Bistability in myo-Inositol Utilization by Salmonella enterica Serovar Typhimurium

The capability of Salmonella enterica serovar Typhimurium strain 14028 (S. Typhimurium 14028) to utilize myo-inositol (MI) is determined by the genomic island GEI4417/4436 carrying the iol genes that encode enzymes, transporters, and a repressor responsible for the MI catabolic pathway. In contrast to all bacteria investigated thus far, S. Typhimurium 14028 growing on MI as the sole carbon source is characterized by a remarkable long lag phase of 40 to 60 h. We report here that on solid medium with MI as the sole carbon source, this human pathogen exhibits a bistable phenotype characterized by a dissection into large colonies and a slow-growing bacterial background. This heterogeneity is reversible and therefore not caused by mutation, and it is not observed in the absence of the iol gene repressor IolR nor in the presence of at least 0.55% CO2. Bistability is correlated with the activity of the iolE promoter (P-iolE), but not of P-iolC or P-iolD, as shown by promoter-gfp fusions. On the single-cell level, fluorescence microscopy and flow cytometry analysis revealed a gradual switch of P-iolE from the "off" to the "on" status during the late lag phase and the transition to the log phase. Deletion of iolR or the addition of 0.1% NaHCO3 induced an early growth start of S. Typhimurium 14028 in minimal medium with MI. The addition of ethoxyzolamide, an inhibitor of carboanhydrases, elongated the lag phase in the presence of bicarbonate. The positive-feedback loop via repressor release and positive induction by bicarbonate-CO2 might allow S. Typhimurium 14028 to adapt to rapidly changing environments. The phenomenon described here is a novel example of bistability in substrate degradation, and, to our knowledge, is the first demonstration of gene regulation by bicarbonate-CO2 in Salmonella.