Assessment of Biofilm Formation and Shiga-like Toxin Genes in ESBL-producing E. coli Isolates Recovered from Retail Salad Vegetables in Mirzapur District, Uttar Pradesh, India

Background: The present study assessed the occurrence and biofilm forming potential of ESBL-producing Escherichia coli isolates recovered from raw salad vegetables collected from 32 retail vegetable shops of Mirzapur district, Uttar Pradesh, India (n=224). Methods: Standard bacteriological culture methods using cefotaxime-supplemented EMB agar were used for initial isolation of cefotaxime-resistant E. coli which were subjected to phenotypic detection of ESBL production by Kirby-Bauer disc diffusion test. Subsequently, all the phenotypically positive ESBL-producing E. coli isolates were subjected to determine their biofilm-producing ability by Congo red agar (CRA) assay and microtiter plate assay (MPA) followed by genotypic confirmation of biofilm production by PCR assay targeted at fim-H, Sfa and papC genes. Additionally, the isolates were genotypically explored for a potential presence of Shiga-like toxin genes (stx1 and stx2). Result: Altogether, 276 cefotaxime-resistant E. coli isolates were recovered, of which, 99.27% (274/276) were observed to be phenotypically positive for ESBL production. The brown, dry and rough (BDAR) and red, dry and rough (RDAR) colony morphotypes were observed by 70.80% (194/274) and 29.19% (80/274) of the isolates, respectively by CRA method revealing their potential to form biofilms. Biofilm formation was evident in 94.16% (258/274) of the isolates by MPA method, of which 79.45% (205/258), 17.05% (44/258) and 3.48% (9/258) were considered as weak, moderate and strong biofilm producers, respectively. The fimH gene was found to be the predominant genetic determinant of biofilm formation which was detected in 77% (211/274) of the tested isolates followed by Sfa gene (11.31%) and papC gene (4.01%). A significant difference (p less than 0.05) was evident between CRA and MPA methods as well as phenotypic and genotypic methods for detection of biofilm formation among the recovered isolates. In addition, significant differences (p less than 0.05) were also observed upon vegetable-wise comparison of ESBL-producing E. coli isolates and the presence of biofilm producing genes. The stx1 and stx2 genes were found to be present in one isolate recovered from a carrot sample and two isolates from coriander samples, respectively. . The findings of this study highlight the widespread occurrence of ESBL-producing E. coli from raw salad vegetables with significant biofilm-forming potential posing a public health risk. The detection of Shiga-like toxin genes in some isolates further underscores the need for stringent surveillance and improved hygiene practices to mitigate contamination risk in fresh produce.