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Episode 8-
Professor Philip Marsh, Part 1

Professor Philip D. Marsh on Challenging the Orthodoxy Pt.1

If you're going to be a trailblazer, then you're going to have to take down a few trees along the way. You have to be willing to follow your nose and believe in what you're doing enough to embrace your critics with open arms.


Today’s guest is Philip Marsh, Professor Emeritus of Oral Microbiology at the School of Dentistry at the University of Leeds in the UK. In his impressive career, he's published over 300 research papers and review articles and is co-author of a leading textbook on oral microbiology. 


He has won numerous awards including the ORCA-Rolex Award for Caries Research in 1991, the IADR Distinguished Science Award (Research in Dental Caries) in 1998, the Society for General Microbiology (UK) Colworth Prize Award in 2001, and a Lifetime Achievement Award in 2008 from the World Congress of Minimally Invasive Dentistry. The only one missing is a Noble Prize!


Join us next time for Part 2 of Phil’s interview.



Follow your curiosity, connect, and join our ever-growing community of extraordinary minds.

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University of Leeds School of Dentistry



What's In This Episode

  • Phil’s journey to the field of oral microbiology

  • The confidence shattering criticism he faced early in his career

  • How Phil braved the wilderness and stood up for what he believed in

  • The science behind Phil’s groundbreaking theories














Dr. Kim Kutsch:

There is a place where technology and art meet, where work and play are one and the same. When the threads of curiosity are pulled in this place, the spark of innovation ripples across industries. Those who make this place their home are giants. Titans, who pursue creative passion while leaving their mark.







Dr. Kim Kutsch:

They are courageous thought leaders set on changing the practice of dentistry and their corner of the world. More than the sum of their parts, we deconstruct the traits that bind these uncommon innovators...









Dr. Kim Kutsch:

... to discover what makes them Contrary to Ordinary, where we explore the extraordinary.

Hi there. I'm Dr. Kim Kutsch, host and founder at CariFree. I'm fascinated by what makes the paradigm shifters, world shakers and art makers tick. Let's embark on a journey. Extraordinary is a place where ordinary people choose to exist. Together, we will trek the peaks of possibility, illuminate the depths of resilience, and navigate the boundless landscape of innovation to discover how some of the most innovative dentists and thought leaders unlocked their potential and became extraordinary. On this season of Contrary to Ordinary, we explore the motivation, lives and character of the innovators who see limitless potential around them, the people behind some of the largest paradigm shifts in the practice of dentistry.

On this season of Contrary to Ordinary, we explore the motivation, lives, and character of the innovators who see limitless potential around them, the people behind some of the largest paradigm shifts in the practice of dentistry. If you're going to be a trailblazer, then you're going to have to take down a few trees along the way. You have to be willing to follow your nose and believe in what you're doing enough to embrace your critics with open arms. My guest today is Philip D. Marsh, professor Emeritus of Oral Microbiology at the School of Dentistry at the University of Leeds in the UK. In his impressive career, he has published over 300 research papers and review articles, and is co-author of the leading textbook on oral microbiology.

He has so many awards under his belt that it's difficult to list them all, but I'll list just a few. He was the recipient of the ORCA-Rolex Award for Caries Research in 1991; the IADR Distinguished Science Award, Research in Dental Caries in 1998; the Society for General Microbiology UK Colworth Prize Award in 2001, and a lifetime achievement award in 2008 from the World Congress of Minimally Invasive Dentistry. The only one missing is a Nobel Prize, and frankly, I think he deserves one. This will be part one of Bill's interview and we're going to start where he grew up, the industrial heartland of the UK.

Phil Marsh:

It's known as the West Midlands, and people there were very ordinary but very, very skilled workers. So they made everything from nails, nuts and bolts, metal work, cars, refrigerators, anything that could be made that involved manufacturing. I went to a school, we call them in this country grammar schools, and that puts you on a track that you would generally get some qualifications and not be going into industry but going to some sort of profession.

I enjoyed the science. I enjoyed science more than some of the other subjects and so I selected three science subjects to move forward with - physics, chemistry, and biology, so it was very standard, and noticeably, not maths because I found mathematics very challenging. And I always joke that I ended up doing biology because I also enjoyed art and I liked drawing pictures of amoebae and coloring in the nucleus and paramecium and all those. So I like drawing bones. I found some old pictures that I did of bones from my biology course and it's all shaded in so you get the 3D effect. So I think my art was a big influence on me liking biology, plus the fact there wasn't so many numbers in biology at that time.

Dr Kim Kutsch:

Somewhere along the way, you picked at microbiology. Where did that stem from?

Phil Marsh:

There was a page or half a page in our zoology type book on microorganisms, and what fascinated me was how something so small could have such a profound influence on complex organisms like man and animals. So I was intrigued by that, even though there was almost no information to base a decision that was for the rest of my life on. My kids wouldn't be allowed to get away with that. You would say, "Are you sure this is a good idea?" But I was quite keen. That's what shaped me, was just a little bit of knowledge and understanding that microbes were significant.

Dr Kim Kutsch:

Who were some of your mentors then, Phil?

Phil Marsh:

Well, some of those teachers that steered me to have some ambition in that direction were very important. The mentors really came a little bit later once I'd graduated, and certainly shaped some of my critical thinking. Up to that point, everything was serendipity. It wasn't following a course like dentistry where you are going to be a dentist. When you're going into something like microbiology, the number of avenues you could end up, going from brewing to industrial microbiology to medical microbiology, veterinary microbiology, endless.

Dr Kim Kutsch:

It is endless.

Phil Marsh:

So in a sense, I had no preconceived idea, except the health orientated aspects were definitely a strong feature, and in fact, the course I chose to do for my degree had very little on that. I was just not aware. When you are looking at the description in a booklet of all the university courses, you just see some words and they all sound so very interesting and exciting, but I didn't realize that that meant you were perhaps being lent in a particular direction and there were probably other courses that were dealing with pathogenicity in more detail. After I got my degree, I was looking for a PhD and I had an agreement that I was going to do one on influenza in the same city. This was the University of Sheffield in the north of England, and we'd had some lectures on flu.

I was just fascinated in how you have these waves of pandemics and know, again, something absolutely tiny could have so much impact, not just on an individual but on populations. However, at the very last minute, the guy had to say the money that he got to fund the PhDs, he couldn't have two concurrent grants or one grant that followed the other. There had to be a gap from the funding body, so he said either come back in a year or sorry. And then there was one at a medical school in London and this guy was very keen, so he invited me down and I just assumed that all PhDs were similar. But in fact, I was his first PhD student and I was in a lab where there were no other PhD students, and in fact, they didn't do much research. So he was phoning me up all the time saying, "I need a decision," and I was still waiting for the other application, and so I thought, "Well, I'll take this one."

Well, going to London on a PhD stipend was challenging because literally after paying for some accommodation, there was almost nothing left to eat with. I was the thinnest I'd ever been, hungry for knowledge and hungry for food. London was a great place, but everything's big. There's distances, there's travel, there's expense, but I learned to ride a motorbike in the end and hammered around London on a tiny motorcycle, which would absolutely frighten the life out of me now when I realize how I weaved in and out of the traffic. And the fact there was no equipment meant that I had to improvise and had to use my wits more than I should have done.

So I was studying interactions among skin bacteria, so the person who was my supervisor, Sydney Selwyn, he had published some papers where he and a surgeon had identified antibiotic producing staphylococci that are present naturally on skin. When the surgeon operated, he found fewer infections post-surgery if the patient by chance naturally had these antibiotic producing harmless staphylococci, so these were classified then as staphylococcus epidermidis, compared to people who had a similar organism but he didn't produce any inhibitors.

So he wanted to pursue this so we set up some rudimentary continuous culture studies where we could grow these staphylococci with susceptible organisms and see how they went on. And as you might expect, the antibiotic producing organism would knock out competitors compared to the non-antibiotic producing ones, and then on simple agar plates, you could see the range of inhibition. It was very broad spectrum and it was also very tolerant to all sorts of stresses. You could boil it, you could immerse it at different pH values, so it was a very robust molecule. And eventually, I did some simple biochemistry and determined it was probably a circular polypeptide. It was amino acid and the resistance to environmental stresses was probably because it was circular. So anyway, that was that.

Dr Kim Kutsch:

I love Phil's description of his time as a penniless PhD student weaving in and out of traffic on his tiny motorcycle on the mean London streets. In contrast, many of Phil's peers who hadn't chosen the academic path were living lives full of travel and fun. Once Phil had finished his studies, he turned his attention to finding work. That search took him across the city to what was then called the London Hospital Medical College.

Phil Marsh:

They were doing studies into the etiology of dental caries in adolescent children, so it was an ambitious longitudinal study where you take a certain tooth that was known to have some caries in a three to four year period. The teeth were shown to be sound before the study started, and then we took samples three times a year for up to three and a half years, so 10 samples were taken. And we did, using culture, as there were no molecular techniques in those days - you had to grow everything and then do tests to identify. We would analyze the predominant organisms that were growing there and some ones of interest in great detail like streptococcus mutans as was known in those days, and they actually offered me the job. So the reason was they thought the fact that I'd worked on mixtures of skin organisms would put me in a good shape to be studying mixtures of oral bacteria, even though I couldn't have named an oral bacterium in those days because there was almost no knowledge. Nothing was taught about oral microbiology in the early to mid 1970s. It was very niche.

And so I joined a team of about 10, 12 people. There were five microbiologists doing the analyzing, supervised by two people who were influential - Jeremy Hardy, but in particular, George Bowden was a major influence. He was a real, real thinker. So in terms of a mentor, I was lucky to sit opposite George for five, six years, and he was someone who really understood science. He had come through the technical route rather than the academic route, so he could dismantle a microscope, put it back together and understand why everything was blurred, whereas, if you've come through university, you ask someone to set it up for you and say, "Why isn't this focusing sharply?"

Dr Kim Kutsch:

If you could pick a couple of traits or values that George had that you could identify that had an impact on you, what would you say those were?

Phil Marsh:

I think it's more that you don't just accept what is published out there. You read stuff that's in the literature or you're talking to people at conferences, but you have to process it and weigh it up against what you are finding and see whether you think that holds true or not. So not just to accept everything that's written down as the absolute truth, but he taught me how to break down the problems and start to analyze them, and initially, how to start writing papers because in a big study like that, not many papers were written. You have to wait till the end to get a big study, and little bits and pieces pop out on the journey because he encouraged us to have our own little research projects. And I would hand over a manuscript that had been typed, hand-typed, and then he'd go... Actually, no. I would've handwritten it at that stage and then he'd go through it and cross most of it out, and I found that very disconcerting.

But slowly, you begin to learn how you construct manuscripts, how you construct your arguments, how you justify them. I just learnt a lot from George, which in a sense, I hadn't and should have learnt from my PhD. What the PhD taught me was how to be self-reliant. I couldn't just assume, "Oh, there's a big machine that's going to sort this out." You have to work out how you're going to analyze something using your initiative.

Dr Kim Kutsch:

I think one of the things I'm hearing from you is that you learned from George was to be open-minded, to be okay with challenging the status quo, and then having the courage to stand there with it and speak your truth.

Phil Marsh:

And to also have your own evidence. If you're going to put something there where you are challenging, you need to have your evidence stacked up and think it through, and you're correct, not necessarily going with the absolute dogma. Be prepared to challenge, but look for things that don't really hang together well. And so I learned an enormous amount from him, and we did this study and this is where the ideas came from, that not all caries was caused by a single organism, and then if you look in dental plaque from a sample on a tooth, it's not just one or two organisms.

We had pretty good anaerobic microbiology then compared to many groups and we could obviously grow a hundred plus organisms, and some of the organisms that were being talked about, written about, published about, I could see they weren't always very significant in numbers. And you start to ask questions like is 0.5% of a large population of strep mutans more significant than 20% of a different streptococcus? They both make acid at different rates, but no one ever really talks about the numbers, amounts of one organism versus another. The paradigm back then, because we're looking through the mid 1970s to the late 1970s where strep mutans was the causative organism, and everything was based around strep mutans. Vaccines were being made against strep mutans, and if you started questioning this dogma, essentially, you were a heretic.

Dr Kim Kutsch:

It's still so amazing to me that those who set out to move science forward are labeled as troublemakers rather than heroes. This can really affect who will fund your research, something that Phil found in the UK grant system.

Phil Marsh:

What, being in Britain, you were exposed to was a huge US research program. The NIH funded a lot of dental research and there were superb groups, so you had the Walter Lowishes and so on, people who had been at the very start of looking at identifying key organisms and were hugely important and influential. But ultimately, the funding system is built around the sort of paradigm, so if you put in a grant that said, "We're going to study this population of people because we think strep mutans causes caries, so we just want to check it out in this group with some unusual characteristic," you have a good chance of getting funded because on the committee are funders, are people who all work on strep mutans so that's good. But if you were to say, "Do you know what? I'm not sure if this is absolutely the only explanation," well, good luck with that because people are going to say, "Why?"

So in Britain, we had much less funding, and I'd say probably in Europe in general, but in Britain at that stage, not now, every department had a little bit of money that you could decide how you wanted to spend on research. So if you wanted to do some hair-brained scheme like, well, perhaps strep mutans isn't the only organism, well, that's up to you. If you want to spend your money on that, that's your choice. But ultimately, the data you get, you will use to try and get bigger funding, and if you haven't got any data that looked promising, you've wasted time and money.

Ultimately, I got my first grant from our medical research council, which is the equivalent of NIH, by saying, "We know a lot about strep mutans, but actually, we don't know very much in detail about some of the streptococci that aren't supposed to cause caries. So isn't it important that we compare the two groups of organisms, those that we think cause carers and those that must be similar by the nature of being taxonomically related and don't cause carers?" So that's where I got my first grant from, and we might refer back to the data I got from that in the early 1980s.

So George Bowden was a huge influence on me because he also introduced me to microbial ecology. So I was still a relatively junior person in the team, but he would be looking at the data from 50 school children that had run for 10 samplings over three and a half years and we could see the ups and downs of the bacteria over time, but in about a third of the sites, caries developed at some point along that journey. So you can look at the microbes before versus after caries developed and see what changes happen, and you could also compare those sites that stayed completely caries free to those that developed caries at some point, but the fact it was longitudinal, you could see changes over time as well.

Dr Kim Kutsch:

This leads to another topic that I've never shared with you. In fact, I've never shared this with very many people at all, ever, but after Doug Young and I started with taking your data and your research and looking at how pH affects the biofilm, particularly in terms of the disease dental caries, you and I had this conversation 20 years ago. When I asked you, did you think that pH could be a way to treat this disease? And of course, based on your research and data, you told me that you firmly believe that.

So Doug and I started with that, and I have to tell you that it challenged all the existing paradigms because this was still, even 20 years ago, this was still a pathogen driven disease. Maybe we were starting to look at the biofilm and the microecology, but the concept of pH was so contrary to everybody's belief in that whole system at the time that when we did this, we were ridiculed, I mean Doug and I were, and actually got pulled aside by a researcher in the states. Somebody that I really respected patted me on the shoulder and said, "You know, Kim, it's not that simple, and you're a dentist and you should let the real researchers do the research. You're not a real scientist, and so let the real scientists deal with this."

Phil Marsh:


Dr Kim Kutsch:

Yeah, right? Bill, I was really hurt by that. I think still to a degree, the dental profession hasn't really grasped or understood your research, but did you ever deal with any of that?

Phil Marsh:

Sure. Yes, I did. So one characteristic of myself, which if we are being open like we are here, a revelation is I've always been actually very shy and I would say a little lacking in self-confidence. And certainly when I used to go to these conferences and you saw the big hitters on the stage, impressive amounts of research, impressive speakers, but what they were saying just didn't fit with what I was seeing. So I'm not a dentist, I wasn't brought up in the oral microbiology field. I had studied other bacteria and it didn't quite hang true to me, so it was a question of really just starting small.

Dr Kim Kutsch:

It's surprising to hear that someone as intelligent and forward-thinking as Phil is shy. Even when you're further in your career, it can feel difficult to believe that you can stand up and give an alternative to the status quo. Phil remembers a time early in his career when his work was heavily criticized by the orthodoxy of the profession.

Phil Marsh:

We had published a paper where we'd done a local study with school children looking at the microbes in dental plaque, and then we were able to analyze the enamel below using histology, so we could detect the very earliest stages of demineralization and relate which microbes were there and in what numbers and the degree of demineralization. And we published this and one of our conclusions was there was evidence that strep mutans wasn't inevitably associated with demineralization, and therefore, other bacteria could be involved. Sometime later, I got through the mail a letter from the editor which was from a very distinguished Canadian oral microbiologist who wrote a rather forceful letter criticizing publication, and finished with that the failure of me to adhere to some of the principles has inevitably led to the conclusion of microbial non specificity in the initiation of lesions.

Unfortunately, such papers unnecessarily confuse many readers, and in my opinion, have been damaging to the progress of the science. So as a young research worker, this was an unbelievable shock. It was like being hit around the face with a wet kipper, which is a type of fish we have in the UK, and it was, again, one of those blows to your self-confidence. You wonder if you are going in the right direction or not. We did write a robust reply and I said, "On the contrary, we believe that an open mind and the application of novel and more precise approaches to the study of a disease with a complex multifactorial etiology will lead eventually to greater advances in the field of cariology." And hopefully, that's what we've done.

Dr Kim Kutsch:

So standing up against some of the existing paradigms created a challenge for you as a young researcher.

Phil Marsh:

Yeah, a challenge both scientifically but also from a confidence perspective, because I was really just starting on the journey and to get comments like that really is a big dent to your confidence. Because if it's written in the journal, this isn't a private letter to me. This was published in the journal and my reply was subsequently published in the journal next to it, so it's there for everyone to read and you do feel that challenge to your direction of research travel.

Dr Kim Kutsch:

But you have the courage to stand up and go, "What I'm doing is speaking the truth," and it takes courage.

Phil Marsh:

Yeah. Within the limits, any study has limitations depending on what age group, how many people, what techniques you have, but we were doing something novel by looking at demineralization by histology, not by something that could be detected with a probe or an X-ray. So this was very early. It was cross-sectional, which we acknowledge, but yeah, you have to in the end say, "Well, I know there are imperfections like there are in any clinical study, but we are confident that this is right, and this is just part of an increasing buildup of knowledge that says that caries is more complex and it doesn't have a specific microbial etiology."

Dr Kim Kutsch:

I published a paper, oh, God, probably in 1991 in a prominent journal, and it was using lasers in treating periodontologies. And I got this scathing letter to the editor in the next publication of that journal, and from a prominent periodontist in the United States. I didn't even respond to it but I remember the one phrase. He referred to me as the nefarious practices of other aspects of the profession.

Phil Marsh:

Wow. It's very hurtful. It hits you hard, it hits your confidence hard.

Dr Kim Kutsch:

And now, I wear that as a badge of courage, but at the time, I was stunned. Because we were early on in the days of lasers so I know that we were challenging existing paradigms, but to be that attacked by somebody, that was kind of an eyeopener for me. And again, I think it makes you stronger at some point in terms of your determination and your convictions too. Would you agree with that?

Phil Marsh:

Yes, exactly, your determination and convictions. And although research often seems to be a really nice career with nice people going to conferences, it's also very tough. It can be very lonely and you have to be mentally strong because there are people trying to perhaps undermine your views or talk elsewhere that, well, that work isn't so good and et cetera. You have to have that conviction but also the strength just to keep pushing, that determination.

Dr Kim Kutsch:

Social scientist and author, Brené Brown, once said that true belonging is a spiritual practice and it's about the ability to find sacredness in being part of something, but it's also about the courage to stand alone. When you're part of something bigger than yourself, whether it's a profession, a movement or even a family unit, finding the courage to stick with your own convictions can be scary, but for us to find our true selves, we have to brave the wilderness. After writing over 300 papers, you probably won't be surprised to hear that Phil has had a few of his theories quoted over the years, one of them being the ecological plaque hypothesis.

Phil Marsh:

I wasn't totally convinced about that label but I thought it needs something, because I'd spoken in the early nineties to a very senior public health dentist in a Scandinavian country, and we were just chatting over breakfast and I realized he didn't understand that kind of complexity of the microbial etiology, and therefore, his decision-making was influenced by a specific disease type approach that had been applied to other diseases. And I suddenly realized, because talking about all the data, paper here, paper there, it's a complex web, that it needed something that brought it together in a simple message, and I realized I needed to write a sort of overarching, encompassing theory, and that's when I came up with the ecological plaque hypothesis. I used that term in a paper in 1991 in the proceedings of the Finnish Dental Journal because someone I knew slightly was retiring and they devoted an issue to him. And in a way, it was very cowardly because I knew if I put it in there, no one was going to read it, but I could always quote it, which I did.

But then it made its opening on the big stage in 1992 and was published in '94, which was a conference in the UK where it had some of the big hitters there. And it was easy writing the manuscript because I talked about all the evidence for the ecological plaque hypothesis for caries, and then to a certain extent, for periodontal disease. But when I gave it, there were some of the big names from the US sat there. It was a small meeting so you couldn't avoid anyone. You were all together in a hotel for two and a half days, and what did I do? I went straight to my room and locked the door and didn't come out until later on because I was just scared witless.

But you just keep building on it. You keep building, you keep saying the same thing with evidence, you keep showing the evidence and the data, and fortunately, I got invited to other meetings, because if you don't get invited, you have no platform to share this. So fortunately, I was considered a good communicator and I had something interesting to say so you get put out there. Whether it was being put out there so you could get slapped down, I don't know, but gradually, you get your message across. So I read a paper a little while ago from someone who I didn't think would be a big follower, and he said, "The theory that seems the most established is the ecological plaque hypothesis and this explains all the observations and findings." And I suddenly thought, "Wow, people are accepting it," as opposed to it just being, well, it's a bit of a far out hippy-

Dr Kim Kutsch:

Yeah, one more crazy theory.

Thanks for listening to part one of Phil Marsh's fantastic interview. Phil will be joining me next time in part two where we'll be talking about his 38 years at Public Health England, how he strove to be a great communicator and much more. Around here, we aim to inspire and create connections. We can't do it without you. If this conversation moved you, made you smile or scratch that little itch of curiosity today, please share it with the extraordinary people in your life. And if you do one thing today, let it be extraordinary.

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