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Prevalence and Genomic Characterization of mcr-Positive Enterobacteriaceae in Retail Meat in Thailand Following the Colistin Ban.

This study aimed to investigate the prevalence and characteristics of mcr-positive Enterobacteriaceae in retail meat in Thailand following the national ban on prophylactic colistin use in food producing animals. A total of 152 meat samples (103 chicken and 49 pork) were collected from supermarkets and open markets between July and September 2023. Samples were screened for mcr-1 to mcr-5 using multiplex PCR. None of the samples from supermarkets tested positive, whereas mcr genes were detected in 15.4% (6/39) and 13.3% (4/30) of chicken and pork samples, respectively, from open markets, with mcr-1 and/or mcr-3 identified. A total of 21 isolates were recovered from PCR-positive samples (11 from chicken and 10 from pork). Escherichia coli was the predominant species (n = 19), followed by Klebsiella pneumoniae (n = 2). All mcr-positive isolates exhibited multidrug resistance. Whole-genome sequencing was performed for 19 non-clonal isolates. One K. pneumoniae strain from a pork sample co-harbored mcr-1 and mcr-8, representing the first report of this combination in the animal sector in Thailand. In addition, virulence-associated genes, including adhesion factors, toxins, and iron acquisition systems, were identified in selected isolates. Core genome SNP-based phylogenetic analysis revealed substantial genomic diversity among the isolates, suggesting relatedness to strains reported prior to the colistin ban. These findings indicate that retail meat from open markets may serve as an important route for the transmission of mcr-positive bacteria in Thailand and highlight the urgent need to incorporate systematic retail meat surveillance into national antimicrobial resistance monitoring programs.

Animals

Molecular Characterization of Listeria monocytogenes Isolated from Retail Yak Meat in Nyingchi, Xizang, China.

Listeria monocytogenes is a Gram-positive zoonotic pathogen responsible for listeriosis, a severe foodborne disease with high mortality in humans and animals. This study aimed to investigate the molecular epidemiology and genomic characteristics of L. monocytogenes isolated from raw yak meat in Nyingchi, Xizang, China. A total of 231 yak-related samples were collected in Nyingchi, consisting of 214 retail raw yak meat samples, 14 farm environmental samples, and 3 nearby water source samples. L. monocytogenes isolates were identified and characterized using culture-based methods, PCR serotyping, and whole-genome sequencing (WGS). Bioinformatic analyses were performed for virulence, antimicrobial resistance, and functional gene annotation using KEGG and COG databases. The overall contamination rate of Lm was 13.08% (28/214) for retail raw yak meat samples, whereas no isolates were recovered from 14 farm environmental samples (0.00%, 0/14) and 3 nearby water source samples (0.00%, 0/3). The serotypes of isolates were 1/2a (9/28, 32.14%), 1/2b (7/28, 25.00%), and 1/2c (12/28, 42.86%). These 28 isolates exhibited varied antimicrobial resistance profiles, with universal resistance to trimethoprim-sulfamethoxazole, high resistance to erythromycin and clindamycin, and low resistance to vancomycin. MLST analysis revealed seven sequence types (STs): ST9 (12/28, 42.86%), ST619 (6/28, 21.43%), ST8 (6/28, 21.43%), ST7 (1/28, 3.57%), ST87 (1/28, 3.57%), ST121 (1/28, 3.57%), ST297 (1/28, 3.57%). ST619 isolates harbored multiple virulence genes, including those located on Listeria pathogenicity islands LIPI-1, LIPI-3, and LIPI-4, indicating high genomic potential for virulence. Representative isolate Y2 (ST619) possessed a 3,009,858 bp genome with 3036 coding genes, four genomic islands, and two prophages. Functional annotation revealed enrichment of genes involved in carbohydrate transport and metabolism and amino acid biosynthesis pathways. Our findings provide the first genomic insight into L. monocytogenes contamination in yak meat from Nyingchi, Xizang, China, highlighting the urgent need to strengthen food safety monitoring and hygiene management in this region.

Listeria monocytogenes

Emergence of an optrA-positive Enterococcus faecalis ST699 lineage in animal-derived foods in Beijing, China.

Enterococci from animal-derived foods are key reservoirs for antimicrobial resistance (AMR) in the food chain. However, comparative genomic studies investigating the distribution of the oxazolidinone resistance gene optrA among food- and human-derived Enterococci remain limited. This study assessed linezolid-resistant Enterococci from retail meat and healthy humans in Beijing, China (2023-2024). Among 87 isolates, E. faecalis and E. faecium predominated. Food-derived isolates showed broader resistance profiles than human isolates. Fourteen optrA-positive strains were identified, accounting for 92.9% of food isolates. optrA frequently co-localized with erm(A), ant(9)-Ia, and fexA on Tn554-family transposons, suggesting a potentially transferable multidrug resistance module. Notably, an optrA-positive E. faecalis ST699 clone was identified for the first time in Chinese retail meat. This clone formed a distinct lineage and carried a complete Tn554-optrA island. A representative ST699 isolate exhibited enhanced fitness and virulence potential in the Galleria mellonella model. These findings highlight animal-derived foods as important reservoirs of linezolid-resistant Enterococci and provide genomic evidence consistent with their role as potential sources of optrA-mediated resistance. The emergence of a multidrug-resistant E. faecalis ST699 clone with enhanced fitness characteristics underscores the need for continued surveillance of foodborne antimicrobial resistance within the One Health framework.

Enterococcus faecalis

Comparative prevalence of the mercury resistance gene merA in human feces, food, and environmental water from Japan, Vietnam, and Ghana.

In this study, we investigated the prevalence and abundance of the mercury resistance gene merA in human feces, retail chicken meat, and environmental water samples collected from Japan, Vietnam, and Ghana. A real-time PCR assay developed in this study demonstrated high specificity toward merA sequences from more than 12 bacterial species. Using this assay, merA was detected in 6.8% of human fecal samples in Japan (n = 29), in contrast to significantly higher rates observed in Vietnam (70.2%, n = 47) and Ghana (97.4%, n = 39). Similar geographic trends were evident in the chicken meat samples: 18.5% in Japan (n = 27), 66% in Vietnam (n = 91), and 90% in Ghana (n = 10). Environmental water samples showed a consistently high merA detection rate across all countries (75-100%, n = 21), with substantially higher gene copy numbers in Vietnam and Ghana than in Japan. merA was detected in some water samples, even when total mercury concentrations were below the detection limit, indicating that molecular detection may offer greater sensitivity than traditional physicochemical methods. Mercury-resistant bacteria were successfully isolated and cultured, and Citrobacter freundii was identified as the representative strain. Genomic analysis revealed that merA was located on an IncFIB plasmid, flanked by insertion sequences, suggesting its potential for horizontal gene transfer. These findings highlight merA as a promising biomarker for environmental mercury exposure and support the utility of fecal merA analysis as a proxy for assessing mercury-related public health risks.

Humans

Microbial counts and thermal resistance of Salmonella oranienburg in ground turkey meat.

The microbial content of unfrozen and frozen ground turkey meat (GTM) in retail markets and the thermal resistance of Salmonella organienburg in GTM was determined. The unfrozen product had an average total aerobic count of 53 X 10(4) cells per g. These counts reduced slightly when held in a display case for one and two days at 1-2 degrees C. The frozen GTM contained 25% protein, 8% fat, 63% moisture and had a pH of 6.2. Microbial counts averaged 58 X 10(4). The time and temperatures to kill ca 10(4) S. oranienburg cells were determined. A thermal destruction curve can be defined as F60 degrees C. = 3.5 min. with Z = 7.4 degrees C.

Aerobiosis

Control of nonspecific staining in the fluorescent antibody technique for the detection of salmonellae in foods.

A fluorescent antibody conjugate, prepared from the IgG (immunoglobulin G) fraction of Salmonella polyvalent flagellar antiserum, gave better specific staining intensities and significantly lower nonspecific staining than did conjugates prepared from globulin fractions of ammonium sulfate-fractionated Salmonella polyvalent antisera. IgG was purified by affinity chromatography against protein A, a normal cell wall component of Staphylococcus aureus. Affinity chromatography yielded high-purity IgG in a one-step purification procedure. The conjugate prepared from affinity-purified IgG was compared with commercially available fluorescent antibody conjugates for the detection of salmoneallae in retail samplings of meats and poultry and gave better correlations with the cultural method than did the commercial conjugates.

Bacterial Proteins

Genome sequences of fungi isolated from retail ground beef.

Studies on fungi in meat products are very limited. In our previous study, we recovered 10 fungal strains from aerobically packaged ground beef collected from retail stores. We conducted whole-genome sequencing on these strains. This genomic information may help to inform more effective interventions to extend meat shelf life.

fungi

Bacteriological quality and shelf life of ground beef.

The bacteriological quality of unfrozen raw ground beef was evaluated after 0, 3, 6, 9, 12, 15, and 18 days of storage at 29 +/- 1 F (-1.7 +/- 0.6 C). At the time of fabrication, all of the ground beef samples contained 10(6) or fewer total aerobic and psychrotrophic bacteria/g; 81% contained 100 or fewer coliforms/g; 94% contained 100 or fewer Escherichia coli/g; and all of the samples contained 100 or fewer coagulase-positive Staphylococcus aureus and Clostridium perfringens/g. Total aerobic and psychrotrophic bacteria increased by 1 log between 3 and 18 days of storage. Coliform and E. coli counts decreased during storage, whereas coagulase-positive S. aureus and C. perfringens counts did not change significantly. These data indicate that meat processors, wholesalers, and retailers could improve the bacteriological quality and prolong the shelf life of ground beef packaged in oxygen-impermeable film if the temperature of product never exceeded 29 +/- 1 F (-1.7 +/- 0.6 C).

Aerobiosis

Incidence and relationship of group D streptococci with other indicator organisms in meats.

Raw and processed meats were analyzed for presumptive group D streptococci using KF streptococcus agar. Counts were compared with coliform, presumptive Escherichia coli, and Enterobacteriaceae counts but no meaningful relationships were observed. Results indicated that group D streptococci and E. coli type I were principally contaminants from the packing plant, rather than at retail level. The predominating group D streptococcus in both beef and pork cuts was Streptococcus faecalis, while in processed meat (bologna), the predominating group D streptococci were Streptococcus faecium var. durans and Streptococcus faecium. Streptococcus bovis was not detected among the isolates from any meat samples. Marked differences were noted in numbers of group D streptococci in processed meat from different manufacturers. The results did not support the use of group D streptococci as alternative indicator organisms for meats. However, the association of group D streptococci with packing plant contamination may prove to be of value.

Animals

Thin-layer chromatographic screening methods for histamine in tuna fish.

Twelve solvent systems were tested for their ability to separate histamine and histidine on a variety of thin-layer coatings. The best solvent-adsorbent systems were: chloroform-methanol-ammonia (2:2:1), methanol-ammonia (20:1), acetone-ammonia (95:5), and double development with (a) n-butanol-acetone-water (2:2:1) and (b) chloroform-methanol-ammonia (12:7:1), all on silica-gel layers. Ninhydrin was used as the visualization reagent. These four systems were then evaluated for their potential use as rapid screening procedures in the detection of possibly deleterious levels of histamine in tuna fish. Successful separation of histamine from the other ninhydrin-positive components of methanolic tuna fish extracts was achieved with all four systems. A sample from a lot of tuna implicated in human illness was found to have a histamine level considerably higher than tuna purchased from a local retail outlet or an extract spiked to a histamine level considered to be a threshold value for toxicity symptoms. The methanol-ammonia (20:1) and chloroform-methanol-ammonia (2:2:1) systems, used with silica-gel plates, are the most promising for rapid preliminary screening of tuna fish extracts for histamine.

Animals