The aim of this study was to investigate the patterns of antibiotic resistance of Aeromonas spp. bacteria isolated from the sea trout (Salmo trutta) from the state fish hatcheries of the Institute of Food Safety, Animal Health and Environment 'BIOR', Latvia. Bacteriological investigations were performed at four state fish hatcheries located in the drainage basins of the main Latvian rivers -Daugava, Venta and Gauja, during the five-year period (2012 -2016). In fish with visible clinical signs, bacteriological samples were collected from heart, liver, spleen, kidney and ulcer surfaces. Aeromonas hydrophila and Aeromonas salmonicida were isolated from sea trouts. A total of 52 individual sea trouts were examined. Resistance to amoxicillin, ampicillin, cephalexin, colistin, doxycycline, enrofloxacin, erythromycin, florfenicol, gentamycin, kanamycin, lincomycin, neomycin, oxytetracycline, spectinomycin, streptomycin, tetracycline, trimethoprim/sulfamethoxazole was tested. The results of this study suggest a multi-drug resistance pattern among the A. hydrophila isolates. All the isolates were resistant to amoxicillin (100%), ampicillin (100%), cephalexin (100%) and erythromycin (100%). The lowest level of resistance was found against florfenicol (4.55%), gentamycin (4.55%), kanamycin (4.55%), but susceptibility was recorded to enrofloxacin, neomycin and trimethoprim/sulfamethoxazole. A. salmonicida isolates were resistant to oxytetracycline (9.38%) and tetracycline (9.38%). For other antibiotics A. salmonicida isolates were susceptible.

In recent years, the effective immunostimulating properties of β-glucans have been widely proven not only in mammals, but also in aquatic animals. The aim of this study was to determine the effect of dietary β-glucan supplements on the percentage of individual types of leukocytes and the haematocrit of the sea trout (Salmo trutta L.).

This study was performed to determine the effects of dietary supplementation with β-glucan on the growth performance and skin-mucus microbiota of sea trout, Salmo trutta L. in Latvia. The investigations were performed during an eight-month period (September 2018 – April 2019). A total of 15,000 sea trout were divided into five groups. The experimental fish were fed formulated diets enriched with 1 g kg−1 β-glucan (D2), 3 g kg−1 β-glucan (D3), 6 g kg−1 biological product BGN-2 (BGN-2) (D4), and 14 g kg−1 BGN-2 (D5). The control diet (D1) was not supplemented. Our results showed that fish fed diets D4 and D5 achieved significantly (P < 0.05) higher growth parameters compared to those fed the other diets. Pseudomonas and Aeromonas were detected as the main component of fish skin and gill microbiota. Beta-glucan did not affect the skin-mucus microbiota of the sea trout. All isolates were resistant to amoxicillin, ampicillin, cefalexin, and erythromycin and susceptible to gentamicin. The multiple antibiotic resistance index for all isolates was higher than 0.2.