A field guide to pandemic, epidemic and sporadic clones of methicillin-resistant Staphylococcus aureus

Abstract

In recent years, methicillin-resistant Staphylococcus aureus (MRSA) have become a truly global challenge. In addition to the long-known healthcare-associated clones, novel strains have also emerged outside of the hospital settings, in the community as well as in livestock. The emergence and spread of virulent clones expressing Panton-Valentine leukocidin (PVL) is an additional cause for concern. In order to provide an overview of pandemic, epidemic and sporadic strains, more than 3,000 clinical and veterinary isolates of MRSA mainly from Germany, the United Kingdom, Ireland, France, Malta, Abu Dhabi, Hong Kong, Australia, Trinidad & Tobago as well as some reference strains from the United States have been genotyped by DNA microarray analysis. This technique allowed the assignment of the MRSA isolates to 34 distinct lineages which can be clearly defined based on non-mobile genes. The results were in accordance with data from multilocus sequence typing. More than 100 different strains were distinguished based on affiliation to these lineages, SCCmec type and the presence or absence of PVL. These strains are described here mainly with regard to clinically relevant antimicrobial resistance- and virulence-associated markers, but also in relation to epidemiology and geographic distribution. The findings of the study show a high level of biodiversity among MRSA, especially among strains harbouring SCCmec IV and V elements. The data also indicate a high rate of genetic recombination in MRSA involving SCC elements, bacteriophages or other mobile genetic elements and large-scale chromosomal replacements.

Figure 1. Overview of SCCmec elements, their variants and DNA microarray hybridisation patterns.

Black circles, positive; grey circles, present, but yielding weak or ambiguous signals; divided circles, variable genes.

Figure 2. Resistance genes in MRSA strains.

Figure 3. Virulence-associated genes in MRSA strains (*, see text for further explanation).

Figure 4. Network graph visualising relationships between concatenated MLST sequences of all STs mentioned in this study.

Figure 5. Network graph based on hybridisation profiles, visualising similarities and relationships between clonal complexes and the spread of SCCmec elements.

Roman numerals indicate SCCmec types; PVL-negative strains are shown with black letters on white background, PVL-positive in white letters on black. A, ACME; F, SCCfus; M, SCCmer; IRR, irregular SCCmec elements; COM, composite or multiple SCCmec elements.