Welcome to FOCUS In Sound, the podcast series from the FOCUS newsletter published by the Burroughs Wellcome Fund. I’m your host, science writer Ernie Hood.
Staphylococcus aureus is probably the most important bacterial pathogen affecting the public health of Americans. Staph is the leading cause of pus-forming skin and soft tissue infections, the leading cause of infectious heart disease, the number one hospital-acquired infection, and one of the four leading causes of food-borne illness. MRSA, or methicillin-resistant Staph aureus, is a highly virulent form of the infection, and accounts for more deaths annually in the US than HIV/AIDS. And of course the spread of MRSA and other antibiotic-resistant bacterial infections is becoming a major public health crisis in America.
Joining us by phone on this edition of Focus in Sound, my guest, Dr. Eric Skaar, is fighting back. He was named an Investigator in the Pathogenesis of Infectious Disease by the Burroughs Wellcome Fund in 2006. Much of his lab’s research concentrates on Staph aureus, and he and his team have come up with some important new knowledge about Staph and the host-pathogen interface—findings that may lead to new approaches to treatment of Staph infections.
Eric is an associate professor of microbiology and immunology at Vanderbilt University Medical School in Nashville. He earned his BS in bacteriology at the University of Wisconsin, his Ph.D. in immunology and microbial pathogenesis from Northwestern University, and his MPH in epidemiology and bio statistics, also from Northwestern.
Eric, welcome to FOCUS In Sound…
Thanks, thanks for having me.
Your December 2010 publication in the journal Cell Host and Microbe attracted worldwide attention, as you offer solutions to some of the great mysteries about Staph aureus, including why it seems to prefer to infect humans over other animals, and why some people seem to be susceptible to Staph infections, while many others carry the bacteria but seem to be unaffected by it. Can you elaborate on those issues and your related results?
Sure. The interest in this work came about based on our observation that Staph aureus had a preference for hemoglobin as an iron source. The organism needs iron to live, the same reason that you and I need iron and we take multivitamins. Many of the processes in the organism require that element, and we’ve known for quite some time that it acquires iron by binding to hemoglobin, and in the process of learning about hemoglobin we realized that much of the surface of the hemoglobin protein is variable across different animals so the reason that the hemoglobin protein that Staph aureus encounters is different depending on the organism that it’s colonizing so just based on what we know about receptor-ligand interactions, we reasoned that maybe the Staph aureus hemoglobin receptor would have a different ability to bind hemoglobin from one animal versus another, and that was the initial hypothesis that we set out to test. And we knew the Staph aureus hemoglobin receptor called ISDB. Using that knowledge, we grabbed hemoglobin from a bunch of different animal species and tested whether or not Staph could bind to the hemoglobins with a differential affinity and we found that it did in fact bind hemoglobins differently, and it seemed to bind non-human primates and human hemoglobin the best. So that was kind of the key result for us that led us down this path, to test whether or not human hemoglobin was a preferred hemoglobin source for Staph aureus. That was all in test tubes or in vitro, so we next wanted to determine if that had relevance to the host-pathogen interaction using an animal model, so we obtained mice that express human hemoglobin, and mice are notoriously difficult to infect with Staph aureus in the animal model. It’s effective but it takes a very large dose of bacteria to infect a mouse, presumably much more Staph than you would ever need to infect a person. What we found when we took a mouse that had human hemoglobin coursing through its blood was that mouse was much more susceptible to Staph infections, and we can in fact infect the animal with less Staph and get a similar result. Based on that, we concluded that human hemoglobin is a preferred iron source for Staph aureus in a way that impacts the ability of Staph to grow inside its host.
I see. Eric, does this phenomenon seem to hold true of other pathogens as well, or is this unique to Staph aureus?
We’ve only begun to touch on that subject, and all of the experiments we’ve done so far have been in a test tube. With that caveat, it does appear as if organisms that have evolved to live in humans, these are bacterial pathogens that are called obligate human pathogens, organisms that don’t really grow anywhere else but inside people. Those organisms seem to have a similar preference, where they don’t grow and utilize non-human animal hemoglobins nearly as well as they utilize human hemoglobin. One of the strongest examples we have is Corynbacterium diphtheriae, the causative agent of diphtheria. That organism can’t grow on any hemoglobins besides human hemoglobins. So, I don’t have data for it, but I would predict that this would be generalizable across many pathogens that have evolved to grow inside humans.
I’d like to return briefly, Eric, to the mouse model. I understand as you’ve mentioned that it’s been difficult to model Staph infections in mice previous to this work. So what will this new model that you’ve developed allow that hasn’t been possible before?
Well I think from the standpoint of the animal model, what this potentially allows is increased resolution within the model, meaning that maybe smaller differences that would previously have been difficult to identify or unable to be identified, those differences might now be visible or identifiable within this animal model, so the hope would be that small subtle virulence factors that have a small but important effect on the pathogenesis of Staph, those virulence factors now might be uncovered whereas they otherwise wouldn’t be. We can also learn a lot more about how Staph eats or acquires its nutrient iron during infection, because now we have the nutrient iron source that the organism presumably has evolved to use. So from the standpoint of studying how nutrient acquisition occurs in the animal, I think there’s a lot of potential there. The other thing that we haven’t tapped into yet is, there are other animal models of Staph infection. As you mentioned in your introduction, Staph is a leading cause of a number of different types of disease and the take-home point there is that the organism can infect almost any site in the body. So we know that human hemoglobin makes a mouse more susceptible to bloodstream Staph infections. It would be interesting to see what happens with heart infections like endocarditis or pneumonia or bone infections, osteomyelitis. So those are places we’d like to move in the future, to take the animal and move it through a series of different infection models to see if we can find infection models that benefit from this even more. But I think the real value is what it potentially might mean to human infection.
I did want to get you to talk a little about the differential susceptibility to Staph aureus infection and I understand that you’ve been able to elucidate some of why that may be occurring.
I think it’s a little bit of a leap to say that we’ve figured out why that’s occurring, but we now have a hypothesis that’s definitely worth testing, and the idea is fairly simple based on the paper that you mentioned. We very clearly have shown that Staph will differentially bind hemoglobin depending on the animal that it comes from, and that differential recognition is due to the fact that hemoglobin varies across animals in the surface-exposed portion of the protein. The exciting thing about that is that hemoglobin varies in the surface portion of the protein across individual people as well. It’s a highly polymorphic protein and it’s particularly polymorphic in the surface of the protein, so this is not the portion of the protein that complexes heme on the inside, which is what the protein does. It moves oxygen around our body by having oxygen bound to the heme that’s inside it. The surface part of the protein is fairly variable. So if we extrapolate what we know in animals to human hemoglobin, this suggests that maybe the differences in human hemoglobin across the population might also impact the ability of Staph aureus to bind hemoglobins, and if that’s true we might be able to predict people that are at risk for more severe Staph aureus infections than others based on their human hemoglobin sequence.
I understand that you’re going to be actually using some gene bank data that Vanderbilt actually has DNA from thousands of patients to take a look at those questions.
That’s right. There is an excellent resource at Vanderbilt called BioVU and if you’ve been a patient at Vanderbilt University Medical Center within the past 10-15 years, your blood has been taken—assuming you signed a consent form—your blood has been taken and your DNA banked, and now what is available is the DNA for all of the patients that have been through the medical center over this period of time. And in addition to that, you can access the medical records or the reason for why the patient was admitted or was a patient at the hospital. Of course all this is de-identified, we don’t need that information but the information we can get is why any particular person was a patient at Vanderbilt. What we’re doing now in collaboration with Buddy Creech, who is a pediatric infectious disease physician here who specializes in Staph infections, is, Buddy’s helping us mine the ICP-9 codes, which are the code for why all the patients were admitted, to identify people that were admitted with systemic Staph infections. Then we’re sequencing their hemoglobin genes to see if there’s any conserved polymorphisms or conserved amino acid residues in that particular population of patients as compared to otherwise healthy people or people that did not suffer from systemic Staph infections.
That’s a tremendous resource of data you have available to you and that’s going to be very exciting to see where those experiments take you. Eric, if you’re able to eventually suss out the differences in hemoglobins ultimately, between patients who’ve had infections and the seemingly resistant people, what will that mean in terms of potential translation into practice?
I think one of the biggest problems that we face in the field of Staph aureus is the patients who check into hospitals for a variety of reasons tend to acquire Staph infections when they’re in the hospital. So these are patients that were otherwise uninfected that then get admitted to a hospital and acquire a hospital-acquired infection. If we could identify the patients that were at risk for the most severe cases of Staph infection when they check into the emergency room or the intensive care unit, or if they’re about to have surgery, we could potentially design a test where we could take blood samples from these people, determine if they had a hemoglobin sequence that is consistent with a highly aggressive systemic Staph infection. We could then prophylactically treat these people aggressively with antimicrobial therapy in an effort to prevent these hospital-acquired infections. The idea would be that we don’t have the resources available to treat every person that enters the hospital with antimicrobial therapy to prevent infection. However, if we could identify a very small subset of highly susceptible people, we could potentially protect them from infection by treating them with antibiotics.
Eric, in the context of the comments you’ve already made, I’d like to get your take on a couple of interrelated concepts that you’ve already kind of partially delineated. One is a quote I saw on your website that I found intriguing, where you talked about the “battle for metals between bacterial pathogens and their hosts.”
The umbrella program for my research lab is basically that sentence, and it goes back to a comment that I made earlier which is that all bacterial pathogens—all bacteria—need to acquire nutrient metals when they’re inside of the host. They need them for the same reason that we need them. There’s no such thing as an organism that can live without manganese or zinc. Iron is required by almost all organisms on earth. So they just need to get these metals. And the thing about metals that makes them so important to the host-pathogen interaction is that metals have coordination chemistry that allows them to be held onto very tightly by proteins. And because of that, the human body has evolved to bind and hold onto metals exceptionally tightly to prevent bacterial growth. This is the simplest way that we prevent bacteria from making us sick, is we just keep the food away from them, in high-affinity metal-binding proteins, and that process was called, or termed, nutritional immunity by Eugene Weinberg in the 70’s, and I love that term, and we use it a lot. So this is just a very simple idea, exceptionally important, and I think an often underappreciated component of host defense against infection, is the active process of keeping food away from bacteria. If you can’t eat, you obviously can’t grow, and if you can’t grow, for the most part you can’t infect the host. We think that this is an exceptional target for the development of new therapeutics, this very simple idea of, if you can make it so bacteria can’t eat any more then they’re not going to be able to make people sick. So that’s something we’re focused on, trying to learn more about these processes, with the long-term goal being of targeting them.
What form might that targeting take, down the line, as you characterize these processes further?
From a translational standpoint I think we have a lot of potential avenues we could go down, and we just discussed one, so we’ve exploited some host-metal binding properties, that we might be able to exploit that for a diagnostic test for highly susceptible patients. They’re very different than a therapeutic, of course, but yet translational potential. From a therapeutic standpoint, what is needed is some sort of small molecule modifier of the bacterial processes that are required for metal uptake. But the key is that it has to be a modification strategy that does not globally disrupt metal homeostasis in the person. Because as I mentioned, for the same reasons that the bacteria need the metals, we need the metals, so you can’t just completely chelate all the metals, or disrupt metal trafficking throughout the body. You have to specifically target the bacterial metal uptake systems for this to be effective, I think, and that’s where some real innovation is going to have to come in.
Sure. Well again, that looks very promising and it will certainly be keeping you busy for awhile.
I think so. One thing that’s great about metals is that metals are required for something like 40% of all proteins in nature, so it gives us a lot of targets. There’s a lot of area that we can cover and a lot of different things that we can look at.
Eric, what are the implications of this work for the overall problem of antibiotic resistance which is such a huge issue right now? I’ve seen you quoted as saying that “complete and total antibody resistance of Staph aureus seems inevitable at this point.” That’s kind of a scary and alarming conclusion. Where do you think we stand, and how might your work help that situation?
From the standpoint of antibiotic resistance in the long term, I think we potentially are in a lot of trouble. It’s not just Staph aureus, in fact there’s other organisms that are probably a much bigger threat right now; the gram negative rods come to mind, Pseudomonas aeruginosa, Acinetobacter baumanii, these are organisms that are developing resistance at an exceptionally alarming rate, and to the best of my knowledge there are no new antimicrobials in the development pipeline that are focused on targeting these diseases and these infections. So it doesn’t take much to realize the potential and maybe even likelihood that there will be a day in the not-too-distant future when we’ll have infections that are virtually untreatable. I think the solution is to ramp up hugely in the area of anti-infectives research, and how that happens I’m not exactly sure, but that’s something that desperately needs to happen, not just for Staph aureus but for all of these organisms. That’s one of the reasons why I study what I study, is because metal uptake is important for all of these organisms, not just Staph aureus. So if we could find a globalized or a general targeting strategy to inhibit bacterial metal uptake, we might have a new antimicrobial family against a number of different pathogens.
So the drugs you talk about developing would not be new antibiotics, they would be a completely different, novel class to address this metal uptake issue.
That’s right. To the best of my knowledge there are not clinically relevant antimicrobials that target bacterial pathogens based on nutrient metal uptake – targeting nutrient metal uptake systems.
I see. It’s certainly wonderful to know that you are well-supported in your work, because it sounds like there’s a tremendous need for it.
Thank you. I think so, I think this is a really important problem, and unfortunately I think it’s under-appreciated. Today, we have antibiotics that most of the time, work. So it’s very difficult to imagine the future, because today we’re OK. But I think the reality is that the future does not look very bright, and if we’re not aggressive about it, and proactive about it, it could be very bad.
Indeed. Eric, how did you happen to pursue this particular line of research? What attracted you to studying the pathogenesis of infectious disease?
I was very fortunate that I went to a public high school with a microbiology class, so when I was maybe fifteen I took essentially a bacteriology class. And I loved it at that point, so I focused my decisions for college on schools that emphasized bacteriology. I went to the University of Wisconsin at Madison, which has one of few bacteriology Bachelor of Science degrees in the country. I’ve always been fascinated with the process of infection, and how these tiny little organisms can take over a seemingly insurmountable host and wipe them out in rapid order. So that was something that I wanted to learn more about, and decided to focus on researching bacterial pathogens when I was in college. I was fortunate—I think as with most people that are in science as a career, I was fortunate to have excellent mentors, everything from my high school biology teacher to my college research advisor to my graduate thesis mentor to my post-doctoral research advisor, all of them gave me excellent advice, and sort of helped me along the path. I got into pathogens in graduate school. I did my thesis work on Neisseria gonorrhoeae in Hank Seifert’s laboratory, studying DNA recombination. And that was my introduction into the pathogens. I had read about them before, but only worked on environmental microbes. So that was really exciting for me, and after that experience, I wanted to focus on one of the most significant public health problems that I could identify in an organism that was genetically tractable and had an animal model. Because for me, I’m very interested in the interface between the host and the pathogen. I like to study both the host and the causative agent of infection. I wanted an organism that had an animal model, and Staph satisfied that bill. I was fortunate to get a post-doctoral training position in Olaf Schneewind’s lab, who’s one of the foremost researchers in Staph aureus. And he sort of set me on my path to study this organism, and while I was in his lab I made some collaborations with some chemists who got me interested in metals. That was sort of how it worked, I really just had great people helping me along the way, I think.
As you said, that is so often true of well-established scientists, that they stand on those shoulders...
There’s no question about that...all along the way too, from when I was fifteen to post-doc, there was some critical person at each step, which helped me to the next one.
Eric, you’ve already touched on it a bit, but I want to focus you in on the question of where your research is headed from here.
The research that we’re talking about is for hemoglobin, I touched on it a bit. We want to identify human hemoglobin sequences that are recognized more efficiently by Staph aureus—what hemoglobins might make people more susceptible to Staph aureus infection. We’re going to do that in a laboratory by making recombinant versions of hemoglobins with different sequences and seeing how they bind to Staph, and then making mice that have different versions of human hemoglobin and see how those mice change susceptibility to Staph. Also, we’ll do it in a clinical/epidemiological sense by taking patient DNA from Vanderbilt Hospital and seeing if we can make correlations between hemoglobin sequences and susceptibility to infections. In addition, the wonderful thing about hemoglobin is it’s one of the most well-studied proteins in nature, and hemoglobin is sort of a paradigm for genetic susceptibility to infectious diseases. You might be familiar with the sickle cell trait, and how sickle hemoglobin is believed to have evolved to protect against malaria. This is an interesting observation, and we want to now look at the pathologic hemoglobin—so this would be thalassemia and sickle cell—and see if there’s any difference in the ability of Staph aureus to recognize the pathologic hemoglobins that I just mentioned. And then finally, there’s other versions of hemoglobins, there’s neo-natal or fetal hemoglobin, and we have a different hemoglobin in our body when we’re first born and it switches to another kind of adult hemoglobin as we age, and how these different hemoglobin forms impact the host-pathogen interaction are not known. So these are all different areas that we’re thinking about investigating, just trying to get a sense for when, during a person’s life, and what kind of human genetic sequences Staph aureus has evolved to most efficiently colonize.
Eric, your work is just as fascinating as it is important, and we certainly wish you the best of luck for continued success. Thanks so much for joining us today on FOCUS In Sound.
Well thanks, I appreciate the opportunity.
We hope you’ve enjoyed this edition of the FOCUS In Sound podcast. Until next time, this is Ernie Hood. Thanks for listening!