aided image analysis (IA) techniques were used to determine specific growth rates (m) in different regions of Salmonella typhimurium colonies grown on solid medium at 30°C. Captured transmission images were converted to optical density values, allowing calculation of m. Data show that growth was exponential throughout the colony during the period studied (between 13 and 21 h after inoculation), but that m varied from 0·69 to 0·37 h −1 at the periphery and centre, respectively. Results demonstrate regional variation in m values within colonies, and support conceptual models of colony development. The technique described illustrates that kinetic growth data can be derived within bacterial colonies, and this may lead towards an improved understanding of the growth dynamics of these structures.

The growth of Salmonella typhimurium colonies on a model food system (agar solidified culture medium) was followed. Colony radius, determined using computer image analysis (IA) techniques, and viable cell number per colony were measured as indices of colony growth, and the effect of [NaCl] (0.5-3.5% (w/v)) and pH (7.0-5.0) on colony growth at 30 degrees C was observed; colonies were point inoculated from serial dilutions. Colony growth (between 13 and 26 h after inoculation) was linear when expressed in terms of radius, and exponential when expressed in terms of viable cell number per colony. Overall, both increasing the [NaCl] and decreasing the pH had little effect on colony growth, other than to delay the onset of linear radial growth. Initial specific growth rate (mu) ranged from 0.73 to 0.87 h-1. Thin films of agar medium on microscope slides allowed the growth of microcolonies to be observed after just 4 h incubation. A greater understanding of the growth kinetics of bacterial colonies, and the effects of environment on such data, may enable better control of foodborne bacterial pathogens, and consequently an improvement in food product safety.

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