Mucolytic activity and in vitro interaction of extraintestinal pathogenic Escherichia coli with human pulmonary cells

Background Extraintestinal pathogenic Escherichia coli (ExPEC) are major causes of urinary tract infections (UTI), bacteremia, and meningitis, but the virulence traits that may enable lung colonization remain poorly defined. In the respiratory tract, mucus plays a crucial role in protecting against bacterial colonization. As such, mucus degradation is a significant virulence mechanism used by pathogens to establish infections. This process enables bacteria to access epithelial cells, thereby conferring a competitive advantage over other microorganisms. The pathogenic potential of ExPEC strains in the lungs and their interactions with mucus are currently underexplored areas of research. In this study, we aimed to assess the mucinolytic properties of ExPEC strains and to better understand how these strains interact with lung cells in vitro . Therefore, we analyzed 10 ExPEC strains from bacteremia episodes for traits relevant to the pulmonary environment. The strains had their genomes sequenced to define their phylogroups, pathotypes, and virulence gene profiles, with a focus on putative mucinases. Phenotypic assays were performed to assess the strains' capacity to penetrate the mucus barrier, utilize mucin as a carbon source, and degrade the mucus secreted by Calu-3 human bronchial epithelial cells. The assays also evaluated bacterial adherence, invasion, and growth within A549 alveolar cells. Results Most isolates belonged to phylogroups B2 or D and carried multiple virulence-encoding genes, including sslE , which encodes a mucinase. Nine strains breached the mucin barrier and used mucin as a carbon source. One strain lacking all screened mucinase genes (EC071) nonetheless showed significant mucolytic activity and strong growth on mucin, suggesting a mucinase-independent phenotype. Three non-cytotoxic strains tested in Calu-3 cells degraded the mucus layer. In A549 cells, four strains caused cytotoxic effects, and three adhered efficiently to the cells. Two non-cytotoxic strains invaded the cells, one of which (EC028) displayed pronounced invasion and intracellular replication, reaching high intracellular loads. Conclusions In conclusion, ExPEC bloodstream isolates may exhibit mucolytic potential and variable pulmonary cell-interaction traits, consistent with virulence functions that may favor airway colonization, barrier disruption, and intracellular persistence. These findings broaden the recognized pathogenic potential of ExPEC in susceptible hosts.