The importance of the two- and three component systems WalRK and VraTSR for antibiotic resistance and phage susceptibility in Staphylococcus aureus

Sammanfattning: Staphylococcus aureus is a human commensal bacterium, living primarily on the skin and in the nose. It is also a pathogen able to cause a wide range of diseases, including food poisoning, infection of wounds but also more serious infections such as bacteremia and endocarditis. Methicillin-resistant Staphylococcus aureus (MRSA) is resistant to virtually every ß-lactam antibiotic, making infections difficult to treat. Vancomycin is a last-resort antibiotic used for treating MRSA infections; however, increasing reports have been made about resistance development towards vancomycin. S. aureus strains with decreased susceptibility to vancomycin are called vancomycin- intermediate Staphylococcus aureus (VISA). While ß-lactam resistance often arises by acquisition of a gene encoding for an alternative penicillin binding protein (PBP), PBP2a, vancomycin resistance develops through accumulation of mutations in a variety of genes, in particular genes related to the cell wall. These mutations often lead to changed gene expression. For instance, the three-component system VraTSR, involved in regulating the cell wall stress response system is important in VISA, and genes encoding the two-component system WalRK, involved in cell wall maintenance and homeostasis. Furthermore, bacteriophages are viruses that adsorb to the bacterial cell wall in their initial infection stage. Changes in the regulatory systems WalRK and VraTSR might affect the cell wall and consequently phage susceptibility. In this study, we assessed how upregulating individual components of these regulatory systems affected the susceptibility of MRSA strain JE2 to vancomycin and ß-lactam antibiotics, as well as susceptibility to phage infection. Additionally, differences in the response to overexpression of these genes in JE2 compared to JE2-derived VISA strains were investigated. Results showed that overexpression of single genes affected the different resistances in S. aureus. For instance, vraR increased the minimal inhibitory concentration (MIC) of both oxacillin and vancomycin. Furthermore, epistasis seemed to be important since overexpression of several genes had opposite effects in JE2 and vancomycin-adapted JE2 with regards to bacteriophage susceptibility. This suggests prediction of the effect of single genes on susceptibility to vancomycin, ß-lactam antibiotics and phages is difficult, and further research is needed to gain a deeper understanding of resistance mechanisms in VISA. That knowledge is of importance if vancomycin is still going to be an effective treatment for MRSA infections in the future.