It's a germy world out there. Microbes prevail in our environment from cloud top to subterranean urban haunts to the tongue and paws of the family dog, a trio of recent studies reveal.
The research is part of a larger effort to better understand where people pick up the roughly 100 trillion microorganisms that live on and inside each of us, some causing disease, but most harmless or aiding our own biological functions.
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First, NASA-sponsored researchers found that bacteria can thrive from 25,000 feet -- cruising altitude for commercial jets -- up to about 46,000 feet, based on sampling done over the Gulf of Mexico and the Caribbean Sea in 2010.
While humans can't survive at those heights without oxygen and protection against extreme cold, the high, thin air was found to be hospitable to more than 300 different types of bacteria (and a few fungi).
Each cubic meter of air measured contained roughly 150,000 cells, with anywhere from 60 percent to 100 percent of them alive. Samples were taken before, during and after the passage of two major hurricanes -- Earl and Karl -- through the area.
While the diversity of microbes was much greater in samples grabbed during and after the hurricanes' passage, a consistent group of 17 bacteria was found in every sample.
The study, led by scientists at Georgia Tech and published online in January by the Proceedings of the National Academy of Sciences, helps fill in some important gaps about life high in the atmosphere.
Not only do the bacteria join dust and other material in aiding the formation of clouds and precipitation, but the discovery that so many can live at high altitudes also shows germs swept up by large storms may be viable after they've been swept around the planet.
In one of two studies by researchers from the University of Colorado at Boulder, scientists developed what amounts to an air-sampling "Big Gulp" to do a comprehensive test of the microbes inhabiting New York City's subways. Collecting each sample took only 20 minutes, compared to the hours needed for conventional methods.
What they found after analyzing the contents of a couple of cubic meters of air collected a few flights down from the surface is that New York subway air isn't much different from the air at sidewalk level.
The main differences were that subway air had a bit more bacteria normally found on human skin -- no surprise to anyone who's been packed onto a 6 train at rush hour -- and about double the density of fungal material than air "up top." The researchers suspect rotting wood contributes to that discrepancy.
The research, published online last month by the journal Applied and Environmental Microbiology, is particularly helpful in setting a baseline of subway germ counts to possibly aid in monitoring any attempt to mount a biological attack on the transit system.
Another UC-Boulder-led team reported on the canine-human bacterial exchange, sampling 159 people and 36 dogs in 60 families, each with at least one couple between the ages of 26 and 87. Swabs were taken from the tongue, forehead and palms on the people, fur on the head and all four paws on the dogs, plus fecal samples from all.
It turned out that people with both dogs and children had more bacteria in common with their pets. Also, spouses who shared a dog also had more similar skin bacteria between them than those without a pooch in the mix.
There were a number of other age and environmental differences, with parents sharing more tongue and gut microbes with their own children than with other children, but only starting after age 3. The weakest connection was the father-to-infant sharing of germs on the forehead and palms.
The study was published in the online journal eLIFE.
It's estimated that nearly two-thirds of U.S. households have a cat or dog. Scientists say adding household pets to the microbial mix is important as they try to understand such issues as the development of children's immune systems, allergies and, perhaps, autoimmune diseases, as well as the direct spread of some diseases. At the same time, our pets are susceptible to picking up some infections from humans, particularly the flu.