The Xenorhabdus nematophila nilABC Genes Confer the Ability of Xenorhabdus spp. To Colonize Steinernema carpocapsae Nematodes

Abstract

Members of the Steinernema genus of nematodes are colonized mutualistically by members of the Xenorhabdus genus of bacteria. In nature, Steinernema carpocapsae nematodes are always found in association with Xenorhabdus nematophila bacteria. Thus, this interaction, like many microbe-host associations, appears to be species specific. X. nematophila requires the nilA, nilB, and nilC genes to colonize S. carpocapsae. In this work, we showed that of all the Xenorhabdus species examined, only X. nematophila has the nilA, nilB, and nilC genes. By exposing S. carpocapsae to other Xenorhabdus spp., we established that only X. nematophila is able to colonize S. carpocapsae; therefore, the S. carpocapsae-X. nematophila interaction is species specific. Further, we showed that introduction of the nilA, nilB, and nilC genes into other Xenorhabdus species enables them to colonize the same S. carpocapsae host tissue that is normally colonized by X. nematophila. Finally, sequence analysis supported the idea that the nil genes were horizontally acquired. Our findings indicate that a single genetic locus determines host specificity in this bacteria-animal mutualism and that host range expansion can occur through the acquisition of a small genetic element.Microbial associations with plant and animal hosts are ubiquitous in nature and tend to show specificity with regard to the taxonomy of each partner. For example, Salmonella enterica serotype Typhi (29), Helicobacter pylori (5), Neisseria gonorrhoeae (28), and Haemophilus influenzae (22) are pathogens with specificities for human or primate hosts, while Rhizobium leguminosarum forms mutualistic nodules only with Phaseolae legumes (30). The molecular basis underlying host range specificity is well understood in the mutualism between the Rhizobiaceae and their plant hosts: specificity is achieved by cross talk involving bacterial and plant signaling factors and receptors (15). In contrast, species-specific interactions between bacteria and animal hosts are not well understood at the molecular or genetic level. Although myriad traits have been proposed to contribute to host range specificity (16,21,36), none of these have been directly demonstrated to play a role in dictating the animal host range. The increasing global concern over new human pathogens emerging through host range expansion makes understanding the genetic basis of host range specificity in bacteria-animal associations essential (6, 27).Insect parasitic nematodes of the genus Steinernema are mutualistically associated with bacteria of the Xenorhabdus genus. This association is a natural and tractable model for understanding the ecology, evolution, and molecular foundations of bacterial interactions with animal hosts. The soil-inhabiting infective stage of a Steinernema nematode is colonized by symbiotic Xenorhabdus bacteria, which it carries and releases into an insect host. Xenorhabdus bacteria provide activities that suppress insect immunity, kill the insect, and enzymatically degrade the cadaver to support nematode ...

Evolution of host specialization in gut microbes: the bee gut as a model

“…Here, the genes nilABC are unique to strains infecting the nematode Steinernema carpocapsae but are absent in other Xenorhabdus; heterologous expression of nilABC in the other Xenorhabdus enable their colonization of S. carpocapsae. 40 These findings beg the question as to whether single-locus dependent specificity, such as rscS and nilABC, are extreme outlier cases or, rather, represent a more general basis for host specialization. In Salmonella enterica, a widespread pathogen of mammals and birds, adaptation to hosts is thought to be multifactorial, with both gene gain and loss playing a part.…”

“…A recent gene-swapping study of V. fischeri strains hosted by Australian or Hawaiian Euprymna squids suggested that multifactor-mediated host specificity is not incompatible with single loci of large effect: there may in fact be multiple genes in a genome capable of greatly altering host affinity. 43 These studies demonstrate that horizontal gene transfer, whether by an experimenter or by natural processes (as proposed for rscS 35 and nilABC 40 ), can greatly alter a microbe's host range. In the plant pathogens Xanthomonas and Pseudomonas, type III secretion system effectors are likely important determinants of host specificity.…”

“…Reported mechanisms of specificity References 51,52 Streptomyces philanthi beewolf wasps host response or behavior 53 Vibrio fischeri squid, fish, environmental biofilm production, bioluminescence 35,43 Xenorhabdus nematophila nematodes nilABC locus 40 …”

Visualizing Bacteria in Nematodes using Fluorescent Microscopy

“…Microscopic analysis reveals both colonization frequency within a population and localization of bacteria to host tissues 14,16,[19][20][21] . This is an advantage over other methods of monitoring bacteria within nematode populations, such as sonication 22 or grinding 23 , which can provide average levels of colonization, but may not, for example, discriminate populations with a high frequency of low symbiont loads from populations with a low frequency of high symbiont loads.…”

“…This is an advantage over other methods of monitoring bacteria within nematode populations, such as sonication 22 or grinding 23 , which can provide average levels of colonization, but may not, for example, discriminate populations with a high frequency of low symbiont loads from populations with a low frequency of high symbiont loads. Discriminating the frequency and load of colonizing bacteria can be especially important when screening or characterizing bacterial mutants for colonization phenotypes 21,24 . Indeed, fluorescence microscopy has been used in high throughput screening of bacterial mutants for defects in colonization 17,18 , and is less laborious than other methods, including sonication 22,[25][26][27] and individual nematode dissection 28,29 .…”