Biodiversity of Genetic, Metabolic, and Antibiotic Resistance Profiles of Escherichia coli Strains Recovered from the Baltic Sea Region

Marine environments represent a significant reservoir that facilitates the spread of antibiotic resistance. In the present study, 69 samples (seawater, sand, and surface water) were collected from 16 points of high anthropogenic pressure along the Baltic Sea in 2022–2023. Out of these, 184 strains of lactose-fermenting bacteria were isolated, and 24 species were identified by MALDI TOF-MS, with Escherichia coli being the most prevalent (114 strains). Bacterial isolates were assigned to phylogroups and profiled with repetitive sequence PCR and API and evaluated for susceptibility to 16 antibiotics: 61% of E. coli strains belonged to commensal/environmental B1 and A phylogroups, while 20% belonged to potentially pathogenic B2 or D phylogroups. Moreover, 18% of the isolates exhibited resistance to at least one antibiotic, while seven were multidrug-resistant. For amikacin and gentamicin, strains from the B2 and D phylogroups had significantly smaller average growth inhibition zones than strains from the A and B1 phylogroups. Moreover, for six antibiotics belonging to each WHO AWaRe group (Access, Watch and Reserve), average growth inhibition zone diameters were significantly smaller in samples collected in warmer months vs. those obtained in colder months. The results suggest that monitoring for antibiotic-resistant strains should extend from recreational waters to beach sands, prioritizing summer sampling over winter.