Today, I’d like to highlight a fascinating scientific paper that identifies a potential danger in a widespread food additive. The article “Dietary trehalose enhances virulence of epidemic Clostridium difficile” published in Nature last week focuses on a specific pathogen that has become especially virulent in the past decade (1). They find that this virulence may be caused by a mutation that allows the pathogen to digest the food additive trehalose. They propose that the increase use of dietary trehalose could be responsible for the worldwide Clostridium difficile epidemic.

Before I begin, I want to make it clear that this post is not a condemnation of food additives as a whole. I’m covering this paper because it is a cool story about an unusual route a pathogen took to become virulent, but it seems to be a unique instance. I’m not an expert in food safety, and the last thing I want in the comments section is for everyone to vaguely argue whether or not artificial food additives are evil and this paper is proof of it. Please keep this in mind when commenting.

The two main players in this story are bacteria Clostridium difficile and the sugar trehalose. Since neither of these are common knowledge, I’m going to give some background info on them before diving headfirst into the paper.

Trehalose – A common sugar

Trehalose is a sugar that many organisms naturally produce and consume. It consists of two glucose molecules connected by a α,α-1,1-glucoside bond (2). Glucose is a widespread simple sugar (commonly known as table sugar) and the molecular bond serves as a stable unreactive connection between the two glucose molecules. In fact, the bond between the two glucose molecules is so stable that trehalose is very resistant to heat as well as acidic and alkaline conditions. Trehalose is also fairly sweet and does not directly cause any known health issues.

Trehalose’s combination of sweetness and stability has made it desirable for the food industry (3). However, it was not until the early 2000’s that its production cost lowered enough for it to became a ubiquitous food additive. Nowadays, it can be found a variety of foods including pasta, ground beef, and ice cream in concentrations ranging from 2-11% (4).

Now before you throw up whatever trehalose-laden food you’re currently eating, keep in mind that trehalose is a natural sugar that has no negative health consequences by itself. In order to understand its potential health risk, we first must understand the bacteria Clostridium difficile.

Clostridium difficile - A problematic pathogen

Clostridium difficile is a widespread Gram Positive bacteria that can infect humans and farm animals (5). When this bacteria infects a host, it releases toxins that cause severe watery diarrhea. The lovely dirrahea is contaminated with C. difficile allowing the microbe to infect others through fecal contact. If you were infected in a developed country with plenty of access to clean water, you probably wouldn’t die, but you’d have a horrible few days. In a Nevertheless, the pathogen infects millions of people a year resulting in thousands of deaths (6).
Since 2003, Clostridium difficile outbreaks have become increasingly common (7). Scientists have shown that two particular strains of Clostridium difficile, known as RT027 and RT078, are responsible for these outbreaks. However, these strains have been around for a lot longer and no one has been able to determine why these two strains only recently became effective at causing outbreaks.

An unexpected connection

In the paper we’re reviewing, the authors were trying to figure out why the strains RT027 and RT078 were causing so many Clostridium difficile outbreaks (1). They note that the two strains evolved independently so any unusual phenotypes that distinguish them from other Clostridium difficile strains could be important. Their first major clue came when they testing to see whether these strains could utilize unusual carbon sources. They grew strains on minimal media supplemented with different potential sources of energy, sugars in this case, and measured the organism’s growth. They found that RT027 and RT078 grew faster on low levels of trehalose, but other Clostridium difficile strains could not.

The growth rate of R027, RT078, and other Clostridium difficile strains on minimal media supplemented with nothing, glucose, or trehalose. Adapted from (1).

It’s impossible for me to know what the scientists were thinking when they first made this observation, but I imagine it was little more than mild interest. This was probably just one unusual phenotype among many that, at the time, may or may not be related to the organism’s pathogenicity. However, picking up on these little clues is a critical skill that scientists develop and I’m sure it served them well here. Perhaps after this discovery, they started researching trehalose a little bit. When they found out that trehalose became ubiquitous right around the time that Clostridium difficile outbreaks became more common, they had an “Aha” moment and hypothesized that the two were connected.

The scientists also sequenced the genomes of the RT027 and RT078 strains and compared them to those of other less pathogenic Clostridium difficile strains. Since they had observed the trehalose phenotype, they tried to figure out why RT027 and RT078 can grow so well on the sugar. Fortunately, the genes responsible for trehalose metabolism in Clostridium difficile are already known, and they were able to show that both RT027 and RT078 separately evolved mutations that increased their ability to digest trehalose for energy. In the nature paper, this takes only a paragraph to explain, but I’m sure this took months of bioinformatic and genetic work to identify and verify.

The genomic sequencing helped verify the trehalose growth phenotype for these strains, but the authors still needed to show that this phenotype affected the virulence of RT027 and RT078. One way to do this is with mouse models. They fed one group of mice a normal diet and fed another group food supplemented with trehalose. They then infected both groups with RT027 and compared the mortality rates over time. They found that trehalose significantly increased mouse mortality

The authors further showed that deleted genes necessary for trehalose metabolism in RT027 reduced the virulence of RT027 in the trehalose-fed mice. Similar tests with similar results were performed on RT078 as well. All in all, convincing evidence that that ability of these *Clostridium difficile* strains to infect a host is increased if the host consumes trehalose.

To wrap up this story, the authors showed that the levels of trehalose in a normal human gut were sufficient for RT027 to induce trehalose metabolism. The took samples from the intestines of three humans who were eating their normal everyday diets. They then infected these samples with Clostridium difficile strains and tested whether these strains were able to upregulate the express of the genes involved in trehalose metabolism. They found that for two o the three human samples, trehalose levels were strong enough to induce the genes in the RT027 strain, but not in a control strain.

This isn’t extremely compelling evidence because they only looked at 3 humans and only showed that trehalose metabolism was upregulated, not that virulence was increased. However, pesky ethics laws prevent researchers from infecting large groups of people in the name of science and force feeding them trehalose, so this was probably the best the authors could do.

The authors provide lots of supplemental data that further confirms and reinforces these findings, but none of the extra information puts anything completely new on the table. The three figures shown above pretty much sum up the whole story.

Is trehalose causing outbreaks?

The short answer is … maybe.

Let’s recap what the authors have shown

• Since 2003, Clostridium difficile strains RT027 and RT078 are responsible for an rash of recent outbreaks
• These strains are different from other species of Clostridium difficile in that they can easily metabolize trehalose
• Mice are more susceptible to the pathogen if they are fed trehalose
• The levels of trehalose in some humans are sufficient to induce trehalose metabolism in RT027 and RT078

All in all, this makes for reasonable argument that the trehalose in human diets could be facilitating Clostridium difficile outbreaks. The authors drive this point home with a timeline showing how many outbreaks have occurred after trehalose began seeing widespread consumption.

Timeline of recent trehalose outbreaks in relation to the widespread production of trehalose. Adapted from (1).

Yeah, I’ll admit that this chart is compelling, but it only shows a correlation between the two factors. As any stats professor will drill into you, correlation is not causation. Take the equally compelling timeline I created below.

Timeline of the release of Corky Romano starring Chris Kattan in relation to the widespread production of trehalose. Adapted from (1).

Based purely on this chart, you could argue that Chris Kattan’s Corky Romano was such a terrible movie that audiences spontaneously acquired Clostridium difficile infections. Obviously that’s not true (probably), but it’s important to keep in mind that circumstantial evidence isn’t definitive and this nature paper is mostly composed of circumstantial evidence. They’re missing data showing a direct link between trehalose consumption in humans and outbreaks of these strains (9). Smaller issues also must be addressed. For example, the authors looked at trehalose in the intestine, but Clostridium difficile resides in the colon where trehalose levels might be too low to be relevant (10).

Despite my misgivings, this paper makes a compelling argument that the trehalose in human diets facilitate Clostridium difficile outbreaks. I would have to agree that they’re probably correct in their conclusions, but I don’t think that trehalose is the only factor that makes R027 and R078 so virulent. These strains have other important characteristics, such as flouroquinolone resistance (11) that are critical aspects of their pathogenicity. Thus, thought the results of this study demand further investigation into the connection between trehalose and Clostridium difficile, broader studies are needed before we abolish this sugar from our diets.

Images

All images used have been labelled for re-use by Wikipedia or are taken directly from the publication. If any image owner has an issue with this article, please contact me and I will address the issue.

Sources

(1) https://www.nature.com/articles/nature25178
(2) https://en.wikipedia.org/wiki/Trehalose
(3) http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.611.3105&rep=rep1&type=pdf
(4) https://www.nature.com/articles/nature25178
(5) https://en.wikipedia.org/wiki/Clostridium_difficile_(bacteria)
(6) https://en.wikipedia.org/wiki/Clostridium_difficile_infection
(7) https://www.ncbi.nlm.nih.gov/pubmed/17609596?dopt=Abstract
(8) https://en.wikipedia.org/wiki/Corky_Romano
(9) http://www.sciencemediacentre.org/expert-reaction-to-study-looking-at-dietary-trehalose-a-sugar-additive-and-virulence-of-clostridium-difficile-infection-in-a-mouse-model/
(10) https://www.nature.com/articles/d41586-017-08775-4
(11) https://www.nature.com/articles/emi201362

About the Author

I’m a research scientist living in the suburbs of Boston. I recently left academia to work in in industry, but I miss teaching so I decided to start writing articles on interesting discoveries in the world of microbiology. My goal is to uncover subjects that are unusual, unique, and might one day have a major impact on our daily lives.

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