The study investigates the role of CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) in providing acquired resistance against bacteriophages in prokaryotes. Researchers found that following viral infection, bacteria integrate new spacers derived from phage genomic sequences into their CRISPR loci. These spacers, when present in specific numbers and configurations, can modify the phage-resistance phenotype of the cell. The study used Streptococcus thermophilus, a bacterium commonly used in dairy cultures, as a model organism. By generating phage-resistant mutants and analyzing their CRISPR loci, the researchers observed that the addition or deletion of particular spacers correlated with changes in phage sensitivity. They also found that the presence of spacers identical to phage sequences provided resistance against those phages. The study further explored the role of *cas* genes in CRISPR-mediated immunity, suggesting that Cas5 may act as a nuclease and that Cas7 is involved in the synthesis and insertion of new spacers. The findings highlight the dynamic and rapid evolutionary changes in CRISPR loci driven by phage exposure, providing insights into the adaptive immune mechanisms of prokaryotes.The study investigates the role of CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) in providing acquired resistance against bacteriophages in prokaryotes. Researchers found that following viral infection, bacteria integrate new spacers derived from phage genomic sequences into their CRISPR loci. These spacers, when present in specific numbers and configurations, can modify the phage-resistance phenotype of the cell. The study used Streptococcus thermophilus, a bacterium commonly used in dairy cultures, as a model organism. By generating phage-resistant mutants and analyzing their CRISPR loci, the researchers observed that the addition or deletion of particular spacers correlated with changes in phage sensitivity. They also found that the presence of spacers identical to phage sequences provided resistance against those phages. The study further explored the role of *cas* genes in CRISPR-mediated immunity, suggesting that Cas5 may act as a nuclease and that Cas7 is involved in the synthesis and insertion of new spacers. The findings highlight the dynamic and rapid evolutionary changes in CRISPR loci driven by phage exposure, providing insights into the adaptive immune mechanisms of prokaryotes.