Development of a novel antimicrobial combination to target methicillin-resistant Staphylococcus aureus (MRSA)

Objective 1 - In vitro, the combination of ranalexin and lysostaphin displays potent, synergistic inhibition of Staphylococcus aureus MRSA. In order to identify whether this combination treatment can be further developed commercially for use in a clinical setting it is vital that efficacy of the treatment is shown in 'proof of principal' experiments. Thus, the combination treatment will be assessed in a number of key in vivo murine models that mimic clinical conditions of infection by S. aureus/MRSA in humans. (OUT-SOURCED TO HUNTINGDON LIFE SCIENCES).

(i) In a murine model of MRSA septicaemia, treatment with the combination of ranalexin and lysostaphin indicated enhanced efficacy in reducing MRSA kidney burden compared to the individual components alone. The combination treatment completely cleared MRSA from the kidneys of three mice compared to just one in the vancomycin group and none in the negative control of ranalexin or lysostaphin alone treatment groups.

(ii) In a rabbit model of MRSA wound infection, the combination showed significant enhanced efficacy over the individual components alone and demonstrated considerable reductions in MRSA present on the gauze dressings and, more importantly, in the wound itself. In fact, the combination was more effective in treating infected wounds than the positive control, vancomycin.

Objective 2 - Identify whether the combination of ranalexin and lysostaphin inhibits the colonisation of human skin by MRSA using an ex vivo model system utilising real human skin obtained from the Biobank (IN-HOUSE).

(iii) The combination kills MRSA on the surface of human skin and shows enhanced efficacy for clearing skin carriage compared to the individual components alone.

Objective 3 - Preliminary experiments will be carried out to understand the mode of action of the synergistic action of the combination of ranalexin and lysostaphin on MRSA. (IN-HOUSE).

(iv) In vitro studies have revealed that the combination is effective and synergistic: after storage for 5 days at 37 ºC; at low pH and high ion concentration typical for human skin; against lysostaphin-resistant mutants, VISA isolates and non-multiplying cells; in the presence of human serum.

Objective 4 - In vitro experiments will be carried out to determine whether the combination treatment can effectively target biofilms of S. aureus. (IN-HOUSE).

(v) The combination treatment is bactericidal for MRSA once dried on to the surface of medical catheters.

(vi) Using an in vitro model of MRSA biofilm we have shown that the combination shows enhanced efficacy compared to the individual components alone for reducing MRSA biofilm viability and re-growth.