Actinobacillus suis is a common commensal of tonsils of the soft palate of swine, but unknown stimuli can lead to invasive, systemic disease. Its pathogenesis, including the process of host colonization, is poorly understood. Thus, the objective was to measure expression of genes involved in attachment, a key factor in both health and disease.
In healthy animals, A. suis is thought to exist in tonsils in biofilm and planktonic forms. Cells in biofilm likely persist in a lower oxygen/nutrient environment in stationary phase. Cells shed from the biofilm assume a planktonic form, with higher nutrient/oxygen availability. We hypothesize that A. suis will differentially express adhesins in these environments, and that certain signals will lead to an invasive phenotype with a different complement of adhesins.
From 42 adhesin-associated genes (23 adhesins) identified by bioinformatic analysis of the virulent A. suis H91-0380 strain, 9 genes were chosen for RT-qPCR. RNA was prepared from aerobic cultures grown at 37ºC and 200 rpm, and sampled at 60 min post-inoculation (mpi) and 180 mpi for exponential and stationary phase, respectively, and from anoxic static cultures grown at 37ºC and sampled at 60 and 210 mpi.
All genes were up-regulated in one growth phase: type IV pilin ppdD, outer membrane proteins ompA2 and ompP2, and fibronectin-binding ybaV in exponential phase, while biofilm-associated flp, fine-tangled pili ftpA, filamentous hemagglutinin fhaB, ompA1, and autotransporter tibA were up-regulated in stationary phase. Most genes were up-regulated in one growth condition: ftpA, ompA1, and tibA in aerobic, and flp, ppdD, and ompA2 in anoxic growth. Time by treatment interactions were also observed for several genes.
Work is underway to generate knockout mutants of the adhesin genes to better determine their roles in attachment and biofilm formation. Additionally, studies to elucidate host cell types and receptors present in tonsils have begun.