Abstract
Neisseria meningitidis causes meningococcal disease, a global life threatening illness with annual incidences of between 1 and 1000 per 100,000 population. Humans are the only known host with approximately 10% of people having asymptomatic nasopharyngeal carriage at any one time. Thus, the ability of meningococci to attach, invade, and grow in the epithelium is crucial for both its commensal and pathogenic properties. In the rare event that meningococci cross the epithelium into the bloodstream, disease may occur. In order to better understand the mechanisms of meningococcal pathogenesis, transposon mutagenesis was used to identify bacterial genes involved in epithelial cell adherence and internalization as well as traversal of the epithelial barrier. Three epithelial cell lines of respiratory origin, A549 cells, 16HBE14o- cells and Detroit 562 cells were used to examine N. meningitidis L91543 (C:2a:P1.2, ST-11; ET-37) pathogenesis. First, adhesion, invasion and traversal assays were optimized for bacterial uptake to enable the maximum number of mutants to be tested and to avoid stochastic loss from the transposon library. Since the highest level of meningococci adherence and invasion was observed using 16HBE14o- cells, this cell line was chosen for subsequent traversal assays, where an intact epithelial barrier was established on Transwell® membrane inserts. Epithelial barrier integrity was assessed by measuring transepithelial electrical resistance (TEER), permeability of the marker protein, 70 kDa Dextran, and by examining the distribution of the tight junction proteins, occludin and ZO-1, by immunofluorescence. Next, transposon mutagenesis libraries comprising of approx. 14,500 N. meningitidis L91543 mutants, were used to probe meningococcal interactions with 16HBE14o- epithelial cells. Illumina sequencing of amplified transposon junctions was performed on DNA extracted from both input and output pools obtained from the various assays. Comparative analysis of input/output pools showed reduced fitness, not only of genes associated with type IV pili, but mainly of genes involved in metabolism especially nucleotide and amino acid metabolism. Genes involved in membrane transport, regulatory functions and cellular processes also showed reduced fitness. The function of putative genes of interest was validated by generating insertion knockout mutants and testing them independently for their ability to alter meningococcal-epithelial cell interactions. The knockout mutant assays showed 67-100% agreement to the Tn-Seq analysis prediction. Based on the knock out mutant assays as well the Tn-Seq prediction we can conclude that type IV pili, nucleotide biosynthesis, glucose and amino acid metabolism, as well as resistance to antimicrobial peptide are critical for meningococcal interaction with epithelial cells.