Which bacterium was the first microorganism shown to use CRISPR sequences as an adaptive immune system?

Streptococcus thermophilus was the first microorganism in which CRISPR sequences were experimentally shown to provide adaptive immunity against bacteriophages.

In 2007, researchers studying industrial S. thermophilus strains showed that bacteria could acquire short pieces of phage DNA and insert them into the CRISPR array. The added sequences helped the bacteria resist later infection by matching the invading phage genome. This work supplied direct experimental evidence for the immune function of CRISPR systems.

CRISPR arrays act together with CRISPR-associated, or Cas, proteins. In many systems, stored sequence information guides Cas proteins toward matching foreign genetic material. Scientists later adapted these systems into powerful genome-editing tools, but their natural role is defense against genetic invaders such as bacteriophages.

S. thermophilus is also a familiar food bacterium: it is one of the standard starter cultures used with Lactobacillus delbrueckii subsp. bulgaricus in yogurt production. Its food role and its importance in CRISPR research are separate facts.