Since infectious diseases are the most common causes of illness in caged and aviary birds, the microbiology laboratory plays a vital diagnostic role for the avian veterinarian. Samples collected from the patient may yield pathogenic (disease-causing) bacteria or fungi. The major function of the microbiology laboratory is to identify these microorganisms so that the appropriate treatment can be initiated. Once one or more bacterial isolates from a specimen have been identified, the next step is to conduct antimicrobial sensitivity testing. Drug-impregnated discs are placed in contact with colonies of each previously identified microbial isolate. The effectiveness of each antimicrobial is determined by measuring the zone around each disc in which the growth of the organisms have been completely inhibited. Generally speaking, the larger the zone, the more effective the antimicrobial. It is from these "effective" antimicrobials, then, that the veterinarian selects a drug with which to treat the patient. This type of antimicrobial testing is not used with fungal isolates.
Another effective tool available to the avian veterinarian is the Gram stain. Various specimens (feces, discharges, pus, etc) from the patient can be Gram stained and examined microscopically. Such a preparation yields a surprisingly large amount of information: whether or not bacteria and yeasts are present in the sample; the relative numbers of organisms present; their shape and relative size; and their Gram stain reaction (Gram-negative bacteria appear light red or pink and are generally considered the major disease-causing bacteria among caged birds; Gram-positive bacteria appear dark blue or violet and make up most of the normal gastrointestinal and respiratory tract microflora of caged birds). Just knowing this information aids in the tentative identification of the bacteria present. An actual culture would be necessary to make a positive identification. The advantage of the Gram stain is that it represents a relatively inexpensive method of determining the microbiologic status of a patient. More important, it provides a convenient method for monitoring a patient's progress during and after antibiotic therapy.