First Report of blaCTX-M-15 in a Marine Thermotolerant Enterobacter Isolate From Bangladesh: Genomic Insights Into Halotolerance and Antimicrobial Resistance.

Marine ecosystems are becoming important reservoirs for antimicrobial resistance (AMR). However, genomic data on ESBL (Extended-Spectrum Beta-Lactamase)-producing Enterobacter species from marine environments are limited. The widespread blaCTX-M-15 gene, usually found in clinical and wastewater settings, has not been reported in marine Enterobacter from Bangladesh. A thermotolerant, lactose-fermenting isolate (LB01) from the Bay of Bengal seawater near Laboni Beach, Cox's Bazar, was isolated and identified as Enterobacter kobei based on > 99.4% average nucleotide identity and phylogenetic clustering. Its 4.62 Mb genome (55.05% GC) encoded 4410 genes, including blaCTX-M-15, blaACT-9, and qnrS1, with blaCTX-M-15 located next to an ISEc9-like insertion sequence, indicating a mobilizable resistance locus. The genome also contained efflux systems (acrAB, mdtABC, oqxAB), adhesion and iron uptake genes (fim, flh, ent, fep, chu), and comprehensive halotolerance modules (proVWX, betT, nhaA, kdpABC, mscL/S) consistent with its growth in up to 7% NaCl. This is the first genomic characterization of a marine E. kobei carrying blaCTX-M-15 resistant determinant in Bangladesh, showing convergence of resistance, virulence, and halotolerance traits that support persistence in saline, human-impacted environments. The findings broaden the ecological scope of clinically relevant ESBL genes and highlight the importance of coastal ecosystems as overlooked nodes in the global AMR network within a One Health framework.