Abstract
The colony shape of four yeast species growing on agar medium was measured for 116 days by image analysis. Initially, all the colonies are circular, with regular edges. The loss of circularity can be quantitatively estimated by the eccentricity index, Ei , calculated as the ratio between their orthogonal vertical and horizontal diameters. Ei can increase from 1 (complete circularity) to a maximum of 1.17-1.30, depending on the species. One colony inhibits its neighbour only when it has reached a threshold area. Then, Ei of the inhibited colony increases proportionally to the area of the inhibitory colony. The initial distance between colonies affects those threshold values but not the proportionality, Ei /area; this inhibition affects the shape but not the total surface of the colony. The appearance of irregularities in the edges is associated, in all the species, not with age but with nutrient exhaustion. The edge irregularity can be quantified by the Fourier index, Fi , calculated by the minimum number of Fourier coefficients that are needed to describe the colony contour with 99% fitness. An ad hoc function has been developed in Matlab v. 7.0 to automate the computation of the Fourier coefficients. In young colonies, Fi has a value between 2 (circumference) and 3 (ellipse). These values are maintained in mature colonies of Debaryomyces, but can reach values up to 14 in Saccharomyces. All the species studied showed the inhibition of growth in facing colony edges, but only three species showed edge irregularities associated with substrate exhaustion.
Highlights
The colony shape and its edge have been 2003; Banada et al, 2007; Clemmensen et al, morphological features used traditionally in the 2007; den Hertog et al, 2010; Puchkov, 2010). identification process of microbial strains, includ- Differences in colony morphology have been ing yeasts (Kurtzman et al, 2011). This purely attributed to genotypic and phenotypic factors. Their descriptive methodology has been improved lately link to the taxonomic affiliation by the incorporation of n e w techniques of image implies the phylogenetic value attributed to these analysis that automatically produce quick and characters (Kocková-Kratochvilová, 1990)
2001; Vopálenská ef oZ., 2005; Granek and the change in morphology induced in pairs of Magwene, 2010), which can influence the colonies by intercolony signalling; and (b) the production of extracellular structures or pseudohyphae increase in irregularity of the colony edge with (Halme gf oZ., 2004)
They have been associated the age of the colony. This methodology has with the strain origin (Šťovícek ef a/., 2010) and the age of the in four different yeast species: S. cerevisiae, culture (Granek and Magwene, 2010)
Summary
The colony shape and its edge have been 2003; Banada et al, 2007; Clemmensen et al, morphological features used traditionally in the 2007; den Hertog et al, 2010; Puchkov, 2010). identification process of microbial strains, includ- Differences in colony morphology have been ing yeasts (Kurtzman et al, 2011). Identification process of microbial strains, includ- Differences in colony morphology have been ing yeasts (Kurtzman et al, 2011) This purely attributed to genotypic and phenotypic factors. 2001; Vopálenská ef oZ., 2005; Granek and the change in morphology induced in pairs of Magwene, 2010), which can influence the colonies by intercolony signalling; and (b) the production of extracellular structures or pseudohyphae increase in irregularity of the colony edge with (Halme gf oZ., 2004). They have been associated the age of the colony. The formation of Velcro-like interconnections between cells (Váchová ef oZ., 2011)
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