Increasing photoeradication's efficiency of Acinetobacter baumannii by polyphosphonic chelating agents

Ethylenediamine-N,N,N',N'-tetrakis(methylenephosphonic acid (EDTMP), nitrilotri(methylphosphonic acid (ATMP) and zoledronic acid were studied to enhance the methylene blue-mediated photodynamic inactivation of Acinetobacter baumannii. Laser light (wavelength 638 nm; standard light output 40 mW) was used in all experiment. Planktonic cultures were irradiated for 10, 20 and 30 min which corresponded to the light dose of 63 Jcm^{‒ 2}, 126 Jcm^‒2 and 189 Jcm^‒2. Biocidal effect depended on the exposure time and it was shown that MB alone caused the highest reduction in the number of viable cells by 3.10 ± 0.2 log₁₀ units after 30 min of irradiation. A significantly more effective killing effect was achieved when the bacteria were pre-treated with zoledronate, ATMP, or EDTMP (prior to photosensitisation) as the number of viable bacteria was reduced by 4.04±0.2 log₁₀, 3.95±0.2 log₁₀ and 4.01 ± 0.2 log₁₀, respectively. The photo-killing effect caused by MB against biofilm pre-incubated with zoledronate, ATMP, or EDTMP and the number of viable bacteria was reduced by 0.80±0.1 log₁₀, 1.25±0.05 log₁₀ and 0.65±0.05 log₁₀, respectively. Polyphosphonic chelating agents increased the efficiency of photo-destruction of A. baumannii by increasing the amount of bound photosensitizer by planktonic cells and biofilm, and increasing the detachment of live planktonic cells from the biofilm. The presence of glucose in the photosensitizing system significantly affected the bacterial photo-elimination. Pre-incubation of planktonic bacteria with the studied polyphosphonic chelating agents in the presence of glucose, and then exposure to light (with MB) for 30 min caused the lethal effect. This photo-eradication protocol (in the case of biofilms) reduced the number of viable bacteria by 2.05±0.2 log₁₀, 3.2±0.2 log₁₀ and 2.02±0.2 log₁₀ for zoledronic acid, ATMP and EDTMP, respectively.