We've got several gallon jugs of reddish-brownish-purplish liquid sitting here on the kitchen floor. Not sure if these are technically purple bacteria.
They were made by taking leftover
water from rinsing brown rice before cooking - this provides both a source of food for bacteria, and bacteria themselves. Adding sea salt. Shake and bask in the sun.
The method comes from Iyama Ichiro in Japan. He says as long as there are naturally ocurring lactobacillus, the photosynthetic bacteria will also be present - they have a symbiotic(?) relationship. The growing tips of plants cut just before sunrise have the highest concentration of lactobacilli and other beneficial bacteria. Mugwort is supposedly near the top of the list for natural sources.