The effect of parenteral penicillin treatment on the intestinal microbiota was determined
by monitoring the phenotypic antimicrobial resistance among Escherichia coli in 19 calves
(15 calves received treatment and four calves were healthy controls) and by examining
changes in the fecal microbial community structure using molecular fingerprinting
techniques in a subset of eight calves (five treated calves and three control calves). After
five days of penicillin treatment an increased resistance to multiple unrelated
antimicrobial agents, including non-b-lactams, was seen in E. coli from treated calves,
and this was not seen in the controls. Automated ribosomal intergenic spacer analysis
(ARISA) and terminal restriction fragment length polymorphism (TRFLP) revealed that
penicillin treatment causes a significant variation in the microbial structure within an
individual calf. The study shows that parenteral administration of penicillin has an impact
on the composition of the fecal microbiota in calves, and on the antimicrobial resistance
pattern of their fecal E. coli.