Susceptibility of fecal streptococci of poultry origin to nine growth-promoting agents.

The minimal inhibitory concentrations of nine growth-promoting agents were determined by an agar-dilution method against 66 bile-tolerant streptococcal (8 Streptococcus faecalis, 23 Streptococcus faecalis subsp. liquefaciens, 15 Streptococcus faecium, and 20 carboxyphilic streptococci) strains isolated from the ceca of 52 chickens on 19 farms. Avoparcin was equally active on all groups. The natural susceptibilities against the other substances differed among the groups studied. Bacitracin and virginiamycin were more active on S. faecium and S. faecalis than on S. faecalis subsp. liquefaciens; lincomycin and the macrolide antibiotics were more active on S. faecium than on the other groups; and flavomycin was active on all groups except S. faecium. High percentages of acquired resistance were noted in all groups against bacitracin, lincomycin, and the macrolide antibiotics, oleandomycin, spiramycin, and tylosin. Resistance to nitrovin was found only among the S. faecalis and S. faecium groups.