An entomopathogenic bacterium, Xenorhabdus nematophila, is known to have potent antibiotic activities to maintain monoxenic condition in its insect host for effective pathogenesis and ultimately for optimal development of its nematode symbiont, Steinernema carpocapsae. In this study we assess its antibacterial activity against plant-pathogenic bacteria and identify its unknown antibiotics. The bacterial culture broth had significant antibacterial activity that increased with development of the bacteria and reached its maximum at the stationary growth phase. The antibiotic activities were significant against five plant-pathogenic bacterial strains: Agrobacterium vitis, Pectobacterium carotovorum subsp. atrosepticum, P. carotovorum subsp. carotovorum, Pseudomonas syringae pv. tabaci, and Ralstonia solanacearum. The antibacterial factors were extracted with butanol and fractionated using column chromatography with the eluents of different hydrophobic intensities. Two active antibacterial subfractions were purified, and the higher active fraction was further fractionated and identified as a single compound of benzylideneacetone (trans-4-phenyl-3-buten-2-one). With heat stability, the synthetic compound showed equivalent antibiotic activity and spectrum to the purified compound. This study reports a new antibiotic compound synthesized by X. nematophila, which is a monoterpenoid compound and active against some Gram-negative bacteria.

Apoptosis appears to be implicated in the pathogenesis and therapeutic applications of cancer. In this study we investigated the induction of apoptosis by 4-aminopyridine (4-AP), a K⁺ channel blocker, and its mechanism in HepG2 human hepatoblastoma cells. 4-AP reduced cell viability and induced DNA fragmentation, a hallmark of apoptosis, in a dose-dependent manner. In addition, 4-AP induced a sustained increase in intracellular Ca²⁺ concentration, which was completely inhibited by the extracellular Ca²⁺ chelation with EGTA. 4-AP also induced Mn²⁺ influx, indicating that the 4-AP-induced increased intracellular Ca²⁺ levels were due to activation of Ca²⁺ influx pathway. 4-AP also depolarized membrane potential that was measured by using di-O-C₅(3), a voltage-sensitive fluorescent dye. 4-AP-induced Ca²⁺ influx was significantly inhibited not by voltage-operative Ca²⁺ channel blockers (nifedipine or verapamil), but by flufenamic acid (FA), a known nonselective cation channel blocker. Quantitative analysis of apoptosis by the flow cytometry revealed that treatment with either FA or BAPTA, an intracellular Ca²⁺ chelator, significantly inhibited the 4-AP-induced apoptosis. Taken together, these results suggest that the observed 4-AP-induced apoptosis in the HepG2 cells may result from Ca²⁺ influx through the activation of voltage-sensitive Ca²⁺-permeable non-selective cation channels. These results further suggest that membrane potential change by modulation of K⁺ channel activity may be involved in the mechanism of apoptosis in human hepatoma cells.