Rapid adaptation of Bacillus thuringiensis to alkaline environments via the L-lactate metabolism pathway regulated by the CRP/FNR family regulator LtmR

Bacillus thuringiensis (Bt) is an entomopathogenic bacterium. During infection, Bt often encounters alkaline conditions in the insect midgut and adaptation to this alkaline environment is crucial for its survival and establishment. Here, we investigated the mechanisms of rapid adaptation of Bt to an alkaline environment. DNA microarray revealed 739 downregulated genes and 662 upregulated genes in the presence of 28 mM NaOH for 10 min relative to the condition without alkaline treatment. The activities of some primary metabolic pathways of Bt were enhanced under alkaline conditions, and many genes related to synthesis and transportation of amino acids, nucleic acids, and cell surfaces were significantly induced. In particular, ldh2 (HD73_5189) and lpm1 (HD73_0686), which encode lactate dehydrogenase and lactate permease, respectively, were significantly upregulated. Transcription of ldh2 and lpm1 was directly regulated by the CRP/FNR family transcriptional regulator, LtmR (L-lactate transport and metabolism regulator), through binding between LtmR and their promoters. The intracellular concentration of pyruvate increased and that of lactate decreased under alkaline conditions. Following deletion of ldh2, the concentration of pyruvate decreased, and that of lactate increased, suggesting that ldh2 catalyzes the conversion of lactate to pyruvate. ltmR, ldh2, and lpm1 were shown to contribute to Bt virulence in Ostrinia furnacalis. Collectively, these data indicate that the pyruvate-L-lactate metabolic pathway is important for Bt adaptation and virulence in alkaline environments. Homologues of LtmR are highly conserved in B. cereus group, suggesting that they share a similar strategy for pH adaptation.