It has long been known that tissues of healthy plants can be colonized internally by nonpathogenic bacteria. Such microorganisms are often referred to as bacterial endophytes. Defined literally, an endophyte is an organism that lives inside a plant (i.e., the term “endo” is derived from the Greek word “endon,” meaning “within,” and “phyte” from the Greek “phyton,” meaning “plant”) 1. Notwithstanding their discovery almost 50 years ago, comparatively little was known about bacterial endophyte diversity, population dynamics, and effects on host plant growth until recently. However, many common soil bacteria genera possess strains that have been isolated from internal plant tissues (Table 1) (1, 2). In contrast, there are fewer plant genera with species known to harbor bacterial endophytes (Table 2), but this is likely due to a lack of research rather than any special characteristic of these plants that precludes endophyte colonization. In addition to the plant tissues listed in Table 2, endophytic bacteria have also been isolated from within plant seeds of several plant species.

Table 1 Examples of genera of nonpathogenic bacteria that have been isolated from internal plant tissues

Bacterial genera
Acetobacter	Clavibacter	Pasteurella
Achromobacter	Comamonas	Photobacterium
Acidovorax	Corynebacterium	Phyllobacterium
Acinetobacter	Curtobacterium	Providencia
Actinomyces	Deleya	Pseudomonas
Agrobacterium	Enterobacter	Psychrobacter
Alcaligenes	Erwinia	Rahnella
Arthrobacter	Esherichia	Rhizobium
Azoarcus	Flavobacterium	Rhodococcus
Azorhizobium	Herbaspirillum	Rickettsia
Azospirillum	Kingella	Serratia
Bacillus	Klebsiella	Shewanella
Bordetella	Lactobacillus	Sphingomonas
Burkholderia	Leuconostoc	Staphylococcus
Capnocytophaga	Methylobacterium	Stenotrophomonas
Cellulomonas	Micrococcus	Variovorax
Chryseobacterium	Moraxella	Vibrio
Citrobacter	Pantoea	Xanthomonas
		Yersinia