Submitted to: American Society for Microbiology Annual Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 2/20/2013
Publication Date: 5/19/2013
Citation: Schmidt, J.W. 2013. Longitudinal concentrations of antibiotic resistant Escherichia coli in feces do not correspond to the patterns of antibiotic use at a cattle feedlot. American Society for Microbiology Annual Meeting. Poster No. 15 p. 101.
Technical Abstract: Background: Concerns have been raised that therapeutic use of antibiotics at cattle feedlots increases the concentrations of Escherichia coli resistant to antibiotics of critical importance to human medicine. However, the impact of therapeutic use of antibiotics at cattle feedlots on levels of antibiotic resistant E. coli is not well studied and the ecology of antibiotic resistant E. coli at cattle feedlots is poorly understood. Materials: A 698 head cattle herd, which received no sub-therapeutic or herd-wide antibiotic treatments, resided at a feedlot from September 2011 to June 2012. A total of 1,006 fecal samples were taken over the duration residence at the feedlot. Sample periods were: when cattle arrived at the feedlot, during the “Increased Disease Susceptibility” (IDS) period occurring during the six weeks following arrival at the feedlot, in November 2011, in January 2012, in April 2012, and in June 2012 shortly before cattle were harvested. Samples were plated onto MacConkey agar supplemented with either 4 mg/l cefotaxime, 32 mg/l chloramphenicol, 64 mg/l kanamycin, or nalidixic acid 32 mg/l, to determine fecal concentrations of cefotaxime-resistant E. coli (CTXr EC), chloramphenicol-resistant E. coli (CHLr EC), kanamycin-resistant E. coli (KANr EC), and nalidixic acid-resistant E. coli (NALr EC), respectively. Fecal and hide concentrations of CTXr EC, CHLr EC, KANr EC, and NALr EC were evaluated by one-way ANOVA with Tukey multiple comparison test with P values < 0.05 considered significant. Results: Over the lifespan of herd cattle (April 2011 to June 2012) 134 antibiotic injections occurred. The month with the highest frequency of antibiotic use was October 2011 with 63 injections occurring. The month with the second highest frequency of antibiotic use was September 2011 with 31 injections. After October 2011 antibiotic injections declined to 13 in November, 3 in December, 7 in January, 2 in February, and 1 in March. No antibiotics were injected in April, May, and June 2012. When cattle arrived at the feedlot in September 2011 mean fecal concentrations of CTXr EC, CHLr EC, KANr EC, and NALr EC ranged from 1.6 to 1.8 log CFU/swab. During the subsequent IDS, November 2011, January 2012, and April 2012 sample periods mean fecal concentrations of CTXr EC, CHLr EC, KANr EC, and NALr EC ranged from 1.7 to 2.6 log CFU/swab. The highest mean fecal concentrations of CTXr EC, CHLr EC, KANr EC, and NAL r EC, 3.4, 3.3, 2.5, and 1.8 log CFU/swab, respectively, occurred during June 2012. Conclusion: Undefined factors other than therapeutic antibiotic use exert greater influence on the amount of antibiotic resistant E. coli shed by feedlot cattle since the highest levels of CTXr EC, CHLr EC, and KANr EC occurred during June 2012 when no antibiotic had been used in three months.