Phenotypic Characterization of Null Mutants of ΔcbpA and ΔcbpB in S. enterica Serovar Typhimurium

The phenotypic characteristics of the ΔcbpA and ΔcbpB mutants of Salmonella enterica serovar Typhimurium were comprehensively analyzed to elucidate the roles of these nucleoid-associated proteins in metabolism and stress responses. Both ΔcbpA and ΔcbpB mutants exhibited enhanced growth on carbon sources such as ribose, mannose, fructose, glycerol, galactose and sucrose compared to the wild-type strain. Conversely, the wild-type strain demonstrated superior growth on xylose. Notably, the ΔcbpB mutant experienced a prolonged lag phase of ~ 20 h when cultured in M9 medium supplemented with maltose. In M9 medium where alanine or arginine served as the sole nitrogen source, both mutants outperformed the wild-type strain in growth, indicating potential alterations in nitrogen metabolism pathways. The ΔcbpA and ΔcbpB mutants were non-motile and exhibited reduced cell sizes compared to the wild-type. Specifically, the ΔcbpA mutant was approximately 0.8 times smaller than the size of the wild-type, while the ΔcbpB mutant was about 1.3 times smaller. The D₁₀ value, representing the radiation dose required to reduce the bacterial population by 90%, was 113 Gy for the wild-type, 140 Gy for ΔcbpA and 104 Gy for ΔcbpB, suggesting differential radiation sensitivities among the strains. Upon heat exposure, the ΔcbpA mutant exhibited a fivefold reduction in viable cell number compared to the wild-type and ΔcbpB, indicating increased heat sensitivity. No significant differences were observed among the strains in response to oxidative stress induced by hydrogen peroxide. After 72 h in 15% salt conditions, the wild-type strain showed a fivefold reduction in cell numbers, whereas both mutants exhibited only a fourfold reduction, suggesting enhanced salt tolerance. These findings underscore the pivotal roles of CbpA and CbpB proteins in regulating metabolic pathways and mediating stress responses in S. Typhimurium.