Serotype-dependent expression patterns of stabilized lipopolysaccharide aggregates in Aggregatibacter actinomycetemcomitans strains

Abstract

Above a critical concentration, amphiphilic lipopolysaccharide (LPS) molecules in an aqueous environment form aggregate structures, probably because of interactions involving hydrophobic bonds. Ionic bonds involving divalent cations stabilize these aggregate structures, making them resistant to breakdown by detergents. The aim of this study was to examine expression patterns of stabilized LPS aggregates in Aggregatibacter actinomycetemcomitans, a microorganism that causes periodontitis. A. actinomycetemcomitans strains of various serotypes and truncated LPS mutants were prepared for this study. Following treatment with a two-phase separation system using the detergent Triton X-114, crude LPS extracts of the study strains were separated into detergent-phase LPS (DP-LPS) and aqueous-phase LPS (AP-LPS). Repeated treatment of the aqueous phase with the two-phase separation system produced only a slight decrease in AP-LPS, suggesting that AP-LPS was resistant to the detergent and thus distinguishable from DP-LPS. The presence of divalent cations increased the yield of AP-LPS. AP-LPS expression patterns were serotype-dependent; serotypes b and f showing early expression, and serotypes a and c late expression. In addition, highly truncated LPS from a waaD (rfaD) mutant were unable to generate AP-LPS, suggesting involvement of the LPS structure in the generation of AP-LPS. The two-phase separation was able to distinguish two types of LPS with different physical states at the supramolecular structure level. Hence, AP-LPS likely represents stabilized LPS aggregates, whereas DP-LPS might be derived from non-stabilized aggregates. Furthermore, time-dependent expression of stabilized LPS aggregates was found to be serotype-dependent in A. actinomycetemcomitans.