CHARACTERIZATION OF ANTIMICROBIAL RESISTANCE AND BACTERIAL DIVERSITY IN THE GASTROINTESTINAL AND RESPIRATORY TRACT OF CALVES FED DIFFERENT TYPES OF MILK

Abstract

The use of waste milk produced by cows treated with antimicrobials to feed dairy calves is a frequent practice among dairy operators.In this thesis, three studies were conducted to evaluate the effects of feeding waste milk to dairy calves on the development of antimicrobial resistant bacteria and on microbiotaof both, the gastrointestinal and respiratory tract. In the first study antimicrobial resistance profiles in fecal Escherichia coli and nasal Pasteurella multocida isolated from Holstein dairy calves fed waste milk or milk replacer were compared. Feeding waste milk to dairy calves increased the prevalence of resistant fecal E. coli and nasal P. multocidato some of the antimicrobials tested. However, similar profiles of antimicrobial resistance among colonies of E. coli isolated from environmental samples and calves at birth and at 6 week of age suggested other factors, such as environmental contamination, in the acquisition of resistant bacteria. Furthermore, differences on the probabilities to isolate resistant E. coli were also observed depending on calf age. In the second and third study, pasteurized waste milk containing beta-lactam antimicrobial residues was used to feed Holstein dairy calves. Antimicrobial resistance in fecal E. coli isolates was evaluated at both, phenotypic and genotypic level, before and after weaning. Feeding pasteurized waste milk to calves increased the prevalence of phenotypic resistant E. coli to different beta-lactam antimicrobials, as well as the probability to isolate E. coli colonies carrying the cephalosporin resistance gene blaCMY-2. However, once calves were weaned, the prevalence of resistant E. coli to beta-lactams decreased in calves fed pasteurized waste milk, although it continued being greater in such calves than in those fed milk replacer._x000D_ Regarding microbiota of both, the gastrointestinal and respiratory tract of calves, neither diversity nor richness of bacterial populations was affected by the type of milk ingested. However, the analysis of phylogenetic distances among bacteria communities indicated significant differences between feeding regimes in nasal and fecal microbiota, suggesting an effect of the presence of antimicrobial residues or other substances in waster milk but not in milk replacers. On the other hand, feeding pasteurized waste milk to calves also affected taxonomic composition of nasal bacterial communities but nor those from the gastrointestinal tract._x000D_ In conclusion, feeding dairy calves waste milk triggers the presence of resistant bacteria in both, the gastrointestinal and respiratory tract of dairy calves, as well as it influences the structure of bacterial communities colonizing these body regions.

Date of Award	10 Nov 2017
Original language	English
Supervisor	Marta Terre Trulla (Director)