Abstract
This study presents a unifying theory about the morphology of bacterial structures, i.e. filamentous or compact and smooth structures. It is postulated that diffusion micro-gradients of substrate concentration-inside biological floes are the main factor responsible for the morphology of bacterial structures. At low bulk liquid substrate concentrations the micro-gradients of substrates in floes are steep and filamentous bacterial structures, and especially filamentous bacteria due to their preferential unidirectional growth, give easier access to the substrate at the outside of the floes, and thereby proliferate. At high bulk liquid substrate concentration the micro-gradients of substrates in floes are lower and there is no substantial advantage for filamentous organisms to grow outside the floc. This hypothesis is supported by laboratory results, which are summarized in this paper.