LETHAL PHOTOSENSITIZATION OF STAPHYLOCOCCUS-AUREUS IN-VITRO - EFFECT OF GROWTH-PHASE, SERUM, AND PREIRRADATION TIME

Background and objective: Staphylococcus aureus can be killed by low-power laser light in the presence of aluminium disulphonated phthalocyanine (AlPcS(2)). 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 mu g/ml of AlPcS(2) 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, 10(6) 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 AlPcS(2) 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. (C) 1995 Wiley-Liss, Inc.