In the present study, a possible nature immunostimulant and growth promoter—humic substances (HS) originating from Siberian leonardite mineraloid—were tested on juvenile Clarias gariepinus performance. Feed additive was applied onto commercial pelleted feed at four HS levels—0, 1, 3, and 6% w/w (HS0, HS1, HS3, HS6, respectively). Diets were tested in five repetitions (in total, n = 1800 individuals, mean body weight 28.1 ± 6.2 g) for 56 days. Growth and production parameters, fish condition and somatic indices, and overall mortality were evaluated after 14, 28, 42, and 56 days of exposure. Whereas, plasma samples were collected only after 0, 28, and 56 days, when fish health status was assessed with biochemical parameters (total proteins, TP; alanine aminotransferase, ALT; aspartate aminotransferase, AST; lactate dehydrogenase, LDH; cholesterol, CHOL; triglycerides, TAG) and fish antioxidant status with glutathione (reduced glutathione, GSH; oxidized glutathione, GSSG; glutathione ratio GSH/GSSG). Although a significantly positive effect of HS feed additive on growth performance was not found in the present study, moderately positive effects were found regarding biochemical parameters (ALT, AST, LDH, CHOL, and TAG) and antioxidants (GSH/GSSG ratio) that were improved especially in the HS3 group.

Sta phy loc occus aureus strains cause a wide range of infections in humans, often with the potential for complications such as surgical site infections. The production of Panton-Valentin leukocidin (PVL) by certain strains of S. aureus is clinically associated with chronic or recurrent infections, which typically require decolonization, most often with mupirocin. As increased mupirocin use promotes the emergence of resistance, this study investigated the coadministration of mupirocin and therapeutic Kayvirus bacteriophage as a potential strategy to enhance treatment efficacy and prevent the development of new resistance. We collected and evaluated 37 PVL-encoding S. aureus strains from wound samples. Among these, 22% were methicillin-resistant, and 11% were resistant to the tested phage, but all were susceptible to mupirocin. To assess interactions between mupirocin and the phage in PVL-positive strains with varying levels of mupirocin resistance, we used lysogenization by PVL-encoding phage and adaptive laboratory evolution of clinical strains. In mupirocin-susceptible strains, lytic phage efficacy decreased due to altered protein synthesis caused by the interaction of mupirocin with isoleucyl-tRNA synthetase, whereas mupirocin efficacy was unaffected. In contrast, the advantage of combined administration was observed in mupirocin-resistant strains susceptible to phages, as their altered or alternative synthetase allowed protein synthesis to continue, enabling phage proliferation and bacterial lysis, even in the presence of mupirocin. This in vitro study demonstrates that mupirocin in combination with Kayvirus broadens the spectrum of strains susceptible to treatment and that the phage used prevents the development of mupirocin resistance. Key points: • Mupirocin-phage combination broadens the anti-staphylococcal effect in vitro. • Combination treatment reduces the emergence of mupirocin resistance. • Mupirocin action is not inhibited by phage therapy.