Fungistatic and fungicidal effects of the ionophores monensin and tetronasin on the rumen fungus Neocallimastix sp. LM1

The ionophore antibiotics monensin and tetronasin have been reported to inhibit anaerobic fungi in vitro, and are suitable for animal use. In this study, their effectiveness in removing the anaerobic fungus Neocallimastix sp. LM1 from the rumen was investigated in vitro. Both antibiotics were fungistatic: tetronasin at 0.5 microgram/ml and monensin at 1.0 microgram/ml; exposure for 24 h did not inhibit subsequent growth after removal of the ionophore. The ionophores were fungicidal at much higher concentrations, 1 microgram/ml for tetronasin and 16 micrograms/ml for monensin. It seems likely that the combination of relatively high inhibitory dose and the fungistatic nature of monensin would explain difficulties in using this compound to eliminate anaerobic fungi from the rumens of experimental animals.