Bacterial flora of liver abscesses in feedlot cattle fed tylosin or no tylosin

Bacterial flora of liver abscesses from cattle fed tylosin or no tylosin an d susceptibilities of the predominant bacterial isolates to tylosin and oth er antimicrobial compounds were determined. Abscessed livers were collected at slaughter from cattle originating from feedlots that had fed tylosin (n = 36) or no tylosin (n = 41) for at least 2 yr, and segments of livers wit h one or two intact abscesses were transported to the laboratory. Abscesses were cultured for anaerobic and facultative bacteria. Fusobacterium necrop horum, either as single culture or mixed with other bacteria, was isolated from all abscesses. The incidence of subsp. necrophorum, as part of the mix ed infection, was lower (P < .05) in the tylosin group than in the no-tylos in group (33 vs 61%). However, the incidence of Actinomyces pyogenes was hi gher (P < .01) in the tylosin group than in the no-tylosin group (53 vs 10% ). Totals of 119 F. necrophorum and 21 A. pyogenes isolates were used for d eterminations of susceptibilities to bacitracin, oxytetracycline, chlortetr acycline, lasalocid, monensin, tylosin, tilmicosin, and virginiamycin. The minimum inhibitory concentrations (MIC) of antibiotics were determined with a broth microdilution method. The mean MIC of tylosin for F. necrophorum a nd A. pyogenes were not different between isolates from tylosin and no-tylo sin groups. We concluded that continuous feeding of tylosin did not induce resistance in F. necrophorum or A. pyogenes. Also, the higher incidence of mixed infection of F: necrophorum and A. pyogenes in liver abscesses of tyl osin-fed cattle suggests a potential synergistic interaction between the tw o organisms in causing liver abscesses.