Susceptibility of clostridia from farm animals to 21 antimicrobial agents including some used for growth promotion

The minimal inhibitory concentrations of 21 antimicrobial agents were determined by an agar dilution method against 68 strains of 18 Clostridium species isolated from caeca of pigs, cattle and poultry. Of the therapeutically used antibiotics, chloramphenicol was most active in vitro followed by penicillin G, the lincosamides and tetracycline. Penicillin-resistant Cl. butyricum strains produced a beta-lactamase. Avoparcin, carbadox, monensin, nitrovin and virginiamycin were the most effective of the growth promoting antimicrobial agents. Many clostridial species were naturally resistant to flavomycin and Cl. sporogenes was naturally resistant additionally to bacitracin, tiamulin, the lincosamides and virginiamycin component M. The macrolide, lincosamide and streptogramin resistant strains showed four different patterns of resistance to the antibiotics. One of these resistance patterns was the natural resistance of Cl. sporogenes to the lincosamides and virginiamycin component M which is a streptogramin group A antibiotic.