Antimicrobial compounds were screened in vitro in
Trypticase soy broth for antimicrobial activity against a virulent strain of Salmonella enteritidis. Of the several compounds tested,
polymyxin B showed the strongest inhibition in vitro, preventing growth at a concentration of less than or equal to 10 micrograms/ml.
Polymyxin B administered in the
drinking water was effective in vivo for preventing
infections in 1-day-old chickens but did not remove established
infections in 1-week-old chickens. It was found that
trimethoprim, which was not active in vitro, prevented colonization and removed existing
infections in 1-day-old chickens when it was administered together with
polymyxin B sulfate. Enrichment cultures in which
selenite-
cystine and tetrathionate broth media were used showed that chickens given a combination of 100 micrograms of
polymyxin B sulfate per ml and 250 micrograms of
trimethoprim per ml 24 h prior to oral inoculation with 10(8) to 10(9) CFU were negative for S. enteritidis after 7 days. Established
infections (10(5) to 10(6) CFU/g of feces) in 1-week-old chickens were eliminated by treatment with the
polymyxin-
trimethoprim system. This
antimicrobial agent treatment may be useful for preventing colonization in poultry and for eliminating S. enteritidis from infected flocks.