This 1968 study investigates the mechanism of ultraviolet (UV) induction of prophage in lysogenic Staphylococcus aureus strain 111. The researchers found that after UV irradiation, bacteria undergo one or two cell divisions before free phage is released, and the number of infective centers (phage-producing cells) doubles within 30 minutes. When celbenin, a penicillin derivative that inhibits cell wall synthesis, was added immediately after irradiation, phage development was blocked. However, delays of 10-30 minutes before drug addition allowed increasing numbers of infective centers to produce free phage. These results suggest UV irradiation inhibits synthesis of new repressor protein (which normally prevents phage development), while existing repressor molecules persist. Bacterial growth then dilutes the repressor concentration below an effective level, allowing prophage induction. This explains why residual cell growth during the latent period is essential for phage development, rather than repressor being directly destroyed by radiation or being inherently unstable.

Key findings

• UV-irradiated S. aureus cells undergo 1-2 divisions before releasing free phage, with infective centers doubling in 30 minutes while colony-forming units remain constant
• Celbenin (cell wall synthesis inhibitor) blocks phage development when added immediately post-irradiation but cannot prevent development if added after 10-30 minutes, indicating infective centers must divide for phage production
• UV irradiation inhibits new repressor protein synthesis while leaving pre-existing repressor intact; bacterial growth dilutes residual repressor below effective concentration, permitting prophage induction
• The prolonged latent period after UV induction compared to phage infection from outside is explained by the time required for bacterial growth to dilute repressor levels