It is useful to know certain technical information involved with isolation and identification of bacteria. This information may improve physician understanding of the microbiology laboratory, to the mutual benefit of physician and laboratory.

If a culture is ordered, it usually takes at least 48 hours, and often longer, for definitive diagnosis—1 day to culture the organism and 1 day to identify it. Sometimes an additional day must be used to isolate the organism if it is present in a mixed culture. One additional day may be needed for antibiotic sensitivities. A technologist uses knowledge of the site and source of culture material in deciding what media or techniques (e.g., anaerobic conditions) to use for isolation. Some organisms are normal inhabitants of certain body areas but pathogens in other areas, so an experienced technologist knows to some extent what to subculture from a mixture of organisms growing in a specific location and also what special media to use for the pathogens usual in that anatomical location. This knowledge can easily save 1 day’s time. Information that a certain specific organism is suspected may save even more time by permitting original inoculation of the culture material into special test media. Even if definitive isolation is not yet accomplished, the technologist can often provide useful information or even a presumptive diagnosis. For example, among the enteric gramegative rods, Escherichia coli, Enterobacter, and Klebsiella ferment lactose, whereas most of the other pathogens do not. Therefore, a lactose fermenter cannot be Salmonella or Shigella. Pseudomonas, in addition, does not ferment glucose, whereas most of the other pathogens do. Some organisms have a fairly characteristic appearance on isolation media. For example, it may be possible to make a rapid presumptive diagnosis for some of the gram-positive aerobic bacteria based on their appearance on original blood agar culture plates combined with their Gram stain morphology.