Contrasting genes conferring short and long-term biofilm adaptation in Listeria

Listeria monocytogenes is an opportunistic food-borne bacterium that is capable of infecting humans with high rates of hospitalisation and mortality. Natural populations are genotypically and phenotypically variable, with some lineages being responsible for most human infections. The success of L. monocytogenes is linked to its capacity to persist on food and in the environment. Biofilms are an important feature that allow these bacteria to persist and infect humans, therefore, understanding the genetic basis of biofilm formation is key to understanding transmission. We sought to investigate the biofilm forming ability of L. monocytogenes by identifying genetic variation that underlies biofilm formation in natural populations using genome-wide association studies. Changes in gene expression of specific strains during biofilm formation were then investigated using RNAseq. Genetic variation associated with enhanced biofilm formation was identified in 273 genes by GWAS and differential expression in 220 genes by RNAseq. Statistical analyses show that number of overlapping genes flagged by either type of experiment is less than expected by random sampling. This is consistent with an evolutionary scenario where rapid adaptation is driven by variation in gene expression of pioneer genes, and this is followed by slower adaptation driven by nucleotide changes within the core genome. Overall design: To investigate gene expression under biofilm formation in L. monocytogenes we established cultures for L. monocytogenes strains Lm0132 and Lm0134. Two replicates were grown for each strain. Each replicate was grown under two conditions, shaken media and stationary media. We then performed a differential gene expression analysis on Illumina sequence reads containing expression data under the two conditions. Reads from each strain were taken from the shaken media at 0 hours, 24 hours and 48 hours and were compared with reads taken from 24 hours stationary growth.