I can't be totally sure about your particular bottle there, but it sounds to me like this 'arroz vivo' is a form of chicha made from rice. Chicha is a non-alcoholic Latin American beverage made from fermented grains and other high-starch crops. Depending on the type(it can be made by fermenting anything from corn to rice to potatoes), chicha has traditionally been consumed for its health benefits, religious rituals, or simply its taste.
Because it has so many uses, and its ingredients vary so widely, it's impossible to tell you exactly which strain/species of microbial is residing in your particular culture. But I can tell you how it probably got there. Traditionally chicha is made by chewing or sucking on the particular grain you wish to ferment, then spitting it into the fermentation vessel with a fair bit of microbe-containing saliva.