Influence of Calcium Concentration on the Antimicrobial Activity of Lasalocid against Selenomonas ruminantium

Abstract

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 μM, 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 μM 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.