Strain Differences in Bloodstream and Skin Infection: Methicillin-Resistant Staphylococcus aureus Isolated in 2018-2021 in a Single Health System

Abstract

Staphylococcus aureus is a common cause of skin and soft-tissue infections (SSTIs) and has become the most common cause of bloodstream infections (BSIs) in recent years, but whether the strains causing these two clinical syndromes overlap has not been studied adequately. USA300/500 (clonal complex [CC] 8-sequence type [ST] 8) and USA100 (CC5-ST5) have dominated among methicillin-resistant S aureus (MRSA) strains in the United States since the early 2000s. We compared the genomes of unselected MRSA isolates from 131 SSTIs with those from 145 BSIs at a single US center in overlapping periods in 2018-2021. CC8 MRSA was more common among SSTIs, and CC5 was more common among BSIs, consistent with prior literature. Based on clustering genomes with a threshold of 15 single-nucleotide polymorphisms, we identified clusters limited to patients with SSTI and separate clusters exclusively comprising patients with BSIs. However, we also identified eight clusters that included at least one SSTI and one BSI isolate. This suggests that virulent MRSA strains are transmitted from person to person locally in the healthcare setting or the community and that single lineages are often capable of causing both SSTIs and BSIs.

Keywords: CC5; CC8; Staphylococcus aureus; clonal complex; whole-genome sequencing.

Figure 1.

A, Distribution of infection types among skin and soft-tissue infections (SSTIs), stratified by clonal complex (CC). B, Distribution of infection types among bloodstream infections (BSIs), stratified by CC. C, CC distribution comparing SSTI and BSI strains. Abbreviations: AV, arteriovenous; CVC, central venous catheter.

Figure 2.

A, Maximum likelihood tree of all sequenced samples. Heat map depicts clonal complexes (CCs), whether the samples are from a bloodstream infection (BSI), and accessory gene regulator (agr) group. B, CC8 cluster only. C, CC5 cluster only; branch tips indicate clusters <15 single-nucleotide polymorphisms. Clusters with only BSI strains are represented with circles; clusters with only skin and soft-tissue infection (SSTI) strains, with squares; and clusters including both BSI and SSTI strains, with triangles. Abbreviations: ID, identification; NA, not applicable; ST, sequence type. Branch lengths are proportional to the number of nucleotide substitutions per site (scale bars of 2x10⁻⁵ are provided for reference).