Characterization of Helicobacter pylori aggregation reveals a requirement for both AlpA and AlpB

Helicobacter pylori is a gram-negative human pathogen that colonizes the stomach and is a major risk factor for gastric ulcers and cancer. A common but poorly understood characteristic of H. pylori is its propensity to aggregate in liquid culture. To investigate this phenomenon, we developed protocols to form, measure, and disperse aggregates. Using these tools, we determined that H. pylori aggregation is protein-dependent and can occur non-clonally, from the binding of distinct cells. Motility, flagella, quorum sensing, and exogenous host proteins were not necessary for aggregation, but the outer membrane proteins AlpA and AlpB were. H. pylori lacking either alpA , alpB , or both were unable to form aggregates. While bacterial aggregation often confers tolerance to antibiotics, H. pylori aggregates were no more tolerant than dispersed cells to several clinically used antibiotics and human serum. Instead, we found that alpA mutants were highly deficient in biofilm formation, suggesting that aggregation may be a step along the H. pylori biofilm formation pathway.