Occurrence and antimicrobial resistance of Listeria monocytogenes isolated from fish and fish products

Due to the increasing prevalence of listeriosis in fish and fish products, as well as the expanding transmission pathways of Listeria monocytogenes, including antibiotic-resistant populations, from animal- and plant-derived sources to humans, animals, and poultry, enhanced control of raw materials and food products is required. A total of 31 Listeria monocytogenes strains (8.4 % of the examined samples), isolated from fish and fish products, were tested for susceptibility to clinically important antibiotics from different antimicrobial classes using the disk diffusion method. Antibiotic disks were previously quality-controlled for diffusion in agar with the reference strain Streptococcus pneumoniae ATCC 49619 according to EUCAST recommendations. Suspensions of the tested Listeria monocytogenes strains were prepared at a concentration equivalent to 0.5 McFarland standard. The results were interpreted according to the latest valid version of the EUCAST guidelines. Among the tested Listeria strains isolated from samples of lightly salted salmon trimmings and hot-smoked sprats, resistance to erythromycin (15 μg) was confirmed in 2 isolates (6.5 % of the tested strains). One Listeria monocytogenes isolate (3.2 % of the tested strains), recovered from fish cutlets, demonstrated resistance to trimethoprim-sulfamethoxazole (1.25/23.75 μg). Resistance of the Listeria monocytogenes isolates to erythromycin was confirmed by inhibition zone diameters ranging from 23.8 ± 0.2 to 24.0 ± 0.4 mm (P < 0.001), which were below the EUCAST susceptibility breakpoint of ≥25 mm. Following exposure to trimethoprim-sulfamethoxazole, resistance was confirmed by an inhibition zone diameter of 28.6 ± 0.2 mm (P < 0.001), compared with the EUCAST susceptibility breakpoint of ≥29 mm. Particular attention was drawn to seven Listeria monocytogenes isolates (22.6 % of the tested strains), which exhibited very small inhibition zone diameters around disks containing penicillin-class antibiotics, specifically benzylpenicillin (1 IU) and ampicillin (2 μg). These values were only slightly above the EUCAST susceptibility breakpoints, indicating reduced susceptibility. The detection of resistance to erythromycin and trimethoprim-sulfamethoxazole in Listeria monocytogenes isolated from fish and fish products highlights a significant public health concern for humans, animals, and poultry. These findings emphasize the necessity for continuous surveillance to reduce the risk of dissemination of antibiotic-resistant Listeria through fish and fish products.