Phenotypic Identification

To ensure accurate identification, MEI uses classical microbiological phenotypic characterization methods along with genetic methods in determining final identifications of microorganisms. We have found this quality control step to be essential in preventing the false identifications that can arise in any laboratory employing only genetic identification approaches for microbial identification.

Every attempt is made to correctly identify the microorganism by phenotypic/biochemical characterization. If the identifications still differ by the two methods, further specific genetic and phenotypic/biochemical tests are immediately applied as needed, at no extra cost to our customers, to gain definitive identification.

Phenotypic/biochemical characterization begins by sub-culturing samples to verify (or obtain) species purity. Wet-mounted pure culture isolates are examined under a light microscope with appropriate lighting and staining methods.

Using classic taxonomic keys for the microorganism class in question, several morphological characteristics are considered including size, specific morphological structures, surface characteristics and pigmentation, sporulation characteristics and (if present) spore size, shape, ornamentation and color.

For bacteria, slide-mounted specimens are also examined for cell wall characteristics (Gram-staining). Additional rapid microbiological tests are applied as needed using standard codified decision tree logic procedures.

Test examples include determining anaerobic or aerobic growth, oxidase, catalase, indole, coagulase detection, glucose fermentatation/oxidation and urease detection. Further supportive biotyping methods are applied, if required, for certainty.

Mold and yeast cultures are also examined on standardized agar media for colony shape, size, edge morphology, color/pigmentation, exudate droplet formation and growth rate at specific temperatures.