Upregulation of the Outer Membrane Protein OmpV and Its Role in Polymyxin B Stress Adaptation in Vibrio mimicus

OmpV is an outer membrane protein found in a variety of Gram-negative bacteria. In this study, we demonstrated that V. mimicus strain CS-66, isolated from a patient with diarrhea, was resistant to polymyxin B (PL-B) and colistin (CL) but susceptible to chloramphenicol and ciprofloxacin. When strain CS-66 was cultured in Luria–Bertani broth containing a sub-minimal inhibitory concentration of each antibiotic used for the susceptibility tests, ompV expression as assayed by reverse transcription-qPCR, significantly increased regardless of the antibiotic tested. In contrast, cell aggregation during the early log phase was observed only when the strain was grown in the broth supplemented with PL-B or CL. A space-filling model of V. mimicus OmpV was generated. The model showed that, although OmpV adopted a β-barrel conformation, it had closed top and bottom ends and possessed a lateral cavity. Structural analysis further indicated that the interior of this lateral cavity was negatively charged and sufficiently large to accommodate PL-B. These results suggest that V. mimicus may adapt to PL-B stress through physical sequestration within the negatively charged cavity of OmpV.