Electrochemical Impedance and Transcriptomic Responses of Pectobacterium aroidearum SM2 Following Exposure to Cell-Free Filtrate of Kosakonia cowanii Cp1

Extracellular metabolites mediate microbial interactions in plant-associated environments, among which bacterial volatile organic compounds (VOCs) play important roles in regulating competition, colonization, and pathogen establishment. However, the physiological and transcriptional responses of Pectobacterium aroidearum to extracellular compounds produced by other bacteria remain poorly understood. This study aimed to characterize changes in VOC profiles during interactions between P. aroidearum SM2 and Kosakonia cowanii Cp1 and to investigate the electrochemical and transcriptional responses of P. aroidearum SM2 following exposure to the cell-free filtrate of K. cowanii Cp1. Volatile profiling by headspace solid-phase microextraction coupled with gas chromatography–mass spectrometry (HS-SPME-GC-MS) after 2 h of bacterial growth revealed changes in VOC composition during co-culture in tryptic soy broth (TSB), particularly a marked reduction in the relative abundance of 2,5-dimethylpyrazine compared with monocultures. Electrochemical impedance spectroscopy (EIS) revealed numerical variations in impedance magnitude and phase following exposure to the cell-free filtrate obtained from a 2 h culture of K. cowanii Cp1, with the largest numerical difference in impedance magnitude (27.9% relative to the control) observed after 1 h of exposure at 200 kHz. This exposure time was subsequently selected for transcriptomic analysis. RNA-Seq analysis identified significant transcriptional changes, including the downregulation of genes involved in sulfur assimilation and cysteine biosynthesis, together with increased expression of genes associated with galactose utilization and glycerol metabolism. Although these responses cannot be attributed exclusively to VOCs, the results provide a basis for further investigating the contribution of volatile and non-volatile extracellular compounds to bacterial interactions and their potential relevance to the management of soft rot disease.