Antibiotic-resistant gram-negative bacilli in the sewage of the City of Concepcion, Chile.

Counts of total and of antibiotic-resistant heterotrophic Gram-negative bacteria were performed in samples collected from the main sewage effluent of the city of Concepcion, Chile, during an eleven month period in 1991-1992. Antibiotic resistance patterns (ARP) were determined and antibiotic resistance indexes (ARI) were calculated for some fermenting and nonfermenting strains of resistant bacteria. Ampicillin-resistant strains were the most frequent among fermenting bacteria, whereas those cephalothin-resistant were the most frequent among nonfermenting bacilli. These results correlate with the frequency of clinical use of both antibiotics in the community of Concepción. Higher values of ARI for nonfermenting bacilli are probably due to the natural intrinsic resistance of these microorganisms to several beta-lactam compounds. Since plasmid-coded resistance is frequent among enteric bacilli, the large number of resistant bacilli in sewage may be important in the natural dis-