Acid Stress Differentially Affects Virulence-Associated Phenotypes in Atypical Enteropathogenic Escherichia coli

Atypical enteropathogenic Escherichia coli (aEPEC) is an important diarrheagenic pathotype that induces attaching and effacing (AE) lesions in enterocytes while lacking bundle-forming pili. Epidemiological studies have shown an increasing prevalence of aEPEC in children and adults, regardless of diarrheal symptoms. Successful intestinal colonization depends on the capacity of enteric pathogens to withstand gastric acidity, a major host defense barrier. Because gastric pH varies with host age and physiological conditions, acid tolerance may influence pathogen survival and virulence-associated phenotypes. This study evaluated the survival of aEPEC strains under simulated gastric pH conditions and the effects of acid exposure, followed by incubation at pH 6.8, on HeLa cell adherence, invasion, AE lesion formation, and biofilm formation on abiotic surfaces. Marked physiological heterogeneity among the strains was observed. Although all strains survived under moderate acid stress (pH 3.4), only three remained viable under severe acid stress (pH 2.6). Post-stress analyses of strains 0421-1 and 1711-4 revealed strain-dependent responses. In 0421-1, biofilm formation and adherence were maintained, whereas invasion efficiency decreased after both acid treatments and AE lesion formation decreased only at pH 3.4 (p < 0.01). In contrast, 1711-4 showed decreased biofilm formation (p < 0.01) but increased adherence (p < 0.01) and AE lesion formation (p < 0.05) after exposure to pH 2.6, while invasion efficiency remained unchanged; at pH 3.4, AE lesion formation decreased. These findings demonstrate that acid exposure can differentially affect virulence-associated phenotypes in aEPEC, with responses varying according to the strain and the phenotype evaluated.