Comparative antistreptococcal activity of photobactericidal agents

In order to establish a comparative order of efficacy among established photosensitising compounds currently under investigation, the in vitro photobactericidal activities of six commercially available photosensitisers were investigated at equal concentration against Streptococcus sanguis using a Helium Neon (HeNe) laser (632.8 nm). Of the photosensitisers used, the four phenothiazinium compounds were efficient photobactericidal agents as was the protoporphyrin IX salt. However, the zinc phthalocyaninetetrasulfonate was less effective. Of the active agents, 1,9-dimethyl Methylene Blue (DMMB) was notable in achieving complete bacterial kill when used at a concentration of 40.85 microM in conjunction with a light energy dose of 21.8 J cm(-2), although there was inherent dark activity associated with this compound. Since each of the photosensitisers is well known to produce singlet oxygen, the relative activities exhibited are thought to be due to differences in bacterial cell uptake, which in turn are related to the physicochemical properties of the photosensitisers, in particular, to the combination of lipophilicity and ionic character.