The Mechanism of Sensitizing Effect of a Triazine Dye, Cibacron Blue F3GA, on Methicillin-resistant Staphylococcus aureus to Oxacillin

We recently found that a triazinyl dye, cibacron blue F3GA (CB), has a sensitizing effect on the in-vitro susceptibility of methicillin-resistant Staphylococcus aureus (MRSA) to oxacillin (C. Shirai, M. Sugai, H. Komatsuzawa, K. Ohta, M. Yamakido, H. Suginaka, Antimicrob. Agents Chemother. 42: 1278-1280, 1998). Among nine triazinyl dyes tested, CB showed the strongest sensitizing effect. Population analysis demonstrated that CB reduced the resistance level of MRSA. In the presence of oxacillin at subinhibitory concentrations, CB inhibited the growth of MRSA, but its effect on the cells appeared to be bacteriostatic. Under experimental conditions, CB did not affect the amount of PBP2', binding of [14C]benzylpenicillin to PBP2', or peptidoglycan susceptibilities to bacteriolytic enzymes. Autolytic enzyme-deficient MRSA mutants were equally as sensitive as the parent strain to the effect of CB on the susceptibility to oxacillin. CB affected the resistance level of MRSA irrespective of the status of the mecI / mecR1 element and/or penicillinase plasmids. The sensitivities of several bacteriolytic enzymes to heat-inactivated MRSA cells were not affected when the cells were grown in the presence of CB. CB did not stimulate the release of lipoteichoic acid from the cells. These results suggest that the sensitization effect of CB cannot be simply explained by its effect on mecA related products, autolysins, femAB products or the release of lipoteichoic acid.