Evaluation of Colony Growth of Bacteria and the Surface Environment of Beef1

Abstract

Bacterial growth in or on solid and semi-solid foods primarily involves growth of micro-colonies, which are influenced by restrictive factors of little consequence in liquid foods. To evaluate bacterial growth on solid foods, methods were developed to study colony growth with emphasis on the early stages. Pure cultures of Escherichia coli, Lactobacillus bulgaricus. Pseudomonas fluorescens, and Staphylococcus aureus had distinctly different microscopic colony appearance. Growth expressed as numbers of cells per colony was exponential and linear when expressed as diameter. Mature colonies had a population density of approximately 10¹⁰ viable cells. Microcolonies of bacteria were particularly resistant to dispersion and required the presence of physical agents such as sea sand in the dispersion process. Colony growth on small test strips of beef was extremely susceptible to the relative humidity of the environment. Furthermore, availability of nutrients on the surface of beef apparently influenced growth. Pregrowth of P. fluorescens followed by destruction with gamma rays enhanced subsequent growth of L. bulgaricus. Bacterial growth at the micro-colony stage can present some unique considerations in evaluating the microenvironment of solid foods.