This week Tim and Steph talk about Staphylococcus Aureus which is a major pathogen for both humans and animals. We discuss briefly how it is handled in a human medicine setting as well as in agriculture, primarily from a dairy farming perspective. The remainder of the episode is a discussion of the paper “Phagocytosis Escape by a Staphylococcus aureus protein that connects complement and coagulation proteins at the bacterial surface” by Ko and colleagues. The paper can be read for free at PLOS Pathogens by following this link.
Staph aureus is an important bacterium from a disease and economic standpoint costing billions of dollars per year in the United States alone due to costs incurred by hospitals all the way down to things like milk yield loss in cows with mastitis caused by it. It is so prevalent in the world avoiding it isn’t possible and it is even a common organism to have growing peacefully on our skin and some mucous membranes. The issues come about when S. aureus gets introduced to somewhere it doesn’t belong such as a wound or during a medical procedure.
Many people have heard of MRSA or Methicillin Resistant Staph Aureus which is a major problem in human health, causing hospital acquired infections that are very hard to cure. Hospitals take serious steps to prevent infection by S. aureus and slow spread if someone admitted to the hospital has it. Tim tells a story about his family’s close brush with what could have been a staph infection in one of his children and the steps doctors took to ensure the safety of the other patients. MRSA isn’t just an issue in hospitals but has been found on many farms. Treating antibiotic resistant bacteria in animals is just as frustrating as it is in humans and can be deadly for the animals if the right therapy isn’t found soon enough.
Staph aureus mastitis in dairy cattle
The paper we discuss today “Phagocytosis Escape by a Staphylococcus aureus protein that connects complement and coagulation proteins at the bacterial surface” by Ko and colleagues is introduced which can be read for free at PLOS Pathogens by following this link. Tim talks about his experience with Staph aureus mastitis on his farm and how bulk tank cultures are one way to monitor for levels of S. aureus in a herd. A nice factsheet about S. aureus prevention and control can be found here for those interested in reading more on this.
Steph talks about vaccine development for Staph aureus and discusses some of the ways it is able to persist so well in cows’ udders even when treating them with antibiotics. She then talks about her time working in the lab of Dr. Isis Kanevsky-Mullarky a professor in the Dairy Science Department at Virginia Tech who does S. aureus vaccine research. Recently Dr. Kanevsky-Mullarky was awarded the Presidential Early Career Award for Scientists and Engineers and we congratulate her on that achievement.
Extracellular Fibrinogen Binding Protein
Tim introduces everyone to a protein called Extracellular Fibrinogen binding protein or “Efb” that Staph aureus is able to produce and talks about its potential as a vaccine candidate. Some immunological concepts needed to be laid down to understand the next content: We talk about phagocytosis or the “eating” of particles by neutrophils which are an important immune cell for combating potential infections. Next Steph explains opsonization which is when small proteins in the blood that are part of the body’s immune system bind to the bacteria and “flag” it for neutrophils to destroy. Coagulation which is generally thought of in blood clotting is discussed in its role of a way for immune system to trap foreign particles for clearing by immune cells.
Results of Initial Efb Experiments
The researchers first put staph aureus, human neutrophils, the Efb protein, complement into a culture system together and then measured how much bacteria could be phagocytized (broken down). With this combination (the staph, the neutrophils, the Efb and the complement) the phagocytosis by neutrophils was not altered. However, when they added coagulation proteins, in particular fibrinogen, they found that phagocytosis was inhibited. Now they needed to further proof that Efb can also block phagocytosis in a more natural environment. So, they repeated the experiment we just described but instead they tested Efb with staph aureus in whole blood. Similarly, they found that neutrophil phagocytosis was dependent on Efb. Then they tested it in a mouse. They infused the mice with fluorescent Staph aureus with or without Efb. They sacrificed the mice and then looked for the fluorescent Staph aureus inside the neutrophils and found the mice that the neutrophils from mice treated with Efb did not phagocytose the bacteria (phagocytosis was inhibited)
More Experiments to Prove How Staph Tricks the Neutrophils
The next thing the authors wanted to know was whether Efb might bind to the bacteria and then attract fibrinogen to the surface. They wanted to test whether Efb might bind to one of those complement proteins, C3b to help bring in fibrinogen. To do this, they coated specialized plates with the C3b protein and found that the Efb actually binds both C3b and Fg and needed both C3b and Fg to bind. To see if Efb actually attracts fibrinogen Staph aureus was “pre-opsonized” and all that means if before adding any Efb they added the complement protein C3b to pre-flag the bacteria. Then they added the Efb and then the fibrinogen and with microscopy saw that Efb and fibrinogen covered the entirety of the bacteria. When they added Efb that in the lab had been manipulated to lack bindings sites for C3b or fibrinogen you did not see the binding. So now by the authors can suggest that the order of things goes binding of the flag protein C3b to the bacteria to say “hello” I’m here. Then the Efb binds to the C3b follow by the fibrinogen to build a protective shield, kind of a cloak of invisibilty, around the bacteria.
Applicability of the Research
The research we reviewed in this show really illustrates why it is so hard to control a Staph aureus infection. Until we get an effective vaccine for S. aureus the best way to deal with infections is still going to be preventing them. Culturing of microbes causing an infection can help us know what the best plan of attack to deal with an infection is because what particular microbe is causing the problem plays a huge role in determining the correct management practices or antibiotics that will have the biggest impact.
That’s all folks!
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