Comparison of Multilocus Sequence Typing and Pulsed-Field Gel Electrophoresis as Tools for Typing Staphylococcus aureus Isolates in a Microepidemiological Setting

Abstract

Multilocus sequence typing (MLST) of Staphylococcus aureus is well suited to the study of global or long-term epidemiology, but its role in local epidemiology has not been defined. The present study has compared MLST with pulsed-field gel electrophoresis (PFGE) by using S. aureus isolates associated with carriage and disease in a busy regional renal unit. One hundred forty-four patients were prospectively recruited, of whom 103 were receiving hemodialysis and 41 were on continuous ambulatory peritoneal dialysis. Three nasal swab specimens were obtained 1 month apart on entering the study. A nasal swab was positive for S. aureus on at least one occasion in 50 patients (35%). Typing of the 104 carriage isolates demonstrated 21 PFGE types and 21 sequence types (STs). Thirty-one carriers had two or more positive nasal swabs; of these, the isolates in all swabs from a given carrier had identical PFGE types for 29 carriers; the isolates in all of the same 29 swabs had identical STs. The carriage strain in two patients changed both PFGE type and STs during the period of swabbing. Eight patients (6%) had an episode of S. aureus bacteremia during the 12-month study period, and two of these were nasal carriers. One of these invasive isolates had the same PFGE type and ST as the carriage isolate. There were no differences between Simpson's index of diversity for PFGE and Simpson's index of diversity for MLST for both invasive and carriage isolates, suggesting that the two methods have very similar discriminatory abilities. We conclude that PFGE and MLST performed equally in this study.Molecular typing of the important human pathogen Staphylococcus aureus has been used to examine both long-term or global epidemiology and short-term or local epidemiology. Understanding the genetic structure of the global population over time gives insights into the evolution of bacterial lineages and transmission dynamics and also provides a framework with which to study bacterial pathogenesis. This contrasts with the typing of organisms in a defined setting over a short period of time which is used to study nosocomial outbreaks, local transmission and carriage, and the relationship between isolates associated with carriage and infection in a given individual.The attributes required of a typing tool vary depending on the application. For example, studies of nosocomial outbreaks caused by strains such as epidemic methicillin-resistant S. aureus strains are commonly performed in many centers; thus, cost, the technical expertise required, speed, reproducibility, and comparability between centers are all important issues. A range of different techniques have been described and evaluated, but pulsed-field gel electrophoresis (PFGE) is the predominant method in use for studies of outbreaks and local epidemiology (2,5,8,14,17,20). PFGE has also been successfully applied to isolates separated by geographical location (1), but it is generally considered overdiscriminatory for this type of study since the technique detects genetic variation that accum...

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