# Gram-Negative Microbiota Derived from Trout Fished in Slovakian Water Sources and Their Relationship to Postbiotics

## TL;DR

This study explores Gram-negative bacteria in trout from Slovakia and evaluates postbiotics as a way to control harmful strains.

## Contribution

The paper identifies novel Gram-negative bacterial strains in trout and demonstrates the inhibitory potential of specific postbiotics against them.

## Key findings

- 21 Gram-negative bacterial strains were isolated from trout in Slovakia and identified as 13 species across 9 genera.
- Three strains showed susceptibility to nine enterocin postbiotics with inhibitory activity ranging from 100–6400 AU/mL.
- Most strains were antibiotic-susceptible, with inhibitory zones up to 29 mm observed.

## Abstract

Regarding the trout microbiota, most information is focused on lactic acid bacteria, which can show beneficial properties. However, in trout farming, mostly pathogenic Gram-positive species were reported, such as Staphylococcus aureus, Listeria monocytogenes, and/or Clostridium spp. In this study, free-living trout were analyzed for Gram-negative microbiota that can cause loss as disease-stimulating agents. Bacteriocin postbiotics should be one of the approaches used to eliminate these agents. In total, 21 strains of different species isolated from the intestinal tract of 50 trout in Slovakia (Salmo trutta and Salmo gairdnerii) were taxonomically allotted into 13 species and 9 genera. This method showed variability in microbiota identified using MALDI-TOF mass spectrometry with the following species: Acinetobacter calcoaceticus, Citrobacter gillenii, Citrobacter freundii, Escherichia coli, Hafnia alvei, Kluyvera cryocrescens, K. intermedia, Leclercia adecarboxylata, Raoultella ornithinolytica, Pseudomonas fragi, Ps. putida, Ps. lundensis, Ps. teatrolens, and Serratia fonticola. Most strains were susceptible to the antibiotics used, reaching inhibitory zones up to 29 mm. On the other hand, 3 out of 21 strains (14%) were susceptible to nine enterocins- postbiotics (Hafnia alvei Hal281, Pseudomonas putida Pp391, and Ps. fragi Pf 284), with inhibitory activity in the range of 100–6400 AU/mL.