The growth of microorganisms inside cultures can be identified though the manifestation of turbidity, the formation of gas, or the distinct colonies in broth or agar that can be seen by the naked eye, the cytopathic effects of certain microorganism in the culture, or perhaps by discovering certain antigen or nucleotide in samples, the culture medium or he culture system itself, which is exclusive only to a certain genus or species.

The growth characteristics of the cultured specimen are very important in the identification of certain bacteria such as mycobacteria. Growth characteristics may include the time that is needed for growth to appear, the environment in which the certain culture requires to grow, morphology of both the colony and the individual microorganism, the biochemical, physiologic and nucleotide or antigenic sequence. The tests that is used in the identification of bacteria, both in number and its kind, rests largely upon the kind of bacteria present, whether the bacteria is aerobic or not in nature, or it is Gram-positive or negative, or whether it is cocci or bacilli and also upon the expertise and sills of the microbiologist that is doing the microbiologic examination. Each category or kind of microorganism requires a different set and number of tests. For example, Gram-positive cocci which grow on the air without additional carbon dioxide may require only a small battery of tests while the detection of Gram-negative bacilli demands very complicated and complex tests that can reach up to 20 panels to determine its underlying biochemical and physiologic characteristics. Filamentous fungi can be identified using only the characteristics of its growth and the morphology both of the colony and of the individual microorganism. Viruses, on the other hand, can be identified by examining its cytopathic effects on the different cell cultures or on the recognition of the antigens or nucleotide sequences that is either virus specific or species specific.

Interpretation of Culture Results

Some kinds of microorganisms are considered at all times to be clinically significant such as Mycobacterium tuberculosis, Shigella dysenteriae, Coccidioides immites, and influenza virus. Other microorganisms that are usually harmless and can usually be found as part of the local flora of the skin and mucous membranes or that are frequent in the environment can or cannot be clinically significant, which relies on the source of the specimen in which they are isolated. One example is coagulase-negative staphylococci which are normally found on the skin, the gut, the urethra and the vagina, and on the upper respiratory tract. Thus, its extraction from superficial ulcers, sputum and wounds do not usually entail clinical significance. However, this kind of microorganism is a usual reason of infections that is linked with implanted prosthetic materials or intravascular devices.

The part of coagulase-negative staphylococci in triggering the infection can only be known when this microorganism is extracted in large quantities from the surface of an intravascular device, or if on the case of an implanted prosthetic device, from the place that is surrounding the implanted device. This is because of the fact that intravascular devices go through the skin and cultures from an implanted prosthetic device can only be made after the incision.

In addition, Aspergillus fumigatus, which is widely distributed in the natural world, in the hospital, and in the upper respiratory tract of healthy people can be a cause of pulmonary infections that may prove to be fatal to those that have leukemia and those that has been under bone marrow transplant procedures. The extraction of Aspergillus fumigatus from the secretions on the respiratory tract can be a nonspecific finding because it requires histologic examination of tissues to make a definite conclusion of an invasive aspergillosis.

Physicians should also be aware that, sometimes, the composition of microbial species can be changed by circumstances such as an underlying disease, provision of antibiotics, endotracheal or gastric intubation, and also the environment inside the hospital can alter the reluts that can be obtained through gynecological microscopy. Like for example, possible harmful bacteria can be extracted from the pharynx of a very ill patient, yet that bacteria may not cause infection at all.