Lethal photosensitisation of Staphylococcus aureus in vitro: effect of growth phase, serum, and pre-irradiation time.
OBJECTIVE
Staphylococcus aureus can be killed by low-power laser light in the presence of aluminium disulphonated phthalocyanine (AlPcS2). 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.


METHODS
To determine the effect of PIT on killing, suspension of methicillin-resistant S. aureus (MRSA) were incubated in the dark with 12.5 micrograms/ml of AlPcS2 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.


CONCLUSIONS
These results demonstrate that MRSA can be rapidly sensitised by AlPcS2 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.
Wilson
M
M
Department of Microbiology, Eastman Dental Institute, University of London, England.
Pratten
J
J
D000818:Animals; D001769:Blood / Q000382:microbiology; D006736:Horses; D066298:In Vitro Techniques; D007834:Lasers; D016106:Methicillin Resistance; D013211:Staphylococcus aureus / Q000187:drug effects / Q000254:growth & development / Q000528:radiation effects*; D013997:Time Factors