Loneragan, G. H. | Search bioRxiv

bioRxiv2020

Antibiotic resistance among Escherichia coli and Salmonella isolated from dairy cattle feces in Texas

This study was conducted to develop and field-test a low cost protocol to estimate the isolate- and sample-level prevalence of resistance to critically important antibiotic drugs among Escherichia coli and Salmonella isolated from dairy cattle feces. E. coli and Salmonella were isolated from and screened on selective media, with and without antibiotics respectively. Bacterial isolates were further tested for susceptibility to a suite of antibiotics using disk diffusion. Molecular methods were performed on select bacterial isolates to identify and distinguish genetic determinants associated with the observed phenotypes. Among 85 non-type-specific E. coli randomly isolated from MacConkey agar without antibiotics, the isolate-level prevalence of resistance to tetracycline was the highest (8.2%), there was no isolate resistant to third-generation cephalosporin (0.0%) and one isolate was resistant to nalidixic acid (1.2%). Among 37 E. coli recovered from MacConkey agar with cefotaxime at 1.0{micro}g/ml, 100% were resistant to ampicillin and 56.8% were resistant to a third-generation cephalosporin (ceftriaxone). Among 22 E. coli isolates recovered from MacConkey agar with ciprofloxacin at 0.5{micro}g/ml, 90.9% were resistant to tetracycline whereas 77.3% and 54.5% were resistant to nalidixic acid and ciprofloxacin respectively. Sixteen Salmonella were isolated and only one demonstrated any resistance (i.e., single resistance to streptomycin). Among E. coli isolates that were either resistant or intermediate to ceftriaxone, an AmpC phenotype was more common than an extended spectrum beta-lactamase (ESBL) phenotype (29 versus 10 isolates, respectively). Among 24 E. coli isolates that were whole genome sequenced, phenotypic profiles of antibiotic resistance detected were generally substantiated by genotypic profiles. For instance, all isolates with an AmpC phenotype carried a blaCMY2 gene. The protocol used in this study is suited to detecting and estimating prevalence of antimicrobial resistance in bacteria isolated from food animal feces in resource-limited laboratories in the developing world.

microbiology↗

bioRxiv2020

High-resolution genomic comparisons within Salmonella enterica serotypes derived from beef feedlot cattle: parsing the roles of cattle source, pen, animal, sample type and production period

Salmonella enterica is a major foodborne pathogen, and contaminated beef products have been identified as the primary source of Salmonella-related outbreaks. Pathogenicity and antibiotic resistance of Salmonella are highly serotype- and subpopulation-specific, which makes it essential to understand high-resolution Salmonella population dynamics in cattle. Time of year, source of cattle, pen, and sample type(i.e., feces, hide or lymph nodes) have previously been identified as important factors influencing the serotype distribution of Salmonella (e.g., Anatum, Lubbock, Cerro, Montevideo, Kentucky, Newport, and Norwich) that were isolated from a longitudinal sampling design in a research feedlot. In this study, we performed high-resolution genomic comparisons of Salmonella isolates within each serotype using both single-nucleotide polymorphism (SNP)-based maximum likelihood phylogeny and hierarchical clustering of core-genome multi-locus sequence typing. The importance of the aforementioned features on clonal Salmonella expansion was further explored using a supervised machine learning algorithm. In addition, we identified and compared the resistance genes, plasmids, and pathogenicity island profiles of the isolates within each sub-population. Our findings indicate that clonal expansion of Salmonella strains in cattle was mainly influenced by the randomization of block and pen, as well as the origin/source of the cattle; that is, regardless of sampling time and sample type (i.e., feces, lymph node or hide). Further research is needed concerning the role of the feedlot pen environment prior to cattle placement to better understand carry-over contributions of existing strains of Salmonella and their bacteriophages. ImportanceSalmonella serotypes isolated from outbreaks in humans can also be found in beef cattle and feedlots. Virulence factors and antibiotic resistance are among the primary defense mechanisms of Salmonella, and are often associated with clonal expansion. This makes understanding the subpopulation dynamics of Salmonella in cattle critical for effective mitigation. There remains a gap in the literature concerning subpopulation dynamics within Salmonella serotypes in feedlot cattle from the beginning of feeding up until slaughter. Here, we explore Salmonella population dynamics within each serotype using core genome phylogeny and hierarchical classifications. We used machine-learning to quantitatively parse the relative importance of both hierarchical and longitudinal clustering among cattle host samples. Our results reveal that Salmonella populations in cattle are highly clonal over a 6-month study period, and that clonal dissemination of Salmonella in cattle is mainly influenced spatially by experimental block and pen, as well by the geographical origin of the cattle.

microbiology↗