Environmental transition navigates phenotype switching, affecting the virulence and multidrug-resistant profile of pathogenic Morganella morganii.

The bacteria's ability to respond to environmental changes is critical for their survival. This allows them to form intricate communities, withstand stress, and initiate virulence responses in hosts during infection, a phenomenon known as phenotypic switching. In this study, we investigated the role of shaking conditions on phenotype switch in multidrug-resistant and pathogenic Morganella morganii both under in vitro and in vivo conditions. The results demonstrate that M. morganii grown in non-shaking conditions, possibly causing low fluid shear, developed floccules or cellular aggregates, and substantially increased biofilm formation. Meanwhile, the bacterium grown in shaking conditions was non-flocculated and produced less biofilm. This phenotype switch leads to a significant change in the protein secretome and multidrug resistance profile. In the non-shaking condition, M. morganii secretes two main proteins of ∼80 and ∼100 kDa and displays multiple antibiotic resistance (MAR) values of 0.39. In contrast, the bacterial cell in a shaking flask secreted one prominent protein of ∼50 kDa and exhibited a lower MAR value of 0.31. These observations correspond with a significant reduction in both in vitro and in vivo virulence of M. morganii grown in non-shaking conditions, namely haemolysin, swimming motility, histomorphological changes, and survival assay as compared to bacterial cells in a shaking flask displayed higher virulence in both in vitro and in vivo condition. Furthermore, non-shaking tube-grown cells have higher expression of saa, astA, ibeA, papC and papG genes as compared to cells grown in the shaking flask exhibiting higher expression of kpsMT K1, kpsMT "K5", stx1, ireA and cdt genes. Taking together, the study offers strong evidence supporting the presence of two phenotype forms in the multidrug-resistant and pathogenic M. morganii strain, showing differential phenotypes. Additionally, since water flow and movement are prevalent characteristics in aquaculture systems, they can exert fluid shear on the resident microbial communities. Therefore, our study could serve as a foundation for understanding the behavior of M. morganii in aquaculture settings and enable the possibility of monitoring and controlling this multidrug-resistant and pathogenic bacterium by steering phenotypes.