spa Typing of Staphylococcus aureus as a frontline tool in epidemiological typing

Abstract

We determined the value of spa typing in combination with BURP (based upon repeat pattern) grouping analysis as a frontline tool in the epidemiological typing of Staphylococcus aureus, based on a random collection of 1,459 clinical isolates sent to the German Reference Centre for Staphylococci within a 6-month period. The application was found to be helpful for the classification of isolates into the particular clonal lineages currently prevalent in Germany. Due to its major advantages because of the ease of interpretation and the exchangeability of the results, the use of spa typing greatly simplifies communication between laboratories on both the national and the international levels. Thus, it is an excellent tool for national and international surveillance of S. aureus as well as for analysis of the short-term local epidemiology. However, to overcome the limitations of the BURP grouping method in terms of typing accuracy and discriminatory power, the results of the default BURP grouping method must be interpreted with caution. Additional markers, like staphylococcal chromosomal cassette mec, lineage-specific genes, or alternative DNA polymorphisms, are indispensable. They should be selected by dependence on the clonal lineage indicated by spa typing and subsequent BURP analysis as well as on the basis of the particular question to be addressed.

FIG. 1.

Summary of typing results for a selection of 85 isolates, including 14 reference strains (indicated by italics), ordered by the results of BURP grouping. ^a, Groups were defined as described in Material and Methods; Sg, singleton. The degree of typing concordance between different methods in each BURP cluster is color coded: light gray indicates excellent concordance between methods, medium gray indicates moderate concordance, and dark gray represents BURP groups with a high number of falsely classified isolates. Incorrect classifications are demonstrated in boldface type. Underlined characters represent isolates with ambiguous spa types.

FIG. 2.

Neighbor-joining tree based on the distance matrix produced by StaphType software. BURP groups as well as MLST CCs are indicated. Underlined spa types represent putative ancestors within the respective BURP group, as defined by use of the BURP algorithm. Spa types in parentheses were not assigned to a particular BURP group (singletons).

FIG. 3.

SmaI macrorestriction analysis for all isolates with spa types belonging to MLST CC 22, as inferred from BURP analysis. The numbers represent the number of isolates exhibiting the particular spa types among all isolates investigated in this study.

FIG. 4.

Epidemiological investigation of clusters of infection. The genetic relatedness of the isolates is demonstrated by the spa type as well as by SmaI macrorestriction analysis. In hospital A, four identical isolates and one additional isolate were detected. In hospital B, all isolates were identical, as demonstrated by both methods applied. ICU, intensive care unit.