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Hospital Healthcare Europe

Beating one of our most dangerous ‘superbugs’

Ruth Massey 
and Andrew Edwards
16 June, 2011  

Dr Ruth Massey
Senior Lecturer 
Department of Biology and Biochemistry, University of Bath, UK

Dr Andrew Edwards

Postdoctoral Researcher 
Department of Biology 
and Biochemistry, University of Bath, UK

Staphylococcus aureus is a commensal organism found persistently colonising 20% of the human population and intermittently colonising a further 40% throughout their lifetime.1 It is also a major human pathogen.2

The vast majority of infections caused by S. aureus are skin and soft tissue infections (SSTIs), where bacteria colonising the nose of a carrier enter the tissue by direct contact. As such, a host becomes infected with their own carried strain, or by that carried by someone they come into direct contact with, such as a health care worker, sport colleague, and is usually dependent upon a breach in the host’s skin caused by cuts, grazes, catheter insertion site.

Thankfully, the majority of SSTIs are easily treated. However, serious consequences can occur when an infection spreads and gains access to the blood-vessels surround the SSTI and then disseminates throughout the body via the circulatory system.3 S. aureus sepsis is sufficient to cause death as a result of septic shock, but a further complication of having bacteria in the bloodstream is that they can spread to remote anatomical sites and cause secondary (metastatic) abscesses in organs such as the brain, kidney or liver.2-4 This metastatic spread is a relatively common complication of bacteraemia and is associated with greatly increased morbidity and mortality.4

Understanding its escape routes
Central to the ability of S. aureus to initiate secondary abscesses is the ability to exit the bloodstream and enter the surrounding tissues, followed by multiplication and abscess formation. Despite the importance of this process, we understand little about how S. aureus escapes circulatory flow and enters surrounding tissues. In Figure 1 we have illustrated the route S. aureus is believed to take from colonisation of the nose of a carrier to the development of an abscess in an organ such as the kidney.

Invading the blood vessels
We recently published a study characterising the ability of S. aureus to invade endothelial cells (cells lining blood vessels) and how this was directly associated with their ability to leave the bloodstream and develop secondary abscesses.5 One of the many proteins expressed on the surface of S. aureus is fibronectin-binding protein A (FnBPA). This bacterial protein is essential for S. aureus to adhere to and invade many human cell types, including the endothelial cells.6 We believe that this ability to intimately associate themselves with endothelial cells is crucial to their ability to subsequently leave the bloodstream and develop secondary abscesses.

Illustrated in Figure 2, FnBPA contains three N terminal regions capable of binding to elastin and fibrinogen, and 11 non-identical fibronectin binding repeats (FnBRs). The FnBRs bind fibronectin (Fn), which is part of the extracellular matrix surrounding human cells. Fn is attached to human cells via cell-surface integrins (α5β1) and, as such, the bacteria uses fibronectin as a bridging molecule, triggering cytoskeletal rearrangements, resulting in invasion of the human cell by the bacterium (see Figure 3).7

We had previously reported that although S. aureus FnBPA had eleven FnBRs, a single FnBR was sufficient to confer that ability to invade cell.7 As such, it was unclear why the bacterium had evolved to express eleven when one would do. To examine this, we constructed a collection of S. aureus strains each expressing different numbers of FnBRs and performed a series of detailed in vitro experiments to compare their abilities to attach to Fn and invade endothelial cells. We found that the more FnBRs the bacterium expressed on its surface, the quicker it invaded cells.5

Testing out the theory
To correlate the ability to invade endothelial cells with escape from the bloodstream and the development of secondary abscesses, we worked with colleagues at the University of Gothenburg. We initially compared the ability of four of our constructed S. aureus strains expressing FnBPA containing varying numbers of FnBRs to cause infection.

To mimic sepsis and post-sepsis events, our collaborators intravenously inject bacteria into mice and monitor disease. Only 35% of the mice infected with S. aureus expressing all eleven FnBRs (the most efficient cell-invader) survived over 15 days, whereas 80%–100% of mice infected with S. aureus expressing either none, one or three FnBRs survived.5 Weight loss is used as an indicator of disease severity and, again, the S. aureus strains expressing all eleven FnBRs caused the most significant weight loss.5

We had found a clear correlation between the ability to invade cells and the ability to cause disease, but we wanted to look more specifically at the ability of S. aureus to use its cell-invading capacity to leave the bloodstream. To test this, mice were infected intravenously with one of the four S. aureus strains and euthanised three days later. We then determined the number of bacteria that had left the bloodstream and infected the kidneys. Significantly higher numbers of the S. aureus strain expressing all eleven FnBRs were found in the kidneys, demonstrating a link between the ability to efficiently invade the cells lining the bloodstream with the ability 
to leave the bloodstream and infect secondary sites in the body.5

Route to a novel therapy
With the evolution and worldwide spread of antibiotic-resistant bacterial strains, it is crucial that we fully understand their pathogenic mechanisms if novel therapies are to be developed. The work described here is an example of how such an understanding may lead to the development of a novel therapy. We found that multiple FnBRs were important in allowing the bacteria leave the bloodstream and develop into a stage of disease with significantly increased morbidity and mortality.

If we could treat bacteraemic patients with either bacterial protein mimic or a human-target protein mimic, we could theoretically block the interaction of S. aureus with the endothelium and thus prevent the bacteria from leaving the bloodstream. Although such a therapy would have to be administered alongside conventional antibiotics, it has the potential to prevent the development of metastatic S. aureus infections.

Future work
While S. aureus has long been considered an extracellular pathogen, its ability to enter host cells appears to play a key role in pathogenesis. Relative to the study of well-characterised intracellular pathogens such as Listeria, Yersinnia and Salmonella, studies on S. aureus are in their infancy. We have a lot more to learn about the diverse pathogenic processes it utilises to cause disease.

While the mechanism by which S. aureus enters host cells is well characterised, the subsequent fate of both the bacterium and the cell is unclear. We are currently seeking funding to continue this work, which will allow us to develop an in vitro system that more closely reflects the in vivo situation. This will allow us to look in greater detail at the cell invasion process and provide an excellent screening platform for the development of peptide or small molecule inhibitors of S. aureus invasion prior to animal studies.

Jennifer Potts, University of York, and Elisabet Josefsson, University of Gothenburg.


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