Nematode Bacteria Symbiosis

The symbiotic association between entomopathogenic nematode and its bacteria have been reported by several workers (Kaya 1990; Kaya and Gaugler 1993; Tanada and Kaya 1993; Sicard et al. 2005; Somavanshi et al. 2006; Wang et al. 2007a). Infective juveniles of entomopathogenic nematode carry the bacteria Xenorhabdus (in case of steinernematids) or Photorhabdus (in case of heterorhabditids) belonging to Enterobacteriaceae (Forst et al. 1997; Nagesh et al. 2002). These bacteria are Gram-negative, anaerobes, nonspore former and do not have resistant stage. Infective juveniles of Steinernema sp. harbour Xenorhabdus sp. in a special intestinal vesicle, whereas those of Heterorhabditis sp. carry Photorhabdus sp. in the anterior two third part of the intestine (Forst and Clarke 2002).

Entomopathogenic nematodes, Steinernema and Heterorhabditis, belonging to different species harbour different species of bacteria (Table 13.1). The life cycle of nematode-bacteria association is composed of two stages: (i) a free stage in the soil, where the infective juveniles carry bacteria in their guts and search for new insect host, and (ii) a parasitic stage, where the infective juveniles infect insect, release their bacterial symbionts and reproduce in order to produce new infective juveniles (Emelianoff et al. 2007). Both partners benefit from the association. The bacteria provide a nutritive medium for the growth and reproduction of nema-todes. These bacteria are also useful in other two ways: (i) largely responsible for the rapid death of the host, as well as (ii) suppressing other competing organisms by the production of antibiotics. On the other hand, nematode protects the bacteria from the external environment, carries them into the insect haemocoel and in some cases inhibits the insect immune response. Martens et al. (2003) suggested that

Table 13.1 Entomopathogenic nematodes and their symbiotically associated bacteria (Reproduced from Ganguly 2006)

Xenorhabdus nematophila initiates infective juvenile colonization of S. carpocapsae by competing for limited colonization sites or resources within the nematode intestine. Mahar et al. (2008) isolated the bacterial cells and metabolites of entomopathogenic bacterium Pseudomonas luminescens from H. bacteriophora and compared their effectiveness to the larvae of diamondback moth, Plutella xylostella. All different instars of diamondback moth were susceptible to lethal effect of bacterium and its metabolites. However, bacterial cells of Pp. luminescens suspended in broth were slightly more lethal to diamondback moth larvae. Jan et al. (2008) in an experiment found that cells of the bacterial symbiont X. nematophila isolated from S. car-pocapsae are lethal to the pupae of greater wax moth, G. mellonella, beet army-worm, Spodoptera exigua, diamondback moth, Pp. xylostella and blackvine weevil, Otiorhynchus sulcatus in the absence of nematode vectors. The cells of X. nemato-phila were found to enter the haemocoel of the pupae.

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