Role of Efflux Pumps in the in vitro Development of Ciprofloxacin Resistance in Listeria monocytogenes

Cross-Phosphorylation between AgrC Histidine Kinase and the Noncognate Response Regulator Lmo1172 in Listeria monocytogenes under Benzalkonium Chloride Stress

Benzalkonium chloride (BC) is widely used for disinfection in the food industry. However, Listeria monocytogenes strains with resistance to BC have been reported recently. In L. monocytogenes, the Agr communication system consists of a membrane-bound peptidase AgrB, a precursor peptide AgrD, a histidine kinase (HK) AgrC, and a response regulator (RR) AgrA. Our previous study showed that the agr genes are significantly upregulated by BC adaptation. This study aimed to investigate the role of the Agr system in BC resistance in L. monocytogenes. Our results showed that the Agr system was involved in BC resistance. However, a direct interaction between BC and AgrC was not observed, nor between BC and AgrA. These results indicated that BC could induce the Agr system via an indirect action. Both AgrBD and AgrC were required for growth under BC stress. Nevertheless, when exposed to BC, the gene deletion mutant ∆agrA strain exhibited better growth performance than its parental strain. The RR Lmo1172 played a role in BC resistance in the ∆agrA strain, suggesting that Lmo1172 may be an alternative to AgrA in the phosphotransfer pathway. Phosphorylation of Lmo1172 by AgrC was observed in vitro. The cognate HK Lmo1173 of Lmo1172 was not involved in BC stress, regardless of whether it was as the wild-type or the ∆agrA mutant strain. Our evidence suggests that the HK AgrC cross-phosphorylates its noncognate RR Lmo1172 to cope with BC stress when the cognate RR AgrA is absent. In vivo, further studies will be required to detect phosphotransfer of AgrC/AgrA and AgrC/Lmo1172.

Genetic Characterization of Listeria from Food of Non-Animal Origin Products and from Producing and Processing Companies in Bavaria, Germany

Reported cases of listeriosis from food of non-animal origin (FNAO) are increasing. In order to assess the risk of exposure to Listeria monocytogenes from FNAO, the genetic characterization of the pathogen in FNAO products and in primary production and processing plants needs to be investigated. For this, 123 samples of fresh and frozen soft fruit and 407 samples of 39 plants in Bavaria, Germany that produce and process FNAO were investigated for Listeria contamination. As a result, 64 Listeria spp. isolates were detected using ISO 11290-1:2017. Environmental swabs and water and food samples were investigated. L. seeligeri (36/64, 56.25%) was the most frequently identified species, followed by L. monocytogenes (8/64, 12.50%), L. innocua (8/64, 12.50%), L. ivanovii (6/64, 9.38%), L. newyorkensis (5/64, 7.81%), and L. grayi (1/64, 1.56%). Those isolates were subsequently sequenced by whole-genome sequencing and subjected to pangenome analysis to retrieve data on the genotype, serotype, antimicrobial resistance (AMR), and virulence markers. Eight out of sixty-four Listeria spp. isolates were identified as L. monocytogenes. The serogroup analysis detected that 62.5% of the L. monocytogenes isolates belonged to serogroup IIa (1/2a and 3a) and 37.5% to serogroup IVb (4b, 4d, and 4e). Furthermore, the MLST (multilocus sequence typing) analysis of the eight detected L. monocytogenes isolates identified seven different sequence types (STs) and clonal complexes (CCs), i.e., ST1/CC1, ST2/CC2, ST6/CC6, ST7/CC7, ST21/CC21, ST504/CC475, and ST1413/CC739. The core genome MLST analysis also showed high allelic differences and suggests plant-specific isolates. Regarding the AMR, we detected phenotypic resistance against benzylpenicillin, fosfomycin, and moxifloxacin in all eight L. monocytogenes isolates. Moreover, virulence factors, such as prfA, hly, plcA, plcB, hpt, actA, inlA, inlB, and mpl, were identified in pathogenic and nonpathogenic Listeria species. The significance of L. monocytogenes in FNAO is growing and should receive increasing levels of attention.

Molecular Dynamics Investigation of MFS Efflux Pump MdfA Reveals an Intermediate State between Its Inward and Outward Conformations

Multidrug resistance poses a major challenge to antibiotic therapy. A principal cause of antibiotic resistance is through active export by efflux pumps embedded in the bacterial membrane. Major facilitator superfamily (MFS) efflux pumps constitute a major group of transporters, which are often related to quinolone resistance in clinical settings. Although a rocker-switch model is proposed for description of their conformational transitions, detailed changes in this process remain poorly understood. Here we used MdfA from E. coli as a representative MFS efflux pump to investigate factors that can affect its conformational transition in silico. Molecular dynamics (MD) simulations of MdfA's inward and outward conformations revealed an intermediate state between these two conformations. By comparison of the subtle differences between the intermediate state and the average state, we indicated that conformational transition from outward to inward was initiated by protonation of the periplasmic side. Subsequently, hydrophilic interaction of the periplasmic side with water was promoted and the regional structure of helix 1 was altered to favor this process. As the hydrophobic interaction between MdfA and membrane was also increased, energy was concentrated and stored for the opposite transition. In parallel, salt bridges at the cytoplasmic side were altered to lower probabilities to facilitate the entrance of substrate. In summary, we described the total and local changes during MdfA's conformational transition, providing insights for the development of potential inhibitors.

Enhanced internal ionic interaction of MFS efflux pump MdfA contributes to its elevated antibiotic export

Infections caused by Gram-negative pathogens are difficult to manage due to their antibiotic resistance. Efflux pumps, which transport intracellular toxins out of the cytoplasm, play an important role in the detoxification of bacteria when treated with antibiotics. The major facilitator superfamily (MFS) is a kind of widely distributed efflux pumps and can actively export clinically important antibiotics such as ciprofloxacin, while the role of internal ionic interactions in regulating drug export remains poorly understood. Herein we used a representative MFS efflux pump MdfA to investigate the impact of internal ionic interactions on the antibiotic resistance of E. coli. First, we identified the internal salt bridges of MdfA and searched their natural variants across all the sequenced E. coli isolates. By constructing these variants, we discovered that extending the salt bridge on the cytoplasmic side (E136D) conferred an elevated antibiotic resistance level of E. coli, and the level was further enhanced by combining it with an artificial mutation K346R. By analyzing the trajectories of MdfA's variants in molecular dynamics (MD) simulations, we revealed that ionic interaction strengths on the two sides were proportionally enhanced, while the protein flexibility was not affected. Moreover, enhanced interactions resulted in a larger surface for MdfA's protonation, suggesting a higher possibility for its activation. Collectively, our data revealed the importance of internal interactions on the drug export of MdfA, offering insights for the development of novel inhibitors against MFS efflux pumps.

Ring-fused 2-pyridones effective against multidrug-resistant Gram-positive pathogens and synergistic with standard-of-care antibiotics

The alarming rise of multidrug-resistant Gram-positive bacteria has precipitated a healthcare crisis, necessitating the development of new antimicrobial therapies. Here we describe a new class of antibiotics based on a ring-fused 2-pyridone backbone, which are active against vancomycin-resistant enterococci (VRE), a serious threat as classified by the Centers for Disease Control and Prevention, and other multidrug-resistant Gram-positive bacteria. Ring-fused 2-pyridone antibiotics have bacteriostatic activity against actively dividing exponential phase enterococcal cells and bactericidal activity against nondividing stationary phase enterococcal cells. The molecular mechanism of drug-induced killing of stationary phase cells mimics aspects of fratricide observed in enterococcal biofilms, where both are mediated by the Atn autolysin and the GelE protease. In addition, combinations of sublethal concentrations of ring-fused 2-pyridones and standard-of-care antibiotics, such as vancomycin, were found to synergize to kill clinical strains of VRE. Furthermore, a broad range of antibiotic resistant Gram-positive pathogens, including those responsible for the increasing incidence of antibiotic resistant healthcare-associated infections, are susceptible to this new class of 2-pyridone antibiotics. Given the broad antibacterial activities of ring-fused 2-pyridone compounds against Gram-positive (GmP) bacteria we term these compounds GmPcides, which hold promise in combating the rising tide of antibiotic resistant Gram-positive pathogens.

Listeria monocytogenes genes supporting growth under standard laboratory cultivation conditions and during macrophage infection

The Gram-positive bacterium Listeria monocytogenes occurs widespread in the environment and infects humans when ingested along with contaminated food. Such infections are particularly dangerous for risk group patients, for whom they represent a life-threatening disease. To invent novel strategies to control contamination and disease, it is important to identify those cellular processes that maintain pathogen growth inside and outside the host. Here, we have applied transposon insertion sequencing (Tn-Seq) to L. monocytogenes for the identification of such processes on a genome-wide scale. Our approach identified 394 open reading frames that are required for growth under standard laboratory conditions and 42 further genes, which become necessary during intracellular growth in macrophages. Most of these genes encode components of the translation machinery and act in chromosome-related processes, cell division, and biosynthesis of the cellular envelope. Several cofactor biosynthesis pathways and 29 genes with unknown functions are also required for growth, suggesting novel options for the development of antilisterial drugs. Among the genes specifically required during intracellular growth are known virulence factors, genes compensating intracellular auxotrophies, and several cell division genes. Our experiments also highlight the importance of PASTA kinase signaling for general viability and of glycine metabolism and chromosome segregation for efficient intracellular growth of L. monocytogenes.

Assessment of Bacteriocin-Antibiotic Synergy for the Inhibition and Disruption of Biofilms of Listeria monocytogenes and Vancomycin-Resistant Enterococcus

In this study, we have evaluated the effects of previously characterized bacteriocins produced by E. faecium strains ST651ea, ST7119ea, and ST7319ea, against biofilm formation and biofilms formed by L. monocytogenes ATCC15313 and vancomycin-resistant E. faecium VRE19. The effects of bacteriocins on the biofilms formed by L. monocytogenes ATCC151313 were evaluated by crystal violet assay and further confirmed by quantifying viable cells and cell metabolic activities through flow cytometry and TTC assay, respectively, indicating that bacteriocin activities required to completely eradicate biofilms are at least 1600 AU mL−1, 3200 AU mL−1, and 6400 AU mL−1, respectively for each bacteriocin evaluated. Furthermore, bacteriocins ST651ea and ST7119ea require at least 6400 AU mL−1 to completely eradicate the viability of cells within the biofilms formed by E. faecium VRE19, while bacteriocin ST7319ea requires at least 12800 AU mL−1 to obtain the same observations. Assessment of synergistic activities between selected conventional antibiotics (ciprofloxacin and vancomycin) with these bacteriocins was carried out to evaluate their effects on biofilm formation and pre-formed biofilms of both test microorganisms. Results showed that higher concentrations are needed to completely eradicate metabolic activities of cells within pre-formed biofilms in contrast with the biofilm formation abilities of the strains. Furthermore, synergistic activities of bacteriocins with both ciprofloxacin and vancomycin are more evident against vancomycin-resistant E. faecium VRE19 rather than L. monocytogenes ATCC15313. These observations can be further explored for possible applications of these combinations of antibiotics as a possible treatment of clinically relevant pathogens.

Nonsynonymous Mutations in fepR Are Associated with Adaptation of Listeria monocytogenes and Other Listeria spp. to Low Concentrations of Benzalkonium Chloride but Do Not Increase Survival of L. monocytogenes and Other Listeria spp. after Exposure to Benzalkonium Chloride Concentrations Recommended for Use in Food Processing Environments

Selection for Listeria monocytogenes strains that are tolerant to quaternary ammonium compounds (such as benzalkonium chloride [BC]) is a concern across the food industry, including in fresh produce processing environments. This study evaluated the ability of 67 strains of produce-associated L. monocytogenes and other Listeria spp. (“parent strains”) to show enhanced BC tolerance after serial passaging in increasing BC concentrations and to maintain this tolerance after substreaking in the absence of BC. After serial passaging in BC, 62/67 “BC passaged cultures” showed higher MICs (4 to 20 mg/L) than parent strains (2 to 6 mg/L). After the substreaking of two isolates from BC passaged cultures for each parent strain, 105/134 “adapted isolates” maintained MICs (4 to 6 mg/L) higher than parent strain MICs. These results suggested that adapted isolates acquired heritable adaptations that confer BC tolerance. Whole-genome sequencing and Sanger sequencing of fepR, a local repressor of the MATE family efflux pump FepA, identified nonsynonymous fepR mutations in 48/67 adapted isolates. The mean inactivation of adapted isolates after exposure to use-level concentrations of BC (300 mg/L) was 4.48 log, which was not significantly different from inactivation observed in parent strains. Serial passaging of cocultures of L. monocytogenes strains containing bcrABC or qacH did not yield adapted isolates that showed enhanced BC tolerance in comparison to that of monocultures. These results suggest that horizontal gene transfer either did not occur or did not yield isolates with enhanced BC tolerance. Overall, this study provides new insights into selection of BC tolerance among L. monocytogenes and other Listeria spp.

IMPORTANCEListeria monocytogenes tolerance to quaternary ammonium compounds has been raised as a concern with regard to L. monocytogenes persistence in food processing environments, including in fresh produce packing and processing environments. Persistence of L. monocytogenes can increase the risk of product contamination, food recalls, and foodborne illness outbreaks. Our study shows that strains of L. monocytogenes and other Listeria spp. can acquire heritable adaptations that confer enhanced tolerance to low concentrations of benzalkonium chloride, but these adaptations do not increase survival of L. monocytogenes and other Listeria spp. when exposed to concentrations of benzalkonium chloride used for food contact surface sanitation (300 mg/L). Overall, these findings suggest that the emergence of benzalkonium chloride-tolerant Listeria strains in food processing environments is of limited concern, as even strains adapted to gain higher MICs in vitro maintain full sensitivity to the concentrations of benzalkonium chloride used for food contact surface sanitation.

An Exploration of Listeria monocytogenes, Its Influence on the UK Food Industry and Future Public Health Strategies

Listeria monocytogenes is a Gram-positive intracellular pathogen that can cause listeriosis, an invasive disease affecting pregnant women, neonates, the elderly, and immunocompromised individuals. Principally foodborne, the pathogen is transmitted typically through contaminated foods. As a result, food manufacturers exert considerable efforts to eliminate L. monocytogenes from foodstuffs and the environment through food processing and disinfection. However, L. monocytogenes demonstrates a range of environmental stress tolerances, resulting in persistent colonies that act as reservoirs for the reintroduction of L. monocytogenes to food contact surfaces and food. Novel technologies for the rapid detection of L. monocytogenes and disinfection of food manufacturing industries have been developed to overcome these obstacles to minimise the risk of outbreaks and sporadic cases of listeriosis. This review is aimed at exploring L. monocytogenes in the UK, providing a summary of outbreaks, current routine microbiological testing and the increasing awareness of biocide tolerances. Recommendations for future research in the UK are made, pertaining to expanding the understanding of L. monocytogenes dissemination in the UK food industry and the continuation of novel technological developments for disinfection of food and the food manufacturing environment.

Contamination by Listeria monocytogenes in Latin American Meat Products and Its Consequences

Background and objective:

Listeria monocytogenes is one of the most important bacteria in food technology, causing listeriosis, a disease with high mortality rates, important especially in developing countries. Thus, the objective of this review was to gather recent work on the presence of L. monocytogenes in meat and meat products in Latin America, in addition to pointing out control methods and resistance genes that can be disseminated.

Methods:

Original research articles in Portuguese, Spanish and English published since 2017 were selected, reporting the presence of L. monocytogenes in meat and meat products in Latin American countries. Articles were also reviewed on innovative methods for controlling the bacteria in food, such as intelligent packaging and the use of essential oils, and on resistance genes found in L. monocytogenes, pointing out the possible implications of this occurrence.

Results and conclusion:

Some negligence was observed in determining the prevalence of this bacterium in several countries in Latin America. Although studies on L. monocytogenes have been found in milk and dairy products, demonstrating the existence of the necessary structure and knowledge for research development, studies on meat and meat products have not been found in most countries. In control methods developed against L. monocytogenes, the versatility of the approaches used stands out, enabling their use in different types of meat products, according to their technological characteristics. Several resistance genes have been determined to be carried and possibly disseminated by L. monocytogenes, which adds more importance in the establishment of methods for its control.

Adaptation to a Commercial Quaternary Ammonium Compound Sanitizer Leads to Cross-Resistance to Select Antibiotics in Listeria monocytogenes Isolated From Fresh Produce Environments

The effective elimination of Listeria monocytogenes through cleaning and sanitation is of great importance to the food processing industry. Specifically in fresh produce operations, the lack of a kill step requires effective cleaning and sanitation to mitigate the risk of cross-contamination from the environment. As facilities rely on sanitizers to control L. monocytogenes, reports of the development of tolerance to sanitizers and other antimicrobials through cross-resistance is of particular concern. We investigated the potential for six L. monocytogenes isolates from fresh produce handling and processing facilities and packinghouses to develop cross-resistance between a commercial sanitizer and antibiotics. Experimental adaptation of isolates belonging to hypervirulent clonal complexes (CC2, CC4, and CC6) to a commercial quaternary ammonium compound sanitizer (cQAC) resulted in elevated minimum inhibitory concentrations (2–3 ppm) and minimum bactericidal concentrations (3–4 ppm). Susceptibility to cQAC was restored for all adapted (qAD) isolates in the presence of reserpine, a known efflux pump inhibitor. Reduced sensitivity to 7/17 tested antibiotics (chloramphenicol, ciprofloxacin, clindamycin, kanamycin, novobiocin, penicillin, and streptomycin) was observed in all tested isolates. qAD isolates remained susceptible to antibiotics commonly used in the treatment of listeriosis (i.e., ampicillin and gentamicin). The whole genome sequencing of qAD strains, followed by comparative genomic analysis, revealed several mutations in fepR, the regulator for FepA fluoroquinolone efflux pump. The results suggest that mutations in fepR play a role in the reduction in antibiotic susceptibility following low level adaptation to cQAC. Further investigation into the cross-resistance mechanisms and pressures leading to the development of this phenomenon among L. monocytogenes isolates recovered from different sources is needed to better understand the likelihood of cross-resistance development in food chain isolates and the implications for the food industry.

Role of the VirSR-VirAB system in biofilm formation of Listeria monocytogenes EGD-e

The ability of Listeria monocytogenes, an important foodborne pathogen, to form biofilms in food processing environments leads to increased opportunity for contamination of food products, which is a major concern for food safety. In this study, the role of a complex system composed of the VirSR two-component signal transduction system (TCS) and the ATP-binding cassette (ABC) transporter VirAB in biofilm formation of L. monocytogenes EGD-e was investigated. Biofilm formation was measured using the microplate assay with crystal violet staining, confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM), and attachment and swarming motility were compared between strain EGD-e and its isogenic deletion mutants. Additionally, the relative expression levels of genes associated with the early steps of biofilm development in the wild-type and mutant strains were also determined by RT-qPCR. Results from microplate assay, CLSM and SEM showed that VirR is not required for biofilm formation in L. monocytogenes EGD-e. A central finding of this study is that both VirAB and VirS are essential for biofilm formation and they could function as a whole in biofilm formation of L. monocytogenes EGD-e. The results also demonstrated that both VirAB and VirS are involved in attachment, but they are not associated with swarming motility. Results from RT-qPCR showed that flaA, motA and motB were downregulated in the mutant strains ΔvirAB and ΔvirS, which could be the possible reason for reduced attachment and biofilm formation in these mutants. This study provides a better understanding of the mechanisms involved in biofilm formation of L. monocytogenes, leading to improved processes to control this biofilm-forming foodborne pathogen.

Tannic Acid-Stabilized Silver Nanoparticles Used in Biomedical Application as an Effective Antimelioidosis and Prolonged Efflux Pump Inhibitor against Melioidosis Causative Pathogen

Burkholderia pseudomallei is the causative pathogen of melioidosis and this bacterium is resistant to several antibiotics. Silver nanoparticles (AgNPs) are an interesting agent to develop to solve this bacterial resistance. Here, we characterize and assess the antimelioidosis activity of AgNPs against these pathogenic bacteria. AgNPs were characterized and displayed a maximum absorption band at 420 nm with a spherical shape, being well-monodispersed and having high stability in solution. The average size of AgNPs is 7.99 ± 1.46 nm. The antibacterial efficacy of AgNPs was evaluated by broth microdilution. The bactericidal effect of AgNPs was further assessed by time-kill kinetics assay. Moreover, the effect of AgNPs on the inhibition of the established biofilm was investigated by the crystal violet method. In parallel, a study of the resistance induction development of B. pseudomallei towards AgNPs with efflux pump inhibiting effect was performed. We first found that AgNPs had strong antibacterial activity against both susceptible and ceftazidime-resistant (CAZ-resistant) strains, as well as being efficiently active against B. pseudomallei CAZ-resistant strains with a fast-killing mode via a bactericidal effect within 30 min. These AgNPs did not only kill planktonic bacteria in broth conditions, but also in established biofilm. Our findings first documented that the resistance development was not induced in B. pseudomallei toward AgNPs in the 30th passage. We found that AgNPs still showed an effective efflux pump inhibiting effect against these bacteria after prolonged exposure to AgNPs at sublethal concentrations. Thus, AgNPs have valuable properties for being a potent antimicrobial agent to solve the antibiotic resistance problem in pathogens.

Ecogenetics of antibiotic resistance in Listeria monocytogenes

The acquisition process of antibiotic resistance in an otherwise susceptible organism is shaped by the ecology of the species. Unlike other relevant human pathogens, Listeria monocytogenes has maintained a high rate of susceptibility to the antibiotics used for decades to treat human and animal infections. However, L. monocytogenes can acquire antibiotic resistance genes from other organisms' plasmids and conjugative transposons. Ecological factors could account for its susceptibility. L. monocytogenes is ubiquitous in nature, most frequently including reservoirs unexposed to antibiotics, including intracellular sanctuaries. L. monocytogenes has a remarkably closed genome, reflecting limited community interactions, small population sizes and high niche specialization. The L. monocytogenes species is divided into variants that are specialized in small specific niches, which reduces the possibility of coexistence with potential donors of antibiotic resistance. Interactions with potential donors are also hampered by interspecies antagonism. However, occasional increases in population sizes (and thus the possibility of acquiring antibiotic resistance) can derive from selection of the species based on intrinsic or acquired resistance to antibiotics, biocides, heavy metals or by a natural tolerance to extreme conditions. High-quality surveillance of the emergence of resistance to the key drugs used in primary therapy is mandatory.

Listeria monocytogenes Virulence, Antimicrobial Resistance and Environmental Persistence: A Review

Listeria monocytogenes is a ubiquitous opportunistic pathogen responsible for the well-known listeriosis disease. This bacterium has become a common contaminant of food, threatening the food processing industry. Once consumed, the pathogen is capable of traversing epithelial barriers, cellular invasion, and intracellular replication through the modulation of virulence factors such as internalins and haemolysins. Mobile genetic elements (plasmids and transposons) and other sophisticated mechanisms are thought to contribute to the increasing antimicrobial resistance of L. monocytogenes. The environmental persistence of the pathogen is aided by its ability to withstand environmental stresses such as acidity, cold stress, osmotic stress, and oxidative stress. This review seeks to give an insight into L. monocytogenes biology, with emphasis on its virulence factors, antimicrobial resistance, and adaptations to environmental stresses.

Antimicrobial Susceptibility and Multilocus Sequence Typing of Listeria monocytogenes Isolated Over 11 Years from Food, Humans, and the Environment in Italy

Due to the increasing number of studies reporting the detection of antimicrobial-resistant isolates of Listeria monocytogenes, we sought to determine the antimicrobial susceptibility of L. monocytogenes isolates collected in Italy and find potential correlations to their serotypes and multilocus sequence types (MLST). The antimicrobial susceptibility of 317 L. monocytogenes isolates collected from food, humans, and the environment from 1998 to 2009 was assessed by minimum inhibitory concentration (MIC). Serotyping and MLST was also performed on all isolates. Potential correlations among antimicrobial resistance profiles, serotyping, and MLST were statistically evaluated. Twenty-four percent of L. monocytogenes isolates were resistant to oxacillin, 28.7% intermediate to clindamycin, and 24.3% to ciprofloxacin. The majority of isolates with elevated MIC to oxacillin was of environmental origin and belonged to serotype 4b/4e and ST2. Isolates with intermediate MIC values to clindamycin and ciprofloxacin were mostly of food and human origin and belonged to serotype 4b/4e and ST9. Regarding the time frame of isolate collection, comparing the last 3 years (2007-2009) to previous years (1998-2006), an increase was observed in the percentage of resistant and intermediate isolates per year. This trend strongly suggests the need for increasing attention on the prevalence of antimicrobial resistance in L. monocytogenes in Italy. To predict future resistance trends, the monitoring of clinical intermediate resistance might represent a useful tool especially for antibiotics associated to multiple-step mechanisms of acquired resistance. A specific focus should be addressed to antimicrobial-resistant isolates of serotype 4b, repeatedly associated with food-borne outbreaks.

The VirAB-VirSR-AnrAB Multicomponent System Is Involved in Resistance of Listeria monocytogenes EGD-e to Cephalosporins, Bacitracin, Nisin, Benzalkonium Chloride, and Ethidium Bromide

In Listeria monocytogenes, it has been proposed that the VirSR two-component signal transduction systems (TCSs) and two ATP-binding cassette (ABC) transporters, VirAB and AnrAB, constitute a complex TCS/ABC transporter system which has been recognized as a unique resistance mode. The role of the putative VirAB-VirSR-AnrAB system in antimicrobial resistance and the respective contributions of the members of the system to resistance were investigated in this study. We constructed gene deletion mutants of L. monocytogenes EGD-e and complemented strains of the mutants and determined MICs of antimicrobial agents against these strains against using the agar dilution method. We assessed the relative expression levels of target genes by reverse transcription-quantitative PCR (RT-qPCR) and measured promoter activity levels by β-galactosidase assays. Our results showed that the VirAB-VirSR-AnrAB system mediates not only nisin and bacitracin resistance but also resistance to cephalosporins, ethidium bromide (EtBr), and benzalkonium chloride (BC). In this system, two ABC transporters, VirAB and AnrAB, play distinct roles in cefotaxime resistance: the former is responsible only for antimicrobial sensing and signaling by VirSR, while the latter contributes to transportation of antimicrobials. Notably, VirAB itself, rather than the VirAB-VirSR-AnrAB system as a whole, contributes to kanamycin and tetracycline resistance. On the basis of the results obtained from this study, we speculate that VirAB acts as a sensor for VirSR in response to cephalosporins, bacitracin, nisin, EtBr, and BC, while VirAB itself plays a role in response to kanamycin and tetracycline in L. monocytogenes EGD-e.IMPORTANCE This report describes TCS/ABC transporter modules characterized in Listeria monocytogenes EGD-e. The modules consist of the VirSR TCS and the VirAB and AnrAB ABC transporters. Our results showed that this system is involved in nisin and bacitracin resistance, as well as resistance to cephalosporins, ethidium bromide (EtBr), and benzalkonium chloride (BC). In this system, VirAB is responsible only for antimicrobial sensing and signaling by VirSR, while AnrAB contributes to transportation of antimicrobials. Interestingly, VirAB itself, rather than the VirAB-VirSR-AnrAB system as a whole, contributes to kanamycin and tetracycline resistance.

Reversing resistance to counter antimicrobial resistance in the World Health Organisation's critical priority of most dangerous pathogens

The speed at which bacteria develop antimicrobial resistance far outpace drug discovery and development efforts resulting in untreatable infections. The World Health Organisation recently released a list of pathogens in urgent need for the development of new antimicrobials. The organisms that are listed as the most critical priority are all Gram-negative bacteria resistant to the carbapenem class of antibiotics. Carbapenem resistance in these organisms is typified by intrinsic resistance due to the expression of antibiotic efflux pumps and the permeability barrier presented by the outer membrane, as well as by acquired resistance due to the acquisition of enzymes able to degrade β-lactam antibiotics. In this perspective article we argue the case for reversing resistance by targeting these resistance mechanisms - to increase our arsenal of available antibiotics and drastically reduce antibiotic discovery times - as the most effective way to combat antimicrobial resistance in these high priority pathogens.