THE EFFECT OF SODIUM-CHLORIDE CONCENTRATION AND PH ON THE GROWTH OF SALMONELLA-TYPHIMURIUM COLONIES ON SOLID MEDIUM

The growth of Salmonella typhimurium colonies on a model food system ( agar solidified culture medium) was followed. Colony radius, determine d using computer image analysis (IA) techniques, and viable cell numbe r 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 de grees C was observed; colonies were point inoculated from serial dilut ions. 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 grow th 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 observ ed after just 4 h incubation. A greater understanding of the growth ki netics of bacterial colonies, and the effects of environment on such d ata, may enable better control of foodborne bacterial pathogens, and c onsequently an improvement in food product safety.