Identification and molecular characterization of symbiotic bacteria (Xenorhabdus sp.) associated with indigenous entomopathogenic nematode (Steinernema sp.)

Abstract

Members of the genus Xenorhabdus and Photorhabdus are symbiotically associated with the infective juveniles (IJs) of the entomopathogenic nematodes belonging to the genera Steinernema and Heterorhabditis. These bacteria are instrumental in killing the insects by causing septicemia within 24 to 48 hours, and also providing nutrition for the nematodes. These nematodes along with their bacteria are soil dwelling lethal parasites of insects, which are being used as effective biopesticides for promoting organic farming and for managing several insect pests of crops as well as some household pests. An investigation was undertaken with the objectives to (i) isolate the symbiotic bacterium associated with indigenous Entomopathogenic nematode species, (ii) characterize them based on morphological, biological, biochemical and molecular parameters and (iii) establish the species identity of the bacterium isolated from Entomopathogenic nematodes. newlineExtensive surveys were carried out throughout Meghalaya and Kerala, the two states of India and collected 86 soil samples. Soil samples were baited with Galleria mellonella for isolating entomopathogenic nematodes (EPNs). Bacteria associated with EPNs were isolated from the hemolymph of insect on NBTA medium. The symbiotic bacterium associated with S. meghalayensis and Steinernema sp strain IARI-EPN-SGkr, were characterized and proposed as new species of the genus Xenorhabdus, i.e. X . meghalayensis sp. nov. and X. keralensis sp. nov. on the basis of 16s rRNA gene sequence comparison, RFLPs, cultural, biochemical and morphological characteristics.The nearest phylogenetic relatives of bacterium X. meghalayensis were X. kozodoii (97%), X. stockiae (95%) and X. hominickii (95%) based on similarity in 16s rDNA sequence. It is different from other known species of the genus in their ability to cause distinct starch hydrolysis, being positive for Tween 80, Arabinose and for N-Acetyl-D-Glucosamine and D-Gluconic Acid utilization.