Survey of entomopathogenic nematodes and associate bacteria in Thailand and their potential to control Aedes aegypti

Aedes aegypti is an insect vector that transmits several viruses affecting humans worldwide. Entomopathogenic nematodes (EPNs) and their symbiotic bacteria are organisms with the potential to control many insects. In this study, we did a survey aimed to identify EPNs and their symbiotic bacteria and evaluate the larvicidal activity of bacteria against Ae. aegypti. We collected 540 soil samples from 108 sites in Phitsanulok Province, lower northern Thailand. Baiting techniques and White traps were used to isolate EPNs from soil samples. By sequencing of 28S rDNA and internal transcribed spacer regions, 51 EPN isolates were identified as Steinernema surkhetense (35 isolates), Heterorhabditis indica (14 isolates) and Heterorhabditis sp. SGmg3 (two isolates). Based on sequencing of a partial region of the recA gene, 35 isolates of Xenorhabdus were identified as Xenorhabdus stockiae, and 20 Photorhabdus isolates were identified as Photorhabdus luminescens subsp. akhurstii (10 isolates), P. luminescens subsp. hainanensis (seven isolates) and P. asymbiotica subsp. australis (three isolates). Screening for larvicidal activity of bacteria against Ae. aegypti was performed in the laboratory. Xenorhabdus WB5.4 and Xenorhabdus WB12.5, which were closely related to X. stockiae, resulted in high mortality of Ae. aegypti (99.99% and 70%, respectively) at 96 hr after exposure. Comparing with control groups, mortality of Ae. aegypti larvae was low (1.11%–6.67%) after exposure for 24–96 hr. Our findings showed the potential of X. stockiae for controlling Ae. aegypti. Further studies are needed to elucidate the mechanisms through which these bacteria kill Ae. aegypti larvae.     

Bioproduction and anticancer activity of biosurfactant produced by the dematiaceous fungus Exophiala dermatitidis SK80

A new biosurfactant producer was isolated from palm-oilcontaminated soil and later identified through morphology and DNA sequencing as the yeast-like fungus Exophiala dermatitidis. Biosurfactant production was catalyzed by vegetable oil, supplemented with a basal medium. The culture conditions that provided the biosurfactant with the highest surface activity were found to be 5% palm oil with 0.08% NH4NO3, at a pH of 5.3, with shaking at 200 rpm, and a temperature of 30 degrees C for a 14-day period of incubation. The biosurfactant was purified, in accordance with surfactant properties, by solvent fractionation using silica gel column chromatography. The chemical structure of the strongest surface-active compound was elucidated through the use of NMR and mass spectroscopy, and noted to be monoolein, which then went on to demonstrate antiproliferative activity against cervical cancer (HeLa) and leukemia (U937) cell lines in a dose-dependent manner. Interestingly, no cytotoxicity was observed with normal cells even when high concentrations were used. Cell and DNA morphological changes, in both cancer cell lines, were observed to be cell shrinkage, membrane blebbling, and DNA fragmentation.