Background 

The threat of antimicrobial resistant bacteria and the implications for human health is an area of research that has been receiving a great deal of attention in both the media and scientific community. In the Centers for Disease Control and Prevention report, “Antibiotic Resistance Threats in the United States, 2013,” Salmonella is listed as a microorganism of serious threat. Salmonella outbreaks are most commonly associated with poultry and eggs; however, beef has also been implicated as a source of infection. The use of antibiotics in food animals may contribute to dissemination of antimicrobial resistant bacteria through the food chain. One of the major concerns is that many antibiotics used in food animals are also important in human medicine. Strict regulations or bans on the use of antimicrobials in agriculture could have devastating effects on animal health and create food safety issues.   

The objective of this study was to determine the effects of chlortetracycline and ceftiofur treatment on the prevalence, quantity and antimicrobial resistance of Salmonella in feedlot cattle. 

Methodology   

Individual fecal samples were collected at an experimental feedlot at West Texas A&M University in Canyon, Texas. The randomized and controlled field trial was comprised of 176 steers in four different treatment groups to assess the impact of ceftiofur and chlortetracycline use. Sixteen pens (11 steers/pen) were subjected to 4 treatment regimens; 1) all 11 steers in 8 pens were treated with ceftiofur on day 0; 2) 4 of these pens followed with intermittent chlortetracycline from day 4 through day 19; 3) 1 out of 11 steers in the remaining 8 pens were treated with ceftiofur on day 0; 4) 4 of these pens received chlortetracycline as above. Cattle fecal samples from days 0, 4, 14 and 26 were enriched and plated to selective media to identify Salmonella positive samples. Spiral plating onto brilliant green agar was used to obtain crude estimates of Salmonella quantities. Antibiotic susceptibility was performed for 11 different antimicrobials by the micro‐broth dilution method using the Sensititreâ automated inoculation delivery and fluorometric plate reading system. Genotypic antimicrobial susceptibility profiles were determined by whole genome sequencing of Salmonella isolates. Sequencing data was analyzed for the presence of antimicrobial resistance genes using the online databases Resfinder and CARD. Salmonella serotypes were determined using whole genome sequencing data and short read sequence typing for bacterial pathogens. Two samples of each serotype determined by SRST2 were sent to National Veterinary Services Laboratory for confirmation using standard serotyping methods. 

Findings

Implications