Properties of ionophore-resistant Bacteroides ruminicola enriched by cultivation in the presence of tetronasin

Bacteroides ruminicola M384 was grown in the presence of increasing concentrations of tetronasin, an ionophore that has been developed as a feed additive for ruminants. The resulting culture, B. ruminicola M384/Tn(R), was then maintained in medium containing 0.1 µg tetronasin/ml. Growth of the parent strain was eliminated by the addition of 0.1 µg tetronasin/ml, but the growth rate of B. ruminicola M384/Tn(R), which grew more slowly than the parent strain, was unaffected by adding tetronasin. Bacteroides ruminicola M384/Tn(R) retained its resistance to tetronasin even after repeated subculture in the absence of the ionophore, suggesting that a mutation had occurred. The absence of plasmids in individual colonies of B. ruminicola M384/Tn(R) implied that the mutation was chromosomal. Bacteroides ruminicola M384/Tn(R) was also more resistant to the ionophores monensin and lasalocid and, to a lesser degree, to the antibiotic avoparcin than B. ruminicola M384. Binding of [C]tetronasin to B. ruminicola M384/Tn(R) was lower than binding of the ionophore to the parent stain, and this difference was eliminated by washing cells with EDTA. The peptidolytic activity of B. ruminicola M384 towards triphenylalanine (M(r) = 460) was unaffected in B. ruminicola M384/Tn(R), but the rate of breakdown of tetra-phenylalanine (M(r) = 607) was decreased. This difference was also abolished by EDTA. It was concluded that growth of B. ruminicola in the presence of tetronasin resulted in a mutation affecting the permeability of the cell envelope, such that permeation of tetronasin and molecules of a similar size (M(r) = 628) was decreased.