Bacteriologic studies attempt to identify the specific organism causing an infection (Table 7.1). This organism may be specific to one disease, such as M. tuberculosis for TB, or it may cause a variety of infections, such as those associated with S. aureus. Antibiotic susceptibility studies then determine the responses of the specific organism to various classes and types of antibiotic drugs. An antibiotic drug that inhibits bacterial growth is the logical choice for treating the infection.

Some questions that need to be asked when searching for bacteria as the cause of a disease process include the following: (1) Are bacteria responsible for this disease? (2) Is antimicrobial therapy indicated? Most bacteria-related diseases have a febrile course. From a practical standpoint during evaluation of the febrile patient, the sooner a diagnosis can be reached and the sooner a decision can be made concerning antimicrobial therapy, the less protracted the period of 
recovery.

Anaerobic bacterial infections are commonly associated with localized necrotic abscesses; they may yield several different strains of bacteria. Because of this, the term polymicrobic disease is sometimes used to refer to anaerobic bacterial diseases. This view is in sharp contrast to the “one organism, one disease” concept that characterizes other infections, such as typhoid fever, cholera, or diphtheria. Isolation and identification of the different strains of anaerobic bacteria and susceptibility studies may be desirable so that appropriate therapy may be given.