Use of derivatives of tetrakis (N-alkylpyridinium) -porphyrin for killing microbes or preventing their growth

A topical composition comprising the porphyrin of formula I: ** ** Formula wherein R1 is a C12 alkyl group, and R2-R4 are methyl; and X- is an anion; for use in the treatment or prevention of a disease caused by a microbe in a mammal; wherein the microbe is selected from a bacterium, a fungus, and the topical composition comprising a pharmaceutical carrier.