<i>Klebsiella pneumoniae</i> Volatile Organic Compounds (VOCs) Protect <i>Artemia salina</i> from Fish Pathogen <i>Aeromonas</i> sp.: A Combined In Vitro, In Vivo, and In Silico Approach

Antibiotic resistance is an alarming threat all over the world, and the biofilm formation efficacy of bacteria is making the situation worse. The antagonistic efficacy of <i>Klebsiella pneumoniae</i> against one of the known fish pathogens, <i>Aeromonas</i> sp., is examined in this study. Moreover, <i>Aeromonas</i> sp.’s biofilm formation ability and in vivo pathogenicity on <i>Artemia salina</i> are also justified here. Firstly, six selected bacterial strains were used to obtain antimicrobial compounds against this pathogenic strain. Among those, <i>Klebsiella pneumoniae</i>, another pathogenic bacterium, surprisingly demonstrated remarkable antagonistic activity against <i>Aeromonas</i> sp. in both in vitro and in vivo assays. The biofilm distrusting potentiality of <i>Klebsiella pneumoniae</i>’s cell-free supernatants (CFSs) was likewise found to be around 56%. Furthermore, the volatile compounds of <i>Klebsiella pneumoniae</i> were identified by GC-MS in order to explore compounds with antibacterial efficacy against <i>Aeromonas</i> sp. through an in silico study, where 5′-methylthioadenosine/S-adenosylhomocysteine nucleosidase (MTAN) (PDB: 5B7P) was chosen as a target protein for its unique characteristics and pathogenicity. Several volatile compounds, such as oxime- methoxy-phenyl-, fluoren-9-ol, 3,6-dimethoxy-9-(2-phenylethynyl)-, and 2H-indol-2-one, 1,3-dihydro- showed a strong binding affinity, with free energy of −6.7, −7.1, and −6.4 Kcal/mol, respectively, in complexes with the protein MTAN. Moreover, the root-mean-square deviation, solvent-accessible surface area, radius of gyration, root-mean-square fluctuations, and hydrogen bonds were used to ensure the binding stability of the docked complexes in the atomistic simulation. Thus, <i>Klebsiella pneumoniae</i> and its potential compounds can be employed as an alternative to antibiotics for aquaculture, demonstrating their effectiveness in suppressing <i>Aeromonas</i> sp.