Bioenergetic mechanism for nisin resistance, induced by the acid tolerance response of Listeria monocytogenes

Scramblase activity of proteorhodopsin confers physiological advantages to Escherichia coli in the absence of light

Microbial rhodopsins are widely distributed in the aqua-ecosystem due to their simple structure and multifaceted functions. Conventionally, microbial rhodopsins are considered to be exclusively light active. Here, we report the discovery of light-independent function of a proteorhodopsin from a psychrophile Psychroflexus torquis (ptqPR). ptqPR could improve the growth and viability of Escherichia coli cells under stressful conditions in the absence of light, and this was achieved by improving the energy maintenance, membrane potential, membrane fluidity, and membrane integrity. We further show that this non-canonical function of PR is related to its scramblase activity. PR mutants which lost scramblase activities also lost their ability to confer physiological advantages in E. coli. These findings shed light on why microbial rhodopsins are widely distributed in ecological systems where light is inaccessible.

Nisin resistance in Gram-positive bacteria and approaches to circumvent resistance for successful therapeutic use

Antibiotic resistance among bacterial pathogens is one of the most worrying problems in health systems today. To solve this problem, bacteriocins from lactic acid bacteria, especially nisin, have been proposed as an alternative for controlling multidrug-resistant bacteria. Bacteriocins are antimicrobial peptides that have activity mainly against Gram-positive strains. Nisin is one of the most studied bacteriocins and is already approved for use in food preservation. Nisin is still not approved for human clinical use, but many in vitro studies have shown its therapeutic effectiveness, especially for the control of antibiotic-resistant strains. Results from in vitro studies show the emergence of nisin-resistant bacteria after exposure to nisin. Considering that nisin has shown promising results for clinical use, studies to elucidate nisin-resistant mechanisms and the development of approaches to circumvent nisin-resistance are important. Thus, the objectives of this review are to identify the Gram-positive bacterial strains that have shown resistance to nisin, describe their resistance mechanisms and propose ways to overcome the development of nisin-resistance for its successful clinical application.

Resistance Mechanisms to Antimicrobial Peptides in Gram-Positive Bacteria

With the alarming increase of infections caused by pathogenic multidrug-resistant bacteria over the last decades, antimicrobial peptides (AMPs) have been investigated as a potential treatment for those infections, directly through their lytic effect or indirectly, due to their ability to modulate the immune system. There are still concerns regarding the use of such molecules in the treatment of infections, such as cell toxicity and host factors that lead to peptide inhibition. To overcome these limitations, different approaches like peptide modification to reduce toxicity and peptide combinations to improve therapeutic efficacy are being tested. Human defense peptides consist of an important part of the innate immune system, against a myriad of potential aggressors, which have in turn developed different ways to overcome the AMPs microbicidal activities. Since the antimicrobial activity of AMPs vary between Gram-positive and Gram-negative species, so do the bacterial resistance arsenal. This review discusses the mechanisms exploited by Gram-positive bacteria to circumvent killing by antimicrobial peptides. Specifically, the most clinically relevant genera, Streptococcus spp., Staphylococcus spp., Enterococcus spp. and Gram-positive bacilli, have been explored.

Genome-wide mutant profiling predicts the mechanism of a Lipid II binding antibiotic

Identifying targets of antibacterial compounds remains a challenging step in antibiotic development. We have developed a two-pronged functional genomics approach to predict mechanism of action that uses mutant fitness data from antibiotic-treated transposon libraries containing both upregulation and inactivation mutants. We treated a Staphylococcus aureus transposon library containing 690,000 unique insertions with 32 antibiotics. Upregulation signatures, identified from directional biases in insertions, revealed known molecular targets and resistance mechanisms for the majority of these. Because single gene upregulation does not always confer resistance, we used a complementary machine learning approach to predict mechanism from inactivation mutant fitness profiles. This approach suggested the cell wall precursor Lipid II as the molecular target of the lysocins, a mechanism we have confirmed. We conclude that docking to membrane-anchored Lipid II precedes the selective bacteriolysis that distinguishes these lytic natural products, showing the utility of our approach for nominating antibiotic mechanism of action.

The Acid Tolerance Response Alters Membrane Fluidity and Induces Nisin Resistance in Listeria monocytogenes

The ability of L. monocytogenes cells to adapt to a variety of stressors contributes to its growth in a wide range of foods. The present study examines the effect of acid and of the acid tolerance response (ATR) on membrane fluidity and on the organism's resistance to acid and to the bacteriocin nisin. When ATR was induced in wild-type cells, these cells also became resistant to nisin. ATR(+) cells also had lower membrane rigidities than control ATR(-) cells that had not been subjected to the acid tolerance response. However, cells that were genetically resistant to nisin did not show any significant (P < 0.05) change in rigidity when grown in the presence of nisin. These studies suggest that the use of acid and nisin for L. monocytogenes control in ready-to-eat foods may be compromised if cross-resistance emerges.

The Pepsin Hydrolysate of Bovine Lactoferrin Causes a Collapse of the Membrane Potential in Escherichia coli O157:H7

In the present study, the ability of bovine lactoferrin hydrolysate (LfH) to disrupt the cytoplasmic membrane of Escherichia coli O157:H7 was investigated. Lactoferrin and LfH antimicrobial activities were compared against E. coli O157:H7 and E. coli O157:H7 spheroplasts. The effect of LfH on the cytoplasmic membrane of E. coli O157:H7 cells was determined by evaluating potassium efflux (K(+)), dissipation of ATP and membrane potential (ΔΨ). LfH produced a rapid efflux of potassium ions, a decrease in intracellular levels of ATP coupled with a substantial increase in extracellular ATP levels and a complete dissipation of the ΔΨ. The results suggest that LfH causes a collapse of the membrane integrity by pore formation in the inner membrane, leading to the death of the cell. Moreover, the mechanism of action of LfH on E. coli O157:H7 appears to involve an interference with the inner membrane integrity based on experiments using E. coli O157:H7 spheroplasts.

Zinc Lactate and Sapindin Act Synergistically with Lactocin 160 Against Gardnerella vaginalis

Lactocin 160 is a vaginal probiotic-derived bacteriocin shown to selectively inhibit the growth of Gardenerella vaginalis and some other pathogens commonly associated with bacterial vaginosis. The natural origin of this peptide, its safety, and selective antimicrobial properties make it a promising candidate for successful treatment and prophylaxis of bacterial vaginosis (BV). This study evaluated interactions between lactocin 160 and four other natural antimicrobials in the ability to inhibit G. vaginalis. We report that zinc lactate and soapnut extract act synergistically with lactocin 160 against this pathogen and therefore have a potential to be successfully used as the components of the multiple-hurdle antimicrobial formulation for the treatment of BV.

Single cell swimming dynamics of Listeria monocytogenes using a nanoporous microfluidic platform

Listeria monocytogenes remains a significant foodborne pathogen due to its virulence and ability to become established in food processing facilities. The pathogen is characterized by its ability to grow over a wide temperature range and withstand a broad range of stresses. The following reports on the chemotaxis and motility of the L. monocytogenes when exposed to relatively small concentrations of acetic acid. Using the developed nanoporous microfluidic device to precisely modulate the cellular environment, we exposed the individual Listeria cells to acetic acid and, in real time and with high resolution, observed how the cells reacted to the change in their surroundings. Our results showed that concentrations of acetic acid below 10 mM had very little, if any, effect on the motility. However, when exposed to 100 mM acetic acid, the cells exhibited a sharp drop in velocity and displayed a more random pattern of motion. These results indicate that at appropriate concentrations, acetic acid has the ability to disable the flagellum of the cells, thus impairing their motility. This drop in motility has numerous effects on the cell; its main effects being the obstruction of the cell's ability to properly form biofilms and a reduction in the overall infectivity of the cells. Since these characteristics are especially useful in controlling the proliferation of L. monocytogenes, acetic acid shows potential for application in the food industry as an active compound in designing a food packaging environment and as an antimicrobial agent.

Nisin resistance of Listeria monocytogenes is increased by exposure to salt stress and is mediated via LiaR

Growth of Listeria monocytogenes on refrigerated, ready-to-eat food is a significant food safety concern. Natural antimicrobials, such as nisin, can be used to control this pathogen on food, but little is known about how other food-related stresses may impact how the pathogen responds to these compounds. Prior work demonstrated that exposure of L. monocytogenes to salt stress at 7°C led to increased expression of genes involved in nisin resistance, including the response regulator liaR. We hypothesized that exposure to salt stress would increase subsequent resistance to nisin and that LiaR would contribute to increased nisin resistance. Isogenic deletion mutations in liaR were constructed in 7 strains of L. monocytogenes, and strains were exposed to 6% NaCl in brain heart infusion broth and then tested for resistance to nisin (2 mg/ml Nisaplin) at 7°C. For the wild-type strains, exposure to salt significantly increased subsequent nisin resistance (P < 0.0001) over innate levels of resistance. Compared to the salt-induced nisin resistance of wild-type strains, ΔliaR strains were significantly more sensitive to nisin (P < 0.001), indicating that induction of LiaFSR led to cross-protection of L. monocytogenes against subsequent inactivation by nisin. Transcript levels of LiaR-regulated genes were induced by salt stress, and lmo1746 and telA were found to contribute to LiaR-mediated salt-induced nisin resistance. These data suggest that environmental stresses similar to those on foods can influence the resistance of L. monocytogenes to antimicrobials such as nisin, and potential cross-protective effects should be considered when selecting and applying control measures for this pathogen on ready-to-eat foods.

Inflammation-induced acid tolerance genes gadAB in luminal commensal Escherichia coli attenuate experimental colitis

Dysregulated immune responses to commensal intestinal bacteria, including Escherichia coli, contribute to the development of inflammatory bowel diseases (IBDs) and experimental colitis. Reciprocally, E. coli responds to chronic intestinal inflammation by upregulating expression of stress response genes, including gadA and gadB. GadAB encode glutamate decarboxylase and protect E. coli from the toxic effects of low pH and fermentation acids, factors present in the intestinal lumen in patients with active IBDs. We hypothesized that E. coli upregulates gadAB during inflammation to enhance its survival and virulence. Using real-time PCR, we determined gadAB expression in luminal E. coli from ex-germfree wild-type (WT) and interleukin-10 (IL-10) knockout (KO) (IL-10(-/-)) mice selectively colonized with a commensal E. coli isolate (NC101) that causes colitis in KO mice in isolation or in combination with 7 other commensal intestinal bacterial strains. E. coli survival and host inflammatory responses were measured in WT and KO mice colonized with NC101 or a mutant lacking the gadAB genes (NC101ΔgadAB). The susceptibility of NC101 and NC101ΔgadAB to killing by host antimicrobial peptides and their translocation across intestinal epithelial cells were evaluated using bacterial killing assays and transwell experiments, respectively. We show that expression of gadAB in luminal E. coli increases proportionately with intestinal inflammation in KO mice and enhances the susceptibility of NC101 to killing by the host antimicrobial peptide cryptdin-4 but decreases bacterial transmigration across intestinal epithelial cells, colonic inflammation, and mucosal immune responses. Chronic intestinal inflammation upregulates acid tolerance pathways in commensal E. coli isolates, which, contrary to our original hypothesis, limits their survival and colitogenic potential. Further investigation of microbial adaptation to immune-mediated inflammation may provide novel insights into the pathogenesis and treatment of IBDs.

Susceptibility of Gardnerella vaginalis biofilms to natural antimicrobials subtilosin, ε-poly-L-lysine, and lauramide arginine ethyl ester

Bacterial vaginosis is a common vaginal infection associated with numerous gynecological and obstetric complications. This condition is characterized by the presence of thick adherent vaginal biofilms, composed mainly of Gardnerella vaginalis. This organism is thought to be the primary aetiological cause of the infection paving the way for various opportunists to colonize the niche. Previously, we reported that the natural antimicrobials subtilosin, ε-poly-L-lysine, and lauramide arginine ethyl ester selectively inhibit the growth of this pathogen. In this study, we used plate counts to evaluate the efficacy of these antimicrobials against established biofilms of G. vaginalis. Additionally, we validated and compared two rapid methods (ATP viability and resazurin assays) for the assessment of cell viability in the antimicrobial-treated G. vaginalis biofilms. Out of the tested antimicrobials, lauramide arginine ethyl ester had the strongest bactericidal effect, followed by subtilosin, with clindamycin and polylysine showing the weakest effect. In comparison to plate counts, ATP viability and resazurin assays considerably underestimated the bactericidal effect of some antimicrobials. Our results indicate that these assays should be validated for every new application.

Mechanisms mediating bactericidal properties and conditions that enhance the potency of a broad-spectrum oligo-acyl-lysyl

Previous studies have established the potential of the oligo-acyl-lysyl (OAK) concept in generating simple chemical mimics of host defense peptides (HDPs) with improved antimicrobial properties. We investigated the antibacterial properties of such an OAK, C(16(ω7))-KK-C(12)-K(amide), to obtain a better understanding of the complex mode(s) of action of cationic antibacterial peptides. The average MIC, determined against a multispecies panel of 50 strains, was 6 ± 5 μg/ml. However, although the OAK exerted an essentially dose-dependent bactericidal effect (time-kill curves typically exhibited 99% death within 2 h), marked differences in the killing rates occurred among inter- and intraspecies strains. Mechanistic comparison between equally sensitive and related strains revealed death of one strain to stem from the OAK's capacity to breach the cell membrane permeability barrier, whereas the death of the related strain resulted from the OAK's direct interference with DNA functions in vivo, without detectable membrane damage. These findings therefore support the notion that the antibacterial mechanism of action of a single HDP can vary among inter- and intraspecies strains. In addition, we present data illustrating the differential effects of environmental conditions (pH, ionic strength and temperature), on the OAK's antibacterial properties, and ultimately demonstrate potency enhancement (by orders of magnitude) through selection of optimal incubation conditions. Such attributes might be useful in a variety of antibacterial applications.

Identification of a genetic locus responsible for antimicrobial peptide resistance in Clostridium difficile

Clostridium difficile causes chronic intestinal disease, yet little is understood about how the bacterium interacts with and survives in the host. To colonize the intestine and cause persistent disease, the bacterium must circumvent killing by host innate immune factors, such as cationic antimicrobial peptides (CAMPs). In this study, we investigated the effect of model CAMPs on growth and found that C. difficile is not only sensitive to these compounds but also responds to low levels of CAMPs by expressing genes that lead to CAMP resistance. By plating the bacterium on medium containing the CAMP nisin, we isolated a mutant capable of growing in three times the inhibitory concentration of CAMPs. This mutant also showed increased resistance to the CAMPs gallidermin and polymyxin B, demonstrating tolerance to different types of antimicrobial peptides. We identified the mutated gene responsible for the resistance phenotype as CD1352. This gene encodes a putative orphan histidine kinase that lies adjacent to a predicted ABC transporter operon (CD1349 to CD1351). Transcriptional analysis of the ABC transporter genes revealed that this operon was upregulated in the presence of nisin in wild-type cells and was more highly expressed in the CD1352 mutant. The insertional disruption of the CD1349 gene resulted in significant decreases in resistance to the CAMPs nisin and gallidermin but not polymyxin B. Because of their role in cationic antimicrobial peptide resistance, we propose the designation cprABC for genes CD1349 to CD1351 and cprK for the CD1352 gene. These results provide the first evidence of a C. difficile gene associated with antimicrobial peptide resistance.

Glutamate decarboxylase-mediated nisin resistance in Listeria monocytogenes

Analysis of a complete set of glutamate decarboxylase (gad) mutants of Listeria monocytogenes strain LO28 (ΔgadD1, ΔgadDT1, ΔgadD2, ΔgadT2, and ΔgadD3 mutants) revealed that the ΔgadD1 mutant is impaired in its ability to tolerate exposure to both sublethal and lethal levels of the lantibiotic nisin. gadD1 is strain variable and is found only in approximately 50% of L. monocytogenes strains. Growth and survival experiments revealed that possession of gadD1 correlates with a higher degree of tolerance to nisin. Significantly, a similar finding using a gadB mutant of L. lactis IL1403 implies that this may be a general phenomenon in Gram-positive bacteria. Our findings thus suggest that the specific inhibition of GAD activity or a reduction in the levels of free glutamate may prevent the growth of otherwise resistant GAD(+) bacteria in foods where low pH and/or nisin is used as a preservative.

sigma(B) and sigma(L) contribute to Listeria monocytogenes 10403S response to the antimicrobial peptides SdpC and nisin

The ability of the foodborne pathogen Listeria monocytogenes to survive antimicrobial treatments is a public health concern; therefore, this study was designed to investigate genetic mechanisms contributing to antimicrobial response in L. monocytogenes. In previous studies, the putative bacteriocin immunity gene lmo2570 was predicted to be regulated by the stress responsive alternative sigma factor, sigma(B). As the alternative sigma factor sigma(L) controls expression of genes important for resistance to some antimicrobial peptides, we hypothesized roles for lmo2570, sigma(B), and sigma(L) in L. monocytogenes antimicrobial response. Results from phenotypic characterization of a L. monocytogenes lmo2570 null mutant suggested that this gene does not contribute to resistance to nisin or to SdpC, an antimicrobial peptide produced by some strains of Bacillus subtilis. While lmo2570 transcript levels were confirmed to be sigma(B) dependent, they were sigma(L) independent and were not affected by the presence of nisin under the conditions used in this study. In spot-on-lawn assays with the SdpC-producing B. subtilis EG351, the L. monocytogenes DeltasigB, DeltasigL, and DeltasigB/DeltasigL strains all showed increased sensitivity to SdpC, indicating that both sigma(B) and sigma(L) regulate genes contributing to SdpC resistance. Nisin survival assays showed that sigma(B) and sigma(L) both affect L. monocytogenes sensitivity to nisin in broth survival assays; that is, a sigB null mutant is more resistant than the parent strain to nisin, while a sigB null mutation in DeltasigL background leads to reduced nisin resistance. In summary, while the sigma(B)-dependent lmo2570 does not contribute to resistance of L. monocytogenes to nisin or SdpC, both sigma(B) and sigma(L) contribute to the L. monocytogenes antimicrobial response.

Isolation of the Bacillus subtilis antimicrobial peptide subtilosin from the dairy product-derived Bacillus amyloliquefaciens

AIMS

To purify and characterize an antimicrobial protein (bacteriocin) isolated from the dairy product-derived Bacillus amyloliquefaciens.

METHODS AND RESULTS

An unknown bacterial species cultured from the Yogu Farm probiotic dairy beverage was identified through 16S ribosomal RNA analysis as B. amyloliquefaciens, a phylogenetically close relative of Bacillus subtilis. The cell-free supernatant (CFS) of overnight cultures was active against Listeria monocytogenes and also against clinical isolates of Gardnerella vaginalis and Streptococcus agalactiae. At the same time, several isolates of vaginal probiotic Lactobacilli were resistant to the CFS. The nature of the compound causing inhibitory activity was confirmed as proteinaceous by enzymatic digestion. The protein was isolated using ammonium sulfate precipitation, and further purified via column chromatography. PCR analysis was conducted to determine relatedness to other bacteriocins produced by Bacillus spp.

CONCLUSION

The antimicrobial protein isolated from B. amyloliquefaciens was shown to be subtilosin, a bacteriocin previously reported as produced only by B. subtilis.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report of intra-species horizontal gene transfer for subtilosin and the first fully characterized bacteriocin isolated from B. amyloliquefaciens. Finally, this is the first report on subtilosin's activity against bacterial vaginosis-associated pathogens.

Brucella suis urease encoded by ure1 but not ure2 is necessary for intestinal infection of BALB/c mice

BACKGROUND

In prokaryotes, the ureases are multi-subunit, nickel-containing enzymes that catalyze the hydrolysis of urea to carbon dioxide and ammonia. The Brucella genomes contain two urease operons designated as ure1 and ure2. We investigated the role of the two Brucella suis urease operons on the infection, intracellular persistence, growth, and resistance to low-pH killing.

RESULTS

The deduced amino acid sequence of urease-alpha subunits of operons-1 and -2 exhibited substantial identity with the structural ureases of alpha- and beta-proteobacteria, Gram-positive and Gram-negative bacteria, fungi, and higher plants. Four ure deficient strains were generated by deleting one or more of the genes encoding urease subunits of B. suis strain 1330 by allelic exchange: strain 1330Deltaure1K (generated by deleting ureD and ureA in ure1 operon), strain 1330Deltaure2K (ureB and ureC in ure2 operon), strain 1330Deltaure2C (ureA, ureB, and ureC in ure2 operon), and strain 1330Deltaure1KDeltaure2C (ureD and ureA in ure1 operon and ureA, ureB, and ureC in ure2 operon). When grown in urease test broth, strains 1330, 1330Deltaure2K and 1330Deltaure2C displayed maximal urease enzyme activity within 24 hours, whereas, strains 1330Deltaure1K and 1330Deltaure1KDeltaure2C exhibited zero urease activity even 96 h after inoculation. Strains 1330Deltaure1K and 1330Deltaure1KDeltaure2C exhibited slower growth rates in tryptic soy broth relative to the wild type strain 1330. When the BALB/c mice were infected intraperitoneally with the strains, six weeks after inoculation, the splenic recovery of the ure deficient strains did not differ from the wild type. In contrast, when the mice were inoculated by gavage, one week after inoculation, strain 1330Deltaure1KDeltaure2C was cleared from livers and spleens while the wild type strain 1330 was still present. All B. suis strains were killed when they were incubated in-vitro at pH 2.0. When the strains were incubated at pH 2.0 supplemented with 10 mM urea, strain 1330Deltaure1K was completely killed, strain 1330Deltaure2C was partially killed, but strains 1330 and 1330Deltaure2K were not killed.

CONCLUSION

These findings suggest that the ure1 operon is necessary for optimal growth in culture, urease activity, resistance against low-pH killing, and in vivo persistence of B. suis when inoculated by gavage. The ure2 operon apparently enhances the resistance to low-pH killing in-vitro.

Insertional mutagenesis to generate lantibiotic resistance in Lactococcus lactis

While the potential emergence of food spoilage and pathogenic bacteria with resistance to lantibiotics is a concern, the creation of derivatives of starter cultures and adjuncts that can grow in the presence of these antimicrobials may have applications in food fermentations. Here a bank of Lactococcus lactis IL1403 mutants was created and screened, and a number of novel genetic loci involved in lantibiotic resistance were identified.

Gene expression analysis of Corynebacterium glutamicum subjected to long-term lactic acid adaptation

Corynebacteria form an important part of the red smear cheese microbial surface consortium. To gain a better understanding of molecular adaptation due to low pH induced by lactose fermentation, the global gene expression profile of Corynebacterium glutamicum adapted to pH 5.7 with lactic acid under continuous growth in a chemostat was characterized by DNA microarray analysis. Expression of a total of 116 genes was increased and that of 90 genes was decreased compared to pH 7.5 without lactic acid, representing 7% of the genes in the genome. The up-regulated genes encode mainly transcriptional regulators, proteins responsible for export, import, and metabolism, and several proteins of unknown function. As much as 45% of the up-regulated open reading frames code for hypothetical proteins. These results were validated using real-time reverse transcription-PCR. To characterize the functions of 38 up-regulated genes, 36 single-crossover disruption mutants were generated and analyzed for their lactic acid sensitivities. However, only a sigB knockout mutant showed a highly significant negative effect on growth at low pH, suggesting a function in organic-acid adaptation. A sigE mutant already displayed growth retardation at neutral pH but grew better at acidic pH than the sigB mutant. The lack of acid-sensitive phenotypes in 34 out of 36 disrupted genes suggests either a considerable redundancy in acid adaptation response or coincidental effects. Other up-regulated genes included genes for ion transporters and metabolic pathways, including carbohydrate and respiratory metabolism. The enhanced expression of the nrd (ribonucleotide reductase) operon and a DNA ATPase repair protein implies a cellular response to combat acid-induced DNA damage. Surprisingly, multiple iron uptake systems (totaling 15% of the genes induced >or=2-fold) were induced at low pH. This induction was shown to be coincidental and could be attributed to iron-sequestering effects in complex media at low pH.

Inhibitory activity of colicin E1 against Listeria monocytogenes

Colicins are gram-negative bacteriocins produced by and effective against Escherichia coli and related species. Colicin E1 (ColE1) is composed of three functional domains, which collectively have a pore-forming effect on targeted bacteria. ColE1 binding and translocation domains are highly specific in contrast to the pore-forming domain, implying that ColE1 could be broadly effective. In this study, the activity of ColE1 against Listeria monocytogenes was evaluated in broth and on surfaces of ready-to-eat products. Individual strains of L. monocytogenes were examined in broth containing ColE1 at 0, 0.1, 1, or 10 microg/ml. Although strain differences in sensitivity to ColE1 existed, growth was significantly reduced in all strains at doses as low as 0.1 microg/ml. Sterilized ham slices were submerged in a five-strain L. monocytogenes cocktail (either 7 or 4 log CFU/ ml) and placed in vacuum packages containing 0, 1, 5, 10, 25, or 50 microg of ColE1. Ham slices were then stored at 4 or 10 degrees C, and samples were removed and examined for L. monocytogenes after 1, 3, 7, and 14 days. Reduction of L. monocytogenes by ColE1 was dependent on initial inoculum concentration and storage temperature. For slices stored at 4 degrees C, treatment with 25 microg reduced Listeria growth below detection limits for the slices inoculated with 4 log CFU/ml for the entire 14 days, whereas for the 7-log CFU/ml slices, growth was detected at 7 days postinoculation. For slices stored at 10 degrees C, 10 microg/ml ColE1 significantly inhibited growth of L. monocytogenes for up to 3 days for both inoculation groups. These data indicate that ColE1 is highly effective against Listeria.