Cell-density-dependent lysis and sporulation of Myxococcus xanthus in agarose microbeads

Vegetative cells of Myxococcus xanthus were immobilized in 25-microns-diameter agarose microbeads and incubated in either growth medium or sporulation buffer. In growth medium, the cells multiplied, glided to the periphery, and then filled the beads. In sporulation buffer, up to 90% of the cells lysed and ca. 50% of the surviving cells formed resistant spores. A strong correlation between sporulation and cell lysis was observed; both phenomena were cell density dependent. Sporulation proficiency was a function of the average number of cells within the bead at the time that sporulation conditions were imposed. A minimum of ca. 4 cells per microbead was necessary for efficient lysis and sporulation to proceed. Increasing this number accelerated the lysis and sporulation process. No lysis occurred when an average of 0.4 cell was entrapped per bead. Entrapping an average of 1.7 cells per bead resulted in 46% lysis and 3% sporulation of survivors, whereas entrapping an average of 4.2 cells per bead yielded 82% lysis and 44% sporulation of the surviving cells. Sporulation and lysis also depended upon the cell density in the culture as a whole. The existence of these two independent cell density parameters (cells per bead and cells per milliliter) suggests that at least two separate cell density signals play a role in controlling sporulation in M. xanthus.