Abstract: Inoculation of ATM-deficient mice with probiotic microorganisms, such as L. johnsonii, changed immune parameters, decreased a marker of DNA damage and increased the lifespan of the mice. Compositions and methods described herein are useful for the treatment and prevention of Ataxia telangiectasia and other cancer-prone diseases, such as p53 deficiency-associated cancers. Compositions and methods of the present invention are also useful for treating and preventing radiation-induced toxicity to normal tissue in a subject being exposed to radiation. Compositions and methods of the invention can increase lifespan in a simple, non-invasive manner.