Stages of Infection during the Tripartite Interaction between Xenorhabdus nematophila , Its Nematode Vector, and Insect Hosts

Abstract

Bacteria of the genus Xenorhabdus are mutually associated with entomopathogenic nematodes of the genus Steinernema and are pathogenic to a broad spectrum of insects. The nematodes act as vectors, transmitting the bacteria to insect larvae, which die within a few days of infection. We characterized the early stages of bacterial infection in the insects by constructing a constitutive green fluorescent protein (GFP)-labeled Xenorhabdus nematophila strain. We injected the GFP-labeled bacteria into insects and monitored infection. We found that the bacteria had an extracellular life cycle in the hemolymph and rapidly colonized the anterior midgut region in Spodoptera littoralis larvae. Electron microscopy showed that the bacteria occupied the extracellular matrix of connective tissues within the muscle layers of the Spodoptera midgut. We confirmed the existence of such a specific infection site in the natural route of infection by infesting Spodoptera littoralis larvae with nematodes harboring GFP-labeled Xenorhabdus. When the infective juvenile (IJ) nematodes reached the insect gut, the bacterial cells were rapidly released from the intestinal vesicle into the nematode intestine. Xenorhabdus began to escape from the anus of the nematodes when IJs were wedged in the insect intestinal wall toward the insect hemolymph. Following their release into the insect hemocoel, GFP-labeled bacteria were found only in the anterior midgut region and hemolymph of Spodoptera larvae. Comparative infection assays conducted with another insect, Locusta migratoria, also showed early bacterial colonization of connective tissues. This work shows that the extracellular matrix acts as a particular colonization site for X. nematophila within insects.

Xenocin Export by the Flagellar Type III Pathway in Xenorhabdus nematophila

“…As a consequence, most of the producer population is killed, leaving a small number to withstand the unfavorable conditions (15). In X. nematophila, since antimicrobials are produced in a nutrient-rich insect carcass to control putrefying competitors (40,41), it may be counterproductive to reduce its own population, considering the requirement of high population density for efficient nematode reproduction (42)(43)(44). This might have driven xenocin export by an alternate route without lysing the cell, as it would be beneficial for the evolutionary success of the symbiotic relationship.…”

Ready or Not: Microbial Adaptive Responses in Dynamic Symbiosis Environments

“…2A) is an obligate step in the reproductive fitness of both the nematode and bacterium and is therefore predictable in their integrated evolutionary life histories. When an IJ infects an insect, it gains entry into the insect hemocoel, into which it releases its Xenorhabdus symbiont (37)(38)(39). The hemocoel cavity is bathed in hemolymph, a relatively nutrient-rich fluid (40).…”

“…The insect cadaver serves as a nutrient source for bacterial growth and nematode reproduction, and Xenorhabdus bacteria are key players in the liberation of the nutrients (carbohydrates, lipids, and amino acids) contained within the insect biomass (26). Events that may precede and indicate impending insect death, and that therefore may serve as signals for preadaptive responses, are the release of nutrients from dying hemocytes (49), the growth of competing microorganisms (50), or changes in the insect intestinal barrier (39).…”

OpnS, an Outer Membrane Porin ofXenorhabdus nematophila, Confers a Competitive Advantage for Growth in the Insect Host

“…The nonfeeding infective juvenile form of the nematode (IJ) exists in the soil and carries the bacteria in a specialized receptacle region in the anterior intestine (4,39). The IJ invades susceptible insect species and enters the hemocoel, where exposure to insect hemolymph stimulates the movement of bacteria down the intestine and out of the anus (36,39). Together, the nematode and bacteria kill the insect host.…”