Survival of rifampin-resistant mutants of Pseudomonas fluorescens and Pseudomonas putida in soil systems

Abstract

The fate of spontaneous chromosomal rifampin-resistant (Rif(r)) mutants of Pseudomonas putida and Pseudomonas fluorescens in sterile and live organic soil from which they were isolated was studied. In sterile native-soil assays, a Rif(r) mutant of P. putida showed no decrease in competitive fitness when compared with the wild-type parent. However, mutants of P. fluorescens were of two general categories. Group 1 showed no difference from the wild type in terms of growth rate, competitive fitness, and membrane protein composition. Group 2 showed a slower growth rate in both minimal and enriched media and an altered membrane protein profile. These mutants also demonstrated decreased competitive fitness compared with the wild-type strain. In live soil, the Rif(r) P. putida strain persisted throughout the 38-day test period with a decay rate of 0.7 log10 CFU/g of soil per 10 days. A group 1 Rif(r) P. fluorescens mutant maintained its inoculated titer for 7 to 10 days and then decayed at a rate of 0.2 to 0.4 log10 CFU/g of soil per 10 days. A group 2 Rif(r) P. fluorescens mutant remained at its titer for 1 to 5 days before decaying at a two- to threefold-faster rate. These findings indicate that rifampin resistance may not be an innocuous mutation in some pseudomonads and that marked strains should be compared with wild-type parents before being used as monitors of parental strain survival. Colonization of sterile soil with either the wild-type or mutant strain precluded normal colonization of the second added strain. These results suggest that there are limited sites for colonization of Pseudomonas species in these soils, a finding which may relate to the eventual fate of the introduced organisms.