Evidence for cell-density-dependent regulation of catalase activity in Rhizobium leguminosarum bv. phaseoli

Pretreatment of Rhizobium leguminosarum bv. 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 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.