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 killing by higher concentrations of this oxidant. The sensitivity of R. leguminosarum to H2O2-mediated oxidative stress varied with the growth phase of the cultures. Stationary phase cells were many times more resistant to killing by 3 mM H2O2 than exponentially growing cultures. Unexpectedly, the catalase activity of cultures was found to rise to a maximum in the early-exponential growth phase and rapidly fall to a minimum during late-exponential growth. Further investigation showed that the induction and subsequent repression of catalase activity in exponential cultures is a cell-density-dependent phenomenon which appears to be controlled by the accumulation of extracellular compound(s) in the growth medium at high cell densities. In this respect, control of catalase in R. leguminosarum resembles a number of other cell-density-regulated phenomena in bacteria which are controlled by the accumulation of extracellular molecules: the best studied example of this quorum sensing is the control of bacterial bioluminescence by the lux autoinducer. Preliminary data indicated that this extracellular component is a non-proteinaceous, heat-stable molecule.