CRISPR adaptation in Streptococcus thermophilus benefits from phage environmental DNA

Abstract

How can a bacterial adaptive immune system (the CRISPR-Cas system) exist at all, when exposure to a virulent phage is so consistently lethal? We proposed that bacteria might actively sample their genetic environment for phage DNA through natural competence. In testing this hypothesis, we revealed that free phage DNA is important to CRISPR immunity—but not as the source of the immunological memory.