Abstract
Entomopathogenic nematodes (EPNs) that are symbiotically associated with Xenorhabdus and Photorhabdus bacteria can kill target insects via direct infection and toxin action. There are limited reports identifying such organisms in the National Park of Thailand. Therefore, the objectives of this study were to identify EPNs and symbiotic bacteria from Nam Nao National Park, Phetchabun Province, Thailand and to evaluate the larvicidal activity of bacteria against Aedes aegypti and Ae. albopictus. A total of 12 EPN isolates belonging to Steinernema and Heterorhabditis were obtained form 940 soil samples between February 2014 and July 2016. EPNs were molecularly identified as S. websteri (10 isolates) and H. baujardi (2 isolates). Symbiotic bacteria were isolated from EPNs and molecularly identified as P. luminescens subsp. akhurstii (13 isolates), X. stockiae (11 isolates), X. vietnamensis (2 isolates) and X. japonica (1 isolate). For the bioassay, bacterial suspensions were evaluated for toxicity against third to early fourth instar larvae of Aedes spp. The larvae of both Aedes species were orally susceptible to symbiotic bacteria. The highest larval mortality of Ae. aegypti was 99% after exposure to X. stockiae (bNN112.3_TH) at 96 h, and the highest mortality of Ae. albopictus was 98% after exposure to P. luminescens subsp. akhurstii (bNN121.4_TH) at 96 h. In contrast to the control groups (Escherichia coli and distilled water), the mortality rate of both mosquito larvae ranged between 0 and 7% at 72 h. Here, we report the first observation of X. vietnamensis in Thailand. Additionally, we report the first observation of P. luminescens subsp. akhurstii associated with H. baujardi in Thailand. X. stockiae has potential to be a biocontrol agent for mosquitoes. This investigation provides a survey of the basic diversity of EPNs and symbiotic bacteria in the National Park of Thailand, and it is a bacterial resource for further studies of bioactive compounds.
Highlights
Xenorhabdus and Photorhabdus are Gram-negative bacteria of the family Enterobacteriaceae and are symbiotically associated with the entomopathogenic nematodes (EPNs), Steinernema and Heterorhabditis respectively [1]
Our findings revealed two genera of EPNs, which consisted of 10 isolates of Steinernema and two isolates of Heterorhabditis
We found X. stockiae was associated with S. websteri, which was consistent with the previous research in Thailand [15, 33]
Summary
Xenorhabdus and Photorhabdus are Gram-negative bacteria of the family Enterobacteriaceae and are symbiotically associated with the entomopathogenic nematodes (EPNs), Steinernema and Heterorhabditis respectively [1]. The infective juvenile stages (IJs) of EPNs containing symbiotic bacteria in their midguts live in the soil of diverse ecological systems [2]. The nematodes can kill the target insects via direct infection through the promotion of the secondary metabolites and toxins produced by symbiotic bacteria. When the IJs of nematodes enter the mouth, anus or spiracles of insect hosts, symbiotic bacteria are released from their intestines into the hemocoel of target insects [3]. The symbiotic bacteria produce several toxins or secondary metabolites, killing the insects by induction of immunosuppression and pervasion of the hemolymph [4]. Several secondary metabolites produced by the symbiotic bacteria have been reported as bioactive compounds with activities including cytotoxic, antimicrobial, antiparasitic and insecticidal ones [6, 7]
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