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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.
Bacterial typing is useful to investigate infection outbreaks by a single species.
Isolates which show identical DNA restriction fragment length polymorphism patterns are considered to be closely related.
The fact that 2 strains share the same pattern does not prove that they are epidemiologically related. Establishment of an epidemiologic relationship depends on: the frequency with which the "indistinguishable" pattern is seen among epidemiologically unrelated isolates and correlation with clinical and epidemiological information.
Obviously, if common contact between 2 patients with strains with the same pulsed-field gel electrophoresis (PFGE) type can be established, the chances are greater that an epidemiologic link can be ascribed. Thus, the greatest power of PFGE typing is in showing strain dissimilarity, not in proving similarity or relatedness.
Reported as isolates from these sources are "indistinguishable" or "different" by pulsed-field gel electrophoresis. Results will be faxed to the client.
1. Arbeit RD, Arthur M, Dunn R, et al: Resolution of recent evolutionary divergence among Escherichia coli from related lineages: the application of pulsed field electrophoresis to molecular epidemiology. J Infect Dis 1990;161:230-235
2. Arbeit RD: Laboratory procedures for the epidemiologic analysis of microorganisms. In Manual of Clinical Microbiology, Seventh edition. Edited by PR Murray, ASM Press, Washington, DC, 1999, pp 116-137
3. Trees E, Rota RA, MacCannell D, Gerner-Smidt P: Molecular epidemiology. In Manual of Clinical Microbiology, 11th edition. Edited by J Jorgensen, ASM Press, Washington, DC, 2015, pp 131-160