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Abstract
A bioassay-guided column chromatographic strategy was adopted in the present study to fractionate the culture extract of Photorhabdus temperata M1021 to identify potential insecticidal and antimicrobial compounds. An ethyl acetate (EtOAc) culture extract of P. temperata was assayed against Galleria mellonella larvae through intra-hemocoel injection and exhibited 100% insect mortality within 60 h. The EtOAc fraction and an isolated compound exhibited phenoloxidase (PO) inhibition of up to 60% and 63%, respectively. The compound was identified as 1,2-benzenedicarboxylic acid (phthalic acid, PA) by gas chromatography-mass spectrometry and nuclear magnetic resonance. PA exhibited insecticidal activity against G. mellonella in a dose-dependent manner, and 100% insect mortality was observed at 108 h after injection of 1 M PA. In a PO inhibition assay, 0.5 and 1 M concentrations of PA were found to inhibit PO activity by 74% and 82%, respectively; and in a melanotic nodule formation assay, nodule formation was significantly inhibited (27 and 10 nodules) by PA (0.5 and 1 M, respectively). PA was furthermore found to have substantial antioxidant activity and maximum antioxidant activity was 64.7% for 0.5 M PA as compare to control. Antibacterial activity was assessed by The MIC values ranged from 0.1 M to 0.5 M of PA. This study reports a multifunctional PA, a potential insecticidal agent, could a factor of insect mortality along with other toxins produced by P. temperata M1021.
Highlights
The genus Photorhabdus consists of nematode-symbiotic, entomophagous Enterobacteria that follow a complex dual-phase life cycle involving mutualistic associations with Heterorhabditis nematodes as well as pathogenic interactions with insects [1]
An initial insecticidal bioassay was performed by injecting 10 μL ethyl acetate (EtOAc) extract of P. temperata M1021 into the hemocoel of fifth instar G. mellonella larvae to assess the insecticidal activity of the extract
These findings are in agreement with those of Jang et al and Salvadori et al [18,19] and suggest that the extracellular metabolites produced by Photorhabdus bacteria are toxic towards a diverse group of insects
Summary
The genus Photorhabdus consists of nematode-symbiotic, entomophagous Enterobacteria that follow a complex dual-phase life cycle involving mutualistic associations with Heterorhabditis nematodes as well as pathogenic interactions with insects [1]. In order to combine symbiosis and pathogenicity, Photorhabdus has to produce an array of metabolites that can facilitate both interactions. The Photorhabdus genome reveals that more than 50% of the genes in the Photorhabdus gene pool are distinct from those of Escherichia coli (commonly used as a model system), suggesting that a large number of novel genes are involved in the pathogenicity and symbiosis of these organisms [2]. [3,4] reveal that nearly 6% of the genome is dedicated to genes predicted to be involved in the production of secondary metabolites [4]. This proportion is greater than the 3.8%. There is significant potential for novel bioactive molecule discovery in Photorhabdus [5]
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