Effects of antimicrobial agents fed to chickens on some gram-negative enteric bacilli

Total and antimicrobial agent-resistant aerobic and facultative anaerobic gram-negative enteric bacilli in fecal samples of broiler chickens fed growth-promotional levels of antimicrobial agents were determined quantitatively. Two 8-week studies were conducted utilizing groups of chickens fed antimicrobial-supplemented rations; the second study involved feed "pasteurization" as a means of minimizing colonization from the feed. Dilution/spread-plating/replica-plating techniques on selective media were used to obtain counts of total organisms and those resistant to tetracycline, chloramphenicol, streptomycin, ampicillin, or kanamycin. The predominant aerobic and facultative anaerobic gram-negative organism was Escherichia coli, which was detected in all samples at levels ranging from 10(5) to over 10(10) CFU/g of feces. Less common were Proteus mirabilis, Klebsiella pneumoniae, and Pseudomonas sp., which varied in occurrence and levels from group to group (range, less than 10(3) to 10(8) CFU/g). Resistance to all antimicrobials (except chloramphenicol in E. coli) was commonly observed at incidences exceeding 10(3) CFU/g in the total populations. Colonization of the chickens' intestinal tracts by susceptible and resistant strains of E. coli, K. pneumoniae, and Pseudomonas sp. appeared to result from their presence in the environment of the newly hatched chickens. Ration pasteurization did affect P. mirabilis, which appeared to colonize from the feed. The results suggest that colonization by, and proliferation of, antimicrobial-resistant aerobic and facultative anaerobic gram-negative enteric bacilli in chicken intestinal tracts may be less dependent on selection through antimicrobial supplementation of the ration than on their prevalence in environments from which they can colonize newborns.