Acid tolerance of Lactobacillus spp. on root carious lesions: a complex and multifaceted response

Lactobacillus spp. are acidogenic and aciduric bacteria and are among the main cariogenic microorganisms associated with the carious process. Objective: This study aimed to identify genes involved in the acid-tolerance of Lactobacillus spp. and potential functions attributed to these genes within the metatranscriptome of sound root surfaces and carious root surfaces. Design: Genomic libraries were built from mRNA isolated from the biofilm samples (10 from sound root and 9 from carious root using Illumina HiSeq 2500). Reads generated by RNA-seq were mapped against 162 oral microbial genomes and genes potentially related to acid tolerance were manually extracted from the Lactobacillus spp. genomes using L. paracasei ATCC 344 as reference genome. The R package DESeq2 was used to calculate the level of differential gene expression between those two clinical conditions. Results: Fifteen Lactobacillus spp. genomes were identified and a total of 653 acid tolerance genes were overexpressed in carious root surfaces. Multiple functions, as translation, ribosomal structure and biogenesis, transport of nucleotides and amino acids, are involved in Lactobacillus spp. acid tolerance. Species-specific functions also seem to be related to the fitness of Lactobacillus spp. in acidified environments such as that of the cariogenic biofilm associated with carious root lesions. Conclusions: The response of Lactobacillus spp. to an acidic environment is complex and multifaceted. This finding suggests several possible avenues for further research into the adaptive mechanisms of these bacteria.