What defines a microbial species?
Your question is relative to the species concept that you are using. Mayr's biological species concept (BSC) is based on the ability to interbreed; a process-based definition. Most biologists use it, but most taxonomists, who are the people who actually describe species, use some variation of the phylogenetic species concept. The phylogenetic species concept is not based on process, but on fixed differences. Fixed differences are evidence of a lack of interbreeding, but the concept is not explicitly based on reproduction. Fixed morphological differences have been used to classify microbes, most often in culture. But currently, microbes are most often classified with a combination of morphological and genetic characters. The small subunit rRNA is sequenced and put through an algorithm for estimating species based on genetic distances. In short, your question depends a lot on which species concept you are using, and whether you would accept a distance-based DNA definition. But species need not be defined reproductively.