Decolonisation of nasal MRSA with the novel antimicrobial peptide ranalexin in combination with lysostaphin

MRSA can colonise a range of anatomical locales including the anterior nares and it is known that MRSA infection occurs in 20-60% of colonised patients. Decolonisation therapies such as mupirocin are not always effective and recent data from bacteraemias in Scotland show that 7% of MRSA isolates are resistant to mupirocin.
The aim of this study was to evaluate the effect of the antimicrobial peptide, ranalexin, in combination with lysostaphin, for use in decolonisation of nasal MRSA. The minimum inhibitory concentration (MIC) of ranalexin/lysostaphin (ratio of 100:1) and mupirocin was tested, according to BSAC guidelines, against 12 planktonic cultures of healthcare and community-associated MRSA. Biofilms, grown for 48h, were treated with 1x, 4x and 10x the MIC concentration of ranalexin/lysostaphin and mupirocin. Cell viability was quantified using either CellTitre Blue, which is a metabolic dye measured by fluorescence or sonication.
The planktonic MIC range of ranalexin/lysostaphin and mupirocin was 2-4mg/L and 0.25->512mg/L, respectively. One HA-MRSA and one CA-MRSA strain tested displayed resistance to mupirocin with MICs of 32mg/L and >512mg/L, respectively. The viable cell number remaining within biofilms treated with ranalexin/ lysostaphin was reduced ~75% when exposed to 10x MIC (p=<0.001). By comparison a ~50% (p=>0.05) reduction in cell viability was achieved at 10x MIC of mupirocin. These results show that there is potential in this novel combination therapy as an additional agent for the decolonisation of patients who are nasal carriers of MRSA