| Popis: |
CRISPR-Cas is an adaptive immune system that allows bacteria to inactivate mobile genetic elements. Approximately 50% of bacteria harbor CRISPR-cas, however in the human pathogenStaphylococcus aureus, CRISPR-casloci are less common and often studied in heterologous systems. We analyzed the prevalence of CRISPR-casin genomes of methicillin resistantStaphylococcus aureus(MRSA) isolated in Denmark. Only 2.9 % of the strains carried CRISPR-cassystems, but for strains of sequence type ST630 over half were positive. All CRISPR-casloci were type III-A and located within the staphylococcal chromosomal cassette (SCCmec) type V(5C2&5) conferring β-lactam resistance. Curiously, only 23 different CRISPR spacers were identified in 69 CRISPR-positive strains and almost identical SCCmeccassettes, CRISPR arrays andcasgenes, are present in staphylococcal species other thanaureus, suggesting that these were transferred horizontally. For the ST630 strain 110900, we demonstrate that the SCCmeccassette containing CRISPR-casexcises from the chromosome at high frequency. However, the cassette was not transferable under the conditions investigated. One of the CRISPR spacers targets a late gene in the lytic bacteriophage (phage) virus philPLA-RODI, and we show that the system protects against phage infection by reducing phage burst size. However, CRISPR-Cas can be overloaded or bypassed by CRISPR escape mutants. Our results imply that the endogenous type III-A CRISPR-Cas system inS. aureusis active against targeted phages, albeit with low efficacy. This suggests nativeS. aureusCRISPR-Cas offers only partial immunity, and in nature may work in tandem with other defense systems.ImportanceCRISPR-Cas is an adaptive immune system enabling bacteria and archaea to protect themselves against mobile genetic elements such as phages. In strains ofStaphylococcus aureus, CRISPR-casis rare, but when present, it is located within the SCCmecelement encoding resistance to methicillin and other β-lactam antibiotics. We show that the entire module is excisable, with almost identical versions found in different species ofnon-aureusstaphylococci suggesting that the system only rarely acquires new spacers inS. aureus. Additionally, we show that in its endogenous form, theS. aureusCRISPR-Cas is active but inefficient against lytic phages, with phages being able to form escape mutants or overload the system. This leads us to propose that CRISPR-Cas inS. aureusoffers only partial immunity in native systems, and so may work together with other defense systems to prevent phage-mediated killing. |
