Antimicrobial activity of 2-aminophenoxazin-3-one under anaerobic conditions

Potential toxicity of 2-aminophenoxazin-3-one to 20 bacterial species and two fungi and its inactivation under anaerobic conditions were investigated. Minimum inhibitory concentration for cellulolytic bacteria was in the range of 50-100 micrograms.mL-1, but at 100 micrograms.mL-1 of 2-aminophenoxazin-3-one, there was no effect on the growth of any of the noncellulolytic bacteria. Four noncellulolytic bacterial strains showed no inhibition of growth, even at 200 micrograms.mL-1 of this compound. Under anaerobic conditions and in the presence of cysteine, the long wavelength absorption band of this compound slowly shifted from about 434 to 320 nm, and its inhibitory effect on RNA synthesis was relieved after one-half of a generation time in cultures of cellulolytic ruminal bacteria. A similar shift of absorption band was observed in rumen fluid filtered through a 0.22-microns Millipore filter. It was concluded that protonation of 2-aminophenoxazin-3-one under anaerobic conditions present in the rumen would considerably reduce its potential toxicity to cellulolytic bacteria.