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Appl Environ Microbiol. 1992 April; 58(4): 1095-1101
Copyright © 1992, American Society for Microbiology. All Rights Reserved.

Extracellular Polysaccharide Is Not Responsible for Aluminum Tolerance of Rhizobium leguminosarum bv. Phaseoli CIAT899

Mark T. Kingsley^,* and B. Ben Bohlool

Department of Microbiology, University of Hawaii, Honolulu, Hawaii 96822

ABSTRACT

Strain UHM-5, a pSym^- Exo^- derivative of the aluminum-tolerant Rhizobium leguminosarum bv. phaseoli strain CIAT899, was equally tolerant of aluminum (Al) as the parental culture. Dialyzed culture supernatants of the wild-type cells grown in YEM broth (10⁹ cells ml^-1) contained 185 µg of glucose equivalents ml^-1 whereas UHM-5 culture supernatants yielded 2 µg of glucose ml^-1. The Exo^- derivative and the parental strain gave essentially similar growth in medium containing from 0 to 300 µM Al, indicating that the pSym of CIAT899, and extracellular polysaccharide, were not involved in the aluminum tolerance of this strain. However, increasing the level of Al from 80 to 150 µM increased the lag phase, induced a slight killing of the inoculum, and depressed the final populations by about fivefold. Doubling the aluminum concentration from 150 to 300 µM presented a severe aluminum stress to CIAT899 and UHM-5: the inoculum level dropped 10-fold, indicating killing of the inoculum, and remained depressed for ca. 4 days before continuing to grow slowly; the final population was decreased 15-fold relative to that of cultures grown in medium containing 80 µM Al. The production by CIAT899 of other extracellular or intracellular aluminum tolerance factors was investigated in culture by using aluminum-sensitive rhizobia as stress indicators. These experiments, conducted at 80 µM Al, demonstrated that CIAT899 produced neither extracellular nor intracellular products that could alleviate toxicity for the Al-sensitive indicator rhizobia.

^* Corresponding author.

Present address: Environmental Microbiology, Battelle Pacific Northwest Laboratory, P.O. Box 999, P7-54, Richland, WA 99352.

While this manuscript was in preparation, Ben Bohlool was killed, tragically, in an automobile accident. His unique approach to microbial ecology brought great enjoyment to learning about and appreciating the tremendous diversity of our microbial world. His students and colleagues, the staff of the NifTAL Project, and especially his wife, Carol, and daughter, Kiaora, will miss his presence greatly.

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