Doug Powell, renowned for his food safety efforts and Barfblog, often uses the phrase "don’t eat poop" when it comes to food safety. I’ve stolen that line and I now use it a lot too when talking about zoonotic disease. However, over time, I’ve started to wonder whether the line always applies.

I was giving a talk last week and the question of "how clean is too clean?" came up. It’s not the first time, and I have a tough time answering it these days.  The response relates to a few things, such as the hugely important role of our commensal bacterial population and the "hygiene hypothesis."

While some people might be turned off by the concept, we are outnumbered by bacteria in our own bodies. We have approximately 10 bacterial cells in or on us for every one of our own cells. When you compare the number of bacterial genes to our genes, the difference gets even bigger. While bacteria can cause disease, they are also critical to our health – we actually can’t live without them. They help our immune system develop and function. They help with digestion. The help us fight off other more harmful microorganisms. They produce vitamins and other compounds. They interact with us in ways that we don’t full understand, and probably in ways we’ve never even thought about. We know clearly the intestinal bacterial population plays a role in things like allergic diseases, and there’s increasing evidence of interaction between our intestinal bacteria and our brain.

A key part of our development is learning to how to live with and tolerate our bacterial microbiota. If our bodies recognized all bacteria as bad foreign invaders, we’d kill ourselves trying to kill them. Instead, we develop tolerance to certain bacterial populations. Developing tolerance is a critical aspect of healthy life, and things that interfere with development of tolerance might set the scene for future diseases, particularly allergic and inflammatory diseases. That’s where the hygiene hypothesis comes in: are we now too clean?

So, the concept that all bugs are bad is clearly wrong. Which bugs are good and how to live with the abundant microbial world in and around us is the tougher question to answer. Previous approaches to infectious diseases, based on "find bacterium… must kill…", are too simplistic and potential harmful in some situations. There’s new research indicating that the best treatment for recurrent Clostridium difficile infection may be administration of feces from a healthy donor by enema. So, clearly exposure to feces is not always bad.

Back to the original question (I was killing time on a plane as I wrote this so I had a chance to ramble on….): How clean to we want things to be, and can we be too clean? Furthermore, does reduction in our exposure to microorganisms predispose us to various diseases, such as allergic and inflammatory diseases? The answer to both of theseis presumably yes. However, what level of clean is good and what level is excessive?

In a hospital, we want clean… very clean. We have a highly susceptible population and lots of bad bugs in circulation. We want close attention paid to disinfection and thorough hand hygiene in hospitals, no doubt about it. But what about in the general population? Antibacterial soaps are not generally recommended for households because there’s no evidence they are needed and they might increase the likelihood of antibiotic resistance (since bacteria that become resistant to antibacterial agents in soaps can also be resistant to some antibiotics). We don’t need high level disinfection as a routine practice all over the house. At certain times and in certain areas, sure, it’s certainly still a good idea. For example, if you’re working with raw chicken, careful attention to hygiene and surface disinfection is important because of the high likelihood of exposure to some important pathogens (e.g. Salmonella). But do we need to be spraying disinfectants around the rest of the house on a routine basis (as some TV commercials indicate)? Probably not.

Being a germaphobe can be good, but maybe it can also be bad. We need to think about the role of this complex and massive (yet still poorly defined) microbial population that lives with us. How much exposure to bacteria from different sources is actually needed for health, especially in kids? How much is harmful? There has to be a middle ground, and hopefully we’ll find it.

I’m not trying to say never wash your hands, just like I’d never say wash your hands after you touch anything, anytime. In certain locations (e.g. hospitals, food preparation areas) we need to pay extra attention to hygiene and disinfection. But what about the rest of the time? How do we find that balance? No one knows, but it’s an important question to consider.