Isolation and characterization of a visibly luminous variant ofVibrio fischeri strain ES114 from the sepiolid squidEuprymna scolopes

Abstract

Vibrio fischeri strains isolated from light organs of the sepiolid squidEuprymna scolpes are non-visibly luminous and fast growing in laboratory culture, whereas in the symbiosis they are visibly luminous and slow growing. A spontaneous, visibly luminous, slowgrowing variant was isolated from a laboratory culture of the squid-symbioticV. fischeri strain ES114. Taxonomic and DNA-homology analyses demonstrated that the variant wasV. fischeri and was very similar to the original form. However, the variant grew at one-fourth the rate of the original form, produced 30,000-fold more luminescence, induced luminescence at a lower cell density, and produced a higher level ofV. fischeri luminescence autoinducer. Regulation of luminescence, nonetheless, was similar in the two forms and typical ofV. fischeri with respect to responses to autoinducer, glucose, the iron chelator ethylenediamine-di(o-hydroxyphenyl acetic acid), and 3′:5′-cyclic AMP. Compared to the original form, cells of the variant were smaller, exhibited from zero to two polar, sheathed flagella instead of a tuft of three to eight flagella, produced a deeper yellow-orange pigment, did not acidify media containing glycerol, and produced a more distinct pellicle. The two forms also differed in the levels of several outer membrane and soluble proteins. These results establish a distinctive physiological, morphological, and biochemical dimorphism inV. fischeri ES114 in which the variant exhibits several traits similar toV. fischeri cells in the symbiotic state. The variant and its conversion from the original form in laboratory culture may provide insight into the properties ofV. fischeri cells in the symbiosis and may serve as a model for elucidating the mechanism for their pleiotropic conversion upon colonization of the squid.