Course of antibiotic sensitivities in Escherichia coli and Staphylococcus aureus from animals.

In order to establish the evolution of resistance against the common antibiotics from the beginning of their use up to the present time, the sensitivity of Escherichia coli isolated from the cecum of wild boars was determined. The MIC's of E. coli from these animals in a zoological garden was examined for comparison. The samples from the wild boars were collected in five forests in the Belgian Ardennes. The MIC's obtained (Table 1) are comparable to the mean values given in the early literature concerning strains of human origin (Table 7). E. coli from wild boars in a zoological garden has less sensitivity than from domesticated animals. However, the MIC's of staphylococci from fecal matter of the animals in captivity were not markedly enhanced except for streptomycin (Table 3). It was striking to find that staphylococci were absent in all the fecal samples from the wild boars living in their natural environment. The specific habitat of staphylococci is in man. Wild boars are the ancestors of domestic pigs. The MIC's of E. coli from the latter were determined for evaluation of altered resistance. All isolated strains were fairly resistant to the six substances tested (Table 2). Chloramphenicol was still the most active antibiotic. Staphylococci from some domestic animals (pigs, cattle, chicken) were sensitive to all the substances tested except the aminoglucosides and ampicillin (Table 3). E. coli from pigs bred on large farms in Spain (Toledo and Badajoz Provinces) were also highly resistant (Table 4) to five compounds and sensitive to cephalotin, polymyxin B, furoxone, and Carbadox. The samples obtained from family farms also contained unsensitive strains (Avila Province). These animals had never been given antibacterial feed additives as was confirmed by examination of feed samples (Table 6). Staphylococci when present in the Spanish samples can be considered to be antibiotic sensitive with the same exceptions as in the Belgian samples and also unsensitive to chloramphenicol. The use of antibiotics in animal feed was very common in Spain as was shown by feed analysis (Table 6). The risk of transferable resistance by plasmids between animal species must not be very high. In some cases where resistance formation could be mediated by R-factors, sensitive E. coli or staphylococci were isolated. Resistance was not generalized among animals from the same farm or in adjacent sties. No explanation for this discrepancy was found.