Reference system for characterization of Bordetella pertussis pulsed-field gel electrophoresis profiles

Pulsed-field gel electrophoresis (PFGE) has been used as an epidemiological tool for surveillance studies of Bordetella pertussis since the early 1990s. To date there is no standardized procedure for comparison of results, and therefore it has been difficult to directly compare PFGE results between laboratories. We propose a profile-based reference system for PFGE characterization of B. pertussis strain variation and to establish traceability of B. pertussis PFGE results. We initially suggest 35 Swedish reference strains as reference material for PFGE traceability. This reference material is deposited at the Culture Collection of the University of Gothenburg, Gothenburg, Sweden. Altogether, 1,810 Swedish clinical isolates from between 1970 and 2003 were studied, together with the Swedish Pw vaccine strain, six reference strains, and two U.S. isolates. Our system provides evidence that profiles obtained by using only one enzyme, i.e., XbaI, give enough data to analyze the epidemiological relationship between them. Characterization with one enzyme is far less labor intensive, yielding results in half the time than when a two-enzyme procedure is used. Also, we can see that there is a correlation between PFGE profile and pertactin type. One common PFGE profile, BpSR11 (n = 455), showed 100% prn2 and 100% Fim3 when analyzed for pertactin type and serotype. On the other hand, strains with the same profile may express various serotypes when isolated over longer periods of time. Subculturing of the same isolate eight times or lyophilization caused no change in PFGE profile.