In contrast to LA-ST398 MRSA, where the spread of the clone is limited to farm personnel, the ST398 MSSA clone found in northern Manhattan is readily transmitted among humans independent of animal contact. This ST398 MSSA clone is now encountered among humans in geographically dispersed areas (10
Here we present epidemiological, molecular, and comparative genomic evidence that MSSA isolates of the ST398 lineage are easily transmissible among humans and have a genome that is well adapted to the human host. Specifically, we found that ST398-NM causes a wide range of infections, including invasive bloodstream infections, and is a frequent colonizer within households. These attributes are remarkably different from those of LA-ST398 MRSA, for which transmission is limited to farmers, veterinarians, and their immediate household members (32
). The key genotypic and phenotypic features that distinguish both ST398-NM strains from the LA-ST398 strain S0385 include the following: (i) a different repertoire of MGEs, (ii) accessory genome variation, including a complete repertoire of surface adhesins in the human ST398 strains as opposed to pseudogenes in strain S0385, (iii) an increased ability of human ST398 to adhere to human keratinocytes and immobilized keratin, and (iv) clustering of SNPs in selected functional COGs in the core genomes.
These differences at the genome level suggest several mechanisms of ST398 host-specific adaptation. Both human ST398 MSSA genomes lacked novel toxins or an S. aureus
SaPI, but their repertoire of MGEs was most consistent with those of other known human S. aureus
genomes, such as prophage
3 being integrated into the β-hemolysin locus or the presence of the PVL-encoding prophage
2. Furthermore, there was very limited variation in MGEs among a geographically diverse collection of colonizing and infectious ST398 MSSA isolates. This may be consistent with the idea that there is relatively little pressure to rapidly adapt to new host niches or might indicate recent emergence and rapid dissemination of human ST398.
In contrast, the LA-ST398 strain S0385 contains several unique MGEs, including SaPI5, encoding allelic variants of the von Willebrand factor-binding protein (vWbp
) and staphylococcal complement inhibitor (scn
). Related ruminant and equine vWbps have recently been shown to contain a unique N-terminal region specific for the activation of ruminant and equine prothrombin, thereby contributing to host specificity (33
). However, SaPI5 is absent from many pig ST398 isolates, therefore providing support to the idea that these genes are not essential for pig specificity (29
). It should be noted that S0385 was isolated from a case of human endocarditis in an immunocompromised host, and therefore the acquisition of some of the MGEs in this isolate may have occurred during human infection.
Differences between LA-ST398 and human ST398-NM were not limited to MGEs. The observed gene decay in adhesion proteins in strain S0385 suggests that these genetic factors are likely no longer contributing to virulence in swine and further imply that the most recent common ancestor of the ST398 lineage was human associated. This observation is consistent with studies of S. aureus
isolates causing disease in poultry, where a number of genes involved in pathogenesis of human S. aureus
infections, such as S. aureus
protein A (Spa), were rendered nonfunctional (34
). Accumulation of pseudogenes within predicted surface-expressed structural and adhesion proteins has also been noted in the bovine mastitis strains ET3-1 and ET3-2 (lineage CC151), including in spa
), but not in the clinically less successful strain ET3-3. In the current study, we found that human ST398 MSSA isolates bound more avidly to human keratinocytes and keratin derivatives than pig-derived ST398 isolates. We suggest that this differential binding ability may translate directly into decreased transmission of LA-ST398 among humans lacking close animal contact. In our study, binding of LA-ST398 isolates to human keratinocytes was diminished, but it was not abolished. We propose that this ability to adhere to human cells may be sufficient to initiate transmission from livestock to humans, but it would not be sufficient to maintain spread between humans. The LA-ST398 strain S0385 appears to lack many of the established S. aureus
virulence factors, an attribute that probably limited human infection to an immunocompromised host (20
). This is likely a reflection of the opportunistic nature of S. aureus
and of the importance of the host response in determining susceptibility to infections. Taken together, our findings support host-specific adaptation of human- and animal-tropic S. aureus
strains by accumulation of MGEs and mutation of surface-expressed S. aureus
We also note that human ST398 MSSA isolates were able to adhere to primary porcine keratinocytes in vitro
, and this finding may indicate that human S. aureus
strains in general are able to initiate colonization in exposed livestock. However, the factors that determine the unique success of the ST398 MRSA lineage as colonizers with little disease in livestock remain incompletely understood. It has been speculated that the presence of two tetracycline resistance determinants (tetM
) is responsible for the selection of LA-ST398 MRSA and a reflection of the selective pressure exerted by tetracycline antibiotic use in animal feeds (36
). In contrast, all human isolates tested here lacked tetracycline resistance, but virtually all carried ermT
, consistent with macrolide resistance. The importance of these resistance determinants for ST398 strain virulence and fitness remains to be determined.
The presence of ST398 MSSA (mainly spa type t571) as a source of infections in geographically dispersed regions, including China, many European countries, North America, the Caribbean, and Colombia, supports the notion that MSSA strains constitute potentially pandemic pathogens. Our global knowledge about the molecular epidemiology of MSSA is limited, since much of the surveillance work on S. aureus is centered on MRSA strains. These findings highlight the importance of expanding our knowledge of the molecular epidemiology of MSSA strains globally.
Taken together, our integrated approach of epidemiology and whole-genome analysis of the ST398 lineage has revealed that ST398-NM MSSA constitutes a readily transmissible and clinically important clone that differs significantly at the genome level from its livestock-associated counterpart.