Phenothiaziniums as putative photobactericidal agents for red blood cell concentrates

The antibacterial activities of Methylene Blue and several of its congeners were measured against Yersinia enterocolitica, a Gram-negative pathogen kn own to exhibit significant growth at 4 degreesC and thus constituting a thr eat to red blood cell concentrates which are stored at this temperature. No ne of the derivatives was highly active in dark conditions, as expected, bu t on illumination using a lamp emitting light in the waveband 615-645 nm, c onsiderable bactericidal activity was noted using similar photosensitizer c oncentrations to those used elsewhere to inactivate blood-borne viruses. Tw o novel compounds in this area, the phenothiazinium New Methylene Blue N an d the phenoxazinium Brilliant Cresyl Blue, exhibited bactericidal activity at lower concentrations than both of the established phenothiaziniums, Meth ylene Blue and Toluidine Blue O and the recently published blood photoviruc idal agent 1,9-Dimethyl Methylene Blue. The photoactivity of these compound s was undiminished in the presence of red blood cells.