The objective of this study was to investigate the effect of the antimicrobial drugs (AMD) on the shedding of resistant Enterobacteriaceae in feces of pre-weaned dairy calves. The AMD considered were ceftiofur, administered parenterally, and neomycin sulfate added in milk replacer and fed to calves during the first 20 days of life. Fifty-five calves, aged one to three days, were enrolled and followed to 64 days. Fecal samples were collected three times/week and treatments recorded daily. Enterobacteriaceae were quantified for a subset of 33 calves using spiral plating on plain, ceftiofur supplemented, and neomycin supplemented MacConkey agar. Negative binomial models were used to predict the association between treatment with AMD and the gain and loss of Enterobacteriaceae resistance over time. Acquisition of resistance by the Enterobacteriaceae occurred during treatment and peaked between days three to four post-treatment before decreasing to below treatment levels at days seven to eight post-treatment. Acquisition of neomycin resistance was observed on the first sampling day (day four from the start of feeding medicated milk replacer) to day eight, followed by cyclical peaks until day 29, when the Enterobacteriaceae counts decreased below pre-treatment. Enterobacteriaceae resistance against both AMD increased after AMD administration and didn’t return to pre-therapeutic status until seven or more days after therapy had been discontinued. The study findings provide valuable insights into the dynamics of Enterobacteriaceae under routine AMD use in calves.

Three strains of lactic acid bacteria (LAB) and a commercial probiotic were selected to evaluate their in vitro activity towards Listeria monocytogenes. The strains Lactococcus lactis ssp. lactis, strain 340, L. lactis ssp. lactis, strain 16; Lactobacillus casei ssp. casei, strain 208 and Enterococcus faecium UBEF-41 were inoculated into skim milk and brain heart infusion broth (BHI) to get an initial Lactococcus: Lactobacillus: E. faecium UBEF-41 ratio of 2:1:1 and a concentration of approximately 7 log cfu mL À1 until challenge vs. pathogen. L. monocytogenes ATCC 7644 was also inoculated in same media to get approximately 4 log cfu mL À1 . Growth curves in skim milk and BHI at 4, 10 and 30 C, respectively were studied for: (i) LAB formulation; (ii) L. monocytogenes and (iii) LAB vs. L. monocytogenes. When challenged with LAB, at 30 C in milk, L. monocytogenes was not detectable after day-3 and in BHI it decreased below log cfu mL À1 after day-5. At 10 and 4 C, in both media, L. monocytogenes counts were always significantly lower (p < .001) than the counts of L. monocytogenes alone from day-2 for milk at 4 C and BHI at 10 C and from day-7 for BHI at 4C and milk at 10 C. In conclusion, the proposed formulation was able to limit L. monocytogenes in vitro growth, even at refrigeration temperature. ARTICLE HISTORY

Dairy farm use of antimicrobial drugs (AMD) is a risk for the selection of antimicrobial resistance (AMR); however, these resistance dynamics are not fully understood. A cohort study on two dairy farms enrolled 96 cows with their fecal samples collected three times weekly, for the first 60 days in milk. Enterobacteriaceae were enumerated by spiral plating samples onto MacConkey agar impregnated with 0, 1, 8, 16 and 30 µg/mL ceftiofur. Negative binomial regression analyzed AMR over time. The continuum of ceftiofur concentrations permitted estimation of the minimum inhibitory concentration (MIC) and analysis using interval regression. The most common systemic AMD was ceftiofur, administered in 94% of treatments (15/16 cows). Enterobacteriaceae did not grow in 88% of samples collected from non-AMD treated cows at 8 µg/mL ceftiofur. Samples from AMD treated cows had peak counts of resistant Enterobacteriaceae during AMD treatment and returned to baseline counts by 3–4 days post-treatment at 8 µg/mL. Sensitive Enterobacteriaceae (0–1 µg/mL ceftiofur) were reduced below pre-treated levels for 29–35 days post-AMD treatment. Population MIC peaked during AMD treatment and returned to baseline levels by 7–8 days. We conclude that the effect of systemic ceftiofur on the resistance of Enterobacteriaceae in early lactation dairy cows was limited in duration.