Autoradiographic study of the localization and evolution of growth zones in bacterial colonies

Incorporation of [3H]leucine in the bacteria of 18 to 48 h-old colonies of Pseudomonas aeruginosa, Pseudomonas putida, Bacillus thuringiensis, Staphylococcus aureus and Escherichia coli enabled the localization of bacterial multiplication sites by means of autoradiography of sagittal sections. In colonies where fast diameter expansion occurred, all the bacteria from the peripheral corona contributed to peripheral growth; in colonies where the expansion was slower, the growth rate of the bacteria in this region was heterogeneous. Besides this peripheral growth, a central region of bacterial multiplication was always found, but with variable localization and extension. In aerobic species, such as P. aeruginosa and P. putida, the central growth site was limited to the zone of oxygen penetration into the bacterial mass. However, in facultatively anaerobic species, bacterial multiplication dependend on nutrient supply. For 48 h-old colonies of S. aureus, a more complex localization of growth seemed to be affected simultaneously by nutrient penetration and accumulation of toxic substances.