الفهرس 
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Abstract
It is a common practice to feed milk of dairy cattle treated with antibiotics to calves on the farm. It is therefore expected for the animals’ faecal flora to be subjected to sub-inhibitory concentrations of these antibiotics for an extended period of time.
Since the information available on this matter is limited, it has been decided to carry out a study on the influence of such practices, on the possibility of induction of resistance in commensal faecal flora both in vitro and in vivo. Prior to that, antibiotic resistance patterns of indicator organisms (Enteroccus spp. and Escherichia coli) isolated and identified from different origins, to various antimicrobials were determined. Bulk tank milk, teat swab, and dairy cattle faecal samples were collected from different dairy farms in the northern part of Germany.
The resistance pattern differed – as expected – between enterococci and E. coli isolates. In general, indicator bacteria either enterococci or E. coli were found to be not only resistant but multi-drug resistant to at least two different antibiotics with different antibiotics resistance patterns. Strains isolated from milk regardless of their species showed resistance to most of the antibiotics tested. This probably imposes a humam health hazard due to the possible transmission of resistant strains from animals to the food chain.
Induction of resistance in vitro using sub-inhibitory doses of selected antibiotics in enterococci and E. coli strains was possible at variable extents and increase of resistance differed within strains and antibiotics used. On the other hand, resistance of E. coli faecal flora in calves by feeding them sub-inhibitory concentrations of ampicillin and ciprofloxacin was difficult to detect but could not be definitely excluded.As a conclusive answer for the practice of feeding calves antibiotic containing milk couldn’t be fully elaborated, more experiments including genetic tracing of strains would be helpful.