Sensitivity to UV-B Irradiation as Related to Bacterial Life Cycles

Abstract

Although their life cycles may be relatively simple when compared to eukaryotic organisms, bacteria do have distinct stages of growth. Indeed, some bacterial (prokaryote) species have complex developmental life cycles (spore former, myxobacteria, cytophaga). However, even simple bacteria such as Escherichia coli have sharply different sensitivities to UV-B when they are in different stages of growth (Eisenstark 1970). The various stages in a bacterial growth cycle can be illustrated by the states of its chromosomal replication and cell division (Figure 1). In a resting state, the bacterial cell has a single chromosome, double stranded and circular, without any growing forks. The first stage of cell division is the initiation of DNA replication and the development of a chromosomal growing fork. DNA synthesis zippers along the chromosome until the replication cycle ends with two circular chromosomes, the formation of a cell wall partition and binary fission of the cell into two daughter cells. While this appears to be very simple, the total process is complex; it is highly orchestrated with a number of regulatory programs. In particular, cell division is coupled to chromosome replication. It is important to emphasize that there are vast differences in UV-B sensitivity and repair of UV-B damage depending on the stage of the cycle (Figure 2).