Lethal photosensitisation of Staphylococcus aureus in vitro: Effect of growth phase, serum, and pre‐irradiation time

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Abstract
Background and objective: Staphylococcus aureus can be killed by low‐power laser light in the presence of aluminium disulphonated phthalocyanine (AIPcS2). The purpose of this study was to determine the effect of pre‐irradiation time (PIT), the presence of serum, and the physiological state of the organism on the kills achieved. Study Design/Materials and Methods: To determine the effect of PIT on killing, suspension of methicillin‐resistant S. aureus (MRSA) were incubated in the dark with 12.5 μg/ml of AIPcS2 for 60 s or 300 s, and survivors were enumerated after exposure to 1.2 J of light from an 11‐mW gallium aluminium arsenide laser. The susceptibility of MRSA in its various growth phases was determined in a similar manner using a PIT of 300 s. The effect of serum on killing was determined using stationary phase cells resuspended in horse serum. Results: Using a PIT of either 60 s or 300 s, 106 cfu (99.9%) of MRSA were killed. There was little difference in the susceptibility of lag‐, logarithmic‐, or stationary‐phase cells, the kills being 99.9%, 99.8%, and 99.9%, respectively. Although kills were reduced in the presence of serum, 99.6% of MRSA were killed using a light dose of 1.2 J. Conclusion: These results demonstrate that MRSA can be rapidly sensitised by AIPcS2 to killing by low‐power laser light, that killing is not dependent on the organism's growth phase, and that substantial kills can be achieved in the presence of serum.