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 Selenomon as ruminantium HD-4. Lasalocid, at a dose of 10 mu M, inhibited cell g rowth almost completely after 12 hr incubation in the presence of rela tively high extracellular concentrations of Ca2+ (from 5 to 50 mM), bu t only slightly reduced cell growth in the presence of 0.2 mM Ca2+. Wi th Ca2+ alone, cell growth was also inhibited at 12 hr as a function o f the concentration of Ca2+ over the range 5 to 50 mM. In cultures at mid-exponential phase, growth was markedly inhibited by the simultaneo us addition of 10 mu M lasalocid and 15 mM Ca2+, but only moderately i nhibited by lasalocid in the absence of Ca2+. However, there was no si gnificant effect on bacterial growth at the mid-exponential phase when Ca2+ alone was added to the incubation medium. In thin sections of ce lls treated with lasalocid in the presence of 15 mM Ca2+, abnormal cel ls were found with cytoplasmic voids and with an outer membrane detach ed from the inner membrane layer; this change in the outer membrane wa s also found in cells treated with lasalocid alone. There was no visib le abnormality in the outer membrane in thin sections of Ca2+-treated cells, while most of these cells showed only a slight contraction of c ytoplasmic material or a loss of cytoplasmic contents. These results i ndicate the presence of a synergistic effect between the actions of la salocid and Ca2+ on cell growth, presumably due to induced cytoplasmic alterations.