Abstract

Antimicrobial chemotherapy in human and veterinary medicine is one of the most important therapeutic tool against pathogenic agents causing infectious diseases; nevertheless, the development of multiple resistant strains during the last years has been reported. Some of the measures adopted to control this problem have been the veterinary prescription of antimicrobials for animal use, the permanent rotation of drugs, and the implementation of continuous monitoring programs for bacterial resistance. In the present paper the sensitivity of pathogenic bacteria isolated from dairy cows suffering mastitis in different regions of Chile against antimicrobials most frequently used in dairy herds is reported. The Plate Dilution Method and the Minimum Inhibitory Concentration (MIC) were used to evaluate the bacterial resistance of each isolated strain. A total of 449 bacterial strains were isolated from 963 aseptically collected milk samples in the 5th and Metropolitan Regions of Chile, E. coli being the most frequent mastitis pathogen recovered. In the 10th Region, however, S. aureus was the main pathogen among 1012 bacterial strains isolated from 2000 milk samples. S. aureus, Streptococcus spp and coagulase-negative Staphilococcus (CNS) were shown to be highly resistant to amoxicilin, ampicillin, penicillin, streptomycin and lincomicyn. Resistance to cloxacilin of S. aureus strains isolated in the 5th-Metropolitan and 10th Regions were 6.2% and 3.7% respectively. On the other hand, a high rate of sensitivity was observed in E. coli with resistance values below 25%. According to these results it is possible to conclude that the mastitis pathogens present in the geographical regions under study are resistant to more than one antimicrobial drug and, therefore, usage of these drugs under medical prescription and implementation of permanent monitoring programs for bacterial resistance are strongly recommended.