Furazolidone resistance in Salmonella gallinarum: the relationship between in vitro and in vivo determinations of resistance.

Of 22 strains of Salmonella gallinarum isolated from recent outbreaks of infection in poultry in Greece (15). Amman (3), Kenya (2), Lebanon (1) and Yemen (1), 20 were more resistant to furazolidone in vitro than 6 strains that had been isolated in the U.K. in the 1950s; the minimum inhibitory concentration of furazolidone was approximately 0.3 microgram/ml for the sensitive strains and 1.3 or 2.5 microgram/ml for the more resistant strains. Furazolidone given continuously in the food did not control experimental infections in chickens caused by most of the strains that had been classed as more resistant by the in vitro tests. Chloramphenicol, trimethoprim and sulphadiazine or mixtures of the latter two were the best antibiotics for treating these infections, but they were less satisfactory than furazolidone for treating infections caused by the furazolidone-sensitive strains. As a group, the furazolidone-resistant strains and furazolidone-resistant mutants of one of the sensitive strains were less virulent for chickens than the sensitive strains.