Ionizing Radiation Damage in Micrococcus radiodurans Cell Wall: Release of Polysaccharide

Sublethal⁶⁰ Co γ-irradiation of the bacterium Micrococcus radiodurans in aqueous suspension results in a loss of up to 6% of its cellular dry weight and 30% of its wet weight. In the process some specific cell wall polysaccharides, including a polymer of glucose and N-acylated glucosamine, are released into the surrounding medium. These polysaccharides appear to originate from a hydrophobic site in the middle, lipid-rich, cell wall layer. The damage to this layer which results in the release of these and other polymers may be due to a disruption of this hydrophobic site. The polysaccharide containing glucose and N-acylated glucosamine exists as a high molecular weight polymer in unirradiated cells, but irradiation causes some degradation prior to release. In a free state this polysaccharide is considerably less sensitive to radiolytic degradation than in a bound state. Free radicals generated from surrounding water by ionizing radiation initiate the release, hydroxyl radicals being the most important species. Oxygen protects the cell wall against loss of the polysaccharides, apparently by a mechanism which does not depend on the ability of O₂ to scavenge hydrogen atoms and aqueous electrons.