Growing bacteria in a pure culture so that no other microbes…

Growing bacteria in a pure culture so that no other microbes are present is a mainstay of classical microbiology.  Newer testing techniques are able to identify bacteria without having a pure culture, but can instead identify bacteria growing in a mixed culture.  1. What kind of technique, that doesn’t require growing bacteria on a petri plate, could identify a specific bacterium in a mixed culture, and 2. Why would this technique work but using an enterotube fail with a mixed culture?