Abstract
This article presents the results of a microbiological study of bacterial isolates obtained from fish collected in water bodies of the West Kazakhstan Region using the MALDI-TOF MS method. The main objective of the study was the identification of bacterial isolates associated with different fish species, as well as the assessment of their taxonomic structure and ecological significance as indicators of aquatic environmental conditions. A total of 12 bacterial isolates obtained from fish samples collected in rivers, lakes, and areas with potential anthropogenic impact were used as research material. The analysis revealed that 75.0% of the isolates were successfully identified to the species level, confirming the high efficiency of MALDI-TOF MS for the analysis of environmental bacterial isolates. The remaining 25.0% of isolates could not be reliably identified, which may be related to limitations of the reference database and the specific characteristics of natural strains. Taxonomic analysis showed a predominance of opportunistic Gram-negative bacteria, mainly representatives of the family Enterobacteriaceae, including Serratia liquefaciens and Klebsiella oxytoca. The most frequently detected species was Serratia liquefaciens, accounting for 33.3% of all isolates, indicating its wide distribution in the aquatic environment and its potential incorporation into fish microbiota. The second most common species was Klebsiella oxytoca, whose presence may indicate fecal and anthropogenic contamination of water bodies. Additionally, single representatives of natural microbiota and potentially significant microorganisms were detected in the studied samples, including Rahella bruchi, Lactobacillus sakei, and Lactococcus garvieae. Their presence reflects the complex and mixed nature of fish microbiota formed under the influence of the surrounding aquatic environment. Thus, the study demonstrates that fish from water bodies of the West Kazakhstan Region are characterized by a diverse bacterial microbiota with a predominance of opportunistic bacteria. The use of the MALDI-TOF MS method enabled rapid and efficient identification of isolates and may be recommended for further monitoring of the microbiological state of aquatic ecosystems and assessment of environmental safety in the region.