Spreading versus biomass production by colonies of the fish pathogen Flavobacterium psychrophilum: role of the nutrient concentration.

Colonies of the fish pathogen Flavobacterium psychrophilum have gliding motility in media with low agar concentrations. Although gliding motility, particularly in Flavobacterium johnsoniae, has been well-studied, little is known about its regulation by environmental factors. The work described here shows that the ability of F. psychrophilum to spread over surfaces depends on nutrient availability. In fact, as the nutrient contents of the medium decreased, spreading was favored and the diameter of the colonies increased. Macroscopy examination revealed modifications in colony morphology as nutrient depletion increased: from a dense and defined colony to the formation of microcolonies inside a general colony structure. Additionally, colony expansion dynamics and population density across the colony radius varied inversely with bacterial biomass production. Motility was an immediate response when bacteria were transferred from a rich to a more diluted medium. Our results suggest that, when nutrients are limiting, F. psychrophilum activates a specific growth mode that enables it to colonize surfaces by means of gliding motility. The use of diluted media allowed the differentiation, among previously isolated F. psychrophilum non-gliding mutants, of those completely unable to glide and those with only partially impaired gliding ability.