Introduction: Citrobacter freundii strains, like other representatives of the same genus, are often described as aetiological factors of diseases in humans and animals, including farmed and ornamental fish. The aim of the study was the isolation and identification of C. freundii from ornamental fish in Poland and investigation of the virulence factors and antibiotic resistance among the isolates. Material and Methods: Twenty C. freundii isolates were isolated from swab samples obtained from diseased ornamental fish of 16 different species in Poland. The bacteria were initially identified to the species level by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry, and the results were confirmed based on sequencing of a fragment of the 16S rRNA gene and biochemical profile analysis using the API-20E system. The antibiotic susceptibility profile was evaluated by the disc-diffusion method. Results: Most isolates were resistant to tetracycline (65%), ciprofloxacin (65%), nalidixic acid (60%) and doxycycline (50%). All of them were resistant to ampicillin. As many as 13/20 isolates (65%) carried the viaB gene encoding the Vi antigen and 9/20 (45%) carried the class 1 integron-encoding gene. The hlyA gene encoding the ability to produce α-haemolysins was not detected in any of the isolates tested. Conclusion: This is the first study to describe in detail the identification and occurrence of virulence factors and antibiotic resistance among C. freundii isolates of ornamental fish in Poland. The results indicate the need for further monitoring of the bacterium’s presence.

During the examination of a bicolor angelfish (Centropyge bicolor) imported from the Philippines and intended for sale on the Polish market, clinical signs of weakness and respiratory distress were observed. Mild hyperemia was noted along the lateral sides of the body and around the mouth. Necropsy revealed the presence of five orange-colored trematodes in the intestinal lumen, with an average body length of 3.12 mm. Based on morphological features and molecular analyses, the parasite was identified as Paragyiauchen sp. The following gene loci were amplified: the gene-encoding component of the large ribosomal subunit (28S rRNA), the gene-encoding NADH dehydrogenase subunit 1 (ND1), the gene cluster 5.8S rRNA-ITS2-28S rRNA and the gene cluster ITS2-28S rRNA. Bacterial species identification using MALDI-TOF MS revealed the presence of three species: Shewanella putrefaciens and Brevundimonas diminuta isolated from the head kidney, and Aeromonas caviae isolated from the liver. This study documents the first detection of representatives of Paragyliauchen genus in C. bicolor imported to Europe and highlights the potential risk of introducing new parasites and opportunistic bacterial pathogens through the ornamental fish trade. These findings emphasize the need for parasitological and microbiological screening of imported ornamental fish.