Emergence of a laccase-positive variant of Azospirillum lipoferum occurs via a two-step phenotypic switching process

Two variants have been isolated from the wild-type Azospirillum lipoferum s train 4B. The first variant, 4V(I), spontaneously emerged from the wild-typ e at frequencies in the order of 10(-4) to 10(-3) per cell generation. Comp ared to the wild-type, the 4V(I) variant gained (production of a carotenoid -like pigment, assimilation of certain carbohydrates) and lost (swimming mo tility, reduction of triphenyl tetrazolium chloride, acid production from c ertain sugars) apparently unrelated phenotypic characteristics. Only from t he 4VI variant, a second atypical stable form, variant 4V(II), which acquir ed laccase activity and ability to produce melanin, appeared under very spe cific conditions, namely growth at extremely low oxygen concentrations. Nei ther of the variants was able to revert to the parental phenotype. The resu lts suggest that atypical non-motile laccase-positive isolates of A. lipofe rum that are found in the rice rhizosphere originate from wild-type (motile , laccase-negative) cells via a two-step phenotypic switching event, a non- motile laccase-negative variant being an intermediate phase. (C) 1999 Feder ation of European Microbiological Societies. Published by Elsevier Science B.V. All rights reserved.