ABSTRACT
CRISPR-Cas systems, which obstruct both viral infection and incorporation of mobile genetic elements by horizontal transfer, are a specific immune response common to prokaryotes. Antiviral protection by CRISPR-Cas comes at a cost, as horizontally-acquired genes may increase fitness and provide rapid adaptation to habitat change. To date, investigations into the prevalence of CRISPR have primarily focused on pathogenic and clinical bacteria, while less is known about CRISPR dynamics in commensal and environmental species. We designed PCR primers and coupled these with DNA sequencing of products to detect and characterize the presence of cas1, a universal CRISPR-associated gene and proxy for the Type II CRISPR1-Cas system, in environmental and non-clinical Enterococcus isolates. CRISPR1-cas1 was detected in approximately 33% of the 275 strains examined, and differences in CRISPR1 carriage between species was significant. Incidence of cas1 in E. hirae was 73%, nearly three times that of E. faecalis (23.6%) and 10 times more frequent than in E. durans (7.1%). Also, this is the first report of CRISPR1 presence in E. durans, as well as in the plant-associated species E. casseliflavus and E. sulfureus. Significant differences in CRISPR1-cas1 incidence among Enterococcus species support the hypothesis that there is a tradeoff between protection and adaptability. The differences in the habitats of enterococcal species may exert varying selective pressure that results in a species-dependent distribution of CRISPR-Cas systems.
