Functional analysis of ycfR and ycfQ in Escherichia coli O157:H7 linked to outbreaks of illness associated with fresh produce

Linking Copper-Associated Signal Transduction Systems with Their Environment in Marine Bacteria

Copper is an essential trace element for living cells. However, copper can be potentially toxic for bacterial cells when it is present in excess amounts due to its redox potential. Due to its biocidal properties, copper is prevalent in marine systems due to its use in antifouling paints and as an algaecide. Thus, marine bacteria must possess means of sensing and responding to both high copper levels and those in which it is present at only typical trace metal levels. Bacteria harbor diverse regulatory mechanisms that respond to intracellular and extracellular copper and maintain copper homeostasis in cells. This review presents an overview of the copper-associated signal transduction systems in marine bacteria, including the copper efflux systems, detoxification, and chaperone mechanisms. We performed a comparative genomics study of the copper-regulatory signal transduction system on marine bacteria to examine the influence of the environment on the presence, abundance, and diversity of copper-associated signal transduction systems across representative phyla. Comparative analyses were performed among species isolated from sources, including seawater, sediment, biofilm, and marine pathogens. Overall, we observed many putative homologs of copper-associated signal transduction systems from various copper systems across marine bacteria. While the distribution of the regulatory components is mainly influenced by phylogeny, our analyses identified several intriguing trends: (1) Bacteria isolated from sediment and biofilm displayed an increased number of homolog hits to copper-associated signal transduction systems than those from seawater. (2) A large variability exists for hits to the putative alternate σ factor CorE hits across marine bacteria. (3) Species isolated from seawater and marine pathogens harbored fewer CorE homologs than those isolated from the sediment and biofilm.

Enterohemorrhagic Escherichia coli responds to gut microbiota metabolites by altering metabolism and activating stress responses

Enterohemorrhagic Escherichia coli (EHEC) is a major cause of severe bloody diarrhea, with potentially lethal complications, such as hemolytic uremic syndrome. In humans, EHEC colonizes the colon, which is also home to a diverse community of trillions of microbes known as the gut microbiota. Although these microbes and the metabolites that they produce represent an important component of EHEC's ecological niche, little is known about how EHEC senses and responds to the presence of gut microbiota metabolites. In this study, we used a combined RNA-Seq and Tn-Seq approach to characterize EHEC's response to metabolites from an in vitro culture of 33 human gut microbiota isolates (MET-1), previously demonstrated to effectively resolve recurrent Clostridioides difficile infection in human patients. Collectively, the results revealed that EHEC adjusts to growth in the presence of microbiota metabolites in two major ways: by altering its metabolism and by activating stress responses. Metabolic adaptations to the presence of microbiota metabolites included increased expression of systems for maintaining redox balance and decreased expression of biotin biosynthesis genes, reflecting the high levels of biotin released by the microbiota into the culture medium. In addition, numerous genes related to envelope and oxidative stress responses (including cpxP, spy, soxS, yhcN, and bhsA) were upregulated during EHEC growth in a medium containing microbiota metabolites. Together, these results provide insight into the molecular mechanisms by which pathogens adapt to the presence of competing microbes in the host environment, which ultimately may enable the development of therapies to enhance colonization resistance and prevent infection.

Serratia marcescens DUF1471-Containing Protein SrfN Is Needed for Adaptation to Acid and Oxidative Stresses

Bacteria can quickly adapt to constantly changing environments through a number of mechanisms, including secretion of secondary metabolites, peptides, and proteins. Serratia marcescens, an emerging pathogen with growing clinical importance due to its intrinsic resistance to several classes of antibiotics, can cause an array of infections in immunocompromised individuals. To better control the spread of S. marcescens infections, it is critical to identify additional targets for bacterial growth inhibition. We found that extracellular metabolites produced by the wild-type organism in response to peroxide exposure had a protective effect on an otherwise-H₂O₂-sensitive ΔmacAB indicator strain. Detailed analysis of the conditioned medium demonstrated that the protective effect was associated with a low-molecular-weight heat-sensitive and proteinase K-sensitive metabolite. Furthermore, liquid chromatography-tandem mass spectrometry analysis of the low-molecular-weight proteins present in the conditioned medium led to identification of the previously uncharacterized DUF1471-containing protein TBU67220 (SrfN). We found that loss of the srfN gene did not have an impact on the production of extracellular enzymes. However, the S. marcescens mutant lacking SrfN was significantly more sensitive to growth in medium with a low pH and to exposure to oxidative stress. Both defects were fully rescued by complementation. Thus, our results indicate that SrfN, a low-molecular-weight DUF1471-containing protein, is involved in S. marcescens SM6 adaptation to adverse environmental conditions. IMPORTANCE Serratia marcescens is ubiquitous in the environment and can survive in water, soil, plants, insects, and animals, and it can also cause infections in humans. In the face of disturbances such as oxidative or low-pH stress, bacteria adapt, survive, and recover through several mechanisms, including changes in their secretome. We show that a hydrogen peroxide-exposed S. marcescens milieu contains a small previously uncharacterized DUF1471-containing protein similar to the SrfN protein in Salmonella enterica serovar Typhimurium, and we illustrate the role of this protein in bacterial survival during acid and oxidative stresses.

Integrated metabolomics of "big six" Escherichia coli on pea sprouts to organic acid treatments

Naturally occurring organic acids (OAs) have demonstrated satisfactory effects in inhibiting common pathogens on fresh produce; however, their effectiveness on "big six" Escherichia coli serotypes, comprised of E. coli O26:H11, O45:H2, O103:H11, O111, O121:H19 and O145, remained unaddressed. Regarding this, using nuclear magnetic resonance (NMR) spectroscopy and ultra-high performance liquid chromatography-mass spectrometry (UPLC-MS), the sanitising efficacy and the underlying antimicrobial mechanisms of 10-min treatments with 0.2 mol/L ascorbic acid (AA), citric acid (CA) and malic acid (MA) against the "big six" strains on pea sprouts were thoroughly investigated in this study. Despite the varying antimicrobial efficacy (AA: 0.12-0.99, CA: 0.36-1.72, MA: 0.75-3.28 log CFU/g reductions), the three OAs induced consistent metabolic changes in the E. coli strains, particularly in the metabolism of membrane lipids, nucleotide derivatives and amino acids. Comparing all strains, the most OA-resistant strain, O26 (0.36-1.12 log CFU/g reductions), had the largest total amino acids accumulated to resist osmotic stress; its ulteriorly suppressed cell activity further strengthened its endurance. In contrast, the lowest OA-resistance of O121 (0.99-3.28 log CFU/g reductions) might be explained by the depletion of putrescine, an oxidative stress regulator. Overall, the study sheds light on the effectiveness of a dual-platform metabolomics investigation in elucidating the metabolic responses of "big six" E. coli to OAs. The manifested antimicrobial effects of OAs, especially MA, together with the underlying metabolic perturbations detected in the "big six" strains, provided scientific basis for applying OA treatments to future fresh produce sanitisation.

Plant Bioactive Compounds as an Intrinsic and Sustainable Tool to Enhance the Microbial Safety of Crops

Outbreaks of produce-associated foodborne illness continue to pose a threat to human health worldwide. New approaches are necessary to improve produce safety. Plant innate immunity has potential as a host-based strategy for the deactivation of enteric pathogens. In response to various biotic and abiotic threats, plants mount defense responses that are governed by signaling pathways. Once activated, these result in the release of reactive oxygen and nitrogen species in addition to secondary metabolites that aim at tempering microbial infection and pest attack. These phytochemicals have been investigated as alternatives to chemical sanitization, as many are effective antimicrobial compounds in vitro. Their antagonistic activity toward enteric pathogens may also provide an intrinsic hurdle to their viability and multiplication in planta. Plants can detect and mount basal defenses against enteric pathogens. Evidence supports the role of plant bioactive compounds in the physiology of Salmonella enterica, Escherichia coli, and Listeria monocytogenes as well as their fitness on plants. Here, we review the current state of knowledge of the effect of phytochemicals on enteric pathogens and their colonization of plants. Further understanding of the interplay between foodborne pathogens and the chemical environment on/in host plants may have lasting impacts on crop management for enhanced microbial safety through translational applications in plant breeding, editing technologies, and defense priming.

Persistence of Escherichia coli in the microbiomes of red Romaine lettuce (Lactuca sativa cv. 'Outredgeous') and mizuna mustard (Brassica rapa var. japonica) - does seed sanitization matter?

BACKGROUND

Seed sanitization via chemical processes removes/reduces microbes from the external surfaces of the seed and thereby could have an impact on the plants' health or productivity. To determine the impact of seed sanitization on the plants' microbiome and pathogen persistence, sanitized and unsanitized seeds from two leafy green crops, red Romaine lettuce (Lactuca sativa cv. 'Outredgeous') and mizuna mustard (Brassica rapa var. japonica) were exposed to Escherichia coli and grown in controlled environment growth chambers simulating environmental conditions aboard the International Space Station. Plants were harvested at four intervals from 7 days post-germination to maturity. The bacterial communities of leaf and root were investigated using the 16S rRNA sequencing while quantitative polymerase chain reaction (qPCR) and heterotrophic plate counts were used to reveal the persistence of E. coli.

RESULT

E. coli was detectable for longer periods of time in plants from sanitized versus unsanitized seeds and was identified in root tissue more frequently than in leaf tissue. 16S rRNA sequencing showed dynamic changes in the abundance of members of the phylum Proteobacteria, Firmicutes, and Bacteroidetes in leaf and root samples of both leafy crops. We observed minimal changes in the microbial diversity of lettuce or mizuna leaf tissue with time or between sanitized and unsanitized seeds. Beta-diversity showed that time had more of an influence on all samples versus the E. coli treatment.

CONCLUSION

Our results indicated that the seed surface sanitization, a current requirement for sending seeds to space, could influence the microbiome. Insight into the changes in the crop microbiomes could lead to healthier plants and safer food supplementation.

Transcriptomic Analysis of the Activity and Mechanism of Action of a Ruthenium(II)-Based Antimicrobial That Induces Minimal Evolution of Pathogen Resistance

Increasing concern over rising levels of antibiotic resistance among pathogenic bacteria has prompted significant research into developing efficacious alternatives to antibiotic treatment. Previously, we have reported on the therapeutic activity of a dinuclear ruthenium(II) complex against pathogenic, multi-drug-resistant bacterial pathogens. Herein, we report that the solubility properties of this lead are comparable to those exhibited by orally available therapeutics that in comparison to clinically relevant antibiotics it induces very slow evolution of resistance in the uropathogenic, therapeutically resistant, E. coli strain EC958, and this resistance was lost when exposure to the compound was temporarily removed. With the aim of further investigating the mechanism of action of this compound, the regulation of nine target genes relating to the membrane, DNA damage, and other stress responses provoked by exposure to the compound was also studied. This analysis confirmed that the compound causes a significant transcriptional downregulation of genes involved in membrane transport and the tricarboxylic acid cycle. By contrast, expression of the chaperone protein-coding gene, spy, was significantly increased suggesting a requirement for repair of damaged proteins in the region of the outer membrane. The complex was also found to display activity comparable to that in E. coli in a range of other therapeutically relevant Gram-negative pathogens.

Roles of YcfR in Biofilm Formation in Salmonella Typhimurium ATCC 14028

An increasing number of foodborne diseases are currently attributable to farm produce contaminated with enteric pathogens such as Salmonella enterica. Recent studies have shown that a variety of enteric pathogens are able to colonize plant surfaces by forming biofilms and thereby persist for long periods, which can subsequently lead to human infections. Therefore, biofilm formation by enteric pathogens on plants poses a risk to human health. Here, we deciphered the roles of YcfR in biofilm formation by Salmonella enterica. YcfR is a putative outer membrane protein associated with bacterial stress responses. The lack of YcfR facilitated the formation of multicellular aggregates on cabbage leaves as well as glass surfaces while reducing bacterial motility. ycfR deletion caused extensive structural alterations in the outer membrane, primarily in lipopolysaccharides, outer membrane proteins, cellulose, and curli fimbria, thereby increasing cell surface hydrophobicity. However, the absence of YcfR rendered Salmonella susceptible to stressful treatments, despite the increased multicellular aggregation. These results suggest that YcfR is an essential constituent of Salmonella outer membrane architecture and its absence may cause multifaceted structural changes, thereby compromising bacterial envelope integrity. In this context, YcfR may be further exploited as a potential target for controlling Salmonella persistence on plants.

Bird feathers as potential sources of pathogenic microorganisms: a new look at old diseases

This article describes methods of treatment for avian zoonoses, modern antibiotic therapy and drug resistance of selected pathogens, which pose a threat to the population’s health. A tabular form has been used to present the current data from the European Union from 2011 to 2017 regarding human morbidity and mortality and the costs incurred by national health systems for the treatment of zoonoses occurring in humans and animals. Moreover, the paper includes descriptions of selected diseases, which indirectly affect birds. Scientists can obtain information regarding the occurrence of particular diseases, their aetiology, epidemiology, incubation period and symptoms caused by dangerous microorganisms and parasites. This information should be of particular interest for people who have frequent contact with birds, such as ornithologists, as well as veterinarians, farm staff, owners of accompanying animals and zoological workers. This paper presents a review used for identification and genetic characterization of bacterial strains isolated from a variety of environmental sources, e.g., bird feathers along with their practical application. We describe the bacterial, viral and fungal serotypes present on avian feathers after the slaughter process. This review also enables us to effectively identify several of the early stages of infectious diseases from heterogeneous avian research material.

The KP1_4563 gene is regulated by the cAMP receptor protein and controls type 3 fimbrial function in Klebsiella pneumoniae NTUH-K2044

Klebsiella pneumoniae (K. pneumoniae) is an opportunistic pathogen that can adhere to host cells or extracellular matrix via type 1 and type 3 fimbriae. KP1_4563 is a gene encoding a hypothetical protein in K. pneumoniae NTUH-K2044. KP1_4563 is located between the type 1 and type 3 fimbrial gene clusters and is likely associated with fimbrial function given its putative conserved domains of unknown function (DUF1471). Cyclic AMP receptor protein (CRP) regulates virulence-related gene expression and is a crucial transcriptional regulator in many bacteria. The predicted DNA recognition motif of CRP is present in the KP1_4563 promoter region. This study aimed to investigate the function of KP1_4563 in fimbriae and its transcriptional regulation mechanism by CRP. We generated Kp-Δ4563 mutant and complementation strains. We utilized phenotype and adhesion assays to evaluate the role of KP1_4563 in fimbriae. We conducted quantitative RT-PCR (qRT-PCR), LacZ fusion, electrophoretic mobility shift, and DNase I footprinting assays to study the transcriptional regulation of KP1_4563 gene by CRP. We found that KP1_4563 negatively regulates the function of type 3 fimbriae. Compared with NTUH-K2044, the absence of KP1_4563 enhanced the ability of Kp-Δ4563 to adhere to A549 cells. CRP negatively regulates KP1_4563 by directly binding to its promoter region. KP1_4563 plays an important role in type 3 fimbrial function. This novel insight will assist in the development of strategies for preventing K. pneumoniae infection.

Computational Analysis of Host-Pathogen Protein Interactions between Humans and Different Strains of Enterohemorrhagic Escherichia coli

Serotype O157:H7, an enterohemorrhagic Escherichia coli (EHEC), is known to cause gastrointestinal and systemic illnesses ranging from diarrhea and hemorrhagic colitis to potentially fatal hemolytic uremic syndrome. Specific genetic factors like ompA, nsrR, and LEE genes are known to play roles in EHEC pathogenesis. However, these factors are not specific to EHEC and their presence in several non-pathogenic strains indicates that additional factors are involved in pathogenicity. We propose a comprehensive effort to screen for such potential genetic elements, through investigation of biomolecular interactions between E. coli and their host. In this work, an in silico investigation of the protein–protein interactions (PPIs) between human cells and four EHEC strains (viz., EDL933, Sakai, EC4115, and TW14359) was performed in order to understand the virulence and host-colonization strategies of these strains. Potential host–pathogen interactions (HPIs) between human cells and the “non-pathogenic” E. coli strain MG1655 were also probed to evaluate whether and how the variations in the genomes could translate into altered virulence and host-colonization capabilities of the studied bacterial strains. Results indicate that a small subset of HPIs are unique to the studied pathogens and can be implicated in virulence. This subset of interactions involved E. coli proteins like YhdW, ChuT, EivG, and HlyA. These proteins have previously been reported to be involved in bacterial virulence. In addition, clear differences in lineage and clade-specific HPI profiles could be identified. Furthermore, available gene expression profiles of the HPI-proteins were utilized to estimate the proportion of proteins which may be involved in interactions. We hypothesized that a cumulative score of the ratios of bound:unbound proteins (involved in HPIs) would indicate the extent of colonization. Thus, we designed the Host Colonization Index (HCI) measure to determine the host colonization potential of the E. coli strains. Pathogenic strains of E. coli were observed to have higher HCIs as compared to a non-pathogenic laboratory strain. However, no significant differences among the HCIs of the two pathogenic groups were observed. Overall, our findings are expected to provide additional insights into EHEC pathogenesis and are likely to aid in designing alternate preventive and therapeutic strategies.

Rapid detection of B2-ST131 clonal group of extended-spectrum β-lactamase-producing Escherichia coli by matrix-assisted laser desorption ionization-time-of-flight mass spectrometry: discovery of a peculiar amino acid substitution in B2-ST131 clonal group

One reason for the spread of extended-spectrum β-lactamase (ESBL)-producing Escherichia coli worldwide is the global pandemic of the B2-ST131 clonal group. We searched for the specific biomarker peaks to distinguish between the B2-ST131 clonal group and other sequence type (ST) clonal groups isolated from clinical specimens obtained in our hospital. Biomarker peaks at m/z 7650 in the B2-ST131 group (sensitivity of 100% and specificity of 89.7%) and m/z 7707 in the other ST clonal groups showed the highest discrimination abilities. We further verified reproducibility against other Japanese clinical isolates obtained in another area of Japan. Differences between the molecular mass at the 7650m/z and 7707m/z peaks indicated an E34A amino acid substitution by proteomic and genomic analysis. Matrix-assisted laser desorption ionization-time-of-flight mass spectrometry rapidly and simply identified the B2-ST131 clonal group in routine examinations and will allow for adequate empirical therapy and the possibility to control both hospital infections and the global pandemic.

Metabolic Response of Escherichia coli upon Treatment with Hypochlorite at Sub-Lethal Concentrations

Hypochlorite is a reactive oxygen species that is worldwide as an antibacterial disinfectant. Hypochlorite exposure is known to cause oxidative damage to DNA and proteins. As a response to these effects, the metabolite profiles of organisms treated with sub-lethal doses of hypochlorite are assumed to be severely modified; however, the nature of these changes is hardly understood. Therefore, using nuclear magnetic resonance spectroscopy and gas chromatography-coupled mass spectrometry, we analyzed the time-dependent impact of hypochlorite exposure with a sub-lethal concentration (50 µM) on the metabolite profile of the Escherichia coli strain MG1655. Principle component analysis clearly distinguished between the metabolite profiles of bacteria treated for 0, 5,10, 20, 40, or 60 min. Major changes in the relative amounts of fatty acids, acetic acid, and formic acid occurred within the first 5 min. Comparative gas chromatography-coupled mass spectrometry analyses revealed that the amounts of free methionine and alanine were significantly decreased in the treated cells, demonstrating their susceptibility to hypochlorite exposure. The concentrations of succinate, urea, orotic acid, 2-aminobutyric acid, and 2-hydroxybutyric acid were also severely affected, indicating general changes in the metabolic network by hypochlorite. However, most metabolite levels relaxed to the reference values of untreated cells after 40–60 min, reflecting the capability of E. coli to rapidly adapt to environmental stress factors such as the presence of sub-lethal oxidant levels.

Behavior of Shiga toxigenic Escherichia coli relevant to lettuce washing processes and consideration of factors for evaluating washing process surrogates

Postharvest processes for fresh produce commonly include washing in water containing antimicrobial chemicals, such as chlorine; however, if the antimicrobials are not present in sufficient levels, washing can promote the spread of contamination that might be present. To understand cross-contamination risk during washing, we tested a collection of Shiga toxigenic Escherichia coli (STEC), including O157:H7 and other non-O157 strains, for certain traits during washing of fresh-cut lettuce, i.e., sensitivity to sublethal chlorine levels and ability to cross-contaminate (detach from and attach to) lettuce in the presence of sublethal chlorine levels. Nonpathogenic E. coli Nissle 1917 (EcN) and Pediococcus pentosaceus lactic acid bacterial species (LAB) were included as potential washing process validation surrogates. As measured by extension of the lag phase of growth in media containing 0.15 ppm of chlorine, chlorine sensitivity varied among the STECs. Cross-contamination was assessed by evaluating transfer of bacteria from inoculated to uninoculated leaves during washing. Without chlorine, similar transfer to wash water and uninoculated leaves was shown. In 1 ppm of chlorine, cross-contamination was not detected with most strains, except for the substantial transfer by a STEC O111 strain and EcN in some replicates. Strain O111 and EcN showed less inactivation in 0.25 ppm of chlorine water compared with O157 (P < 0.05). LAB showed similar transfer and similar chlorine inactivation to O157. Considering together the sublethal chlorine sensitivity and detachment/attachment traits, neither EcN nor LAB displayed optimal characteristics as washing process surrogates for the STEC strains, although further evaluation is needed. This work demonstrated a range of behaviors of STEC strains during lettuce washing and may be helpful in hazard characterization, identifying factors to consider for evaluating washing process efficacy, and identifying phenotypic traits to select surrogates to validate washing processes.

Structural and functional characterization of DUF1471 domains of Salmonella proteins SrfN, YdgH/SssB, and YahO

Bacterial species in the Enterobacteriaceae typically contain multiple paralogues of a small domain of unknown function (DUF1471) from a family of conserved proteins also known as YhcN or BhsA/McbA. Proteins containing DUF1471 may have a single or three copies of this domain. Representatives of this family have been demonstrated to play roles in several cellular processes including stress response, biofilm formation, and pathogenesis. We have conducted NMR and X-ray crystallographic studies of four DUF1471 domains from Salmonella representing three different paralogous DUF1471 subfamilies: SrfN, YahO, and SssB/YdgH (two of its three DUF1471 domains: the N-terminal domain I (residues 21–91), and the C-terminal domain III (residues 244–314)). Notably, SrfN has been shown to have a role in intracellular infection by Salmonella Typhimurium. These domains share less than 35% pairwise sequence identity. Structures of all four domains show a mixed α+β fold that is most similar to that of bacterial lipoprotein RcsF. However, all four DUF1471 sequences lack the redox sensitive cysteine residues essential for RcsF activity in a phospho-relay pathway, suggesting that DUF1471 domains perform a different function(s). SrfN forms a dimer in contrast to YahO and SssB domains I and III, which are monomers in solution. A putative binding site for oxyanions such as phosphate and sulfate was identified in SrfN, and an interaction between the SrfN dimer and sulfated polysaccharides was demonstrated, suggesting a direct role for this DUF1471 domain at the host-pathogen interface.

Enteric pathogen-plant interactions: molecular connections leading to colonization and growth and implications for food safety

Leafy green vegetables have been identified as a source of foodborne illnesses worldwide over the past decade. Human enteric pathogens, such as Escherichia coli O157:H7 and Salmonella, have been implicated in numerous food poisoning outbreaks associated with the consumption of fresh produce. An understanding of the mechanisms responsible for the establishment of pathogenic bacteria in or on vegetable plants is critical for understanding and ameliorating this problem as well as ensuring the safety of our food supply. While previous studies have described the growth and survival of enteric pathogens in the environment and also the risk factors associated with the contamination of vegetables, the molecular events involved in the colonization of fresh produce by enteric pathogens are just beginning to be elucidated. This review summarizes recent findings on the interactions of several bacterial pathogens with leafy green vegetables. Changes in gene expression linked to the bacterial attachment and colonization of plant structures are discussed in light of their relevance to plant-microbe interactions. We propose a mechanism for the establishment and association of enteric pathogens with plants and discuss potential strategies to address the problem of foodborne illness linked to the consumption of leafy green vegetables.

Roles of a novel Crp/Fnr family transcription factor Lmo0753 in soil survival, biofilm production and surface attachment to fresh produce of Listeria monocytogenes

Listeria monocytogenes is a foodborne bacterial pathogen and the causative agent of an infectious disease, listeriosis. L. monocytogenes is ubiquitous in nature and has the ability to persist in food processing environments for extended periods of time by forming biofilms and resisting industrial sanitization. Human listeriosis outbreaks are commonly linked to contaminated dairy products, ready-to-eat meats, and in recent years, fresh produce such as lettuce and cantaloupes. We identified a putative Crp/Fnr family transcription factor Lmo0753 that is highly specific to human-associated genetic lineages of L. monocytogenes. Lmo0753 possesses two conserved functional domains similar to the major virulence regulator PrfA in L. monocytogenes. To determine if Lmo0753 is involved in environmental persistence-related mechanisms, we compared lmo0753 deletion mutants with respective wild type and complementation mutants of two fully sequenced L. monocytogenes genetic lineage II strains 10403S and EGDe for the relative ability of growth under different nutrient availability and temperatures, soil survival, biofilm productivity and attachment to select fresh produce surfaces including romaine lettuce leaves and cantaloupe rinds. Our results collectively suggested that Lmo0753 plays an important role in L. monocytogenes biofilm production and attachment to fresh produce, which may contribute to the environmental persistence and recent emergence of this pathogen in human listeriosis outbreaks linked to fresh produce.

Transcriptional and functional responses of Escherichia coli O157:H7 growing in the lettuce rhizoplane

Lettuce and spinach are increasingly implicated in foodborne illness outbreaks due to contamination by Escherichia coli O157:H7. While this bacterium has been shown to colonize and survive on lettuce leaf surfaces, little is known about its interaction with the roots of growing lettuce plants. In these studies, a microarray analyses, mutant construction and confocal microscopy were used to gain an understanding of structure and function of bacterial genes involved in the colonization and growth of E. coli O157:H7 on lettuce roots. After three days of interaction with lettuce roots, 94 and 109 E. coli O157:H7 genes were significantly up- and down-regulated at least 1.5 fold, respectively. While genes involved in biofilm modulation (ycfR and ybiM) were significantly up-regulated, 40 of 109 (37%) of genes involved in protein synthesis were significantly repressed. E. coli O157:H7 was 2 logs less efficient in lettuce root colonization than was E. coli K12. We also unambiguously showed that a ΔycfR mutant of E. coli O157:H7 was unable to attach to or colonize lettuce roots. Taken together these results indicate that bacterial genes involved in attachment and biofilm formation are likely important for contamination of lettuce plants with Shiga toxin-producing E. coli strains.

Bacterial responses to reactive chlorine species

Hypochlorous acid (HOCl), the active ingredient of household bleach, is the most common disinfectant in medical, industrial, and domestic use and plays an important role in microbial killing in the innate immune system. Given the critical importance of the antimicrobial properties of chlorine to public health, it is surprising how little is known about the ways in which bacteria sense and respond to reactive chlorine species (RCS). Although the literature on bacterial responses to reactive oxygen species (ROS) is enormous, work addressing bacterial responses to RCS has begun only recently. Transcriptomic and proteomic studies now provide new insights into how bacteria mount defenses against this important class of antimicrobial compounds. In this review, we summarize the current knowledge, emphasizing the overlaps between RCS stress responses and other more well-characterized bacterial defense systems, and identify outstanding questions that represent productive avenues for future research.

Genes ycfR, sirA and yigG contribute to the surface attachment of Salmonella enterica Typhimurium and Saintpaul to fresh produce

Salmonella enterica is a frequent contaminant of minimally-processed fresh produce linked to major foodborne disease outbreaks. The molecular mechanisms underlying the association of this enteric pathogen with fresh produce remain largely unexplored. In our recent study, we showed that the expression of a putative stress regulatory gene, ycfR, was significantly induced in S. enterica upon exposure to chlorine treatment, a common industrial practice for washing and decontaminating fresh produce during minimal processing. Two additional genes, sirA involved in S. enterica biofilm formation and yigG of unknown function, were also found to be differentially regulated under chlorine stress. To further characterize the roles of ycfR, sirA, and yigG in S. enterica attachment and survival on fresh produce, we constructed in-frame deletions of all three genes in two different S. enterica serovars, Typhimurium and Saintpaul, which have been implicated in previous disease outbreaks linked to fresh produce. Bacterial attachment to glass and polystyrene microtiter plates, cell aggregation and hydrophobicity, chlorine resistance, and surface attachment to intact spinach leaf and grape tomato were compared among wild-type strains, single-gene deletion mutants, and their respective complementation mutants. The results showed that deletions of ycfR, sirA, and yigG reduced bacterial attachment to glass and polystyrene as well as fresh produce surface with or without chlorine treatment in both Typhimurium and Saintpaul. Deletion of ycfR in Typhimurium significantly reduced bacterial chlorine resistance and the attachment to the plant surfaces after chlorinated water washes. Deletions of ycfR in Typhimurium and yigG in Saintpaul resulted in significant increase in cell aggregation. Our findings suggest that ycfR, sirA, and yigG collectively contribute to S. enterica surface attachment and survival during post-harvest minimal processing of fresh produce.

The salmonella transcriptome in lettuce and cilantro soft rot reveals a niche overlap with the animal host intestine

Fresh vegetables have been recurrently associated with salmonellosis outbreaks, and Salmonella contamination of retail produce has been correlated positively with the presence of soft rot disease. We observed that population sizes of Salmonella enterica serovar Typhimurium SL1344 increased 56-fold when inoculated alone onto cilantro leaves, versus 2,884-fold when coinoculated with Dickeya dadantii, a prevalent pathogen that macerates plant tissue. A similar trend in S. enterica populations was observed for soft-rotted lettuce leaves. Transcriptome analysis of S. enterica cells that colonized D. dadantii-infected lettuce and cilantro leaves revealed a clear shift toward anaerobic metabolism and catabolism of substrates that are available due to the degradation of plant cells by the pectinolytic pathogen. Twenty-nine percent of the genes that were upregulated in cilantro macerates were also previously observed to have increased expression levels in the chicken intestine. Furthermore, multiple genes induced in soft rot lesions are also involved in the colonization of mouse, pig, and bovine models of host infection. Among those genes, the operons for ethanolamine and propanediol utilization as well as for the synthesis of cobalamin, a cofactor in these pathways, were the most highly upregulated genes in lettuce and cilantro lesions. In S. Typhimurium strain LT2, population sizes of mutants deficient in propanediol utilization or cobalamin synthesis were 10- and 3-fold lower, respectively, than those of the wild-type strain in macerated cilantro (P < 0.0002); in strain SL1344, such mutants behaved similarly to the parental strain. Anaerobic conditions and the utilization of nutrients in macerated plant tissue that are also present in the animal intestine indicate a niche overlap that may explain the high level of adaptation of S. enterica to soft rot lesions, a common postharvest plant disease.