Bacteria, viruses, fungi, and parasites all live around our homes.
When it comes to microorganisms in our living environments, we are bombarded with antibacterial and antiviral soaps, cleaning products of every description, and a general notion that we must keep our houses clean to combat deadly microbial threats.
On the other hand, we are frequently reminded that probiotic microbes have significant health benefits.
Microorganisms are ever-present in our environment and in our bodies, and many are known to be beneficial — or even essential — for our health. However, some are pathogens and can makes us very sick, and they can sometimes even kill us.
Keen to know what microbes might be inhabiting the various parts of my home, I delved into the scientific literature and found out why some of our microscopic roommates are good for us, and why others pose a significant threat to our health.
What microbes lurk around our homes?
Scientists from NSF International — which is based in Ann Arbor, MI — tested 22 households in Southeast Michigan. They found that dishwashing sponges contained the highest number of microorganisms, followed by toothbrush holders, pet bowls, kitchen sinks, coffee reservoirs, kitchen countertops, stove knobs, pet toys, and toilet seats.
In the study, the authors found yeast and molds, bacteria in the coliform family (including Escherichia coli), and Staphylococcus aureus on many of the surfaces tested.
To assess the microbial diversity in house dust, a team of scientists — which was led by Jordan Peccia, a professor of chemical and environmental engineering at Yale University in New Haven, CT — tested samples from 198 homes in Connecticut and Massachusetts.
The researchers found that the most common fungal species were Leptosphaerulina chartarum, Epicoccum nigrum, and Wallemia sebi. The most abundant bacteria were from the Staphylococcus, Streptococcus, and Corynebacteria families.
Homes with pets and those located in suburban areas had more diverse bacterial species, while those with reported water leaks harbored more fungi.
Meanwhile, scientists from Seoul National University in Korea studied the bacteria that inhabit our refrigerators and toilet seats. They found that the many of the bacteria present were also resident on human skin, indicating that we are the source of a lot of the microbes in our living environment.
"In this study, most bacteria detected were probably not pathogens or opportunistic pathogens, and genera belonging to common pathogens were detected in only a very small fraction of communities on the surfaces of refrigerators and toilets," the authors explain.
So, our tiny roommates are everywhere: from our kitchen sinks to our living room floors and toothbrush holders. They key question that remains is what their impact on our health is.
The answer depends on our age, the state of our immune system, and, of course, the individual microorganism in question.
Allergy and hygiene
According to the "hygiene hypothesis" — which was originally proposed by Prof. David Strachan in 1989 — allergic diseases are able to be prevented "by infection in early childhood, transmitted by unhygienic contact with older siblings, or acquired prenatally from a mother infected by contact with her older children."
House dust exposes us to diverse microbes.
In an article published in the October edition of Nature Immunology, Profs. Bart N. Lambrecht and Hamida Hammad — from the VIB Center for Inflammation Research at Ghent University in Belgium — explain that studies in animal models have shown that exposure to some viruses, bacteria, and parasites is linked to lower rates of allergy.
Allergies develop when our bodies mistake an otherwise harmless substance as a threat and react with an immune response. Microbes are known to affect this process in several ways.
Certain bacteria, such as Bacterioides, Bifidobacterium, Faecalibacterium, and Enterobacteria, produce metabolites that promote the generation of regulatory T cells. These cells play a major role in protecting us from developing allergies, but children who are prone to allergy are known to have lower levels of these types of bacteria in their guts.
Microbial components also affect another type of immune cell. Dendritic cells patrol epithelial barriers — namely, the skin, gut, and lungs — where they detect incoming allergens. And if this happens in the absence of microbial components, dendritic cells tend to drive allergic immune reactions, whereas if microbes are present, they do not.
So, where can we find these beneficial microorganisms that might protect us from developing allergies?
Microbes that might give us the edge
Prof. Peccia found that certain beneficial bacteria were preferentially found in homes that housed multiple families and those with more than three children.
One of these, Faecalibacterium prausnitzii, "is anti-inflammatory and protective against Crohn's disease," he explains. Members of the lactobacillus family were also found in higher numbers in such households, and these probiotic bacteria have been implicated in protection against allergies and asthma.
In a separate study, Prof. Peccia's team found that yeasts in the fungal class Kondoa may have a protective effect against severe asthma when present in the home.
Drinking unpasteurized cow's milk is linked to a lower risk of allergy.
Exposure to microorganisms in raw milk and those in the dust of homes located on farms has been strongly implicated in lower allergy rates.
However, bacteria and fungi aren't the only friendly microorganisms.
Although most people tend not to associate parasites with Western nations, in the U.S., millions of individuals are chronically infected with these microorganisms.
But it's not all bad news: there is evidence that tiny parasitic worms called helminths protect their host from allergies.
Many allergens are similar in structure to helminth proteins, which is why in cases of chronic exposure to helminths, these proteins compete with allergens, leading to non-allergic immune responses.
The timing and type of microbe that an individual is exposed to plays a crucial role in the development of allergic diseases.
Not all infections are beneficial. For example, lower respiratory tract infections in children under 3 years of age are a risk factor for wheezing and asthma.
The hygiene hypothesis has been criticized in light of this. Prof. Lambrecht says that new theories suggest that loss of diversity in the human microbiome — a topic I recently explored in a separate article — means that the microbes and parasites that once provided us with protection from allergies no longer fulfill this function.
While microorganisms may play an important role in preventing allergic disease from developing in small children, they can pose a serious threat to the health of others.
When are microbes bad for our health?
For people who have already developed allergic disease, microbes in the living environment spell bad news.
Early life fungal infections, especially those of the airways, are linked to the worsening of existing allergic asthma. Infections of the airways with viruses and bacteria can have similar effects, while fungal skin infections are known to trigger eczema.
Prof. Peccia also found that the homes of severely asthmatic children tended to harbor similar microbial allergens. In particular, high concentrations of fungi were found in the homes of these children, with yeasts in fungal class Volutella standing out.
Our refridgerators are ideal living environments for harmful microorganisms.
In addition to the danger that microbes pose to those already allergic or asthmatic, food-borne household microorganisms contribute to a significant numbers of illnesses each year.
The Centers for Disease Control and Prevention (CDC) estimate that every year, 3,000 people die from food poisoning. Here, the culprits include Salmonella, certain types of E. coli, Listeria, and fungi.
According to the 2013 NSF International Household Germ Study, the refrigerator vegetable compartment was a ready source of Salmonella, Listeria, and fungi, while E. coli were found in the refrigerator meat compartment, as well as on rubber spatulas, blender gaskets, can openers, and pizza cutters.
So, the bottom line is that among the plethora of microorganisms that inhabit our homes and living spaces, some are friends and some are foes. More importantly, how we react to particular microbes depends on our individual immune systems.
Does that mean that I should keep my home meticulously clean? Cleaning my kitchen's hotspots — mostly the dishwashing sponge, refrigerator, kitchen sink, counter, and cooking utensils — will certainly go some way toward protecting my family from contracting food poisoning.
As for the rest of the microbes, it depends. In homes with family members who already have allergies, reducing exposure to any of the culprits that trigger symptoms makes sense.
But are we increasingly becoming trapped in a vicious cycle of cleanliness to prevent allergy symptoms, depriving the next generation from the much-needed early exposure to microbes?
There is no clear answer, but scientists are getting closer to finding out how exposure to microorganisms might have protected us from allergies in the past.
"In the future, we might also see effective and large-scale preventive strategies based on this new knowledge that can restore the lost symbiotic relationships between (micro)organisms and humans without causing disease or requiring a return to an unhygienic lifestyle."
Prof. Bart N. Lambrecht