Tetracycline-Resistant, Third-Generation Cephalosporin-Resistant, and Extended-Spectrum ß-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 ß-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.

Persistence of antibiotic resistance genes in beef cattle backgrounding environment over two years after cessation of operation.

Confined animal feeding operations can facilitate the spread of genes associated with antibiotic resistance. It is not known how cattle removal from beef cattle backgrounding operation affects the persistence of antibiotic resistance genes (ARGs) in the environment. We investigated the effect of cessation of beef cattle backgrounding operation on the persistence and distribution of ARGs in the beef cattle backgrounding environment. The study was conducted at a pasture-feedlot type beef cattle backgrounding operation which consisted of feeding and grazing areas that were separated by a fence with an access gate. Backgrounding occurred for seven years before cattle were removed from the facility. Soil samples (n = 78) from 26 georeferenced locations were collected at the baseline before cattle were removed, and then one year and two years after cattle were removed. Metagenomic DNA was extracted from the soil samples and total bacterial population (16S rRNA), total Enterococcus species and class 1 integrons (intI1), and erythromycin (ermB and ermF), sulfonamide (sul1 and sul2) and tetracycline (tetO, tetW and tetQ) resistance genes were quantified. Concentrations of total bacteria, Enterococcus spp., class 1 integrons, and ARGs were higher in the feeding area and its immediate vicinity (around the fence and the gate) followed by a gradient decline along the grazing area. Although the concentrations of total bacteria, Enterococcus spp., class 1 integrons and ARGs in the feeding area significantly decreased two years after cattle removal, their concentrations were still higher than that observed in the grazing area. Higher concentrations over two years in the feeding area when compared to the grazing area suggest a lasting effect of confined beef cattle production system on the persistence of bacteria and ARGs in the soil.