Abstract
Xenorhabdus and Photorhabdus are gram negative bacteria that can produce several secondary metabolites, including antimicrobial compounds. They have a symbiotic association with entomopathogenic nematodes (EPNs). The aim of this study was to isolate and identify Xenorhabdus and Photorhabdus species and their associated nematode symbionts from Northeastern region of Thailand. We also evaluated the antibacterial activity of these symbiotic bacteria. The recovery rate of EPNs was 7.82% (113/1445). A total of 62 Xenorhabdus and 51 Photorhabdus strains were isolated from the EPNs. Based on recA sequencing and phylogeny, Xenorhabdus isolates were identified as X. stockiae (n = 60), X. indica (n = 1) and X. eapokensis (n = 1). Photorhabdus isolates were identified as P. luminescens subsp. akhurstii (n = 29), P. luminescens subsp. hainanensis (n = 18), P. luminescens subsp. laumondii (n = 2), and P. asymbiotica subsp. australis (n = 2). The EPNs based on 28S rDNA and internal transcribed spacer (ITS) analysis were identified as Steinernema surkhetense (n = 35), S. sangi (n = 1), unidentified Steinernema (n = 1), Heterorhabditis indica (n = 39), H. baujardi (n = 1), and Heterorhabditis sp. SGmg3 (n = 3). Antibacterial activity showed that X. stockiae (bMSK7.5_TH) extract inhibited several antibiotic-resistant bacterial strains. To the best of our knowledge, this is the first report on mutualistic association between P. luminescens subsp. laumondii and Heterorhabditis sp. SGmg3. This study could act as a platform for future studies focusing on the discovery of novel antimicrobial compounds from these bacterial isolates.
Highlights
Xenorhabdus and Photorhabdus are motile, gram-negative rods, facultative anaerobes, nonsporeforming, oxidase-negative, and chemoorganotrophic heterotrophs with respiratory and fermentative metabolism
We found that X. eapokensis was associated with S. sangi, which has been reported as a host for X. vietnamensis and X. thuongxuanensis [25, 51]
We found that the bioactivity (MIC and minimal bactericidal concentration (MBC)) of the crude extracts emphasized that the X. stockiae extracts were active against both gram-positive and gram-negative bacteria
Summary
Xenorhabdus and Photorhabdus are motile, gram-negative rods, facultative anaerobes, nonsporeforming, oxidase-negative, and chemoorganotrophic heterotrophs with respiratory and fermentative metabolism. These bacteria symbiotically inhabit the intestine of the infective juvenile (IJ) stage of entomopathogenic nematodes (EPNs) belonging to the Steinernematidae and Heterorhabditidae families [1]. The IJs of EPNs enter the digestive tract of the insect larvae, penetrate the hemocoel of the insect host, and release the bacteria into the hemolymph. The IJs and bacteria rapidly kill the insect larvae within 24–48 h [2]. Several studies on the bioactive compounds of Xenorhabdus and Photorhabdus against various microorganisms have demonstrated their antibacterial [9], antimicrobial [10], and antiparasitic effects [11]
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