Lactose fermenting enteroinvasive Escherichia coli from diarrhoeal cases confers enhanced virulence.

Enteroinvasive Escherichia coli (EIEC), known for causing bacillary dysentery akin to Shigella species, comprises both lactose-fermenting (LF) and non-lactose-fermenting (NLF) isolates. While NLF-EIEC is a well-established pathogen associated with acute dysentery and harbours classical Shigella-like virulence factors, the role of LF-EIEC in human disease remains underexplored. In this study, we sought to characterize LF-EIEC clinical isolates and assessed their pathogenic potential in comparison to NLF-EIEC. Among 13,682 diarrhoeal stool specimens, six LF and nine NLF-EIEC were isolated, predominantly belonging to serogroups O28ac, O125, O136, and O152. Unlike other E. coli, all the EIEC isolates were non-motile. Both the types of EIEC had multiple plasmids harbouring several virulence encoding genes (ipaBCD, ial, virF, sig, sepA and ipaH). Resistance to recent generation antibiotics were mostly confined to NLF-EIEC but some of the LF-EIEC were resistant only to ceftriaxone. Higher invasion ability and significant increase in the expression of virulence encoding genes by the LF-EIEC (p < 0.05) were noted during infection to Int407 cell-line. Additionally, LF-EIEC exhibited extensive colonization of the mouse intestine and expressed severe keratoconjunctivitis in guinea pigs. Together, our findings highlight LF-EIEC as an emerging pathogenic variant warranting heightened surveillance and comprehensive investigation to better understand its epidemiological and clinical significance.