The Evolving Story of Hygiene Hypothesis

The Evolving Story of Hygiene Hypothesis

Written by Priyanka Dasgupta

Edited by Eleni Koursari

Microbes never fail to amaze you! These tiny creatures can be mighty forces that influence many important aspects of our daily lives. Besides using yeast to bake bread or letting microbial action ferment your wine, microbes living in our body can also help bolster our immunity.  

Collectively, microbes that live in and on our bodies can be referred to as the microbiome. Did you know that the number of genes of all microbes in our body is 200 times more than the number of genes in the human genome? Cool right? In fact, our microbiome is estimated to weigh as high as 5 pounds! 

The presence of certain bacteria or viruses in the body can make someone less or more susceptible to diseases. For example, the microbiome of our hands- in other words the natural microbes found on our hands, can prevent other harmful bacteria from colonising our bodies. Some microbes inside our bodies can even release mediators to help with our immune responses.  

Indeed, research has shown that a healthy and balanced microbiome within our bodies is essential. That is why broad-spectrum antibiotics, those that target multiple species of bacteria at once, are tricky to use!  They target both the harmful microbes along with those naturally found in our bodies. Therefore, broad-spectrum antibiotics can disrupt our natural microbiome.  

In this blog I am going to talk about some theories that might help explain the link between a need for a healthy microbiome and preventing immune-mediated diseases.  

HYGIENE HYPOTHESIS

The Hygiene hypothesis at its core states that, the lack of exposure to necessary microbes early on in childhood can hinder the proper development of the immune system. This can make one more susceptible to allergies. The lack of microbe exposure leads to an ‘underdeveloped’ microbiome, so removing it from the equation, hampers the development of the immune system. 

Rubber gloves, yellow and green bottle of detergent and sponge on white background , Source: Storyblocks Images

To really see where the Hygiene Hypothesis is applicable, one must understand the asterisked terms and conditions. At the time of its formulation by David Strachan in 1989, this hypothesis was an attempt to explain the rapid increase in allergic maladies like asthma or hay fever. The proposal came largely based on his observation of smaller families. These families had reduced exposure to infectious agents, mainly for two reasons- fewer people/siblings for germs to pass on from; and in small families there was a greater possibility for increased hygienic standards and cleaner amenities. It is because of this reasoning, that the hypothesis came to be popularly known as the Hygiene Hypothesis. 

But don’t mistake excessive hygiene to be the cause for all allergies. In other words, giving up on hygiene is not the solution to allergies. That’s like putting up the ‘free for all sign’: the harmful infectious agents are going to come running alongside! 

Old Friend’s Hypothesis

In 2003, the Graham Rook proposed a more refined hypothesis, referred to as the Old Friend’s Hypothesis, that tackled some of the flaws of the hygiene hypothesis. In the hygiene hypothesis, reduced exposure to the microbes that were important for modulating the immune system, was the cause of a rise in allergic susceptibility. 

Blue, yellow and red bacteria illustration, Source: Storyblocks Images

However, Rook pointed out that the microbes that the hygiene hypothesis was referring to were those that had evolved only in the past 1000 years. Plus, these were crowd infections that could not have evolved in the isolated hunter-gatherer groups humans started out as almost 12000 years ago. The Hygiene hypothesis missed something very crucial! It wasn’t the reduced exposure to infectious agents that caused the spike in allergies. Instead, it was the older friends, tolerated in human bodies through raw hunter gatherer environments, that the human immune system got used to. 

When the exposure to harmful infectious agents decreased, so did the exposure to these beneficial ‘old friends.’ This collateral reduction led to the rise in allergies and immune mediated diseases. 

In today’s times (ever since industrialization) there has been a marked change in the living conditions – in terms of ‘hygiene’. This is more evident in the developed countries where antibiotics and disinfectants used at a mass scale have eliminated not only harmful infectious agents, but also the other ‘old friends’. 

Expanded Hygiene Hypothesis

So, after reviewing the evidence, it is now hypothesized that a reduced microbial diversity can lead to a defect in the natural development of the immune system (immune tolerance). This in turn might lead to certain autoimmune diseases of varying degrees and relatedness. 

Keeping the above discussions in mind, the hygiene hypothesis in itself, has evolved over time. It is now clear that it is not the harmful infectious agents’ exposure or family size but the old friends that are necessary. 

Other terms like ‘Biome Depletion Theory’ have also been suggested. 

In fact, as our understanding of which microbes are beneficial evolves, this knowledge can be used for treating certain diseases. For instance, in some cases fecal microbial transplants can be performed to replenish the populations of bacteria in a patient’s microbiome, helping them recover from diarrhea or abdominal cramping.  

The story doesn’t end here. Does a healthy microbiome affect our immunity? The answer is, definitely yes! But, if you ask us, how exactly? - the answer to that is, we are constantly learning more about this as the theories behind keep evolving. With the advances in scientific exploration, who knows- perhaps in the next decade there might be a new theory in town!