Hi Luke,
In general the lowering of the temperature has been suggested as a cure in some cases of Aeromonas infection based on behaviors observed in experimentally infected amphibians. This has been only observed for this specific infection as far as I know as it lowers the temperature below the best growth rate for that bacteria. However many of the other pathnogenic bacteria that infect amphibians grow very well down to 5 C. In general the lower the temperature the less effective the immune system of the amphibian.
Depending on the cause of the lesion there are a number of excellant treatments available. At work we use Silvadine cream, which you will need a prescription from vet to buy. Some people use over the counter antibiotic medications but if you choose this route make sure you do not purchase any containing benzocaine as this anaelgesic is a good euthanasia agent for salamanders.
Ed