4. Results: Survival curves during PDT

E. coli survival curve:

Each survival curve (3 experiments plus 3 controls, see figure below) was exponentially fitted and projected to the inactivation threshold, counts = 10 (survival = 0.01), to specify the exposure time and hence the threshold radiant exposure, H [J/cm2], that yielded inactivation (see Table 1).

About 2 orders of magnitude killing is achieved by 75 s of light exposure, for the conditions of the experiment.

Comparison with other studies

The table below summarizes two literature reports and this study on PDT inactivation of E. coli. The table calculates the threshold radiant exposure Hth required for inactivation, defined as SURVIVAL = exp(-H/Hth).

This study roughly indicates that a radiant exposure H = 15.6 J/cm2 (130 mW/cm2 for 2 minutes) achieved 99% killing. This observation indicates the threshold exposure Hth is 1.69 J/cm2. The Univ. of Central Lancashire report indicates a threshold of Hth = 1.47 J/cm2. The Guelph report indicates a Hth < 2.4 J/cm2. 1. "The phototoxicity of phenothiazinium derivatives against Escherichia coli and Staphylococcus aureus, FEMS Immunology and Medical Microbiology 39(2003) pp17-22

2. "Assessment of Photodynamic Destruction of Escherichia coli O157:H7 and Listeria monocytogenes by Using ATP Bioluminescence," Applied and Environmental Microbiology, Nov. 2003, pp. 6393-6398

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