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Kinetics and Strain Specificity of Rhizosphere
and Endophytic Colonization by Enteric Bacteria on Seedlings of Medicago sativa
and Medicago truncatula.
Appl Environ Microbiol 2003 Mar;69(3):1783-90
Dong Y, Iniguez AL, Ahmer BM, Triplett EW.
Department of Agronomy, University of Wisconsin-Madison, Madison, Wisconsin
53706. Department of Microbiology, The Ohio State University, Columbus, Ohio
43210.
The presence of human-pathogenic, enteric bacteria on the surface and in the
interior of raw produce is a significant health concern. Several aspects of the
biology of the interaction between these bacteria and alfalfa (Medicago sativa)
seedlings are addressed here. A collection of enteric bacteria associated with
alfalfa sprout contaminations, along with Escherichia coli K-12, Salmonella
enterica serotype Typhimurium strain ATCC 14028, and an endophyte of maize,
Klebsiella pneumoniae 342, were labeled with green fluorescent protein, and
their abilities to colonize the rhizosphere and the interior of the plant were
compared. These strains differed widely in their endophytic colonization
abilities, with K. pneumoniae 342 and E. coli K-12 being the best and worst
colonizers, respectively. The abilities of the pathogens were between those of
K. pneumoniae 342 and E. coli K-12. All Salmonella bacteria colonized the
interiors of the seedlings in high numbers with an inoculum of 10(2) CFU,
although infection characteristics were different for each strain. For most
strains, a strong correlation between endophytic colonization and rhizosphere
colonization was observed. These results show significant strain specificity for
plant entry by these strains. Significant colonization of lateral root cracks
was observed, suggesting that this may be the site of entry into the plant for
these bacteria. At low inoculum levels, a symbiosis mutant of Medicago
truncatula, dmi1, was colonized in higher numbers on the rhizosphere and in the
interior by a Salmonella endophyte than was the wild-type host. Endophytic entry
of M. truncatula appears to occur by a mechanism independent of the symbiotic
infections by Sinorhizobium meliloti or mycorrhizal fungi.
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