Classical methods are based on phenotypic or morphological laboratory tests. The results of these tests are compared against the test results determined by taxonomic studies on large numbers of strains. In order to perform classical laboratory tests, a sample must first be obtained and cultured. However, there are some cases where growing a culture of the bacteria is difficult, or deterministic tests on a bacterial culture are not available (Naber). In such cases, infection must be inferred through other indirect methods (e.g. antibody detection) (Fry & Pitcher).
Bacillus anthracis is an example of an infectious agent that has unfortunately been the topic of many news articles recently. Anthrax is mentioned not because of its recent appearances in the headlines, but because B. anthracis is one of the most studied biological warfare agents. The "vast [amount of] information available for B. anthracis" makes it an excellent example of the use of classical bacterial identification methods (Dixon et. al.).
A common method of identification is the use of B. anthracis-specific gamma phage (Qi et. al.). Colonies of the sample are grown and infected with the phage, and the presence of plaques indicates the presence of B. anthracis. Another test often used is Gram staining. In Gram staining, the bacteria are stained with a purple dye, stained again with potassium iodide, washed with alcohol, and stained with a pink dye. After completion of the procedure, Gram negative bacteria will be pink and Gram positive bacteria will be purple. Yet Gram staining is not extremely useful, because it only separates bacteria into two categories.
B. anthracis is a member of the B. cereus family, which contains B. thuringiensis in addition to the two bacteria just mentioned. On the basis of genetic evidence B. anthracis, B. cereus, and B. thuringiensis can be considered one species (Helgason, et. al.). In order to separate B. anthracis from the other bacteria in the B. cereus family using classical methods, it is necessary to observe the morphological features of a colony grown on a sheep blood agar plate. Anthrax colonies grown on blood agar are "non-hemolytic and are white to gray, often looking like ground glass" and also form a characteristic poly-d-glutamic capsule. The capsule can be demonstrated microscopically by using either McFadyean's polychrome methylene blue or India ink. Blood agar cultures are useful diagnostically in cases of systemic infection, but not in cases of cutaneous infection (Dixon, et. al.).
Classical methods can have several drawbacks. One drawback is the amount of time that it takes to perform laboratory tests, which can take from days to weeks. For example, bacterial meningitis is a potentially fatal childhood disease without a standard rapid diagnostic. Gram staining of cerebrospinal fluid and conventional culture are the traditional gold standard for diagnosis. Quick diagnosis and treatment are necessary to improve the patient's condition, but culture identification requires at least 48 hours (Garcia-De-Lomas & Navarro). Another drawback is that sometimes cultures cannot be obtained efficiently, as is the case with lower respiratory tract infections, which are common in children. It is not easy to verify diagnoses of lower respiratory tract infections with laboratory tests because of the difficulty in obtaining cultures. A third drawback of classical methods is that they may not show sufficient sensitivity. Less than 30% of Tuberculosis cases are confirmed with laboratory tests (Garcia-De-Lomas & Navarro).