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The numbers that describe the human microbiome are overwhelming. There are between three and ten times more bacterial cells in and on “you’ (and in and on every other person, too) than there are cells of all of your tissues and organs. These bacteria help the body to resist infections by pathogens from the environment, assist in the digestion and absorption of food, synthesize a number of vitamins, and produce a number of hormones that modulate and coordinate the organ systems of the body (including the immune system!). Many aspects of our individual microbiomes are established at birth (especially our gut microbiomes), but the growing science of probiotics indicates that vital microbial systems can be modified by diet.
In a recent paper published in Molecular Psychiatry (May 16, 2017) researchers at the University College Cork in Ireland found that fear is influenced by a organism’s gut microorganisms. Comparing bacteria-free mice to mice with normal gut microflora, they found that the bacteria-free mice had reduced fear responses to stressful stimuli. This study correlates well with work published more than a decade ago by researchers at Kyushu University in Japan that showed that gut bacteria can modulate stress responses through vagus nerve influences and also other mechanisms.
Work at UCLA (published in Gastroenterology in 2013) showed that the ingestion of a probiotic yogurt twice a day for four weeks led to a reduced brain response (as measured by functional MRI’s) to negative images.
A number of these researchers are hopeful that diet and microflora manipulation may become a tool for clinicians to treat fear and anxiety-based syndromes including post-traumatic stress!
Our environments contain many bacteria, too, and many of these come into contact with us via some expected and also some unexpected routes. Most of these environmental bacteria are quite harmless, but some are powerful human pathogens. Think of the sponge you use in your kitchen, for example. Sponges are ideal bacterial habitats, and, as they are used to wipe up messes and clean surfaces, they get infused with a rich diversity and abundance of bacteria. In a paper published in Scientific Reports this past summer (July 19, 2017) a research team at the University of Furtwangen in Germany identified 362 species of bacteria living in a kitchen sponge at densities of 82 billion bacteria per cubic inch of sponge! Typical methods of disinfecting a sponge (microwaving it, washing it in the dishwasher or laundry, soaking it in vinegar or other cleaning solutions, or cooking it in hot water) do initially reduce the overall numbers of bacteria, but they also select for some of the most pathogenically dangerous species in the sponge microbial system!
“When people at home try to clean their sponges,” stated the lead researcher on the project,” they make it worse.”
What should you do with your kitchen sponge? It quickly becomes a dispersal agent for bacteria and the more you try to clean it, the more pathogenic it becomes! The researchers recommend that you either throw the sponge away and replace it with a new one every week or so, or disinfect it and then use in some other house location where food is not handled (like a bathroom or a basement).
Organisms around us also have microbiomes and many of these come into contact with us. A paper published this fall in Scientific Reports (November 24, 2017) by a group of scientists at Penn State and Nanyang Technical University of Singapore precisely quantified the surface bacterial populations on the bodies, wings and feet of houseflies (Musca domestica) and blowflies (Chrysomya megacephala). The hundreds of bacterial species carried by these flies include many human pathogens. These researchers also determined that when one of these flies lands on a surface, they leave behind in their footprints potential bacterial colonies that can, depending on the receptivity of the surface, grow into significant inoculates. Included in the human pathogens transported by these flies was Helicobacter pylori, the bacterium that can cause stomach ulcers in humans. Fly transmission of H. pylori was unknown before this study and was not included in the epidemiological models of H. pylori dispersion. Interestingly, this study also found that flies collected in urban environments tend to carry more pathogenic bacteria than flies collected in rural environments. Even the proximity of stables and barns did not bring the pathogenic bacteria loads on the flies up to “city” levels!
(I guess we know where to have our picnics this summer!)
And, finally, I come to some observations from the Science section of the New York Times that I have been trying to work into a blog for almost a year! The topic is the bacterial microbiome of a chicken egg.
A Times reader wrote in to ask “why do Americans refrigerate their eggs?” making the observation that in Europe and many Asian countries eggs are kept in bowls out on the kitchen counter (February 13, 2017). The answer has to do with the microbial and protein components of the egg shell’s surface. In the United States large egg producing facilities (defined as those that have more than 3000 hens) are required to wash their eggs before they can be sold to consumers. The reason is the bacterium, Salmonella. Salmonella is quite common in the digestive and reproductive tracts of chickens, and the bacterium can get incorporated into the forming egg in the chicken’s ovary or smeared on the outside of the egg shell from the chicken’s feces. The Center for Disease Control (CDC) estimates that there are 1.2 million cases of Salmonella food poisoning each year in the United States and 450 resulting deaths. While eggs are not the only source of Salmonella, they are a very controllable one.
So in the Unites States most commercially produced eggs are washed. This washing does remove the surface Salmonella, but it also removes a thin, protective coating that seals and waterproofs the egg. The lack of this coating means that the egg will lose its internal moisture much more rapidly and allow environmental bacteria to much more easily cross into the egg. Both of these consequences greatly reduce the shelf-life of an egg unless, of course, it is refrigerated.
The European Union, though, prohibits the washing of commercially produced eggs. They feel that the potential benefit gained by the removal of Salmonella does not justify the loss of egg freshness and quality caused by the washing away of the protective egg shell seal. It would be interesting to see if there are more (or fewer) cases of Salmonella poisoning in Europe compared to the United States.
So, bacteria are in us, around us, and moving through us. Wash your hands and enjoy your eggs!