MECHANISM OF NITROFURAN RESISTANCE IN SALMONELLA-ENTERITIDIS PHAGE TYPE-4 AND INTERPRETATION OF NITROFURAN SUSCEPTIBILITY TESTS

The mechanism of nitrofuran resistance in Salmonella enteritidis phage type 4 was studied. Nitrofuran reductase ativity was inversely related to the furazolidone MIC for the organism. Strains with low-level nitrofuran resistance, typically found in almost all isolates of S. enteritidis PT4, had intermediate nitrofuran reductase activity. Disc diffusion tests with furazolidone, 15 or 50 μg discs. and nitrofurantion, 50 or 300 μg discs, failed to distinguish reliably between susceptible popilations and those wiht low-level resistance. In order to detect low-level resistance to nitrofurans a dilution method should be used with a furazolidone breakpoint of 1 mg/l or a nitrofurantoin breakpoint of 16 mg/l. © 1991 The British Society for Antimicrobial Chemotherapy.