Evaluation of antimicrobial activity of gut bacteria from Brown trout (Salmo trutta fario) against fish pathogens, with emphasis on the emerging aquatic pathogen, Stenotrophomonas maltophilia.

The growing threat of antibiotic-resistant pathogens and the limitations on antibiotic use in aquaculture have driven interest in probiotic alternatives for disease prevention and management. Probiotics represent a promising, eco-friendly strategy for enhancing fish health and controlling infectious diseases. This study aimed to evaluate the probiotic potential of bacterial isolates from the gastrointestinal tract of brown trout (Salmo trutta fario), with a special focus on their antimicrobial activity against Stenotrophomonas maltophilia, an emerging pathogen in aquaculture. A total of 48 bacterial colonies were isolated, among which 18 acid producing strains were screened for antimicrobial activity. All isolates were Gram-positive, catalase- and oxidase-negative, and non-hemolytic. Among them, NM12 and NM06 demonstrated strong inhibitory effects against common fish pathogens, particularly S. maltophilia, with inhibition zones of 17.7 ± 1.21 mm and 12.07 ± 1.90 mm, respectively. While NM06 inhibited S. aureus, NM12 inhibited S. aureus, E. coli, A. veronii, and K. pneumoniae. Other isolates, ARL14 and NM951, showed antimicrobial activity against a range of Gram-positive and Gram-negative bacteria but not against S. maltophilia. Acid and bile tolerance assays confirmed their survivability under simulated gastrointestinal conditions. NM951 showed high acid tolerance at pH 2.0, whereas NM12, NM06, and ARL14 exhibited a time-dependent decline at pH 2.0 but remained viable from pH 3.0 onwards. NM12 and NM951 tolerated bile concentrations of 0.15 % and 0.3 % with minimal growth delays (≤15 min), while NM06 and ARL14 resisted 0.15 % bile with ≤15 min delay and tolerated 0.3 % bile with 15-40 min delay. In adhesion, hydrophobicity, and auto-aggregation assays, NM06 exhibited the highest auto-aggregation (72.35 % ± 2.57 %), hydrophobicity (55.17 % ± 2.95 %) and adhesion to intestinal mucus (51.72 % ± 4.75). Similarly, NM12 showed auto-aggregation (65.02 % ± 1.17 %), hydrophobicity (60.54 % ± 4.17 %), and adhesion to intestinal mucus (39.77 % ± 2.57). Antibiogram profiling revealed some resistance, particularly in NM12, but other isolates demonstrated sensitivity to key antibiotics. Using 16S rRNA sequencing analysis, the isolates were identified as Leuconostoc mesenteroides NM12, Mammaliicoccus sciuri NM06, Carnobacterium maltaromaticum ARL14, and Lactococcus lactis NM951. The findings highlight M. sciuri NM06 and L. mesenteroides NM12 as promising probiotic candidates with notable antagonistic activity against S. maltophilia, supporting their potential for use in aquaculture health management.