10-Year Molecular Surveillance of Listeria monocytogenes Using Whole-Genome Sequencing in Shanghai, China, 2009-2019

Current methodologies available to evaluate the virulence potential among Listeria monocytogenes clonal complexes

Listeria monocytogenes is a foodborne pathogen that causes listeriosis in humans, the severity of which depends on multiple factors, including intrinsic characteristics of the affected individuals and the pathogen itself. Additionally, emerging evidence suggests that epigenetic modifications may also modulate host susceptibility to infection. Therefore, different clinical outcomes can be expected, ranging from self-limiting gastroenteritis to severe central nervous system and maternal-neonatal infections, and bacteremia. Furthermore, L. monocytogenes is a genetically and phenotypically diverse species, resulting in a large variation in virulence potential between strains. Multilocus sequence typing (MLST) has been widely used to categorize the clonal structure of bacterial species and to define clonal complexes (CCs) of genetically related isolates. The combination of MLST and epidemiological data allows to distinguish hypervirulent CCs, which are notably more prevalent in clinical cases and typically associated with severe forms of the disease. Conversely, other CCs, termed hypovirulent, are predominantly isolated from food and food processing environments and are associated with the occurrence of listeriosis in immunosuppressed individuals. Reports of genetic traits associated with this diversity have been described. The Food and Agriculture Organization (FAO) is encouraging the search for virulence biomarkers to rapidly identify the main strains of concern to reduce food waste and economical losses. The aim of this review is to comprehensively collect, describe and discuss the methodologies used to discriminate the virulence potential of L. monocytogenes CCs. From the exploration of in vitro and in vivo models to the study of expression of virulence genes, each approach is critically explored to better understand its applicability and efficiency in distinguishing the virulence potential of the pathogen.

Genetic diversity of Listeria monocytogenes from seafood products, its processing environment, and clinical origin in the Western Cape, South Africa using whole genome sequencing

Listeria monocytogenes is a concern in seafood and its food processing environment (FPE). Several outbreaks globally have been linked to various types of seafood. Genetic profiling of L. monocytogenes is valuable to track bacterial contamination throughout the FPE and in understanding persistence mechanisms, with limited studies from South Africa. Forty-six L. monocytogenes isolates from origins: Fish/seafood products (n = 32) (salmon, smoked trout, fresh hake, oysters), the FPE (n = 6), and clinical (n = 8) were included in this study. Lineage typing, antibiotic susceptibility testing, and screening for two genes (bcrABC and emrC) conferring sanitizer tolerance was conducted. The seafood and FPE isolates originated from seven different factories processing various seafood products with undetermined origin. All clinical isolates were categorized as lineage I, and seafood and FPE isolates were mostly categorized into lineage II (p < 0.01). Seafood and FPE isolates (53%) carried the bcrABC gene cassette and one fish isolate, the emrC gene. A subset, n = 24, was grouped into serotypes, sequence types (STs), and clonal complexes (CCs) with whole genome sequencing (WGS). Eight CCs and ten STs were identified. All clinical isolates belonged to serogroup 4b, hypervirulent CC1. CC121 was the most prevalent in isolates from food and the FPE. All isolates carried Listeria pathogenicity islands (LIPI) 1 and 2. LIPI-3 and LIPI-4 were found in certain isolates. We identified genetic determinants linked to enhanced survival in the FPE, including stress survival islets (SSI) and genes conferring tolerance to sanitizers. SSI-1 was found in 44% isolates from seafood and the FPE. SSI-2 was found in all the ST121 seafood isolates. Isolates (42%) harbored transposon Tn1688_qac (ermC), conferring tolerance to quaternary ammonium compounds. Five plasmids were identified in 13 isolates from seafood and the FPE. This is the first One Health study reporting on L. monocytogenes genetic diversity, virulence and resistance profiles from various types of seafood and its FPE in South Africa.

Prevalence, Genotypic Characteristics, and Antibiotic Resistance of Listeria monocytogenes From Retail Foods in Huzhou, China

Listeria monocytogenes are considered to be the major foodborne pathogen worldwide. To understand the prevalence and potential risk of L. monocytogenes in retail foods, a total of 1243 retail foods in 12 food categories were sampled and screened for L. monocytogenes from 2020 to 2022 in Huzhou, China. A total of 46 out of 1234 samples were confirmed to be L. monocytogenes positive with a total rate of 3.7%. The contamination rate of seasoned raw meat (15.2%) was the highest, followed by raw poultry meat and raw livestock meat (9.9%) and salmon sashimi (9.5%). The L. monocytogenes isolates belonged to four serotypes, 1/2a,1/2b, 1/2c, and 4b, with the most prevalent serotype being 1/2a (47.9%). All isolates were grouped into 15 sequence types (STs) belonging to 14 clonal complexes (CCs) via multilocus sequence typing (MLST). The most prevalent ST was ST9/CC9 (23.9%), followed by ST3/CC3 (19.6%) and ST121/CC121 (17.4%). Notably, 11 STs were detected from ready-to-eat (RTE) foods, some of them have been verified to be strongly associated with clinical origin listeriosis cases, such as ST3, ST2, ST5, ST8, and ST87. Listeria pathogenicity islands 1 (LIPI-1) and LIPI-2 were detected in approximately all L. monocytogenes isolates, whereas the distribution of both LIPI-3 genes and LIPI-4 genes exhibited association with specific ST, with LIPI-3 in ST3 and ST288, and LIPI-4 in ST87. The strains carrying LIPI-3 and LIPI-4 virulence genes in this study were all isolated from RTE foods. Antimicrobial susceptibility tests showed that >90% of isolates were susceptible to PEN, AMP, ERY, CIP, SXT, VAN, CHL, and GEN, indicating the antibiotic treatment might be still efficient for most of the L. monocytogenes strains. However, for the three clinical first-line antibiotics (PEN, AMP, and GEN), we also observed three and four strains showing MIC values greater than the susceptibility standards for PEN and AMP, respectively, and one strain showing resistance to GEN.

Molecular Characteristics and Virulence Profile of Clinical Listeria monocytogenes Isolates in Northern Taiwan, 2009-2019

Listeria monocytogenes is a critical foodborne pathogen that causes severe invasive and noninvasive diseases and is associated with high mortality. Information on the prevalence of L. monocytogenes infections in Taiwan is very limited. This study aimed to analyze the molecular epidemiological surveillance and virulence gene distribution of 176 human clinical L. monocytogenes isolates collected between 2009 and 2019 in northern Taiwan. Our results showed that the isolates belonged to 4 serogroups (IIa, IIb, IVb, and IIc), with most isolates in serogroups IIa (81/176, 46%) and IIb (71/176, 40.3%). Multilocus sequence typing analysis revealed 18 sequence types (STs) and 13 clonal complexes (CCs). Eighty-four percent of all isolates belonged to six STs: CC87-ST87 (40/176, 22.7%), CC19-ST378 (36/176, 19.9%), CC155-ST155 (28/176, 15.5%), CC1-ST710 (16/176, 8.8%), CC5-ST5 (16/176, 8.8%), and CC101-ST101 (11/176, 6.1%). Furthermore, our analysis showed the distributions of four Listeria pathogenicity islands (LIPI) among all isolates. LIPI-1 and LIPI-2 existed in all isolates, whereas LIPI-3 and LIPI-4 only existed in specific STs and CCs. LIPI-3 existed in the STs, CC1-ST710, CC3-ST3, CC288-ST295, and CC191-ST1458, whereas LIPI-4 could be found in the STs, CC87-ST87 and CC87-ST1459. Strains containing LIPI-3 and LIPI-4 are potentially hypervirulent; thus, 68/176 isolates (39.1%) collected in this study were potentially hypervirulent. Since L. monocytogenes infections are considered highly correlated with diet, molecular epidemiological surveillance of Listeria in food is important; continued surveillance will provide critical information to prevent foodborne diseases.

Updated Parameters for Listeria monocytogenes Dose-Response Model Considering Pathogen Virulence and Age and Sex of Consumer

Better knowledge regarding the Listeria monocytogenes dose-response (DR) model is needed to refine the assessment of the risk of foodborne listeriosis. In 2018, the European Food Safety Agency (EFSA) derived a lognormal Poisson DR model for 14 different age-sex sub-groups, marginally to strain virulence. In the present study, new sets of parameters are developed by integrating the EFSA model for these sub-groups together with three classes of strain virulence characteristics ("less virulent", "virulent", and "more virulent"). Considering classes of virulence leads to estimated relative risks (RRs) of listeriosis following the ingestion of 1000 bacteria of "less virulent" vs. "more virulent" strains ranging from 21.6 to 24.1, depending on the sub-group. These relatively low RRs when compared with RRs linked to comorbidities described in the literature suggest that the influence of comorbidity on the occurrence of invasive listeriosis for a given exposure is much more important than the influence of the virulence of the strains. The updated model parameters allow better prediction of the risk of invasive listeriosis across a population of interest, provided the necessary data on population demographics and the proportional contribution of strain virulence classes in food products of interest are available. An R package is made available to facilitate the use of these dose-response models.

A novel multiplex PCR based method for the detection of Listeria monocytogenes clonal complex 8

Listeria monocytogenes is an important foodborne pathogen worldwide, which could cause listeriosis with a 20-30 % fatality rate in immunocompromised individuals. Listeria monocytogenes MLST clonal complex (CC) 8 strain is a common clone in food and clinical cases. The aim of this study was to develop multiplex PCR (mPCR) and high-resolution melting (HRM) qPCR to simultaneously detect L. monocytogenes CC8 and the other L. monocytogenes strains based on pan-genome analysis. A novel multiplex PCR and HRM qPCR targeted for the genes LM5578_1180 (specific for CC8) and LM5578_2262 (for L. monocytogenes) were developed. The specificity of this multiplex PCR and HRM qPCR were verified with other CCs of L. monocytogenes and other species strains. The detection limit of this multiplex PCR and HRM qPCR is 2.1 × 103 CFU/mL and 2.1 × 100 CFU/mL, respectively. This multiplex PCR and HRM qPCR could accurately detect CC8 strains with the interference of different ratios of L. monocytogenes CC9, CC87, CC121, CC155, and L. innocua strains. Subsequently, the detection ability of mPCR and HRM qPCR were also evaluated in spiked samples. The mPCR method could successfully detect 6.2 × 103 CFU/mL of CC8 L. monocytogenes after 6 h enrichment while the multiplex HRM qPCR method could successfully detect 6.2 × 104 CFU/mL of CC8 L. monocytogenes after 3 h enrichment. The feasibility of these methods were satisfactory in terms of sensitivity, specificity, and efficiency after evaluating 12 mushroom samples and was consistent with that of the National Standard Detection Method (GB4789.30-2016). In conclusion, the developed assays could be applied for rapid screening and detection of L. monocytogenes CC8 strains both in food and food production environments, providing accurate results to adopt monitoring measures to improve microbiological safety.

Listeria monocytogenes, a silent foodborne pathogen in Ecuador

Listeria monocytogenes is a foodborne pathogen that can produce serious, even fatal, infections. Among other foods, it can be found in unpasteurized dairy and ready-to-eat products. Surveillance of L. monocytogenes is of great interest since sources of infection are difficult to determine due to the long incubation period, and because the symptoms of listeriosis are similar to other diseases. We performed a genomic study of L. monocytogenes isolated from fresh cheeses and clinical samples from Ecuador. Sixty-five isolates were evaluated and sequenced, 14 isolates from cheese samples and 20 from clinical listeriosis cases from the National Institute of National Institute of Public Health Research, and 31 isolates from artisanal cheese samples from 8 provinces. All isolates exhibited heterogeneous patterns of the presence of pathogenicity islands. All isolates exhibited at least 4 genes from LIPI-1, but all references (26 L. monocytogenes closed genomes available in the NCBI database) showed the complete island, which encompasses 5 genes but is present in only two Ecuadorian isolates. Most isolates lacked gene actA. Genes from LIPI-2 were absent in all isolates. LIPI-3 and LIPI-4 were present in only a few references and isolates. With respect to the stress survival islets, our samples either presented SSI-1 or SSI-F2365, except for one isolate that presented SSI-F2365 and also one gene from SSI-1. None of the samples presented SSI-2. The predominant ST (sequence type) was ST2 (84.62% 55/65), and the only ST found in food (93.33% 42/45) and clinical samples (65% 13/20). Isolates were not grouped according to their sampling origin, date, or place in a phylogenetic tree obtained from the core alignment. The presence of ST2 in food and clinical samples, with high genomic similarity, suggests a foodborne infection risk linked to the consumption of fresh cheeses in Ecuador.

Molecular characterization of Listeria monocytogenes strains isolated from imported food in China from 14 countries/regions, 2003-2018

Listeria monocytogenes (Lm) is associated with severe foodborne infections and ubiquitous in the nature. Identification of characteristics of Lm transmission through trading of food products is essential for rapidly tracking Lm sources and controlling dissemination of listeriosis. In this study, a total of 44 Lm strains were isolated from food products originating from 14 countries/regions during 2003-2018 at the Shanghai port. The genomes of these Lm strains were sequenced by high-throughput sequencing. Multilocus sequence typing (MLST) analysis showed that 43 isolates were divided into 17 sequence types (STs). The distribution of STs was decentralized, with the dominant ST2 accounting for only 18.18% of the strains. The LM63 strain did not match with any of the existing STs. Core-genome MLST (cgMLST) analysis based on 1748 core genes categorized the 44 strains into 30 cgMLST types (CTs), with CT10153 and CT7892 as the most predominant CTs. Notably, LM63 and LM67 shared the same CT in the cgMLST analysis. The phylogenetic analysis based on single-copy homologous genes revealed that the 44 Lm strains were primarily classified into two lineages. The SNP analysis also indicated that these strains were roughly divided into two clades, with strains in the first clade mainly collected earlier than those in the second clade, which were predominantly collected from 2010 onwards. The analysis using the virulence factor database (VFDB) indicated that the virulence gene inlJ was the most prevalent among these 44 strains. Notably, ddrA, msbA, and sugC were enriched in this dataset, requiring further clarification of their roles in Listeria through future studies. These results might provide a clue for understanding of the global epidemiology and surveillance of Lm and present insights for implementing effective measures to reduce or prevent Listeria contamination outbreaks in imported food products.

Assessment of the relationship between the MLST genetic diversity of Listeria monocytogenes and growth under selective and non-selective conditions

Listeria monocytogenes can grow under stressful conditions and contaminate various food categories. Progresss in DNA sequencing-based identification methods, such as multi-locus sequence typing (MLST) now allow for more accurate characterization of pathogens. L. monocytogenes MLST genetic diversity is reflected by the different prevalence of the "clonal complexes" (CCs) in foods or infections. Better understanding of the growth potentials of L. monocytogenes is essential for quantitative risk assessment and efficient detection across CCs genetic diversity. Using optical density measurements taken with an automated spectrophotometer, we compared the maximal growth rate and lag phase of 39 strains from 13 different CCs and various food origins, in 3 broths mimicking stresful food conditions (8 °C, aw 0.95 and pH5) and in ISO Standard enrichment broths (Half Fraser and Fraser). This is important as growth could influence risk through pathogen multiplication in food. Besides, enrichment problems could lead to a lack of detection of some CCs. Despite small differences highlighting natural intraspecific variability, our results show that growth performances of L. monocytogenes strains under the conditions tested in selective and non-selective broth do not appear to be strongly correlated to CCs and cannot explain higher CC "virulence" or prevalence.

The Isolation, Genetic Analysis and Biofilm Characteristics of Listeria spp. from the Marine Environment in China

Listeria monocytogenes is an important pathogen that can cause listeriosis. Despite the growing recognition of Listeria spp. as a foodborne and environmental pathogen, the understanding of its prevalence and characteristics of Listeria spp. in the marine environment remains unknown. In this study, we first investigated the genetic and phenotypic characteristics of Listeria species isolated in a coastal city in China. The findings revealed that the sequence type 87 (ST87) L. monocytogenes, a prevalent clinical and seafood strain in China, dominates in recreational beach sands and possesses a notable biofilm-forming capacity in seawater. The presence of ST87 L. monocytogenes in coastal environments indicates the potential health risks for both recreational activities and seafood consumption. Moreover, the ST121 isolates from sand had a versatile plasmid encoding multifunctional genes, including uvrX for UV resistance, gbuC for salt resistance, and npx for oxidative resistance and multiple transposases, which potentially aid in survival under natural environments. Black-headed gulls potentially facilitate the spread of L. monocytogenes, with similar ST35 strains found in gulls and beach sand. As a reservoir of microbes from marine environments and human/animal excrement, coastal sand would play an important role in the spread of L. monocytogenes and is an environmental risk for human listeriosis.

Cinnamaldehyde Targets the LytTR DNA-Binding Domain of the Response Regulator AgrA to Attenuate Biofilm Formation of Listeria monocytogenes

The Agr quorum sensing (QS) system is known to contribute to biofilm formation in Listeria monocytogenes. Cinnamaldehyde, a natural food preservative, is considered an inhibitor of Agr-mediated QS in L. monocytogenes. However, the exact mechanism by which cinnamaldehyde acts on Agr remains unclear. In this study, we assessed the effects of cinnamaldehyde on the histidine kinase AgrC and the response regulator AgrA in the Agr system. AgrC kinase activity was not influenced by cinnamaldehyde, and binding between AgrC and cinnamaldehyde was not observed when microscale thermophoresis (MST) was performed, indicating that AgrC was not the target of cinnamaldehyde. AgrA is specifically bound to the agr promoter (P2) to activate the transcription of the Agr system. However, AgrA-P2 binding was prevented by cinnamaldehyde. The interaction between cinnamaldehyde and AgrA was further confirmed with MST. Two conserved amino acids, Asn-178 and Arg-179, located in the LytTR DNA-binding domain of AgrA, were identified as the key sites for cinnamaldehyde-AgrA binding by alanine mutagenesis and MST. Coincidentally, Asn-178 was also involved in the AgrA-P2 interaction. Taken together, these results suggest that cinnamaldehyde acts as a competitive inhibitor of AgrA in AgrA-P2 binding, which leads to suppressed transcription of the Agr system and reduced biofilm formation in L. monocytogenes. IMPORTANCE Listeria monocytogenes can form biofilms on various food contact surfaces, posing a serious threat to food safety. Biofilm formation of L. monocytogenes is positively regulated by the Agr quorum sensing system. Thus, an alternative strategy for controlling L. monocytogenes biofilms is interfering with the Agr system. Cinnamaldehyde is considered an inhibitor of the L. monocytogenes Agr system; however, its exact mechanism of action is still unclear. Here, we found that AgrA (response regulator), rather than AgrC (histidine kinase), was the target of cinnamaldehyde. The conserved Asn-178 in the LytTR DNA-binding domain of AgrA was involved in cinnamaldehyde-AgrA and AgrA-P2 binding. Therefore, the occupation of Asn-178 by cinnamaldehyde suppressed transcription of the Agr system and reduced biofilm formation in L. monocytogenes. Our findings could provide a better understanding of the mechanism by which cinnamaldehyde inhibits L. monocytogenes biofilm formation.

Genomic and pathogenicity islands of Listeria monocytogenes-overview of selected aspects

Listeria monocytogenes causes listeriosis, a disease characterized by a high mortality rate (up to 30%). Since the pathogen is highly tolerant to changing conditions (high and low temperature, wide pH range, low availability of nutrients), it is widespread in the environment, e.g., water, soil, or food. L. monocytogenes possess a number of genes that determine its high virulence potential, i.e., genes involved in the intracellular cycle (e.g., prfA, hly, plcA, plcB, inlA, inlB), response to stress conditions (e.g., sigB, gadA, caspD, clpB, lmo1138), biofilm formation (e.g., agr, luxS), or resistance to disinfectants (e.g., emrELm, bcrABC, mdrL). Some genes are organized into genomic and pathogenicity islands. The islands LIPI-1 and LIPI-3 contain genes related to the infectious life cycle and survival in the food processing environment, while LGI-1 and LGI-2 potentially ensure survival and durability in the production environment. Researchers constantly have been searching for new genes determining the virulence of L. monocytogenes. Understanding the virulence potential of L. monocytogenes is an important element of public health protection, as highly pathogenic strains may be associated with outbreaks and the severity of listeriosis. This review summarizes the selected aspects of L. monocytogenes genomic and pathogenicity islands, and the importance of whole genome sequencing for epidemiological purposes.

The Slaughterhouse as Hotspot of CC1 and CC6 Listeria monocytogenes Strains with Hypervirulent Profiles in an Integrated Poultry Chain of Italy

In Europe, very few studies are available regarding the diversity of Listeria monocytogenes (L. monocytogenes) clonal complexes (CCs) and sequence types (ST) in poultry and on the related typing of isolates using whole genome sequencing (WGS). In this study, we used a WGS approach to type 122 L. monocytogenes strains isolated from chicken neck skin samples collected in two different slaughterhouses of an integrated Italian poultry company. The studied strains were classified into five CCs: CC1-ST1 (21.3%), CC6-ST6 (22.9%), CC9-ST9 (44.2%), CC121-ST121 (10.6%) and CC193-ST193 (0.8%). CC1 and CC6 strains presented a virulence gene profile composed of 60 virulence genes and including the Listeria Pathogenicity Island 3, aut_IVb, gltA and gltB. According to cgMLST and SNPs analysis, long-term persistent clusters belonging to CC1 and CC6 were found in one of the two slaughterhouses. The reasons mediating the persistence of these CCs (up to 20 months) remain to be elucidated, and may involve the presence and the expression of stress response and environmental adaptation genes including heavy metals resistance genes (cadAC, arsBC, CsoR-copA-copZ), multidrug efflux pumps (mrpABCEF, EmrB, mepA, bmrA, bmr3, norm), cold-shock tolerance (cspD) and biofilm-formation determinants (lmo0673, lmo2504, luxS, recO). These findings indicated a serious risk of poultry finished products contamination with hypervirulent L. monocytogenes clones and raised concern for the consumer health. In addition to the AMR genes norB, mprF, lin and fosX, ubiquitous in L. monocytogenes strains, we also identified parC for quinolones, msrA for macrolides and tetA for tetracyclines. Although the phenotypical expression of these AMR genes was not tested, none of them is known to confer resistance to the primary antibiotics used to treat listeriosis The obtained results increase the data on the L. monocytogenes clones circulating in Italy and in particular in the poultry chain.

Whole-genome sequencing reveals genomic characterization of Listeria monocytogenes from food in China

Introduction

Listeria monocytogenes is a foodborne bacterium that could persist in food and food processing environments for a long time. Understanding the population structure and genomic characterization of foodborne L. monocytogenes is essential for the prevention and control of listeriosis.

Methods

A total of 322 foodborne L. monocytogenes isolates from 13 geographical locations and four food sources in China between 2000 and 2018 were selected for whole-genome sequencing.

Results

In silico subtyping divided the 322 isolates into five serogroups, 35 sequence types (STs), 26 clonal complexes (CCs) and four lineages. Serogroup IIa was the most prevalent serogroup and ST9 was the most prevalent ST of foodborne L. monocytogenes strains isolated in China. The in-depth phylogenetic analysis on CC9 revealed that ST122 clone might be original from ST9 clone. Furthermore, 23 potentially relevant clusters were identified by pair-wised whole-genome single nucleotide polymorphism analysis, indicating that persistent- and/or cross-contamination had occurred in markets in China. ST8 and ST121 were the second and third top STs of L. monocytogenes in China, which had heterogeneity with that of L. monocytogenes isolates from other countries. The antibiotic resistance genes aacA4, tetM, tetS, dfrG carried by different mobile elements were found in L. monocytogenes strains. One lineage II strain carrying Listeria Pathogenicity Island 3 was first reported. In addition, a novel type of premature stop codon in inlA gene was identified in this study.

Discussion

These findings revealed the genomic characteristics and evolutionary relationship of foodborne L. monocytogenes in China on a scale larger than previous studies, which further confirmed that whole-genome sequencing analysis would be a helpful tool for routine surveillance and source-tracing investigation.

Wholegenome sequencing as the gold standard approach for control of Listeria monocytogenes in the food chain

Listeria monocytogenes has been implicated in numerous outbreaks and related deaths of listeriosis. In food production, L. monocytogenes occurs in raw food material and above all, through postprocessing contamination. The use of next-generation sequencing technologies such as whole-genome sequencing (WGS) facilitates foodborne outbreak investigations, pathogen source tracking and tracing geographic distributions of different clonal complexes, routine microbiological/epidemiological surveillance of listeriosis, and quantitative microbial risk assessment. WGS can also be used to predict various genetic traits related to virulence, stress, or antimicrobial resistance, which can be of great benefit for improving food safety management as well as public health.

Quantification of Total and Viable Cells and Determination of Serogroups and Antibiotic Resistance Patterns of Listeria monocytogenes in Chicken Meat from the North-Western Iberian Peninsula

Twenty samples of minced chicken meat procured from butcher’s shops in León (Spain; 10 samples) and Vila Real (Portugal; 10 samples) were analyzed. Microbial concentrations (log10 cfu/g) of 7.53 ± 1.02 (viable aerobic microbiota), 7.13 ± 1.07 (psychrotrophic microorganisms), and 4.23 ± 0.88 (enterobacteria) were found. The detection method described in the UNE-EN ISO 11290-1 standard (based on isolation from the chromogenic medium OCLA) with confirmation by the polymerase chain reaction (PCR; lmo1030) (OCLA−PCR), revealed Listeria monocytogenes in 14 samples (70.0% of the total), nine of Spanish origin and five of Portuguese (p > 0.05). The levels of viable and inactivated L. monocytogenes in the samples were determined with a q-PCR using propidium monoazide (PMAxx) as a viability marker. Seven samples tested positive both with the OCLA−PCR and with the q-PCR, with estimated concentrations of viable cells varying between 2.15 log10 cfu/g (detection limit) and 2.94 log10 cfu/g. Three samples tested negative both with the OCLA−PCR and with the q-PCR. Seven samples were positive with the OCLA−PCR, but negative with the q-PCR, and three samples tested negative with the OCLA−PCR and positive with the q-PCR. The percentage of viable cells relative to the total ranged between 2.4% and 86.0%. Seventy isolates of L. monocytogenes (five from each positive sample) were classified in PCR serogroups with a multiplex PCR assay. L. monocytogenes isolates belonged to serogroups IIa (52 isolates; 74.3%), IIc (7; 10.0%), IVa (2; 2.9%), and IVb (9; 12.9%). The susceptibility of the 70 isolates to 15 antibiotics of clinical interest was tested. The strains presented resistance to between three and eight antibiotics. The average number of resistances was greater (p < 0.001) among strains isolated from Spanish samples (6.20 ± 1.08), than in those from Portugal (5.00 ± 1.08). In both groups of strains, a prevalence of resistance higher than 95% was observed for oxacillin, cefoxitin, cefotaxime, and cefepime. The need to handle minced chicken meat correctly, taking care to cook it sufficiently and to avoid cross-contamination, so as to reduce the danger of listeriosis, is emphasized. A combination of culture-dependent and culture-independent methods offers complementary routes for the detection in food of the cells of L. monocytogenes in various different physiological states.

Characterisation of Listeria monocytogenes Isolates from Hunted Game and Game Meat from Finland

Listeria monocytogenes is an important foodborne zoonotic bacterium. It is a heterogeneous species that can be classified into lineages, serogroups, clonal complexes, and sequence types. Only scarce information exists on the properties of L. monocytogenes from game and game meat. We characterised 75 L. monocytogenes isolates from various game sources found in Finland between 2012 and 2020. The genetic diversity, presence of virulence and antimicrobial genes were studied with whole genome sequencing. Most (89%) of the isolates belonged to phylogenetic lineage (Lin) II and serogroup (SG) IIa. SGs IVb (8%) and IIb (3%) of Lin I were sporadically identified. In total, 18 clonal complexes and 21 sequence types (STs) were obtained. The most frequent STs were ST451 (21%), ST585 (12%) and ST37 (11%) found in different sample types between 2012 and 2020. We observed 10 clusters, formed by closely related isolates with 0-10 allelic differences. Most (79%) of the virulence genes were found in all of the L. monocytogenes isolates. Only fosX and lin were found out of 46 antimicrobial resistance genes. Our results demonstrate that potentially virulent and antimicrobial-sensitive L. monocytogenes isolates associated with human listeriosis are commonly found in hunted game and game meat in Finland.

Sugar Modification of Wall Teichoic Acids Determines Serotype-Dependent Strong Biofilm Production in Listeria monocytogenes

Biofilm production is responsible for persistent food contamination by Listeria monocytogenes, threatening food safety and public health. Human infection and food contamination with L. monocytogenes are caused primarily by serotypes 1/2a, 1/2b, and 4b. However, the association of biofilm production with phylogenic lineage and serotype has not yet been fully understood. In this study, we measured the levels of biofilm production in 98 clinical strains of L. monocytogenes at 37°C, 25°C, and 4°C. The phylogenetic clusters grouped by core genome multilocus sequence typing (cgMLST) exhibited association between biofilm production and phylogenetic lineage and serotype. Whereas clusters 1 and 3 consisting of serotype 4b strains exhibited weak biofilm production, clusters 2 (serotype 1/2b) and 4 (serotype 1/2a) were composed of strong biofilm formers. Particularly, cluster 2 (serotype 1/2b) strains exhibited the highest levels of biofilm production at 37°C, and the levels of biofilm production of cluster 4 (serotype 1/2a) strains were significantly elevated at all tested temperatures. Pan-genome analysis identified 22 genes unique to strong biofilm producers, most of which are related to the synthesis and modification of teichoic acids. Notably, a knockout mutation of the rml genes related to the modification of wall teichoic acids with l-rhamnose, which is specific to serogroup 1/2, significantly reduced the level of biofilm production by preventing biofilm maturation. Here, the results of our study show that biofilm production in L. monocytogenes is related to phylogeny and serotype and that the modification of wall teichoic acids with l-rhamnose is responsible for serotype-specific strong biofilm formation in L. monocytogenes. IMPORTANCE Biofilm formation on the surface of foods or food-processing facilities by L. monocytogenes is a serious food safety concern. Here, our data demonstrate that the level of biofilm production differs among serotypes 1/2a, 1/2b, and 4b depending on the temperature. Furthermore, sugar decoration of bacterial cell walls with l-rhamnose is responsible for strong biofilm production in serotypes 1/2a and 1/2b, commonly isolated from foods and listeriosis cases. The findings in this study improve our understanding of the association of biofilm production with phylogenetic lineage and serotype in L. monocytogenes.

Genetic diversity of Listeria monocytogenes isolates from farm products in Shanghai, China

Farm products are considered important vehicles for the transmission of Listeria monocytogenes. The typing of L. monocytogenes from farm products contributes to the surveillance and source tracing of the pathogen. In this study, 77 L. monocytogenes strains from seven farm product categories in Shanghai were analyzed by serological typing, multilocus sequence typing (MLST), pulsed-field gel electrophoresis (PFGE) and whole-genome nucleotide polymorphism (wgSNP). The results showed that the 77 isolates were classified into four serovars (1/2a, 1/2c, 1/2b and 4b), and of these, 1/2a (n=47) was the most prevalent. Seventeen sequence types (STs) were generated by MLST with two novel STs (ST1402 and ST1403), and 20 of 77 L. monocytogenes isolates had high genetic identity with previously documented outbreak isolates according to the minimum spanning tree from the MLST results. Moreover, 34 PFGE patterns (PF1-PF34) were differentiated, and based on a similarity value higher than 80% by the UPGMA dendrogram, the discriminatory index was relatively low (equal to 0.775). Furthermore, 14 isolates were chosen and further analyzed by wgSNP based on the previous typing results, which demonstrated that wgSNP and MLST yielded mostly consistent typing results but higher resolution than PFGE. In conclusion, 77 L. monocytogenes isolates from farm products collected in nine districts in Shanghai were highly genetically diverse, and 20 of these isolates had high relatedness with previously documented outbreak strains worldwide. The results indicate a possible cross-contamination risk of L. monocytogenes and a potential public health concern resulting from farm products during the supply chain in Shanghai, China.

PulseNet International Survey on the Implementation of Whole Genome Sequencing in Low and Middle-Income Countries for Foodborne Disease Surveillance

PulseNet International (PNI) is a global network of 88 countries who work together through their regional and national public health laboratories to track foodborne disease around the world. The vision of PNI is to implement globally standardized surveillance using whole genome sequencing (WGS) for real-time identification and subtyping of foodborne pathogens to strengthen preparedness and response and lower the burden of disease. Several countries in North America and Europe have experienced significant benefits in disease mitigation after implementing WGS. To broaden the routine use of WGS around the world, challenges and barriers must be overcome. We conducted this study to determine the challenges and barriers countries are encountering in their attempts to implement WGS and to identify how PNI can provide support to improve and become a better integrated system overall. A survey was designed with a set of qualitative questions to capture the status, challenges, barriers, and successes of countries in the implementation of WGS and was administered to laboratories in Africa, Asia-Pacific, Latin America and the Caribbean, and Middle East. One-third of respondents do not use WGS, and only 8% reported using WGS for routine, real-time surveillance. The main barriers for implementation of WGS were lack of funding, gaps in expertise, and training, especially for data analysis and interpretation. Features of an ideal system to facilitate implementation and global surveillance were identified as an all-in-one software that is free, accessible, standardized and validated. This survey highlights the minimal use of WGS for foodborne disease surveillance outside the United States, Canada, and Europe to date. Although funding remains a major barrier to WGS-based surveillance, critical gaps in expertise and availability of tools must be overcome. Opportunities to seek sustainable funding, provide training, and identify solutions for a globally standardized surveillance platform will accelerate implementation of WGS worldwide.

Listeria monocytogenes isolates from Cornu aspersum snails: Whole genome-based characterization and host-pathogen interactions in a snail infection model

Even though Listeria monocytogenes is an extensive-studied foodborne pathogen, genome analysis of isolates from snails that may represent a reservoir of L. monocytogenes are still scarce. Here, we use whole-genome sequencing (WGS) to assess the genomic diversity of hypervirulent, virulent and non-virulent phenotypes of 15 L. monocytogenes isolated from snails to unveil their survival, virulence, and host-pathogen mechanisms of interactions in a snail infection model. Most of isolates (66.7%) were characterized as multidrug resistant (MDR) and belonged to clonal complexes (CCs) which are strongly associated with cases of human infection. All isolates contained intact genes associated with invasion and infection while hypervirulent isolates are adapted to host environment, possessing genes which are involved in teichoic acid biosynthesis, peptidoglycan modification and biofilm formation, correlating with their tolerance to haemolymph plasma phenotype and biofilm formation ability. A snail infection model showed that hypervirulent isolates triggered programmed host cell death pathway by increasing up to 30% the circulating apoptotic hemocytes in combination with induced nitrate production and reactive oxygen species (ROS) generation in snails' haemolymph. In contrast, the administration of the non-virulent strain which possesses a truncated mogR gene that regulates flagellar motility gene expression led only to an increase of necrotic non-apoptotic cells. Overall, this study provides significant insights into the genetic diversity of L. monocytogenes from snails, the genomic features of them linked to their hypervirulent/non-virulent phenotype, and the mechanisms of host-pathogen interactions.

Phylogenetic and Evolutionary Genomic Analysis of Listeria monocytogenes Clinical Strains in the Framework of Foodborne Listeriosis Risk Assessment

Listeria monocytogenes is one of the most important foodborne pathogens responsible for listeriosis, a severe disease with symptoms ranging from septicemia, meningoencephalitis, and abortion. Given the strong impact of listeriosis on human health and the difficulty of controlling L. monocytogenes along the food production chain, listeriosis has become a priority subjected to molecular surveillance in European Union/European Economic Area since 2007. From 2018, surveillance is based on whole-genome sequence using the core genome multilocus sequence type. The complete sequences of 132 clinical strains were used to define the evolutionary relatedness among subtypes of L. monocytogenes isolated in Italy from 2010 to 2016, allowing the identification of clades and/or clusters associated with outbreaks or sporadic cases of listeriosis. All the strains analyzed are clustered in lineages I and II, and the majority of the strains were classified as lineage II. A probable epidemic entrance in different years for every clade and cluster from each different region was defined. The persistence of the same specific clonal complexes of L. monocytogenes has been found over long periods; this may be related to the fact that some strains are able to survive better than others in a food production environment. Phylogenic studies, using whole-genome sequence data, are able to identify the emergence of highly persistent pathogenic variants, contributing to improving the hazard characterization of L. monocytogenes.

Systematic review of Listeria monocytogenes from food and clinical samples in Chinese mainland from 2010 to 2019

Listeria monocytogenes, a foodborne pathogen, can cause human listeriosis. Listeriosis is a potentially fatal gastrointestinal illness, which is closely related to the spread of food to humans. We review the literature published during 2010 to 2019 to better understand the prevalence of L. monocytogenes in food products, incidence of human listeriosis, and their characteristics in Chinese mainland. We found the main sequence types (STs) strains from foods are similar globally, and the prevalence of L. monocytogenes from raw meat was the highest among all food products. The most common STs in food products and clinical cases were ST9 (serogroup Ⅰ.2) strains and ST87 (serogroup Ⅱ.2) strains, respectively. The ST87 strains being the most common STs of clinical cases might be related to the exist of Listeria pathogenicity islands 4 genes and Chinese eating habits for ready to eat foods, among which the prevalence of ST87 strain was the highest in ready to eat food. Therefore, more research should be conducted to explore the reasons for the L. monocytogenes isolates differences in food and clinic sources. Meanwhile, more research should be conducted to explore the reasons for differences among the L. monocytogenes isolates in food and clinical sources.

Genetic Diversity and Relationships of Listeria monocytogenes Serogroup IIa Isolated in Poland

In the present study, 100 L. monocytogenes isolates of serogroup IIa from food and food production environments in Poland were characterized towards the presence of virulence, resistance, and stress response genes using whole-genome sequencing (WGS). The strains were also molecularly typed and compared with multi-locus sequence typing (MLST) and core genome MLST analyses. The present isolates were grouped into 6 sublineages (SLs), with the most prevalent SL155 (33 isolates), SL121 (32 isolates), and SL8 (28 isolates) and classified into six clonal complexes, with the most prevalent CC155 (33 strains), CC121 (32 isolates), and CC8 (28 strains). Furthermore, the strains were grouped to eight sequence types, with the most prevalent ST155 (33 strains), ST121 (30 isolates), and ST8 (28; strains) followed by 60 cgMLST types (CTs). WGS data showed the presence of several virulence genes or putative molecular markers playing a role in pathogenesis of listeriosis and involved in survival of L. monocytogenes in adverse environmental conditions. Some of the present strains were molecularly closely related to L. monocytogenes previously isolated in Poland. The results of the study showed that food and food production environments may be a source of L. monocytogenes of serogroup IIa with pathogenic potential.

Landscape of Stress Response and Virulence Genes Among Listeria monocytogenes Strains

The pathogenic microorganism Listeria monocytogenes is ubiquitous and responsible for listeriosis, a disease with a high mortality rate in susceptible people. It can persist in different habitats, including the farm environment, the food production environments, and in foods. This pathogen can grow under challenging conditions, such as low pH, low temperatures, and high salt concentrations. However, L. monocytogenes has a high degree of strain divergence regarding virulence potential, environmental adaption, and stress response. This review seeks to provide the reader with an up-to-date overview of clonal and serotype-specific differences among L. monocytogenes strains. Emphasis on the genes and genomic islands responsible for virulence and resistance to environmental stresses is given to explain the complex adaptation among L. monocytogenes strains. Moreover, we highlight the use of advanced diagnostic technologies, such as whole-genome sequencing, to fine-tune quantitative microbiological risk assessment for better control of listeriosis.

Whole-Genome Sequencing Characterization of Virulence Profiles of Listeria monocytogenes Food and Human Isolates and In Vitro Adhesion/Invasion Assessment

Listeria monocytogenes (Lm) is the causative agent of human listeriosis. Lm strains have different virulence potential. For this reason, we preliminarily characterised via Whole-Genome Sequencing (WGS) some Lm strains for their key genomic features and virulence-associated determinants, assigning the clonal complex (CC). Moreover, the ability of the same strains to adhere to and invade human colon carcinoma cell line Caco-2, evaluating the possible correspondence with their genetic virulence profile, was also assessed. The clinical strains typed belonged to clonal complex (CC)1, CC31, and CC101 and showed a very low invasiveness. The Lm strains isolated from food were assigned to CC1, CC7, CC9, and CC121. All CC1 carried the hypervirulence pathogenicity island LIPI-3 in addition to LIPI-1. Premature stop codons in the inlA gene were found only in Lm of food origin belonging to CC9 and CC121. The presence of LIPI2_inlII was observed in all the CCs except CC1. The CC7 strain, belonging to an epidemic cluster, also carried the internalin genes inlG and inlL and showed the highest level of invasion. In contrast, the human CC31 strain lacked the lapB and vip genes and presented the lowest level of invasiveness. In Lm, the genetic determinants of hypo- or hypervirulence are not necessarily predictive of a cell adhesion and/or invasion ability in vitro. Moreover, since listeriosis results from the interplay between host and virulence features of the pathogen, even hypovirulent clones are able to cause infection in immunocompromised people.

Ruminant-associated Listeria monocytogenes isolates belong preferentially to dairy-associated hypervirulent clones: a longitudinal study in 19 farms

Studies have shown that ruminants constitute reservoirs of Listeria monocytogenes, but little is known about the epidemiology and genetic diversity of this pathogen within farms. Here we conducted a large-scale longitudinal study to monitor Listeria spp. in 19 dairy farms during three consecutive seasons (N = 3251 samples). L. innocua was the most prevalent species, followed by L. monocytogenes. Listeria monocytogenes was detected in 52.6% of farms and more frequently in cattle (4.1%) and sheep (4.5%) than in goat farms (0.2%). Lineage I accounted for 69% of L. monocytogenes isolates. Among animal samples, the most prevalent sublineages (SL) and clonal complexes (CC) were SL1/CC1, SL219/CC4, SL26/CC26 and SL87/CC87, whereas SL666/CC666 was most prevalent in environmental samples. Sixty-one different L. monocytogenes cgMLST types were found, 28% common to different animals and/or surfaces within the same farm and 21% previously reported elsewhere in the context of food and human surveillance. Listeria monocytogenes prevalence was not affected by farm hygiene but by season: higher prevalence was observed during winter in cattle, and during winter and spring in sheep farms. Cows in their second lactation had a higher probability of L. monocytogenes faecal shedding. This study highlights dairy farms as a reservoir for hypervirulent L. monocytogenes.

Ruminant-associated Listeria monocytogenes isolates belong preferentially to dairy-associated hypervirulent clones: a longitudinal study in 19 farms

Studies have shown that ruminants constitute reservoirs of Listeria monocytogenes, but little is known about the epidemiology and genetic diversity of this pathogen within farms. Here we conducted a large-scale longitudinal study to monitor Listeria spp. in 19 dairy farms during three consecutive seasons (N = 3251 samples). L. innocua was the most prevalent species, followed by L. monocytogenes. Listeria monocytogenes was detected in 52.6% of farms and more frequently in cattle (4.1%) and sheep (4.5%) than in goat farms (0.2%). Lineage I accounted for 69% of L. monocytogenes isolates. Among animal samples, the most prevalent sublineages (SL) and clonal complexes (CC) were SL1/CC1, SL219/CC4, SL26/CC26 and SL87/CC87, whereas SL666/CC666 was most prevalent in environmental samples. Sixty-one different L. monocytogenes cgMLST types were found, 28% common to different animals and/or surfaces within the same farm and 21% previously reported elsewhere in the context of food and human surveillance. Listeria monocytogenes prevalence was not affected by farm hygiene but by season: higher prevalence was observed during winter in cattle, and during winter and spring in sheep farms. Cows in their second lactation had a higher probability of L. monocytogenes faecal shedding. This study highlights dairy farms as a reservoir for hypervirulent L. monocytogenes.

ON-rep-seq as a rapid and cost-effective alternative to whole-genome sequencing for species-level identification and strain-level discrimination of Listeria monocytogenes contamination in a salmon processing plant

Identification, source tracking, and surveillance of food pathogens are crucial factors for the food-producing industry. Over the last decade, the techniques used for this have moved from conventional enrichment methods, through species-specific detection by PCR to sequencing-based methods, whole-genome sequencing (WGS) being the ultimate method. However, using WGS requires the right infrastructure, high computational power, and bioinformatics expertise. Therefore, there is a need for faster, more cost-effective, and more user-friendly methods. A newly developed method, ON-rep-seq, combines the classical rep-PCR method with nanopore sequencing, resulting in a highly discriminating set of sequences that can be used for species identification and also strain discrimination. This study is essentially a real industry case from a salmon processing plant. Twenty Listeria monocytogenes isolates were analyzed both by ON-rep-seq and WGS to identify and differentiate putative L. monocytogenes from a routine sampling of processing equipment and products, and finally, compare the strain-level discriminatory power of ON-rep-seq to different analyzing levels delivered from the WGS data. The analyses revealed that among the isolates tested there were three different strains. The isolates of the most frequently detected strain (n = 15) were all detected in the problematic area in the processing plant. The strain level discrimination done by ON-rep-seq was in full accordance with the interpretation of WGS data. Our findings also demonstrate that ON-rep-seq may serve as a primary screening method alternative to WGS for identification and strain-level differentiation for surveillance of potential pathogens in a food-producing environment.

Listeria monocytogenes Contamination Characteristics in Two Ready-to-Eat Meat Plants From 2019 to 2020 in Shanghai

Listeria monocytogenes is a ubiquitous foodborne pathogen that causes listeriosis and is mostly linked to consumption of ready-to-eat (RTE) foods. Lack of hygiene in food processing environments may be a primary reason for contamination by L. monocytogenes isolates. In this study, L. monocytogenes strains isolated from two RTE meat processing plants in the Shanghai municipality, China, were characterized during 2019–2020 using pulsed-field gel electrophoresis and whole-genome sequencing. Results showed that 29 samples (12.2%) out of 239 were positive for L. monocytogenes, with 21 (18.9%) and 8 (6.25%) isolates from plants A and B, respectively. The packaging room at plant A had the most contamination (14, 48.3%; p < 0.05), with a peak occurrence of 76.5% in processing environments. Nineteen L. monocytogenes isolates belonging to the pulsotype (PT) 7 group were indistinguishable (≥ 95.7%). Furthermore, core-genome multiple loci sequencing typing identified up to nine allelic differences, and the closet pairwise differences among these ST5 isolates included 0–16 small nucleotide polymorphisms. Therefore, L. monocytogenes likely persisted at plant A during 2019–2020 with ongoing clone transmission. In contrast, no L. monocytogenes isolates were identified from processing environments at plant B. Most L. monocytogenes isolates were sampled from raw materials (62.5%). Several isolates (ST378, ST8, and ST120) were detected only once in 2020 and were considered as transient isolates. However, three ST121 isolates with the same PT (PT2) were detected in 2020 and should be noted for their stronger survival ability in harsh environments. These results suggest that continuous monitoring, stringent surveillance, and source tracking are crucial to guaranteeing food safety in RTE food plants.

Intensive Environmental Surveillance Plan for Listeria monocytogenes in Food Producing Plants and Retail Stores of Central Italy: Prevalence and Genetic Diversity

Listeria monocytogenes (Lm) can persist in food processing environments (FPEs), surviving environmental stresses and disinfectants. We described an intensive environmental monitoring plan performed in Central Italy and involving food producing plants (FPPs) and retail grocery stores (RSs). The aim of the study was to provide a snapshot of the Lm circulation in different FPEs during a severe listeriosis outbreak, using whole genome sequencing (WGS) to investigate the genetic diversity of the Lm isolated, evaluating their virulence and stress resistance profiles. A total of 1217 samples were collected in 86 FPEs with 12.0% of positive surfaces at FPPs level and 7.5% at RSs level; 133 Lm isolates were typed by multilocus sequencing typing (MLST) and core genome MLST (cgMLST). Clonal complex (CC) 121 (25.6%), CC9 (22.6%), CC1 (11.3%), CC3 (10.5%), CC191 (4.5%), CC7 (4.5%) and CC31 (3.8%) were the most frequent MLST clones. Among the 26 cgMLST clusters obtained, 5 of them persisted after sanitization and were re-isolated during the follow-up sampling. All the CC121 harboured the Tn6188_qac gene for tolerance to benzalkonium chloride and the stress survival islet SSI-2. The CC3, CC7, CC9, CC31 and CC191 carried the SSI-1. All the CC9 and CC121 strains presented a premature stop codon in the inlA gene. In addition to the Lm Pathogenicity Island 1 (LIPI-1), CC1, CC3 and CC191 harboured the LIPI-3. The application of intensive environmental sampling plans for the detection and WGS analysis of Lm isolates could improve surveillance and early detection of outbreaks.

Whole Genome Sequence Analysis of Phage-Resistant Listeria monocytogenes Serotype 1/2a Strains from Turkey Processing Plants

Listeria monocytogenes is a Gram-positive bacterial pathogen and the causative agent of listeriosis, a severe foodborne infection. L. monocytogenes is notorious for its ability to persist in food processing environments (FPEs) via a variety of adaptive traits. Even though traits such as cold tolerance, biofilm formation and sanitizer resistance have been extensively investigated for their roles in persistence of L. monocytogenes in FPEs, much less is known about resistance to bacteriophages. Previous studies explored phage resistance mechanisms in laboratory-created mutants but it is imperative to investigate phage resistance that is naturally exhibited in FPE-derived strains. Here, we integrated the analysis of whole genome sequence data from a panel of serotype 1/2a strains of sequence types 321 and 391 from turkey processing plants, with the determination of cell surface substituents required for phage adsorption and phage infection assays with the four wide-host-range phages A511, P100, 20422-1 and 805405-1. Using a specific set of recombinant phage protein probes, we discovered that phage-resistant strains lacked one or both of the serogroup 1/2-specific wall teichoic acid carbohydrate decorations, N-acetylglucosamine and rhamnose. Furthermore, these phage-resistant strains harbored substitutions in lmo1080, lmo1081, and lmo2550, which mediate carbohydrate decoration of the wall teichoic acids.