Distribution of Genetic Determinants Associated with CRISPR-Cas Systems and Resistance to Antibiotics in the Genomes of Archaea and Bacteria

The CRISPR-Cas system represents an adaptive immune mechanism found across diverse Archaea and Bacteria, allowing them to defend against invading genetic elements such as viruses and plasmids. Despite its broad distribution, the prevalence and complexity of CRISPR-Cas systems differ significantly between these domains. This study aimed to characterize and compare the genomic distribution, structural features, and functional implications of CRISPR-Cas systems and associated antibiotic resistance genes in 30 archaeal and 30 bacterial genomes. Through bioinformatic analyses of CRISPR arrays, <i>cas</i> gene architectures, direct repeats (DRs), and thermodynamic properties, we observed that Archaea exhibit a higher number and greater complexity of CRISPR loci, with more diverse cas gene subtypes exclusively of Class 1. Bacteria, in contrast, showed fewer CRISPR loci, comprising a mix of Class 1 and Class 2 systems, with Class 1 representing the majority (~75%) of the detected systems. Notably, Bacteria lacking CRISPR-Cas systems displayed a higher prevalence of antibiotic resistance genes, suggesting a possible inverse correlation between the presence of these immune systems and the acquisition of such genes. Phylogenetic and thermodynamic analyses further highlighted domain-specific adaptations and conservation patterns. These findings support the hypothesis that CRISPR-Cas systems play a dual role: first, as a defense mechanism preventing the integration of foreign genetic material-reflected in the higher complexity and diversity of CRISPR loci in Archaea-and second, as a regulator of horizontal gene transfer, evidenced by the lower frequency of antibiotic resistance genes in organisms with active CRISPR-Cas systems. Together, these results underscore the evolutionary and functional diversification of CRISPR-Cas systems in response to environmental and selective pressures.