Potential for misidentification of a spore-forming Paenibacillus polymyxa isolate as an endophyte by using culture-based methods

Abstract

While Paenibacillus polymyxa strain Pw-2 has been identified as an endophyte of lodgepole pine (M. Shishido, B. M. Loeb, and C. P. Chanway, Can. J. Microbiol. 41:707-713, 1995), P. polymyxa strain L6 has not, a distinction that could be explained by the differential abilities of these isolates to form spores, rather than the differential abilities to colonize the interior tissues of lodgepole pine. Chemical disinfection was used to destroy bacteria on the root exterior, but bacterial endospores are known for their ability to withstand chemical disinfection, and strain Pw-2 was found to produce 300 to 11,000 times more germinating endospores than strain L6 under the experimental conditions used by Shishido et al. (Can. J. Microbiol. 41:707-713, 1995). Attempts to identify strain Pw-2 within lodgepole pine root tissues by using confocal microscopy techniques failed. We discuss the possibility that spore-forming bacteria can be mistakenly identified as endophytes when culture-based methods alone are used.

FIG. 1.

Recovery of P. polymyxa strains L6 and Pw-2 from washes of lodgepole pine seedling roots before (a) and after (b) heat treatment (55°C, 30 min), determined for roots after 6, 9, or 12 weeks of incubation. Standard errors are indicated by bars. Black bars represent data for strain Pw2; gray bars represent data for strain L6. The average CFU of L6 recovered per root after heat treatment (±standard error) for weeks 6, 9, and 12 were 12 (±9), 520 (±380), and 4,200 (±2,000), respectively.