by Elio | You may not think of sex as having a microbial basis, but it does. That is true for many invertebrates that are colonized by the endosymbiotic bacterium Wolbachia. Members of this genus are probably the most common obligate intracellular bacteria on Earth and, for reasons discussed below, among the most remarkable ones. They infect half or more of all insects, as well as isopods (crustaceans), spiders, and even worms. In some of their hosts, they influence how sex is determined, making sex itself a sexually-transmitted affair.