Clinical Information

Bacterial-typing techniques are useful for determining strain relatedness in the setting of nosocomial outbreaks or apparent outbreaks. Serial isolates obtained from the same patient can be typed to determine whether they are the same or different. Typing often allows the physician to discriminate between 2 species, recognize an outbreak, or identify the source of infection.

In the past, strain typing was accomplished by testing for different biochemical, phage, or antibiotic resistance patterns. Antibiograms are often unreliable because they are easy to over-interpret or under-interpret. Other strain-typing methods are often organism-specific and each requires a unique set of reagents and procedures. The availability of classical strain-typing techniques has been limited.

An excellent example of the power of the technique was in the analysis of a large number of clustered isolates of methicillin-resistant Staphylococcus aureus obtained from patients and staff at a Mayo Rochester hospital during September and October, 1992. Although the high frequency with which this organism was isolated suggested a nosocomial outbreak, molecular typing of the isolates showed: only 3 of the 14 were identical; the remaining isolates were most likely the result of a surge in the number of random isolates of this organism. Thus, the 14 isolates were not part of a nosocomial epidemic due to a single strain, and radical measures for control of a nosocomial outbreak were unnecessary.