ANTIMICROBIAL ACTIVITY OF LASALOCID AGAINST SELENOMONAS-RUMINANTIUM -EFFECT OF CHANGES IN PH INDUCED BY CHANGING GLUCOSE-CONCENTRATION

A significant decrease in pH occurred in the culture medium when cells of Selenomonas ruminantium HD-4 were incubated in the presence of rel atively high concentrations of glucose (0.4 and 1.0%). Forty mu M lasa locid reduced cell growth to 35.5 and 35.7% of control growth, respect ively, for 0.05 and 0.4% glucose, while growth was completely inhibite d by 40 mu M lasalocid in the presence of 1.0% glucose. In the presenc e of 80 mu M lasalocid, cells were unable to grow within 24 hr at any glucose concentration. In the case of treatment with 10 mu M lasalocid , growth was inhibited when pH decreased below 6.0 in the presence of 0.4 and 1.0% glucose, With 0.05% glucose, pH never reduced to below 6. 4 within 24 hr and there was no significant effect of lasalocid on eit her growth or pH. When cultures were treated with 10 mu M lasalocid at the mid-exponential phase, growth was moderately inhibited and abnorm al cells which had outer membrane detached from the inner membrane lay er resulting in cell lysis were found in thin sections. These results indicate that the presence of large amounts of glucose decreases pH in culture medium and that the decrease in pH to below about 6.0 causes the increased effect of lasalocid on the growth of S. ruminantium. The se results suggest that the antimicrobial activity of lasalocid is pot entiated by a decrease in extracellular pH.