Differential virulence of Aggregatibacter actinomycetemcomitans serotypes explained by exoproteome heterogeneity

Aggregatibacter actinomycetemcomitans (Aa) is a Gram-negative bacterial pathogen associated with periodontitis and non-oral diseases like rheumatoid arthritis and Alzheimer´s disease. Aa isolates with the serotypes a, b and c are globally most prevalent. Importantly, isolates displaying these serotypes have different clinical presentations. While serotype b isolates are predominant in severe periodontitis, serotypes a and c are generally encountered in mild periodontitis or healthy individuals. It was so far not known how these differences are reflected in the overall secretion of virulence factors. Therefore, this study was aimed at a comparative analysis of exoproteomes from different clinical Aa isolates with serotypes a, b or c by mass spectrometry, and a subsequent correlation of the recorded exoproteome profiles with virulence. Overall, we identified 425 extracellular proteins. Significant differences in the exoproteome composition of isolates with different serotypes were observed in terms of protein identification and abundance. In particular, serotype a isolates presented more extracellular proteins than serotype b or c isolates. These differences are mirrored in their virulence in infection models based on human salivary gland epithelial cells and neutrophils. Remarkably, serotype a isolates displayed stronger adhesive capabilities and induced more lysis of epithelial cells and neutrophils than serotype b or c isolates. Conversely, serotype c isolates showed relatively low leukotoxicity, while provoking NETosis to similar extents as serotype a and b isolates. Altogether, we conclude that the differential virulence presentation by Aa isolates with the dominant serotypes a, b or c can be explained by their exoproteome heterogeneity.