EVIDENCE FOR CELL-DENSITY-DEPENDENT REGULATION OF CATALASE ACTIVITY IN RHIZOBIUM-LEGUMINOSARUM BY. PHASEOLI

Pretreatment of Rhizobium leguminosarum by. phaseoli cultures with low , non-lethal levels of H2O2 led to them becoming more resistant to kil ling by higher concentrations of this oxidant. The sensitivity of R. l eguminosarum to H2O2-mediated oxidative stress varied with the growth phase of the cultures. Stationary phase cells were many times more res istant to killing by 3 mM H2O2 than exponentially growing cultures. Un expectedly, the catalase activity of cultures was found to rise to a m aximum in the early-exponential growth phase and rapidly fall to a min imum during late-exponential growth. Further investigation showed that the induction and subsequent repression of catalase activity in expon ential cultures is a cell-density-dependent phenomenon which appears t o be controlled by the accumulation of extracellular compound(s) in th e growth medium at high cell densities. In this respect, control of ca talase in R. leguminosarum resembles a number of other cell-density-re gulated phenomena in bacteria which are controlled by the accumulation of extracellular molecules: the best studied example of this quorum s ensing is the control of bacterial bioluminescence by the lux autoindu cer. Preliminary data indicated that this extracellular component is a non-proteinaceous, heat-stable molecule.