sequence of Escherichia coli strain K12 revealed tens of thousands of tandem simple sequence repeat (SSR) tracts, with motifs ranging from 1 to 6 nucleotides. SSRs were well distributed throughout the genome. Mononucleotide SSRs were over-represented in noncoding regions and under-represented in open reading frames (ORFs). Nucleotide composition of mono- and dinucleotide SSRs, both in ORFs and in noncoding regions, differed from that of the genomic region in which they occurred, with 93% of all mononucleotide SSRs proving to be of A or T. Computer-based analysis of the fine position of every SSR locus in the noncoding portion of the genome relative to downstream ORFs showed SSRs located in areas that could affect gene regulation. DNA sequences at 14 arbitrarily chosen SSR tracts were compared among E. coli strains. Polymorphisms of SSR copy number were observed at four of seven mononucleotide SSR tracts screened, with all polymorphisms occurring in noncoding regions. SSR polymorphism could prove important as a genome-wide source of variation, both for practical applications (including rapid detection, strain identification, and detection of loci affecting key phenotypes) and for evolutionary adaptation of microbes.

Fate of pGFP-bearing Escherichia coli O157:H7 in ground beef at 2 and 10 degrees C and effects of lactate, diacetate, and citrate.

Although beef has been implicated in the largest outbreaks of Escherichia coli O157:H7 infection in the United States, studies on the fate of this pathogen have been limited. Problems in such studies are associated with detection of the pathogen at levels considerably lower than the levels of the competing microorganisms. In the present study, a green fluorescent protein-expressing E. coli O157:H7 strain was used, and the stable marker allowed us to monitor the behavior of the pathogen in ground beef stored aerobically from freshness to spoilage at 2 and 10 degrees C. In addition, the effects of sodium salts of lactate (SL) (0.9 and 1.8%), diacetate (SDA) (0.1 and 0.2%), and buffered citrate (SC) (1 and 2%) and combinations of SL and SDA were evaluated. SC had negligible antimicrobial activity, and SL delayed microbial growth, while SDA and SL plus SDA were most inhibitory to the total-aerobe population in the meat. At 2 degrees C, the initial numbers of E. coli O157:H7 (3 and 5 log(10) CFU/g) decreased by approximately 1 log(10) CFU/g when spoilage was manifest (>7 log(10) CFU of total aerobes/g), irrespective of the treatment. There was no decline in the numbers of the pathogen during storage at 10 degrees C. Our results showed that the pathogen was resistant to the salts tested and confirmed that refrigerated meat contaminated with the pathogen remains hazardous.

Automated rapid screening of foods for the presence of salmonellae.

We previously reported an accelerated method to detect salmonellae in foods. It consisted of a preenrichment step (6 h) followed by immunomagnetic separation (IMS) and automated detection, based on positive biochemical characteristics, during overnight incubation. We describe here a simplified procedure with a detection limit of 2 CFU/25 g of food, a procedure that does not require the IMS step. Liquid eggs, milk, ice cream, or poultry were inoculated with Salmonella or non-Salmonella Enterobacteriaceae organisms (2 to 100 CFU/25 g). Following preenrichment for 6 h at 35 degrees C, samples (3 ml) were added to selective liquid media (6 ml). Automated light transmittance was recorded during incubation at 42 degrees C in the BioSys instrument. Salmonella-positive samples were identified by a sharp drop in transmittance (caused by black discoloration of the media) within 18 h of incubation following the formation of hydrogen sulfide and its reaction with iron ions. Detection of 2 to 10 CFU/25 g of the tested foods was completed in 24 h. A total of 24 Salmonella spp., including heat-injured cells, and 37 non-Salmonella Enterobacteriaceae were tested using the procedure.

Automated detection of Salmonella spp. in foods.

An automated method to detect salmonellae in foods was developed and tested in food samples intentionally contaminated with the test organisms. Liquid eggs, shell eggs, dry eggs, skim milk and chicken were spiked with Salmonella enteritidis, S. typhimurium or S. newport to yield 2 to 25 CFU per 25 g or ml of sample. Following pre-enrichment in universal pre-enrichment broth at 42 degrees C for 6 h (eggs and milk) or 16 h (chicken), Salmonella cells were captured by immunomagnetic beads coated with Salmonella antibody (Vicam, Watertown, MA). The beads were transferred to selective liquid media containing carbohydrate (dulcitol or xylose), amino acid (lysine or ornithine), and H2S indicator, and incubated at 42 degrees C in the BioSys instrument (MicroSys, Ann Arbor, MI). Salmonella positive samples were identified by black discoloration of the media during incubation, while negative samples remained colorless. These color changes were recorded by the instrument. All the artificially contaminated samples tested positive within 15-18 h, while control samples remained negative during 24 h incubation. The results agreed with standard identification procedures. A total of 24 h was required to detect 2 to 25 CFU of the pathogen in 25 g or ml of eggs and milk, and up to 36 h in chicken, compared to 72 h in the standard methods.

Automated detection of hydrogen sulfide release from thiosulfate by Salmonella spp.

Hydrogen sulfide production is used in conventional tests for identification and differentiation of Salmonella spp. from other species of Enterobacteriaceae, and a black precipitate on agar media is the indicator of the reaction. Selective liquid media were formulated for automated optical detection of H2S in salmonellae using the BioSys instrument. The media contained thiosulfate and ferric ammonium citrate, and production of H2S caused copious black pigmentation of the broth. Combination of the H2S indicators with dulcitol or xylose as fermentable carbohydrate, lysine, ornithine or arginine to induce decarboxylase activity, and Tergitol 4 as inhibitor selectively identified six Salmonella spp. by a sharp drop in transmittance at 585 nm. The time for detection of transmittance changes was inversely proportional to initial numbers of CFU in the media: 10 h for 10(5) CFU/ml and 17 h for 10(1) CFU/ml. No detection was observed in six non-Salmonella species of Enterobacteriaceae tested.

Novel selective and non-selective optical detection of microorganisms.

A new instrument, capable of detecting metabolic changes due to microbiological activity, is described. Optical changes in growth media are monitored in a semi-fluid zone that separates the liquid medium containing the sample. Data demonstrate that common media can be utilized in conjunction with this rapid automated technology. Nutrient broth with the pH dye indicator. bromocresol purple was suitable for total counts. Selective media containing dyes were utilized to assess the presence or absence of specific groups of organisms. Biochemical reactions, such as lysine decarboxylase activity, were identified by the unique generated patterns, and specific enzymatic cleavage reactions with chromogenic substrates, such as 5-bromo-4 chloro-3 indolyl-beta-D-glucuronic acid (X-GLUC), were monitored.

Metabolic activities of Listeria monocytogenes in the presence of sodium propionate, acetate, lactate and citrate.

The effects of sodium propionate, acetate, lactate and citrate on cell proliferation, glucose and oxygen consumption, and ATP production in Listeria monocytogenes were investigated in growing and resting cells. Media pH was 6.7-6.8. Growth inhibition increased while glucose consumption continued in the presence of > or = 1% propionate, > or = 3% acetate and > or = 5% lactate in broth during incubation at 35 degrees C, indicating that glucose consumption was uncoupled from cell proliferation. Acetate and propionate were the most effective antilisterials, whereas citrate (5%) was only slightly inhibitory. Of the four salts, only lactate supported growth, oxygen consumption and ATP production. While concentrations of 1 and 5% propionate, acetate and citrate did not have an effect on oxygen consumption, they inhibited ATP production. ATP production in the presence of the four salts was consistently lower at pH 6.0 than at neutral pH. Lactate served as an alternative energy source for L. monocytogenes in the absence of glucose but became toxic to the organism in the presence of the carbohydrate.

Bioluminescence measurements of the antilisterial activity of nisin: comparison with ampicillin and streptomycin.

The influence of nisin on intracellular ATP and cell numbers of Listeria monocytogenes strain Scott A was determined and compared with the effect of amplicillin and streptomycin under similar conditions. In the presence of nisin (3-12 micrograms/ml), intracellular ATP and cell numbers decreased rapidly during the first hour at 35 degrees C and extracellular ATP increased. Cell numbers and intracellular ATP increased after 3 h of incubation. No effect was observed in cells treated with ampicillin (3-12 micrograms/ml) and streptomycin (15-60 micrograms/ml) during the first hour. However, concentrations of > or = 3 micrograms/ml ampicillin and > or = 30 micrograms/ml streptomycin were listeriostatic after 3 h of incubation. Progressive loss of viability and reduction of intracellular ATP were observed in resting cells in PBS (pH 7.2) containing increasing concentrations of the antimicrobials. Rapid accumulation of extracellular ATP, observed immediately after treatment with nisin but not with the antibiotics, supports the reported collapse of proton motive force in L. monocytogenes by nisin.