|CARTER, BOONE - New Mexico State University|
|HAGEVOORT, G - New Mexico State University|
Submitted to: Agriculture, Food and Analytical Bacteriology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/8/2012
Publication Date: N/A
Interpretive Summary: Dairy cattle may contain the bacteria Salmonella that can make people sick. In some instances, these Salmonella are resistant to antibiotics used to kill them, although it is not fully understood how they acquire this resistance. Young dairy calves contain E. coli that are resistant to multiple antibiotics and may serve as a reservoir for resistance genes for other bacteria such as Salmonella. We sampled dairy calves from one week to six months of age fed either pasteurized or non-pasteurized waste milk and examined the influence of weaning on the prevalence of the resistant E. coli. The prevalence and concentrations of the E. coli decreased with increasing age and tended to be higher in calves fed pasteurized waste milk. Weaning had no effect. Understanding the development and transfer of antimicrobial resistance may lead to interventions to reduce these important bacteria before they become a problem in livestock or humans.
Technical Abstract: The use of antimicrobial drugs is reported to increase the prevalence of resistant bacteria, including commensals. Dairy calves are colonized at a very young age by a multi-drug-resistant E. coli (MDR EC), and research indicates that the prevalence is not related to recent use of antimicrobials but that diet and other environmental factors are likely involved. To further investigate the occurrence of this bacterium, we sampled dairy calves on several southwestern United States farms at one week of age through 6 months of age and determined not only prevalence, but fecal concentrations of the MDR EC. The influence of feeding pasteurized (PWM) versus non-pasteurized (NPWM) waste milk was examined, and the effect of weaning was additionally investigated by sampling two groups of calves 2 d pre-weaning and again 2 d post weaning. Antimicrobial susceptibilities were compared among MDR EC and Salmonella isolates cultured from these calves. The number of fecal samples positive for MDR EC as well as their concentrations decreased (P < 0.01) with increasing calf age during both collections, with the decrease being more pronounced when comparing feeding NPWM versus PWM. There did not appear to be any correlation between MDR EC and Salmonella prevalence. Antimicrobial susceptibility was examined in MDR EC isolates (six isolates/age group/collection; n = 72 total MDR EC isolates). In general, in the first collection, more resistance was observed in calves fed NPWM compared to the second collection, when the opposite trend was observed. All EC isolates were resistant to chloramphenicol and tetracycline and all but one resistant to sulfisoxazole, whereas the majority of the isolates were susceptible to ciprofloxacin and ceftriaxone. The number of isolates resistant to all other antibiotics examined decreased with increasing calf age. Thirty-eight percent of the isolates were resistant to 10 or more antibiotics with three primary resistance patterns observed: CSSuT, ACSSuT, and MDR-AmpC. Twenty-one Salmonella isolates were examined, more than half of which were cultured from calves one week old or less. All of the isolates were susceptible to amikacin, gentamicin, and ciprofloxacin, and only one isolate showed resistance to ceftriaxone. Most of the observed resistance was to ampicillin, chloramphenicol, tetracycline, and sulfisoxazole, and 57% of the Salmonella isolates were susceptible to all of the antibiotics examined. These isolates were resistant to a range of 3 to 11 antibiotics, with the most prevalent resistance the ACSSuTAuCF and MDR-AmpC patterns. No significant differences were observed in the fecal concentrations or in the number of MDR EC positive pens due to weaning. There was a tendency (P = 0.06) for fewer MDR EC positive pens in the second group of calves post weaning. Weaning did not appear to have any influence on antimicrobial resistance in these isolates. Based on the results herein, it appears that diet has minimal effect on the prevalence or concentration of MDR EC and that they survive only in an immature digestive system with limited bacterial diversity. However, it cannot be ruled out that these bacteria could serve as a reservoir for resistance elements for pathogenic bacteria. Salmonella and MDR EC isolates largely shared the SSuT resistance phenotype, which may be an indication of resitance transfer among these two species.