A simple, direct plating method, alternative to dilution plating, for estimation of the abundance of Penicillium verrucosum on incubated cereal grain

Because dilution plating is more labor intensive than direct plating, we would like to propose the use of a direct plating technique for estimation of Penicillium verrucosum abundance in cereal grain in incubation studies, with use of very selective and indicative for the fungus agar DYSG medium. The proposed method is based on the measurement of the diameter of P. verrucosum colonies grown around cereal kernels placed on DYSG medium. In three different experiments wheat grain contained a great range of P. verrucosum CFU number (from <25 to 77 x 10⁶ per 1 g). When P. verrucosum CFU number was at least as high as 10² per 1 g of the grain, 100% of the wheat kernels, placed on the surface of DYSG medium, were surrounded by colonies of P. verrucosum. The diameter (x, mm) of P. verrucosum colonies surrounding wheat kernels on DYSG medium was correlated with the fungal CFU number (y) on the wheat grain. The relationship is described by the exponential regression equation (y = 0.1258 e 0.9309x, R²=0.96). The relationship became linear (y = 0.404 x-0.901, R² = 0.96) after transformation of P. verrucosum CFU numbers to logarithms to base 10.