CELL-DENSITY REGULATES CELLULAR REVERSAL FREQUENCY IN MYXOCOCCUS-XANTHUS

Myxococcus xanthus is a Gram-negative bacterium that aggregates to for m fruiting bodies when nutrients are limiting. Previous studies showed that the frz mutants that are defective in chemotaxis exhibited irreg ular and infrequent patterns of cellular reversal. In contrast, wild-t ype cells, when examined individually, reverse relatively frequently, about once every 6 min. It is not known how the change of reversal fre quency effects cellular aggregation during fruiting body formation in M. xanthus. In this study, we stained cells with a tetrazolium dye so that we could track the reversal frequencies of single cells and cells in groups. We found that developmental cells in large groups reverse much less than cells in small groups or as single cells, This reduced cellular reversal frequency is related to the frz signal transduction system and correlated with the methylation of FrzCD (a methyl-acceptin g chemotaxis protein). Cells containing a mutation in the frz genes or in the genes required for social motility do not respond in this way. The reduction in cellular reversals as developmental cells accumulate in groups suggests a simple hypothesis for the aggregation of cells i nto discrete mounds during fruiting body formation. We also found that M. xanthus cells glide with equal frequency in the forward or reverse directions, indicating that cells do not contain a ''head'' or ''tail .''