Certain bacteria that ordinarily are nonpathogenic (such as normal inhabitants of certain areas) frequently appear in cultures and are traditionally considered contaminants, presumably introduced by faulty culture technique. The major species are Staphylococcus epidermidis and the diphtheroids. Occasionally there are others, such as Bacillus subtilis. However, under certain circumstances, especially when the patient is immunocompromised, these organisms may produce disease. Three or more species of bacteria appearing in culture from ordinarily sterile areas (e.g., blood, spinal fluid, body cavity fluid, urine without a long-term indwelling catheter) suggest contamination. However, true polymicrobial infections may occur.

Bacterial species omitted. Discussion of bacterial infections in this chapter has been limited to the common organisms in clinical practice and a few that enter the differential diagnosis of certain common situations, such as hepatitis or fever of unknown origin. Many others have been omitted, such as Haemophilus ducreyi, which produces the venereal disease chancroid, and Borrelia spirilis, which causes relapsing fever. In general, most are diagnosed through culture of appropriate specimens. The reader is referred to standard textbooks on microbiology for additional information on these organisms.