When grown on solid surface under optimal growth conditions, S. cerevisiae and C. albicans cells form concentric rounded colonies. Under particular conditions such cultures modify their growth patterns and form filaments (pseudohyphae) that are capable of invading agar and adhering to surfaces. Under some conditions S. cerevisiae and C. albicans cells form very large colonies (biofilm/mat formation). The capability to modify growth pattern, primarily formation of pseudoyphae is strongly associated with the pathogenicity of the culture. The biological role of filamentous/invasive growth is not clear. As this growth pattern is observed primarily in response to nitrogen (ammonium) starvation, the filaments are considered as a way through which the culture forages for nutrients. We shall describe a series of observation suggesting that the modifications in growth patterns are not related to nitrogen starvation. For example, high concentrations of NaCl, KCl, or sorbitol block completely pseudohyphal growth or mat formation. Also, cultures that are forming filaments or biofilm have no apparent advantage, in terms of survival under starvation, over cultures that form common colonies. We propose that modifying the colony organization is a protective response against hypo-osmolarity and dryness, imposed by reduced ammonium concentrations, rather than a response to starvation.
