Downloaded from www.microbiologyresearch.org by IP: 54.162.133.179 On: Tue, 09 Feb 2016 19:01:50 Journal of General Microbiology (1987), 133, 361-367. Printed in Great Britain 36 1 Glucose and Penicillin Concentrations in Agar Medium Below Fungal Colonies By GEOFFREY D. ROBSON,’* STEPHEN D. BELL,’ PAUL J. KUHN2 AND ANTHONY P. J. TRINCI’ Microbiology Group, Department of Cell and Structural Biology, School of Biological Sciences, Williamson Building, University of Manchester, Manchester M13 9PL, UK Shell Research Limited, Sittingbourne Research Centre, Sittingbourne, Kent ME9 8AG, UK (Received 18 August 1986) ~ The growth of colonies of Rhizoctonia cerealis and Penicillium chrysogenum on solid media in plate cultures was studied. When grown on defined media containing 10-50 mM-glucose, R. cerealis did not cause a significant reduction in the glucose concentration of the medium in advance of colonization, but did cause the formation of a steep glucose concentration gradient in the substrate below the colony; the medium directly below the centre of a 7 cm diameter colony of R . cerealis was exhausted of glucose even when the fungus was grown on medium containing 50 mwglucose. Penicillin produced by colonies of P . chrysogenum accumulated in the medium in advance of fungal colonization. For a period up to about 18 d after inoculation, the concentration of penicillin in the medium throughout the plate increased with colony development and thereafter, except at the margins of the plate, decreased. INTRODUCTION Pirt (1967) considered the formation of a microbial colony on nutrient agar from a small inoculum and suggested that (1) cells would grow at the maximum exponential rate as long as the nutrient concentrations remained much above the saturation constants (K, values) and no condition inhibitory to growth was developed ; (2) nutrient concentration gradients would be formed beneath the colony; (3) eventually the concentration of some ‘growth-limiting’nutrient would fall virtually to zero in the medium under the centre of the colony and that therefore the central cells would stop growing; (4) growth of the colony would eventually be restricted to a peripheral annulus, the width of which would remain constant and would be determined by the balance between nutrient consumption and nutrient diffusion into the growing zone; and (5) after the establishment of a peripheral growing zone of constant width, the colony would expand in radius at a linear rate. The initial exponential phase of growth predicted by Pirt has been confirmed experimentally for unicellular (Wimpenny & Lewis, 1977; Wimpenny, 1979) and filamentous (Schuhmann & Bergter, 1976) bacteria, and for moulds (Trinci, 1974). Also, Yanagita & Kogane (1963) showed that the specific rate of [32P]orthophosphate uptake (= growth rate) at the centre of a colony of Aspergillus niger occurred at only 3% of the rate in the peripheral growth zone. Slow growth of cells has also been observed at the centre of bacterial colonies and has been attributed to oxygen limitation (Reyrolle & Letellier, 1979; Wimpenny & Parr, 1979). As predicted by Pirt, colonies of most fungi and some bacteria (Burchard, 1974) increase in radius at a linear rate until they have completely colonized the plate. Trinci (1971) showed for a number of fungi that this linear growth rate (K,) was a function of the width of a peripheral annulus (w, the peripheral growth zone) and the organism’s specific growth rate (p), thus K, = wp. However, colonies of most unicellular micro-organisms (Palumbo et al., 1971 ; Rieck et al., 1973; Gray & Kirwan, 1974) and some fungal colonies, e.g. those of Penicillium spp., initially 0001-3659 0 1987 SGM