Advanced Techniques in Diagnostic Microbiology pp Cite as. The first step in microbial identification is the phenotypic assessment of the growing colony. In many cases, the colonial morphology such as color, shape, size, hemolytic reaction, and growth characteristics on various selective and differential media can place an organism in a single family, genus, or even species level. In fact, assessing the ability of an organism to grow in various laboratory media and their oxygen requirement coupled with Gram-stain morphology and a few rapid tests such as catalase, oxidase, coagulase, and indole can provide preliminary identification for most of the clinically significant isolates. For example, it is very likely that an organism that grows on MacConkey agar plate and ferments lactose is a member of the family Enterobacteriaceae or that an oxidase-positive non-lactose fermenting Gram-negative rod that has distinct grape odor is Pseudomonas aeruginosa.