Susceptibility of Listeria monocytogenes isolated from food in Italy to antibiotics

Selection of Listeria monocytogenes InlA-Binding Peptides Using Phage Display—Novel Compounds for Diagnostic Applications?

Listeria monocytogenes is a pathogenic, gram-positive bacterium causing foodborne infections and listeriosis, an infection responsible for serious medical conditions, especially for pregnant women, newborns, or people with a weak immune system. Even after antibiotic treatment, 30% of clinical infections result in death. L. monocytogenes is able to enter and multiply in mammalian cells. Invasion into epithelial cells in the human intestine is mediated by the interaction of the bacterial surface protein internalin A (InlA) with the host cell receptor E-cadherin (E-cad). We have used phage display to select InlA-specific peptides consisting of 12 amino acids using a randomized, recombinant peptide library. We could demonstrate that the selected peptides bound to recombinant InlA protein as well as to L. monocytogenes cells. In vitro, some of the peptides inhibited the interaction between recombinant InlA and human E-cad. As far as we know, this is the first publication on the development of InlA-specific peptide ligands. In the future, our peptides might be used for the development of innovative diagnostic tools or even therapeutic approaches.

New Insights into Listeria monocytogenes Antimicrobial Resistance, Virulence Attributes and Their Prospective Correlation

Listeriosis is one of the most common foodborne diseases caused by Listeria monocytogenes (L. monocytogenes). A poor prognosis has been recorded for the invasive listeriosis, especially neurolisteriosis. In several countries throughout the world, foodborne infections with L. monocytogenes exceeded the legal safety limits in animal sourced foods. Therefore, we decided to investigate the variability, virulence and antimicrobial resistance profiles of this pathogen. Both phenotypic and genotypic methods were used for identifying L. monocytogenes isolates and confirming their virulence profiles. The antimicrobial resistances and their correlation analysis with the existence of virulence genes were detected. Additionally, sequencing and phylogenetic analysis based on L. monocytogenes inlA and inlB genes were undertaken. The prevalence rate (11.9%) and the resistance profiles of L. monocytogenes were shocking. The multi-drug resistance (MDR) phenotypes were common among our isolates (64.9%). Fortunately, the resistance phenotypes were always associated with low virulence arrays and the MDR strains possessed low virulence fitness. Herein, the high genotypic and phenotypic diversity of L. monocytogenes isolates and their weak clonality and adaptability highlighted the difficulty in controlling and managing this pathogen. Therefore, it is important to add more restriction guidelines from national authorities on the consumption of ready to eat foods.

A global perspective of antibiotic-resistant Listeria monocytogenes prevalence in assorted ready to eat foods: A systematic review

BACKGROUND AND AIM

Listeria monocytogenes in ready-to-eat (RTE) foods remains consistently under-reported globally. Nevertheless, several independent studies conducted to investigate have elucidated the prevalence and antibiotic resistance profiles of L. monocytogenes in RTE-associated foods and their antibiotic resistance profiles. Given the rapid increase in consumption of RTE foods of both animal and plant origin, it is imperative to know the prevalence deductive data focusing on how much of L. monocytogenes is present in RTE foods, which is critical for food safety managers and retailers to assess the possible risk posed to end-users. In addition, valuable insight and another angle to the depth of the problem, we conducted a systematic review and meta-analysis to synthesize available data regarding the prevalence of L. monocytogenes in RTE foods and antibiotic resistance profiles.

MATERIALS AND METHODS

We conducted a meta-analysis study of L. monocytogenes and antibiotic resistance to clinically relevant antibiotics to determine the extent of L. monocytogenes contamination in RTE foods and antibiotic resistance profiles. The primary search terms, also known as keywords used, were restricted to peer-reviewed and review articles, and databases, including Google Scholars, Science-Direct, and Scopus, were searched. The inclusion of articles meeting eligibility criteria published between 2010 and 2020 after title, abstract, and full article screening. Data analysis was performed at multiple stages using quantitative meta-analysis reviews.

RESULTS

L. monocytogenes pooled proportion/prevalence was highest in chicken products determined at (22%) followed by various but uncategorized RTE foods at 21%. Regarding antibiotic resistance, profiling's highest pooled prevalence resistance was observed in penicillin at 80% resistance, followed by cephalosporin at 47%.

CONCLUSION

Within its limitations, this study has attempted to provide insight into the pooled proportion/prevalence of L. monocytogenes in RTE foods and the antibiotic resistance profile at the global level. Determining the proportion/prevalence of L. monocytogenes in RTE foods across the globe and antibiotic resistance profile is essential for providing quality food and reducing public health problems due to unsuccessful treatment of foodborne illness. This study provides insight into the pooled prevalence of L. monocytogenes in RTE foods and the antibiotic resistance profile. The results of this study partly endeavored to help appropriate authorities strengthen their preventive measures on specific RTE foods that are most likely to be contaminated with L. monocytogenes and antibiotic resistance profiles.

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.

Antimicrobial resistance profiles of Listeria monocytogenes isolated from chicken meat in Fukuoka, Japan

In this study, the antimicrobial resistance profiles of L. monocytogenes isolated from chicken meat in Fukuoka in 2017 were compared with the isolates of 2012. A total of 85 and 50 chicken meat samples, including different body parts, were collected from different supermarkets in Fukuoka in 2012 and 2017, respectively. Detection, isolation, identification, and characterization of L. monocytogenes were performed according to the conventional methods. Forty-five among 85 samples (53%) were positive for L. monocytogenes in 2012, while 12 among 50 samples in 2017 (24%) tested positive. One hundred fifty-three and 29 L. monocytogenes strains were isolated in 2012 and 2017, respectively. The serotypes of isolates in 2012 were 1/2a (21.5%), 1/2b (73.9%), 1/2c (1.5%), and 4b/4e (3.1%). In contrast, the 2017 isolates showed 1/2a (48.3%) and 1/2b (51.7%) serotypes. While all isolates in 2012 were positive for hlyA (listeriolysin O) in the PCR assay with hlyA primer set 7, only 17 hlyA positive isolates were seen in 2017. Moreover, 75 isolates with different ribotypes in 2012 and 29 isolates in 2017, respectively, were tested for antimicrobial susceptibility by broth microdilution for 18 different antimicrobial agents. Most of the 2012 and 2017 isolates displayed antimicrobial susceptibility. However, among the 2012 and 2017 isolates, 98.7% and 100% of the isolates were resistant to cefoxitin, 57.3% and 95.7% to fosfomycin, 72.0% and 82.6% to oxacillin, 8.0% and 17.4% to clindamycin, respectively. In addition, 2.7% of the isolates in 2012 were resistant to flomoxef and 4.3% of the isolates in 2017 to linezolid. Multidrug resistance (MDR) to 3 or more antimicrobials was observed in 35/75 (46.7%) isolates of 2012 and 19/23 (82.6%) in 2017. Detection of antimicrobial resistance (AMR) genes by PCR showed that the resistant isolates of 2012 were positive for mecA (96.3%) and ermC (83.3%), whereas the resistant isolates in 2017 screened positive for mecA (94.7%) and mefA (25.0%). Other cfxA, ermA, ermB, fosA, fosB, and fosC genes were absent in the PCR assay for any of the isolates. This study investigated for the first time the change in the L. monocytogenes contamination of chicken meat and antibiotic resistance of the isolated L. monocytogenes strains in Fukuoka, Japan, in the course of 5 years. Although the contamination rate of L. monocytogenes in 2017 was found to be lower than that in 2012, AMR of the isolates in 2017 was higher.

A Synthetic Derivative of Antimicrobial Peptide Holothuroidin 2 from Mediterranean Sea Cucumber (Holothuria tubulosa) in the Control of Listeria monocytogenes

Due to the limited number of available antibiotics, antimicrobial peptides (AMPs) are considered antimicrobial candidates to fight difficult-to-treat infections such as those associated with biofilms. Marine environments are precious sources of AMPs, as shown by the recent discovery of antibiofilm properties of Holothuroidin 2 (H2), an AMP produced by the Mediterranean sea cucumber Holothuria tubulosa. In this study, we considered the properties of a new H2 derivative, named H2d, and we tested it against seven strains of the dangerous foodborne pathogen Listeria monocytogenes. This peptide was more active than H2 in inhibiting the growth of planktonic L. monocytogenes and was able to interfere with biofilm formation at sub-minimum inhibitory concentrations (MICs). Atomic-level molecular dynamics (MD) simulations revealed insights related to the enhanced inhibitory activity of H2d, showing that the peptide is characterized by a more defined tertiary structure with respect to its ancestor. This allows the peptide to better exhibit an amphipathic character, which is an essential requirement for the interaction with cell membranes, similarly to other AMPs. Altogether, these results support the potential use of our synthetic peptide, H2d, as a template for the development of novel AMP-based drugs able to fight foodborne that are resistant to conventional antibiotics.

Molecular diversity and antimicrobial susceptibility of Listeria monocytogenes isolates from invasive infections in Poland (1997-2013)

The epidemiology of invasive listeriosis in humans appears to be weakly characterized in Poland, the sixth most populous member state of the European Union. We obtained antimicrobial susceptibility data, PCR-serogroups and genotypic profiles for 344 invasive isolates of Listeria monocytogenes, collected between 1997 and 2013 in Poland. All isolates were susceptible to the 10 tested antimicrobials, except one that was resistant to tetracycline and minocycline and harbored the tet(M), tet(A) and tet(C) genes. Overall, no increasing MIC values were observed during the study period. Four PCR-serogroups were observed: IVb (55.8%), IIa (34.3%), IIb (8.1%) and IIc (1.8%). We identified clonal complexes (CCs) and epidemic clones (ECs) previously involved in outbreaks worldwide, with the most prevalent CCs/ECs being: CC6/ECII (32.6%), CC1/ECI (17.2%), CC8/ECV (6.1%) and CC2/ECIV (5.5%). The present study is the first extensive analysis of Polish L. monocytogenes isolates from invasive infections.

Emergence of Antibiotic Resistance in Listeria monocytogenes Isolated from Food Products: A Comprehensive Review

Listeria monocytogenes is an opportunistic pathogen that has been involved in several deadly illness outbreaks. Future outbreaks may be more difficult to manage because of the emergence of antibiotic resistance among L. monocytogenes strains isolated from food products. The present review summarizes the available evidence on the emergence of antibiotic resistance among L. monocytogenes strains isolated from food products and the possible ways this resistance has developed. Furthermore, the resistance of food L. monocytogenes isolates to antibiotics currently used in the treatment of human listeriosis such as penicillin, ampicillin, tetracycline, and gentamicin, has been documented. Acquisition of movable genetic elements is considered the major mechanism of antibiotic resistance development in L. monocytogenes. Efflux pumps have also been linked with resistance of L. monocytogenes to some antibiotics including fluoroquinolones. Some L. monocytogenes strains isolated from food products are intrinsically resistant to several antibiotics. However, factors in food processing chains and environments (from farm to table) including extensive or sub-inhibitory antibiotics use, horizontal gene transfer, exposure to environmental stresses, biofilm formation, and presence of persister cells play crucial roles in the development of antibiotic resistance by L. monocytogenes.

Phenotypic and Genotypic Analysis of Antimicrobial Resistance among Listeria monocytogenes Isolated from Australian Food Production Chains

The current global crisis of antimicrobial resistance (AMR) among important human bacterial pathogens has been amplified by an increased resistance prevalence. In recent years, a number of studies have reported higher resistance levels among Listeria monocytogenes isolates, which may have implications for treatment of listeriosis infection where resistance to key treatment antimicrobials is noted. This study examined the genotypic and phenotypic AMR patterns of 100 L. monocytogenes isolates originating from food production supplies in Australia and examined this in the context of global population trends. Low levels of resistance were noted to ciprofloxacin (2%) and erythromycin (1%); however, no resistance was observed to penicillin G or tetracycline. Resistance to ciprofloxacin was associated with a mutation in the fepR gene in one isolate; however, no genetic basis for resistance in the other isolate was identified. Resistance to erythromycin was correlated with the presence of the ermB resistance gene. Both resistant isolates belonged to clonal complex 1 (CC1), and analysis of these in the context of global CC1 isolates suggested that they were more similar to isolates from India rather than the other CC1 isolates included in this study. This study provides baseline AMR data for L. monocytogenes isolated in Australia, identifies key genetic markers underlying this resistance, and highlights the need for global molecular surveillance of resistance patterns to maintain control over the potential dissemination of AMR isolates.

Antibiotic susceptibility of 259 Listeria monocytogenes strains isolated from food, food-processing plants and human samples in Germany

BACKGROUND

The objective of this study was to evaluate the susceptibility of 259 Listeria monocytogenes strains isolated from food and food-processing environments and patient samples in Germany to 14 antibiotics widely used in veterinary and human medicine. L. monocytogenes strains were isolated mainly from milk and milk products and classified according to their molecular serotypes IIa (n=112), IIb (n=41), IIc (n=36), IVa (n=1), IVb (n=66), and IVb-v1 (n=3).

METHODS

Susceptibility tests were performed by using the automated 96-well based microdilution system Micronaut-S. Ampicillin, benzylpenicillin, ceftriaxone, ciprofloxacin, daptomycin, erythromcyin, gentamicin, linezolid, meropenem, rifampicin, tetracycline, tigecycline, trimethoprim/sulfamethoxazole and vancomycin were tested in at least five different concentrations.

RESULTS

Among the 259 strains under study, 145 strains revealed multidrug-resistance (resistance to ≥3 antibiotics) and predominantly belonged to serotype IV (59%). Strains were mainly resistant to daptomycin, tigecycline, tetracycline, ciprofloxacin, ceftriaxone, trimethropim/sulfamethoxazole and gentamicin.

CONCLUSIONS

Antibiotic resistance in general and multidrug-resistance in particular were more prevalent in L. monocytogenes strains isolated in Germany compared to similar reference stocks from other European countries and the USA but similar to stocks from China.

Prevalence, Virulence Potential, and Antibiotic Susceptibility Profile of Listeria monocytogenes Isolated From Bovine Raw Milk Samples Obtained From Rajasthan, India

Listeriosis is a serious foodborne disease of a global concern, and can effectively be controlled by a continuous surveillance of the virulent and multidrug-resistant strains of Listeria monocytogenes. This study was planned to investigate prevalence of L. monocytogenes in bovine raw milk samples. A total of 457 raw milk samples collected from 15 major cities in Rajasthan, India, were analyzed for the presence of L. monocytogenes by using standard microbiological and molecular methods. Five of the 457 samples screen tested positive for L. monocytogenes. Multiplex serotyping showed that 3/5 strains belonged to serotype 4b followed by one strain each to 1/2a and to 1/2c. Further virulence potential assessment indicated that all strains possessed inlA and inlC internalins, and, in addition, two strains also possessed the gene for inlB. All strains were positive for Listeriolysin O (LLO) and showed phosphatidylinositol-specific phospholipase C (PI-PLC) activity on an in vitro agar medium with variations in production levels among the strains. A good correlation between the in vitro pathogenicity test and the chick embryo test was observed, as the strains showing higher LLO and PI-PLC activity were found to be lethal to fertilized chick embryos. All strains were resistant to the majority of antibiotics and were designated as multidrug-resistant strains. However, these strains were susceptible to 9 of the 22 tested antibiotics. The maximum zone of inhibition (mm) and acceptable minimum inhibitory concentration were observed with azithromycin, and thus it could be the first choice of a treatment. Overall, the presence of multidrug-resistant L. monocytogenes strains in the raw milk of Rajasthan region is an indicator of public health hazard and highlighting the need of consumer awareness in place and implementation of stricter food safety regulations at all levels of milk production.

Prevalence and Genetic Characteristics of Meatborne Listeria monocytogenes Isolates from Livestock Farms in Korea

This study aimed to evaluate the prevalence of Listeria monocytogenes on livestock farms in Korea and determine their serotypes and genetic correlations. Twenty-five livestock farms in Korea (central: 15, south west: 7, south east: 3) were visited 2-3 times, and 2,018 samples (feces: 677, soil: 680, silage: 647, sludge: 14) were collected. Samples were enriched in LEB (Listeria enrichment broth) and Fraser broth media, and then plated on Palcam agar. The isolates were identified by PCR and 16S rRNA gene sequencing. Then, the serotypes, presence of virulence genes (actA, inlA, inlB, plcB, and hlyA), and antibiotic resistance were determined. Genetic correlations among the isolates were evaluated by analyzing the restriction digest pattern with AscI. Of the 2,018 samples, only 3 (0.15%) soil samples (FI-1-FI-3) from 1 farm in the south east region were positive for L. monocytogenes. Based on biochemical tests and multiplex PCR, the serotype of the isolates were 4ab (FI-1 and FI-3) and 3a (FI-2), which are not common in foodborne L. monocytogenes. The 3a serotype isolate was positive for all tested virulence genes, whereas the 4ab serotype isolates were only positive for hlyA, actA, and inlA. The isolates were resistant to all 12 tested antibiotics, especially FI-3. The genetic correlations among the isolates were 100% for those of the same serotype and 26.3% for those of different serotypes. These results indicate that the prevalence of L. monocytogenes on livestock farms in Korea is low; however, the isolates are pathogenic and antibiotic resistant.

Inhibition of multidrug resistant Listeria monocytogenes by peptides isolated from combinatorial phage display libraries

The aim of the study was to isolate and characterize novel antimicrobial peptides from peptide phage library with antimicrobial activity against multidrug resistant Listeria monocytogenes. Combinatorial phage-display library was used to affinity select peptides binding to the cell surface of multidrug resistant L. monocytogenes. After several rounds of affinity selection followed by sequencing, three peptides were revealed as the most promising candidates. Peptide L2 exhibited features common to antimicrobial peptides (AMPs), and was rich in Asp, His and Lys residues. Peptide L3 (NSWIQAPDTKSI), like peptide L2, inhibited bacterial growth in vitro, without any hemolytic or cytotoxic effects on eukaryotic cells. L1 peptide showed no inhibitory effect on Listeria. Structurally, peptides L2 and L3 formed random coils composed of α-helix and β-sheet units. Peptides L2 and L3 exhibited antimicrobial activity against multidrug resistant isolates of L. monocytogenes with no haemolytic or toxic effects. Both peptides identified in this study have the potential to be beneficial in human and veterinary medicine.

The microbiological quality of ready-to-eat salads in Turkey: a focus on Salmonella spp. and Listeria monocytogenes

The microbiological safety of ready-to-eat (RTE) foods is of special concern as they are not exposed to further processing before consumption. In the present study, Listeria monocytogenes and Salmonella spp. were isolated from 15(6%) and 21(8%) samples respectively out of 261 RTE foods commercialized in Turkey. Escherichia coli was present in 10(4%) samples analyzed. Psychrotrophic aerobic populations >6logCFU/g were found in 36 (14%) of the samples, while total coliforms were detected in 155 (59%) of samples analyzed. All of the Salmonella spp. and L. monocytogenes isolates tested, exhibited resistance to one or more antimicrobial agents used. For Salmonella spp. isolates, resistance to penicillin (69%), erythromycin (38%), gentamicin (36%), tetracycline (36%) neomycin (33%), ampicillin (33%), amikacin (33%), vancomycin (33%), streptomycin (29%) cefotaxime (9%) and oxacillin (9%) was observed. For L. monocytogenes isolates, resistance to erythromycin (23%) and cephalothin (20%) was evident. The presence of pathogens and the relatively high resistance among the bacteria tested in RTE foods could pose public health and therapeutic problems in consumers. These results indicate the need of implementing hygienic rules in the production chain of RTE foods to ensure microbiological safety and to improve shelf life.

Virulence aspects of Listeria monocytogenes LO28 high pressure-resistant variants

High pressure treatment is a novel food processing approach for reducing pathogens in foods and food ingredients. However, relatively little is known about the pathogenic potential of organisms that survive the treatment. Twelve previously isolated and characterized variants of Listeria monocytogenes LO28 obtained after a high pressure treatment were assessed for their virulence potential and antibiotic susceptibility. Ten variants showed attenuated virulence while two variants retained full virulence in a mouse model of infection. Seven of the attenuated variants demonstrated a reduction in virulence factor activity. Compared to the wild type, all variants exhibited similar or increased susceptibility to multiple antibiotics commonly used in listeriosis treatment.

An improved susceptibility test based on Amberlite reveals the potential antilisterial activity of fosfomycin in vitro

Listeria monocytogenes is resistant to fosfomycin in vitro but is susceptible in vivo due to increased expression of positive regulator factor A (PrfA) and its dependent factor, hexose phosphate transporter (Hpt), upon infection of host cells. Amberlite, a polymeric adsorbent resin, could induce PrfA-dependent gene expression and thus, in theory, improve the sensitivity of L. monocytogenes to fosfomycin in vitro. In the current study, an improved susceptibility test based on Amberlite was developed using reference strains. Thirty-five clinical isolates were further examined to verify those preliminary results. Briefly, Amberlite increased in vitro fosfomycin sensitivity of all strains. Optimal Amberlite concentrations, as evaluated through the expression of phospholipase B (PlcB) and Hpt, were 10% and 15% (w/v) in agar media and 3% (w/v) in broth media. Mueller–Hinton (MH) medium, tryptone soya (TS) medium, and brain heart infusion (BHI) medium were used to verify the results in the control strains using agar dilution and broth micro- and macro-dilution methods. Better listerial growth was shown in TS and BHI than in MH. Both broth dilution methods yielded lower minimal inhibitory concentration (MIC) of fosfomycin than the agar dilution method. The MIC of fosfomycin for 35 clinical isolates was 2–32 μg/mL, suggesting improved susceptibility. In conclusion, in vitro sensitivity of L. monocytogenes to fosfomycin was substantially improved in the presence of 3% Amberlite-supplemented TSB or BHIB and the broth microdilution method. This improved method revealed the potential antilisterial activity of fosfomycin in vitro and could facilitate the therapy of listeriosis using fosfomycin.

Antimicrobial susceptibilities of Listeria monocytogenes human strains isolated from 1970 to 2008 in Brazil

INTRODUCTION: Listeria monocytogenes is the causative agent of listeriosis, a foodborne illness that affects mainly pregnant women, the elderly and immunocompromised patients. The primary treatment is a combination of ampicillin with an aminoglycoside, in addition to a second-choice drug represented by chloramphenicol, erythromycin, tetracycline and rifampicin. The aim of this study was to analyze the antimicrobial susceptibility profile of strains isolated from human sources in the last four decades. METHODS: Sixty-eight strains were selected from the culture collection of the Laboratory of Bacterial Zoonoses/LABZOO/FIOCRUZ isolated in different regions of Brazil from 1970 to 2008 and primarily isolated from cerebrospinal fluid and blood culture. Susceptibility tests to antimicrobials drugs were evaluated using the criteria established by Soussy using the Kirby-Bauer method and E-Test strips were used to determine the minimum inhibitory concentration (MIC). RESULTS: Among the strains tested, serovar L4b (60.3%) was the most prevalent, followed by serovar 1/2a (20.6%), 1/2b (13.2%) and the more uncommon serovars 1/2c, 3b and 4ab (5.9%). All strains were susceptible to ampicillin, cephalothin, erythromycin, gentamicin, teicoplanin and vancomycin. Only one strain (1.5%) showed resistance to rifampin, and two (3%) were resistant to trimethoprim-sulfamethoxazole. MICs with values up to 2μg/ml reinforce the need for microbiological surveillance. CONCLUSIONS: The study demonstrated low prevalence of strains resistant to the antimicrobial drugs indicated in the treatment of human listeriosis. Monitoring antimicrobial resistance profile is still very important to determine adequate treatment, especially in immunocompromised patients.

Comparative antimicrobial susceptibility of Listeria monocytogenes, L. innocua, and L. welshimeri

The current study compared antimicrobial susceptibility of Listeria innocua, L. welshimeri, and L. monocytogenes isolated from various sources. Antimicrobial susceptibility testing was performed using a microbroth procedure with Sensititre minimum inhibitory concentration plates containing 18 antimicrobials. Resistant isolates were analyzed for the presence of antimicrobial resistance genes using PCR. The majority of L. monocytogenes isolates were resistant to oxacillin (99%, 89/90) and ceftriaxone (72%, 65/90), while few isolates were resistant to clindamycin (21%, 19/90) and ciprofloxacin (2%, 2/90). When selected sources of L. monocytogenes are compared, resistance to ceftriaxone, clindamycin, and oxacillin ranged from 27% to 86%, 7% to 43%, and 96% to 100%, respectively. Resistance to ciprofloxacin (6%, 2/34), quinupristin/dalfopristin (7%, 1/14), and tetracycline (7%, 1/15) was observed with L. monocytogenes isolated from food, animal, and environmental sources, respectively. All L. welshimeri isolates (6/6) were resistant to streptomycin, quinupristin/dalfopristin, ciprofloxacin, rifampin, oxacillin, penicillin, and clindamycin, while most isolates (67%, 4/6) were resistant to trimethoprim/sulfamethoxazole. All L. innocua isolates (4/4) were resistant to oxacillin and penicillin, whereas 75% (3/4) of isolates were resistant to tetracycline, ceftriaxone, and clindamycin. Resistant isolates were negative for aadA, strA-B, sul I-II, penA, vat(A-E), vga(A-B), and vgb(A-B). However, tetM was detected among tetracycline-resistant isolates. L. welshimeri was resistant to more of the tested antimicrobials than the other two Listeria species tested, but resistance was not attributed to selected resistance genes. These data demonstrate the variability in resistance among Listeria species. However, the human pathogen L. monocytogenes appears to be the least resistant among the tested species.

Municipal wastewater effluents as a source of listerial pathogens in the aquatic milieu of the Eastern Cape Province of South Africa: a concern of public health importance

We evaluated the effluent quality of an urban wastewater treatment facility in South Africa and its impact on the receiving watershed for a period of 12 months. The prevalence and antimicrobial susceptibility of potential Listeria pathogens (L. ivanovii and L. innocua) and the physicochemical quality of the treated wastewater effluent was assessed, with a view to ascertain the potential health and environmental hazards of the discharged effluent. Total listerial density varied between 2.9 x 10(0) and 1.2 x 10(5) cfu/mL; free living Listeria species were more prevalent (84%), compared to Listeria species attached to planktons (59-75%). The treated effluent quality fell short of recommended standards for turbidity, dissolved oxygen, chemical oxygen demand, nitrite, phosphate and Listeria density; while pH, temperature, total dissolved solids and nitrate contents were compliant with target quality limits after treatment. The Listeria isolates (23) were sensitive to three (15%) of the 20 test antibiotics, and showed varying (4.5-91%) levels of resistance to 17 antibiotics. Of seven resistance gene markers assayed, only sulII genes were detected in five (22%) Listeria strains. The study demonstrates a potential negative impact of the wastewater effluent on the receiving environment and suggests a serious public health implication for those who depend on the receiving watershed for drinking and other purposes.

Antimicrobial resistance of Listeria monocytogenes strains isolated from humans in France

Susceptibility to antibiotics of 4,816 clinical L. monocytogenes strains isolated since 1926 was studied, and the temporal evolution of susceptibility to antibiotics was analyzed through several decades. The mechanisms of resistance in each resistant strain were studied. The prevalence of resistant strains was estimated at 1.27% among isolates from humans. Resistance to tetracyclines+ and fluoroquinolones was more common and has recently emerged. Although acquired resistance in clinical L. monocytogenes did not implicate clinically relevant antibiotics, the possibility of resistance gene transfers, the description of the first clinical isolate with high-level resistance to trimethoprim, and the recent increase in penicillin MICs up to 2 microg/ml reinforce the need for microbiological surveillance.

Antimicrobial resistance of Listeria monocytogenes strains isolated from humans in France

Susceptibility to antibiotics of 4,816 clinical L. monocytogenes strains isolated since 1926 was studied, and the temporal evolution of susceptibility to antibiotics was analyzed through several decades. The mechanisms of resistance in each resistant strain were studied. The prevalence of resistant strains was estimated at 1.27% among isolates from humans. Resistance to tetracyclines+ and fluoroquinolones was more common and has recently emerged. Although acquired resistance in clinical L. monocytogenes did not implicate clinically relevant antibiotics, the possibility of resistance gene transfers, the description of the first clinical isolate with high-level resistance to trimethoprim, and the recent increase in penicillin MICs up to 2 microg/ml reinforce the need for microbiological surveillance.

Effect of acid stress, antibiotic resistance, and heat shock on the resistance of Listeria monocytogenes to UV light when suspended in distilled water and fresh brine

Exposure to sublethal processing treatments can stimulate bacterial stress responses. The purpose of this research was to determine whether adaptation to common food processing stresses encountered during the preparation of ready-to-eat foods affects the dose of UV light required to significantly reduce Listeria monocytogenes populations in sterile distilled water and a 9% NaCl solution, using uridine as a chemical actinometer. L. monocytogenes strains N1-227 (from hot dog batter), N3-031 (from turkey franks), and R2-499 (from ready-to-eat meat) were acid stressed for 3 h at 35 degrees C in Trypticase soy broth with yeast extract acidified to pH 5.0, heat shocked for 1 h at 48 degrees C in brain heart infusion broth (BHIB), and selected for sulfanilamide resistance (512 microg/ml). These strains were then mixed in equal proportions and suspended in water and 9% NaCl solution, each containing 10(-4) M uridine. Samples were exposed to UV light (253.7 nm) for 0, 5, 10, 15, 20, 25, or 30 min. Inactivation was evaluated by surface plating onto modified Oxford agar and Trypticase soy agar with yeast extract and by enrichment in BHIB followed by incubation at 37 degrees C for 24 h. The absorbance of each sample was measured before and after irradiation to calculate the dose of UV light. There were no significant differences between population estimates based on medium or suspension solution. There were no population differences between acid-stressed and antibiotic-resistant or unstressed and heat-shocked L. monocytogenes strains. However, acid-stressed and antibiotic-resistant strains were significantly more resistant to UV light than were unstressed and heat-shocked strains (P < or = 0.05).

Antimicrobial resistance of Listeria monocytogenes isolated from dairy-based food products

In this study Listeria monocytogenes (L. monocytogenes) was isolated from three traditionally consumed Lebanese dairy-based food products. One hundred and sixty four samples (45 samples of Baladi cheese, 36 samples of Shankleesh and 83 of Kishk) were collected from the Bekaa Valley in the Northeast region of Lebanon. Suspected Listeria colonies were selected and initially identified by using standard biochemical tests. Initial identification of the positive L. monocytogenes colonies was confirmed at the molecular level by Polymerase Chain Reaction (n=30) and the confirmed isolates were evaluated for their susceptibility to 10 commonly used antimicrobials. All of the 30 isolates were confirmed to be L. monocytogenes yielding a PCR product of approximately 660 base pairs (bp). L. monocytogenes was detected in 26.67%, 13.89% and 7.23% of the Baladi cheese, Shankleesh and Kishk samples, respectively. The highest resistance in L. monocytogenes isolates was noted against oxacillin (93.33%) followed by penicillin (90%). The results provide an indication of the contamination levels of dairy-based foods in Lebanon and highlight the emergence of multi-drug resistant Listeria in the environment.

Comparison of antimicrobial resistance in Escherichia coli, Staphylococcus aureus, and Listeria monocytogenes strains isolated from organic and conventional poultry meat

The presence of Escherichia coli, Staphylococcus aureus, and Listeria monocytogenes was determined in 55 samples of organic poultry meat and in 61 samples of conventional poultry meat. A total of 220 E. coli, 192 S. aureus, and 71 L. monocytogenes strains were analyzed by an agar disk diffusion assay for their resistance to ampicillin, cephalothin, chloramphenicol, ciprofloxacin, doxycycline, fosfomycin, gentamicin, nitrofurantoin, streptomycin, and sulfisoxazole (E. coli); chloramphenicol, ciprofloxacin, clindamycin, doxycycline, erythromycin, gentamicin, nitrofurantoin, oxacillin, and sulfisoxazole (S. aureus); and chloramphenicol, doxycycline, erythromycin, gentamicin, sulfisoxazole, and vancomycin (L. monocytogenes). The results indicated a significantly higher (P < 0.0001) prevalence of E. coli but not of S. aureus and L. monocytogenes in organic poultry meat as compared with conventional poultry meat. E. coli isolated from organic poultry meat exhibited lower levels of antimicrobial resistance against 7 of the 10 antimicrobials tested as compared with isolates recovered from conventional meat. In the case of S. aureus and L. monocytogenes isolated from conventional poultry, antimicrobial resistance was significantly higher only for doxycycline as compared with strains isolated from organic poultry. In the case of E. coli, the presence of multiresistant strains was significantly higher (P < 0.0001) in conventional poultry meat as compared with organic poultry meat. Organically farmed poultry samples showed significantly lower development of antimicrobial resistance in intestinal bacteria such as E. coli.

Characterization of antimicrobial resistance of foodborne Listeria monocytogenes

The objective of this study was to evaluate the susceptibility of 120 Listeria monocytogenes strains isolated from food and food-processing environments to 19 antibiotics currently used in veterinary and human therapy. Susceptibility tests were performed by using the automated VITEK2 system. Apart from penicillin, ampicillin and trimethoprim-sulfamethoxazole, for which clinical breakpoints for Listeria susceptibility testing are defined according to the Clinical and Laboratory Standard Institute (CLSI), in the present study the CLSI criteria for staphylococci were applied. Among the 120 tested strains, 14 (11.7%) displayed resistance to at least one antibiotic. In particular, resistance to one antibiotic was more common than multiple resistance, i.e., 10 (8.3%) isolates were resistant to one antibiotic, 3 (2.5%) to two antibiotics and one (0.8%) to five antibiotics. Resistance to clindamycin was the most common, followed by linezolid, ciprofloxacin, ampicillin and rifampicin, trimethoprim/sulphamethoxazole and, finally, vancomycin and tetracycline. This study shows that L. monocytogenes strains from food and food-processing environments are susceptible to the antibiotics commonly used in veterinary and human listeriosis treatment. Considering that L. monocytogenes is slowly becoming antibiotic resistant, a continued surveillance of emerging antimicrobial resistance of this pathogen is important to ensure effective treatment of human listeriosis. These data are useful in improving background data on antibiotic resistance of strains isolated from food and food environment.

Characterization and antibiotic susceptibility of Listeria monocytogenes isolated from poultry and red meat in Morocco

This study was carried out on 426 samples of raw meats collected from butcheries and supermarkets in Casablanca, Morocco. The samples were examined for the occurrence of Listeria species. Strains of Listeria monocytogenes were characterized by several biochemical tests and confirmed by polymerase chain reaction (PCR). β-hemolytic cultures and nonhemolytic isolates were tested for biochemical properties with the Listeria API test. Among the 43 Listeria species isolates; we identified 10 strains for L. monocytogenes (23.3%), 31 strains for L. innocua (72.1%) and 2 strains for L. welshimeri (4.6%). Strains of L. monocytogenes were separated by multiplex PCR; two serogroups IIb and IVb were thus differentiated. Antibiotic susceptibility of L. monocytogenes to 21 antibiotics was determined by the disk diffusion method. All isolates were susceptible to a wide range of the tested antibiotics with the exception of nalidixic acid, colistine and cephalosporins second and third generation for which they were all resistant.

Antimicrobial susceptibility of Listeria monocytogenes isolated from food and clinical cases in Navarra, Spain

The susceptibility of 440 Listeria monocytogenes strains isolated from food (n=401) and clinical cases (n=39) between 1995 and 2005 was determined by standard agar dilution and E-test methods. Antimicrobial drugs currently used in veterinary and human therapy were tested, and they included penicillin G, ampicillin, cephalothin, gentamicin, chloramphenicol, tetracycline, doxycycline, trimethoprim, erythromycin, and clindamycin. The sensitivity of strains was established using Clinical and Laboratory Standards Institute (formerly NCCLS) breakpoints and MIC50 (the MIC for 50% of the strains) to MIC90 values. In general, isolates were susceptible to the majority of the antimicrobials tested, including beta-lactamics and aminoglycosides, which are normally used in the treatment of listeriosis. Resistance to tetracycline and doxycycline was found in five strains isolated from fresh trout belonging to the same fish farm. Molecular analysis by restriction endonuclease analysis showed a similar profile, suggesting the persistence of a strain well adapted to the presence of tetracycline in the environment of a fish farm, which is frequently used in aquaculture in order to prevent infections of fish.

Antimicrobial susceptibility of Listeria monocytogenes from food products

This study was conducted to determine the susceptibility of Listeria monocytogenes isolated from food products to antimicrobial agents commonly used for treatment of infections with gram-positive bacteria, and to disinfectants. A total of 114 L. monocytogenes retail isolates were tested for susceptibility to ceftiofur, chloramphenicol, ciprofloxacin, erythromycin, florfenicol, penicillin, spectinomycin, streptomycin, tetracycline, tiamulin, trimethoprim, and co-trimoxazole, and the disinfectants benzalkonium chloride and triclosan, by determination of minimum inhibitory concentrations (MICs). All isolates were resistant to ceftiofur, but susceptible to the other antibiotics. A single isolate had a MIC of 4 mg/L for ciprofloxacin. For tiamulin. the MIC values were around the breakpoint used. Most isolates had MICs for triclosan at 16 mg/L. The MICs for benzalkonium chloride formed a bimodal distribution, with 105 isolates having a MIC of 4 mg/L and 9 isolates MICs of 16 and 32 mg/L. This study showed that Danish isolates of L. monocytogenes have not developed or acquired resistance to antimicrobial agents used for treatment or disinfection, except for benzalkonium chloride. The MICs for triclosan was high compared to other gram-positive bacteria, suggesting that triclosan might not be useful against L. monocytogenes if incorporated in materials in the food industry.

Distribution and characteristics of Listeria monocytogenes isolates from surface waters of the South Nation River watershed, Ontario, Canada

Listeria monocytogenes is a facultative intracellular pathogen thought to be widely distributed in the environment. We investigated the prevalence and characteristics of L. monocytogenes isolates from surface waters derived from catchments within the South Nation River watershed (Ontario, Canada). This watershed is dominated by urban and rural development, livestock and crop production, and wildlife habitats. From June to November 2005, a total of 314 surface water samples were collected biweekly from 22 discrete sampling sites characterized by various upstream land uses. Presumptive Listeria spp. were isolated using a selective enrichment and isolation procedure, and 75 L. monocytogenes isolates were identified based on colony morphology, hemolytic activity, and amplification of three pathogenicity genes: iap, inlA, and hlyA. Thirty-two of 314 (10%) surface water samples were positive for the presence of L. monocytogenes, but detection ranged between 0 and 27% depending on the sampling date. Isolates belonging to serovar group 1/2a, 3a (50%) and group 4b, 4d, 4e (32%) were dominant. L. monocytogenes populations were resolved into 13 EcoRI ribotypes and 21 ApaI and 21 AscI pulsotypes. These had Simpson indexes of discrimination of up to 0.885. Lineage I-related isolates were dominant (61%) during the summer, whereas lineage II isolates were dominant (77%) in the fall. Isolates were, on average, resistant to 6.1 +/- 2.1 antibiotics out of 17 tested. Half of the L. monocytogenes isolates exhibited potential virulence linked to the production of a functional internalin A, and some isolates were found to be moderately to highly virulent by in vitro Caco-2 plaque formation assay (up to 28% of entry). There was a statistically significant link between the occurrence of L. monocytogenes and proximity to an upstream dairy farm and degree of cropped land. Our data indicate that L. monocytogenes is widespread in the studied catchments, where it could represent a public health issue related to agricultural land use.

Occurrence and antibiotic sensitivity of Listeria monocytogenes strains isolated from oysters, fish, and estuarine water

We analyzed the presence of Listeria spp. in oyster, fish, and seawater samples and tested isolates for antibiotic sensitivity. Listeria monocytogenes was found in 4.5% of fish samples and 8.3% of seawater samples and was not recovered from oysters. Multiresistant environmental strains were found, representing a potential threat to human health.

Antimicrobial susceptibility of nisin resistant Listeria monocytogenes of dairy origin

The antibiotic susceptibility of wild Listeria monocytogenes strains and their corresponding nisin resistant variants was assessed. The resistant strains were more sensitive to most of the tested antibiotics than their wild-type counterparts. A slight increase in MIC was observed for a few antibiotics including the membrane disturbing polymixin B. Cross-resistance was detected with two synthetic antimicrobial peptides. A lower C15/C17 ratio in the membrane fatty acid composition of the nisin resistant strains was found, and one strain pair showed a significant difference in surface hydrophobicity. As judged by these results, no clear correlation could be established between resistance to nisin and to worldwide-used antibiotics.

Molecular analysis of resistance to streptogramin A compounds conferred by the Vga proteins of staphylococci

The Vga and Msr resistance determinants, encoded by mobile genetic elements in various staphylococcal strains, belong to a family of ATP-binding cassette (ABC) proteins whose functions and structures are ill defined. Their amino acid sequences are similar to those of proteins involved in the immunity of streptomycetes to the macrolide-lincosamide-streptogramin antibiotics that they produce. Sequence analysis of the genomes of the gram-positive bacteria with low G+C contents revealed that Lmo0919 from Listeria monocytogenes is more closely related to Vga variants than to Msr variants. In the present study we compared the antibiotic resistance profiles conferred by the Vga-like proteins in two staphylococcal hosts. It was shown that Vga(A), the Vga(A) variant [Vga(A)v], and Lmo0919 can confer resistance to lincosamides and streptogramin A compounds, while only Vga(B) is able to increase the level of resistance to pristinamycin, a mixture of streptogramin A and streptogramin B compounds. By using polyclonal antibodies, we found that the Vga(A) protein colocalized with the beta subunit of the F(1)-F(0) ATPase in the membrane fractions of staphylococcal cells. In order to identify functional units in these atypical ABC proteins, such as regions that might be involved in substrate specificity and/or membrane targeting, we analyzed the resistance phenotypes conferred by various plasmids carrying parts or modified versions of the vga(A) gene and we determined the subcellular localization of the gene products. Only polypeptides composed of two ABC domains were detected in the cell membranes. No region of drug specificity was identified. Resistance properties were dependent on the integrities of both Walker B motifs.