The finding matters because CTX-M-15 is not an obscure resistance factor. It is the most widely disseminated variant of the CTX-M family of extended-spectrum beta-lactamases, enzymes that dismantle the beta-lactam ring at the heart of penicillin-class antibiotics and their modern cephalosporin descendants. In hospitals and communities worldwide, blaCTX-M-15 rides along with epidemic bacterial clones such as Escherichia coli ST131 and Klebsiella pneumoniae ST15, and its spread is fueled by mobile genetic elements, particularly insertion sequences like ISEcp1 and IS26, that shuttle the gene between plasmids and chromosomes. Finding it in a halotolerant marine isolate suggests that clinically relevant resistance has spilled over into an ecosystem that is rarely included in routine antimicrobial resistance surveillance.

The story began in February 2024, when researchers collected seawater from Laboni Beach along the Bay of Bengal. At the time of sampling, the water registered a salinity of 30 practical salinity units, a pH of 7.2, and a chemical oxygen demand of 910 milligrams per liter, alongside measurable concentrations of heavy metals including lead, cadmium, nickel, and zinc. The team plated seawater aliquots onto m-TEC ChromoSelect Agar, a selective medium recommended by the US Environmental Protection Agency for detecting thermotolerant E. coli, and incubated the plates at 44.5 degrees Celsius. Among the colonies that grew was one that looked distinctly different from the characteristic red-magenta appearance of E. coli on this medium.

That odd colony turned out to be something else entirely. On MacConkey agar the isolate produced pink, mucoid, circular colonies consistent with lactose fermentation, and standard biochemical tests, including an indole-negative, citrate-positive profile, pointed toward the genus Enterobacter. Whole-genome sequencing on an Illumina platform generated more than 21 million high-quality paired-end reads, which assembled into a 4.62-megabase draft genome of 129 contigs with a GC content of 55.05 percent. Quality checks indicated 100 percent genome completeness with only 0.2 percent contamination, giving the team a robust foundation for the detailed genomic analyses that followed.

Taxonomic identification required several independent lines of evidence. KmerFinder initially flagged the isolate as Enterobacter kobei, and a Jukes-Cantor species TCS analysis returned its closest matches with very high Z-scores to Enterobacter kobei reference strains. Average nucleotide identity comparisons clinched the case: LB01 shared 99.49 to 99.75 percent identity with E. kobei genomes, while falling below the 95 percent species threshold against Enterobacter cloacae reference strains. A core-genome phylogeny built from 94 Enterobacter genomes and 500 conserved single-copy protein families placed LB01 firmly within the E. kobei clade, with the relevant node supported by a bootstrap value of 100 percent. The tree showed that LB01 clusters with E. kobei strains isolated from both clinical and environmental sources, hinting at the porous boundaries between these ecological compartments.

Phenotypic testing told a story that matched the genome. In disk diffusion assays interpreted under Clinical and Laboratory Standards Institute guidelines, LB01 was resistant to ampicillin and to all three tested third-generation cephalosporins: cefotaxime, ceftazidime, and ceftriaxone. Yet it remained susceptible to ciprofloxacin, nalidixic acid, gentamicin, imipenem, tetracycline, and trimethoprim-sulfamethoxazole. Genome mining with AMRFinderPlus, ResFinder, and the Comprehensive Antibiotic Resistance Database explained the pattern. Beta-lactam resistance was mediated by blaCTX-M-15 and blaACT-9, an AmpC-type enzyme, along with a PBP3 variant, while qnrS1 encoded reduced susceptibility to quinolones and vanG carried a glycopeptide resistance determinant. A dense network of efflux pumps, including AcrAB-TolC, AcrD, MdtBC, EmrAB, and OqxAB, regulated by activators such as MarA, RamA, and LeuO, rounded out a resistome spanning multiple antibiotic classes. Crucially, no carbapenemase genes were present, consistent with the isolate’s susceptibility to imipenem.

The genomic context of blaCTX-M-15 is perhaps the most consequential part of the study. The gene sits on contig00004 embedded in a dense cluster of mobile elements, arranged as tnpM-tnpR-gin-qnrS1-insC1-tnpA-blaCTX-M-15-tnpA-tnpA-tmk-holB. Upstream, the transposases and insertion sequence element insC1 form an ISEc9/ISEcp1-like composite transposon structure, the very class of element documented to mobilize adjacent blaCTX-M genes and mediate their transposition between plasmids and chromosomes. The site-specific recombinase gin and the quinolone resistance gene qnrS1 sit immediately upstream, meaning that fluoroquinolone and beta-lactam resistance determinants are physically linked within the same mobilizable module. This linkage has a subtle but important epidemiological implication: quinolone exposure can co-select for the qnrS1-bearing element, indirectly preserving the linked ESBL gene even when beta-lactam antibiotics are absent from the environment.

Beyond resistance, the genome revealed a battery of traits suited to survival in a harsh coastal habitat. In laboratory assays, LB01 grew in nutrient broth containing up to 7 percent sodium chloride by weight, though it failed at 10 percent, marking it as moderately halotolerant. Optical density measurements declined steadily with rising salinity, from 0.627 at 2 percent NaCl to 0.098 at 10 percent, and plate assays confirmed colony formation up to 7 percent. The genomic basis for this tolerance is comprehensive: compatible solute transporters proVWX and betT for importing proline and choline, biosynthetic genes proA, proB, and proC for making proline and betA and betB for converting choline to glycine betaine, trehalose synthesis genes otsA and otsB for protecting proteins and membranes, the Na+/H+ antiporter nhaA and the high-affinity potassium uptake system kdpABC for ion homeostasis, and mechanosensitive channels mscL and mscS that vent solutes during sudden osmotic downshifts to prevent cell lysis. Oxidative stress defenses, including glutathione biosynthesis, catalase, and superoxide dismutase genes, add another layer of resilience against the reactive oxygen species generated during salt stress.

The virulence profile of LB01 adds a further dimension of concern. The genome carries fimbrial operons fimC, fimD, and fimH and curli fiber genes csgA through csgG for surface attachment and biofilm formation, a full complement of flagellar and chemotaxis genes for motility, and extensive iron acquisition machinery including enterobactin, aerobactin, and heme uptake systems. Outer membrane proteins, lipopolysaccharide biosynthesis genes, and stress response elements complete the picture of an organism equipped to persist in the environment and, if the opportunity arises, interact with animal hosts. Notably, the isolate showed gamma-hemolysis on sheep blood agar, indicating no detectable hemolysin production, but the broader virulence arsenal mirrors reports of environmental Enterobacterales in which resistance and colonization traits co-occur. The authors caution, however, that whether LB01’s halotolerance reflects true marine adaptation or pre-adaptation in wastewater-impacted or estuarine habitats with fluctuating salinity remains an open question.

The study has limitations that the authors acknowledge candidly. It rests on a single isolate, so it cannot quantify how prevalent blaCTX-M-15 is in the Bay of Bengal or pinpoint the dominant sources of contamination. The short-read assembly, with its 129 contigs, prevented full reconstruction of plasmid sequences, and PlasmidFinder detected no plasmid replicons, leaving the precise genomic location of blaCTX-M-15, chromosomal or plasmid-borne, unresolved. The researchers argue that future work should employ long-read sequencing platforms such as Oxford Nanopore or PacBio with hybrid assembly, alongside conjugation assays to demonstrate actual transferability, and chemical analysis of antibiotic residues paired with microbial source tracking to trace the anthropogenic drivers. Even so, the implications are clear. Coastal waters, particularly in regions with dense populations, extensive aquaculture, and limited wastewater treatment, should be incorporated into antimicrobial resistance surveillance frameworks under a One Health approach. The team also points to advanced wastewater treatments, including UV irradiation and ozonation, as tools to curb the flow of resistance genes into the sea. A halotolerant, ESBL-producing bacterium thriving in surf that millions of people touch, swim in, and harvest seafood from is a vivid reminder that the fight against antimicrobial resistance does not stop at the shoreline.

Subject of Research: Genomic detection of the blaCTX-M-15 extended-spectrum beta-lactamase gene in a marine Enterobacter kobei isolate from coastal Bangladesh

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

Article References: Rahman, M. A., Azam, R. M. U., Hossain, M. S., Momtaz, S., Begum, A., & Sultana, M. (2026). First Report of bla CTX ‐ M‐15 in a Marine Thermotolerant Enterobacter Isolate From Bangladesh: Genomic Insights Into Halotolerance and Antimicrobial Resistance. MicrobiologyOpen, 15(5), Article e70425. https://doi.org/10.1002/mbo3.70425

Image Credits: AI Generated

DOI: 10.1002/mbo3.70425

Keywords: antimicrobial resistance, blaCTX-M-15, Enterobacter kobei, ESBL, marine microbiology, Bay of Bengal, whole-genome sequencing, halotolerance, mobile genetic elements, qnrS1, One Health, Bangladesh