Mutation to acriflavine resistance in some myxobacteria

In contrast to other Gram-negative bacteria, several species of fruiting Myxobacteria were found to be extremely sensitive to acriflavine. Depending on the species, acriflavine concentrations of 0.05 to 0.5 micrograms affected growth rate and viability. Spontaneous first-step mutants resistant to acriflavine were isolated. Most of these mutants were cross-resistant to both proflavine and ethidium bromide. Some mutants, however, were not cross-resistant to the latter dye. In two species it was possible to select second-step mutants showing increased resistance to acriflavine. First-step acriflavine-resistant mutants, not cross-resistant to ethidium bromide, acquired cross-resistance to this latter dye as a result of the second mutation. However, in some mutants the second mutation enhancing resistance to acriflavine partially abolished resistance to ethidium bromide. The mechanisms of resistance are not yet known. Resistance in one mutant of a Cystobacter strain is probably not caused by inhibition of acriflavine uptake into the cell. It is possible that more than one gene could be responsible for acriflavine resistance, and that the acriflavine sensitivity of wild-type strains may indicate a higher permeability of the outer membrane of Myxobacteria compared with other Gram-negative bacteria. This high permeability could be a prerequisite for the cell-cell interactions necessary for gliding motility and morphogenesis.