Mixed growth means the plate carries more than one organism. Because every downstream identification method assumes a pure culture, the plate must be subcultured to a fresh medium to separate the organisms into isolated colonies before any identification step is attempted. Subculturing is not a repeat of the original culture; it is a deliberate dilution step that turns a mixture into individual clones.
A wound swab that grows three colony types
A wound swab is streaked on blood agar and MacConkey agar. After incubation, the blood agar shows a heavy growth of large beta-hemolytic colonies, a moderate growth of small white non-hemolytic colonies, and a few pinpoint colonies, while MacConkey agar grows only the large lactose-fermenting colonies. The smear had shown Gram-positive cocci in clusters with many neutrophils. The large beta-hemolytic colonies match the smear finding and are the likely pathogen; the small white and pinpoint colonies are consistent with skin flora. A single beta-hemolytic colony is picked and streaked to a fresh blood agar plate to obtain pure growth before identification proceeds.
Mixed growth from a normally sterile site, such as blood or cerebrospinal fluid, is more often a sign of contamination during collection than a true polymicrobial infection, and it should prompt a review of the collection rather than immediate treatment of every organism grown.