Date Published: November 23, 2009
Publisher: Public Library of Science
Author(s): Vaughn S. Cooper, Wendy A. Carlson, John J. LiPuma, Niyaz Ahmed. http://doi.org/10.1371/journal.pone.0007961
Abstract: The nematode Caenorhabditis elegans may be killed by certain pathogenic bacteria and thus is a model organism for studying interactions between bacteria and animal hosts. However, growing nematodes on prey bacteria may influence their susceptibility to potential pathogens. A method of axenic nematode culture was developed to isolate and quantify interactions between C. elegans and potentially pathogenic strains of the Burkholderia cepacia complex. Studying these dynamics in liquid solution rather than on agar surfaces minimized nematode avoidance behavior and resolved more differences among isolates. Most isolates of B. cenocepacia, B. ambifaria and B. cepacia caused 60–80% mortality of nematodes after 7 days, whereas isolates of B. multivorans caused less mortality (<25%) and supported nematode reproduction. However, some B. cenocepacia isolates recovered from chronic infections were much less virulent (5–28% mortality). As predicted, prior diet altered the outcome of interactions between nematodes and bacteria. When given the choice between Burkholderia and E. coli as prey on agar, axenically raised nematodes initially preferred most lethal Burkholderia isolates to E. coli as a food source, but this was not the case for nematodes fed E. coli, which avoided toxic Burkholderia. This food preference was associated with the cell-free supernatant and thus secreted compounds likely mediated bacterial-nematode interactions. This model, which isolates interactions between bacteria and nematodes from the effects of prior feeding, demonstrates that bacteria can influence nematode behavior and their susceptibility to pathogens.
Partial Text: To understand why only some bacteria are prone to cause disease, it is important to study how they interact with the organisms that are most likely targeted by their offenses or defenses. Nematodes, perhaps the most abundant and diverse metazoans , are excellent candidates. Relationships between bacteria and nematodes span the continuum of parasitism to mutualism: for some nematodes, bacteria are the preferred food source; for others, bacteria are necessary partners for infesting eukaryotic hosts, and for others still, bacteria are pathogenic. However, the conditions that influence how nematodes sense and respond to bacteria are not well understood, so new methods that further this research are needed. More specifically, using nematodes as model hosts for understanding infection requires greater understanding of how nematode physiology and behavior can be affected by bacteria.
Like most animals, nematodes are capable of pathogen avoidance behavior  and such behavior may be influenced by their recent feeding history , . We found that the traditional method of growing C. elegans on its typical food source, E. coli, caused nematodes to avoid lethal bacteria more quickly and to a greater extent, which allowed more nematodes to survive. Thus, axenic nematode culture should be considered for future studies of bacterial pathogenicity because it tends to increase nematode susceptibility to virulent strains. Further, nematode sensitivity to the cues of pathogenicity appears to require training by prior exposure to non-pathogenic bacteria: naïve worms were attracted to the most virulent strains, whereas worms that had been fed bacteria were much more discriminating.