The association of western flower thrips, Frankliniella occidentalis, witha near Erwinia species gut bacteria: Transient or permanent?

Associations between insects and gut bacteria are ubiquitous. It is possibl e to make a distinction between permanent associations (called symbiosis), in which the same type of bacteria is present in more than one generation o f the insect, and transient associations. Transient bacteria are ingested t ogether with food but do not settle in the insect gut in such a way that th ey will be passed on to the next generation. In this study, we describe the permanent association between Western flower thrips (Frankliniella occiden talis), a polyphagous insect species that is a major pest worldwide, and on e type of gut bacteria. On the basis of direct microscopic observations and counts of bacteria, it was found that thrips from the populations studied contained large numbers of bacteria in their hindgut. Bacteria were isolate d from their host and grown on 10 different agar media. The number of bacte ria isolated on agar media equaled the number of direct counts. All isolate s had the same colony morphology. On the basis of their 16S rDNA sequence t hese bacteria were identified as Enterobacteriaceae, closely related to Esc herichia coli. Isolates tested with API 20E biochemical tests were Erwinia species. This was the only type of bacteria found in all thrips individuals on any of the 10 different agar media. Universal primers, which would pote ntially pick up DNA from any bacterium present in the insect, were applied on crude DNA extracts from thrips with bacteria. We only found 16S rDNA seq uences similar to those of the isolated thrips gut bacteria. The same type of bacteria was present in all life stages of the thrips and was found to p ersist in the thrips populations for at least 2 years (more than 50 generat ions). (C) 2001 Academic Press.