Excretion of Antibiotic Resistance Genes by Dairy Calves Fed Milk Replacers with Varying Doses of Antibiotics

Abstract

Elevated levels of antibiotic resistance genes (ARGs) in soil and water have been linked to livestock farms and in some cases feed antibiotics may select for antibiotic resistant gut microbiota. The purpose of this study was to examine the establishment of ARGs in the feces of calves receiving milk replacer containing no antibiotics versus subtherapeutic or therapeutic doses of tetracycline and neomycin. The effect of antibiotics on calf health was also of interest. Twenty-eight male and female dairy calves were assigned to one of the three antibiotic treatment groups at birth and fecal samples were collected at weeks 6, 7 (prior to weaning), and 12 (5 weeks after weaning). ARGs corresponding to the tetracycline (tetC, tetG, tetO, tetW, and tetX), macrolide (ermB, ermF), and sulfonamide (sul1, sul2) classes of antibiotics along with the class I integron gene, intI1, were monitored by quantitative polymerase chain reaction as potential indicators of direct selection, co-selection, or horizontal gene transfer of ARGs. Surprisingly, there was no significant effect of antibiotic treatment on the absolute abundance (gene copies per gram wet manure) of any of the ARGs except ermF, which was lower in the antibiotic-treated calf manure, presumably because a significant portion of host bacterial cells carrying ermF were not resistant to tetracycline or neomycin. However, relative abundance (gene copies normalized to 16S rRNA genes) of tetO was higher in calves fed the highest dose of antibiotic than in the other treatments. All genes, except tetC and intI1, were detectable in feces from 6 weeks onward, and tetW and tetG significantly increased (P < 0.10), even in control calves. Overall, the results provide new insight into the colonization of calf gut flora with ARGs in the early weeks. Although feed antibiotics exerted little effect on the ARGs monitored in this study, the fact that they also provided no health benefit suggests that the greater than conventional nutritional intake applied in this study overrides previously reported health benefits of antibiotics. The results suggest potential benefit of broader management strategies, and that cost and risk may be avoided by minimizing incorporation of antibiotics in milk replacer.

Management Options for Reducing the Release of Antibiotics and Antibiotic Resistance Genes to the Environment

“…Best management practices, such as low animal density and improved nutritional programs, can be developed and adopted to control infectious diseases on farms. In a recent study of antibiotic amendment in dairy calf milk replacer, subtherapeutic antibiotics provided no additional health benefit when the calves were provided a high level of nutrition (Thames et al 2012). In contrast, Quigley and Drew (2000) observed that calves experienced greater incidence of illness when antibiotics were not supplemented, but the calves in their study received a reduced nutritional intake.…”

Antimicrobial‐Resistance Genetic Markers in Potentially Pathogenic Gram Positive Cocci Isolated from Brazilian Soft Cheese

“…With regards to aminoglycoside use in the Brazilian geographical regions from where the cheese samples were manufactured and acquired, no information was retrieved. Cattle farming is associated with a reservoir for potentially pathogenic and drug‐resistant bacteria or even a reservoir of antibiotic resistance genetic markers (Munir and Xagoraraki ; Thames and others ). In general, antimicrobial drugs are widely applied in animal husbandry in an attempt to increase production, related to prophylaxis, infectious disease treatment and/or growth promoters (Heuer and others ).…”

Molecular Methods for Assessment of Antibiotic Resistance in Agricultural Ecosystems: Prospects and Challenges

“…This approach has also provided strong evidence for an increased prevalence in antibiotic resistance associated with agricultural antibiotic use (e.g., Jindal et al, 2006;Peak et al, 2007;Zhou et al, 2009;McKinney et al, 2010). Limitations associated with this approach include variations across agricultural practices that can make it difficult to isolate the specific variable of antibiotic use, evidence that antibiotics (Zhou et al, 2009) and antibiotic-resistant bacteria may be present in the absence of use (Thames et al, 2012), and therapeutic antibiotic use for ill animals on organic farms (McKinney et al, 2010).…”