Genotypic and epidemiologic characterization of extended-spectrum cephalosporin resistant Salmonella enterica from US beef feedlots

Comparison of antimicrobial resistance among Salmonella enterica serovars isolated from Canadian turkey flocks, 2013-2021

The emergence of antimicrobial resistance (AMR) in Salmonella from turkeys has raised a food safety concern in Canada as certain serovars have been implicated in human salmonellosis outbreaks in recent years. While several studies evaluated AMR in broiler chickens in Canada, there are limited studies that assess AMR in turkey flocks. This study analyzed data collected between 2013 and 2021 by the Canadian Integrated Program for Antimicrobial Resistance Surveillance (CIPARS) farm turkey surveillance program to determine the prevalence of AMR and differences in resistance patterns among Salmonella serovars recovered from turkey flocks. Salmonella isolates were tested for susceptibility to 14 antimicrobials using a microbroth dilution method. Hierarchical clustering dendrograms were constructed to compare the individual AMR status of Salmonella serovars. Differences in the probability of resistance between Salmonella serovars were determined using generalized estimating equation logistic regression models to account for farm-level clustering. Of the 1,367 Salmonella isolates detected, 55.3% were resistant to at least one antimicrobial and 25.3% were multidrug resistant (MDR) (resistant to ≥3 antimicrobial classes). The Salmonella isolates exhibited high resistance to tetracycline (43.3%), streptomycin (47.2%), and sulfisoxazole (29.1%). The 3 most frequently occurring serovars were S. Uganda (22.9%), S. Hadar (13.5%), and S. Reading (12.0%). Streptomycin-sulfisoxazole-tetracycline (n = 204) was the most frequent MDR pattern identified. Heatmaps showed that S. Reading exhibited coresistance to the quinolone class antimicrobials, ciprofloxacin, and nalidixic acid; S. Heidelberg to gentamicin and sulfisoxazole; and S. Agona to ampicillin and ceftriaxone. Salmonella Hadar isolates had higher odds of resistance to tetracycline (OR: 152.1, 95% CI: 70.6-327.4) while the probability of being resistant to gentamicin and ampicillin was significantly higher in S. Senftenberg than in all the other serovars. Moreover, S. Uganda had the highest odds of being MDR (OR: 4.7, 95% CI: 3.7-6.1). The high resistance observed warrants a reassessment of the drivers for AMR, including AMU strategies and other production factors. Differences in AMR patterns highlight the need to implement serovar-specific mitigation strategies.

Disease Occurrence in- and the Transferal of Zoonotic Agents by North American Feedlot Cattle

North America is a large producer of beef and contains approximately 12% of the world's cattle inventory. Feedlots are an integral part of modern cattle production in North America, producing a high-quality, wholesome protein food for humans. Cattle, during their final stage, are fed readily digestible high-energy density rations in feedlots. Cattle in feedlots are susceptible to certain zoonotic diseases that impact cattle health, growth performance, and carcass characteristics, as well as human health. Diseases are often transferred amongst pen-mates, but they can also originate from the environment and be spread by vectors or fomites. Pathogen carriage in the gastrointestinal tract of cattle often leads to direct or indirect contamination of foods and the feedlot environment. This leads to the recirculation of these pathogens that have fecal-oral transmission within a feedlot cattle population for an extended time. Salmonella, Shiga toxin-producing Escherichia coli, and Campylobacter are commonly associated with animal-derived foods and can be transferred to humans through several routes such as contact with infected cattle and the consumption of contaminated meat. Brucellosis, anthrax, and leptospirosis, significant but neglected zoonotic diseases with debilitating impacts on human and animal health, are also discussed.

Tetracycline-Resistant, Third-Generation Cephalosporin-Resistant, and Extended-Spectrum β-Lactamase-Producing Escherichia coli in a Beef Cow-Calf Production System

ABSTRACT

Cow-calf production plays a significant role in the beef production chain. However, bacteria in these systems are not typically monitored for antimicrobial resistance (AMR). We determined the baseline level of AMR in fecal bacteria collected from preweaned calves prior to feedlot entry and evaluated the effects of type of graze and age on AMR occurrence. Two grazing experiments (16 cow-calf pairs each) were conducted on tall fescue or wheat. Fecal samples were cultured for the detection of tetracycline-resistant (TETr), third-generation cephalosporin-resistant (3GCr), and extended-spectrum β-lactamase (ESBL)-producing Escherichia coli. Isolates were characterized for resistance to other antibiotics and resistance mechanisms. Concentrations (P < 0.001) and prevalence (P = 0.007) of TETrE. coli isolates were significantly higher in the calves (5.1 log CFU/g and 93%, respectively) than in the cows (4.4 log CFU/g and 80%, respectively). Wheat grazing did not affect TETr isolates phenotypically; however, it significantly expanded (P = 0.005) the resistant population carrying tet(A) over that carrying tet(B). Fecal prevalence of 3GCr and ESBL-producing isolates was 31.3 and 3.4%, respectively, with no significant effects of age (P = 0.340) or wheat grazing (P = 0.597). All 3GCr and ESBL-producing isolates were multidrug resistant (resistant to at least three antimicrobial classes). 3GCr isolates were positive for blaCMY-2 (73%) or blaCTX-M (27%), and blaCTX-M-15 was the most prevalent gene (94%, n = 17) among the CTX-M-positive isolates. Wheat grazing significantly expanded (P < 0.001) the 3GCr population carrying blaCTX-M and reduced the population carrying blaCMY-2. Five of the seven ESBL-producing isolates were positive for blaCTX-M. Our study revealed age-dependent occurrence of TETrE. coli and that wheat grazing expanded the resistant population carrying certain resistance genes. Cow-calf production is a significant reservoir for antibiotic-resistant bacteria of significant public health importance such as 3GCr and CTX-M ESBL-producing E. coli.

HIGHLIGHTS

AmpC- and Extended-Spectrum β-Lactamase-Producing Enterobacteriaceae Detected in Fresh Produce in Central Ohio

ABSTRACT

Salad vegetables purchased from farmer's markets and grocery stores in central Ohio during the summers of 2015 and 2016 were tested for the presence of Enterobacteriaceae resistant to extended-spectrum cephalosporins and carbapenems, Salmonella contamination, and coliform bacterial counts. A total of 364 samples were collected from 36 farmers' markets and 33 grocery stores. Using selective media, we found 23 (6.3%) samples that produced Enterobacteriaceae expressing an AmpC β-lactamase phenotype, with 11 (3.0%) confirmed to contain blaCMY and 6 (1.6%) that produced Enterobacteriaceae with an extended-spectrum β-lactamase (ESBL) phenotype, 1 (0.3%) of which was confirmed to contain blaCTX-M. All blaCMY and blaCTX-M strains were isolated from leafy greens. No Salmonella spp. or carbapenem-resistant Enterobacteriaceae were recovered from fresh produce samples. Adjusting for year, the geometric mean coliform count differed (P < 0.05) between produce types, with the count in tomatoes (15 CFU/mL) being lower than those in cucumbers (77.4 CFU/mL) and leafy greens (75.0 CFU/mL). The coliform counts also differed (P < 0.05) between years, with 19.6 CFU/mL in 2015 and 96.0 CFU/mL in 2016. There was no difference in coliform counts or the prevalences of Enterobacteriaceae expressing AmpC or ESBL phenotypes in produce purchased from farmers' markets and grocery stores.

HIGHLIGHTS

Detection of Cephalosporin and Fluoroquinolone Resistance Genes via Novel Multiplex qPCR in Fecal Salmonella Isolates From Northern Californian Dairy Cattle, 2002-2016

The objectives of this study were to evaluate the prevalence of extended spectrum β-lactamase (ESBL) genes, AmpC-type β-lactamase (ACBL) genes, and plasmid mediated quinolone resistance (PMQR) genes in Salmonella isolated at a Veterinary Medical Teaching Hospital microbiology laboratory, examine trends in presence of these resistance genes, and to explore the correlation between phenotypic resistance and presence of specific genes. The presence of ESBL, ACBL, and PMQR genes were detected using a single, novel multiplex qPCR. Only the genes bla_CMY–2 and bla_TEM were detected in the 110 Salmonella isolates tested. PMQR genes were not detected in isolates screened. Of 94 third-generation cephalosporin resistant isolates, representing eight serotypes, 48% (n = 45) were positive for bla_CMY–2 only and 50% (n = 47) were simultaneously positive for bla_CMY–2 and bla_TEM. Two third-generation cephalosporin resistant isolates were tested negative for all β-lactamase genes in our qPCR assay and likely house ESBL genes not screened for by our qPCR assay. A logistic regression model revealed that for serotype Dublin isolates (n = 38) the odds ratio for testing positive for bla_TEM when compared to all other serotypes was 51.6 (95% CI: 4.01–664.03, p = 0.0029). For serotype Typhimurium (n = 9) the odds ratio for testing positive for bla_TEM when compared to all other serotypes was 43.3 (95% CI: 1.76–1000, p = 0.0216). Overall, our results suggest that the prevalence of resistance to cephalosporins and fluoroquinolones due to ESBLs, ACBLs, and PMQR genes present in bovine nontyphoidal Salmonella enterica isolates has remained relatively constant in the isolates screened over a 14-year period.

Transferable Plasmids of Salmonella enterica Associated With Antibiotic Resistance Genes

Salmonella enterica is a common foodborne illness in the United States and globally. An increasing number of Salmonella infections are resistant to antibiotics, and many of the genes responsible for those resistances are carried by plasmids. Plasmids are important mediators of horizontal gene exchange, which could potentially increase the spread of antibiotic resistance (AR) genes. Twenty-eight different incompatibility groups of plasmids have been described in Enterobacteriaceae. Incompatibility groups differ in their accessory gene content, replication mechanisms, and their associations with Salmonella serotypes and animal sources. Plasmids also differ in their ability to conjugate or be mobilized, essential genes, and conditions required for transfer. It is important to understand the differences in gene content and transfer mechanisms to accurately determine the impact of plasmids on the dissemination and persistence of antibiotic resistance genes. This review will cover the most common plasmid incompatibility groups present in S. enterica with a focus on the transfer mechanisms and associated antibiotic resistance genes.

Emergence of a Novel Salmonella enterica Serotype Reading Clonal Group Is Linked to Its Expansion in Commercial Turkey Production, Resulting in Unanticipated Human Illness in North America

Two separate human outbreaks of Salmonella enterica serotype Reading occurred between 2017 and 2019 in the United States and Canada, and both outbreaks were linked to the consumption of raw turkey products. In this study, a comprehensive genomic investigation was conducted to reconstruct the evolutionary history of S. Reading from turkeys and to determine the genomic context of outbreaks involving this infrequently isolated Salmonella serotype. A total of 988 isolates of U.S. origin were examined using whole-genome-based approaches, including current and historical isolates from humans, meat, and live food animals. Broadly, isolates clustered into three major clades, with one apparently highly adapted turkey clade. Within the turkey clade, isolates clustered into three subclades, including an "emergent" clade that contained only isolates dated 2016 or later, with many of the isolates from these outbreaks. Genomic differences were identified between emergent and other turkey subclades, suggesting that the apparent success of currently circulating subclades is, in part, attributable to plasmid acquisitions conferring antimicrobial resistance, gain of phage-like sequences with cargo virulence factors, and mutations in systems that may be involved in beta-glucuronidase activity and resistance towards colicins. U.S. and Canadian outbreak isolates were found interspersed throughout the emergent subclade and the other circulating subclade. The emergence of a novel S Reading turkey subclade, coinciding temporally with expansion in commercial turkey production and with U.S. and Canadian human outbreaks, indicates that emergent strains with higher potential for niche success were likely vertically transferred and rapidly disseminated from a common source.IMPORTANCE Increasingly, outbreak investigations involving foodborne pathogens are difficult due to the interconnectedness of food animal production and distribution, and homogeneous nature of industry integration, necessitating high-resolution genomic investigations to determine their basis. Fortunately, surveillance and whole-genome sequencing, combined with the public availability of these data, enable comprehensive queries to determine underlying causes of such outbreaks. Utilizing this pipeline, it was determined that a novel clone of Salmonella Reading has emerged that coincided with increased abundance in raw turkey products and two outbreaks of human illness in North America. The rapid dissemination of this highly adapted and conserved clone indicates that it was likely obtained from a common source and rapidly disseminated across turkey production. Key genomic changes may have contributed to its apparent continued success in commercial turkeys and ability to cause illness in humans.

Emerging colistin resistance in Salmonella enterica serovar Newport isolates from human infections

Worldwide emergence of Salmonella enterica serovar Newport (S. Newport) infection in humans, in parallel with a significant increasing prevalence of antimicrobial resistance (AR), is a serious public health concern. However, the prevalence of S. Newport resistance in China remains largely unknown. A retrospective study of 287 S. Newport clinical isolates collected during 1997-2018 was undertaken for characterization of AR profiles using the micro-dilution assay. We found a recent emergence of colistin resistance in four Chinese clinical isolates, including mcr-1-positive isolates. Importantly, phylogenomic and microbiological investigations indicate multiple independent clonal transmission of colistin-resistant S. Newport isolates of different seafood origins. Our study highlights potential reservoirs for transmission of colistin resistance and suggests that the global food supply chain may facilitate this dissemination.

The effect of tylosin on antimicrobial resistance in beef cattle enteric bacteria: A systematic review and meta-analysis

BACKGROUND

Tylosin is a commonly used in-feed antimicrobial and is approved in several countries to reduce the incidence of liver abscesses in beef cattle. Macrolides are critically important antimicrobials in human health and used to treat some foodborne bacterial diseases, such as Campylobacter jejuni and Salmonella. Feeding tylosin could select for resistant enteric bacteria in cattle, which could contaminate beef products at slaughter and potentially cause foodborne illness. We conducted a systematic review and meta-analysis to evaluate the impact of feeding tylosin to cattle on phenotypic and genotypic resistance in several potential zoonotic enteric bacteria: Enterococcus species, Escherichia coli, Salmonella enterica subspecies enterica, and Campylobacter species. This review was registered with PROSPERO (#CRD42018085949).

RESULTS

Eleven databases were searched for primary research studies that fed tylosin at approved doses to feedlot cattle and tested bacteria of interest for phenotypic or genotypic resistance. We screened 1,626 citations and identified 13 studies that met the inclusion criteria. Enterococcus species were tested in seven studies, Escherichia coli was isolated in five studies, three studies reported on Salmonella, and two studies reported on Campylobacter species. Most studies relied on phenotypic antimicrobial susceptibility testing and seven also reported resistance gene testing. A random-effects meta-analyses of erythromycin-resistant enterococci from four studies had significant residual heterogeneity. Only two studies were available for a meta-analysis of tylosin-resistant enterococci. A semi-quantitative analysis demonstrated an increase in macrolide-resistant enterococci after long durations of tylosin administration (>100 days). Semi-quantitative analyses of other bacteria-antimicrobial combinations revealed mixed results, but many comparisons found no effect of tylosin administration. However, about half of these no-effect comparisons did not record the cumulative days of tylosin administration or the time since the last dose.

CONCLUSIONS

When fed at approved dosages for typical durations, tylosin increases the proportion of macrolide-resistant enterococci in the cattle gastrointestinal tract, which could pose a zoonotic risk to human beef consumers. Feeding tylosin for short durations may mitigate the impact on macrolide-resistant enterococci and further studies are encouraged to determine the effect of minimizing or eliminating tylosin use in beef cattle. There may also be an impact on other bacteria and other antimicrobial resistances but additional details or data are needed to strengthen these comparisons. We encourage authors of antimicrobial-resistance studies to follow reporting guidelines and publish details of all comparisons to strengthen future meta-analyses.

Genetic diversity and antimicrobial resistance of Salmonella serotypes recovered throughout the beef production chain and from patients with salmonellosis

Salmonella is one of the major foodborne pathogens worldwide. The antimicrobial resistance (AMR) of this foodborne pathogen has raised a great concern in recent years. Studies on the frequency and characterization of Salmonella serotypes can help to improve our knowledge on the epidemiology of this pathogen. The purpose of this study was to compare the serotypes, AMR and genetic profiles of Salmonella isolates recovered from raw beef throughout the beef production chain and from human feces associated with clinical cases of salmonellosis. The serotype, AMR and pulsed-field gel electrophoresis profile of 243 Salmonella enterica isolates recovered from beef carcasses (n = 78), ground beef (n = 135), and human feces from clinical cases of salmonellosis (n = 30) were compared. Forty-three different Salmonella serotypes were identified and regardless of the source, the top five corresponded to Typhimurium, Give, Group B (partially serotyped), Infantis and Anatum. Twelve serotypes from beef carcasses were also found in ground beef, showing their presence throughout the beef production chain. Salmonella Typhimurium, Infantis, Anatum and Montevideo were the only serotypes identified in all sample types. Resistance to tetracyclines was the most frequent (41.2%) followed by resistance to aminoglycosides (37%), folate pathway inhibitors (21%), quinolones (20.2%), phenicols (17.1%), penicillins (15.6%) and cephems (7%). Multidrug resistance was observed in 28.8% of the isolates, and those from human feces showed resistance to a larger number of antimicrobials. Great concern arises from the resistance and reduced susceptibility observed to quinolones and cephalosporins because these drugs are the first line of treatment for invasive Salmonella infections. Twenty-seven distinct pulse-types were detected among 238 isolates. Clustering analysis for the most frequent serotypes identified groups of isolates with similar AMR profiles. Multidrug resistance spreading throughout the food production chain should be continually monitored and its importance emphasized.

Multiple Food-Animal-Borne Route in Transmission of Antibiotic-Resistant Salmonella Newport to Humans

Characterization of transmission routes of Salmonella among various food-animal reservoirs and their antibiogram is crucial for appropriate intervention and medical treatment. Here, we analyzed 3728 Salmonella enterica serovar Newport (S. Newport) isolates collected from various food-animals, retail meats and humans in the United States between 1996 and 2015, based on their minimum inhibitory concentration (MIC) toward 27 antibiotics. Random Forest and Hierarchical Clustering statistic was used to group the isolates according to their MICs. Classification and Regression Tree (CART) analysis was used to identify the appropriate antibiotic and its cut-off value between human- and animal-population. Two distinct populations were revealed based on the MICs of individual strain by both methods, with the animal population having significantly higher MICs which correlates to antibiotic-resistance (AR) phenotype. Only ∼9.7% (267/2763) human isolates could be attributed to food-animal origins. Furthermore, the isolates of animal origin had less diverse antibiogram than human isolates (P < 0.001), suggesting multiple sources involved in human infections. CART identified trimethoprim-sulfamethoxazole to be the best classifier for differentiating the animal and human isolates. Additionally, two typical AR patterns, MDR-Amp and Tet-SDR dominant in bovine- or turkey-population, were identified, indicating that distinct food-animal sources could be involved in human infections. The AR analysis suggested fluoroquinolones (i.e., ciprofloxacin), but not extended-spectrum cephalosporins (i.e., ceftriaxone, cefoxitin), is the adaptive choice for empirical therapy. Antibiotic-resistant S. Newport from humans has multiple origins, with distinct food-animal-borne route contributing to a significant proportion of heterogeneous isolates.