Influence of calcium concentration on the antimicrobial activity of lasalocid against Selenomonas ruminantium

The present study was designed to examine the interaction between the effects of lasalocid and Ca2+ on the growth and structure of Selenomonas ruminantium HD-4. Lasalocid, at a dose of 10 microM, inhibited cell growth almost completely after 12 hr incubation in the presence of relatively high extracellular concentrations of Ca2+ (from 5 to 50 mM), but only slightly reduced cell growth in the presence of 0.2 mM Ca2+. With Ca2+ alone, cell growth was also inhibited at 12 hr as a function of the concentration of Ca2+ over the range 5 to 50 mM. In cultures at mid-exponential phase, growth was markedly inhibited by the simultaneous addition of 10 microM lasalocid and 15 mM Ca2+, but only moderately inhibited by lasalocid in the absence of Ca2+. However, there was no significant effect on bacterial growth at the mid-exponential phase when Ca2+ alone was added to the incubation medium. In thin sections of cells treated with lasalocid in the presence of 15 mM Ca2+, abnormal cells were found with cytoplasmic voids and with an outer membrane detached from the inner membrane layer, this change in the outer membrane was also found in cells treated with lasalocid alone. There was no visible abnormality in the outer membrane in thin sections of Ca2(+)-treated cells, while most of these cells showed only a slight contraction of cytoplasmic material or a loss of cytoplasmic contents. These results indicate the presence of a synergistic effect between the actions of lasalocid and Ca2+ on cell growth, presumably due to induced cytoplasmic alterations.