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Category: microbes

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Reading this is almost guaranteed to make you want to wash your hands. From Science Daily:

How quickly viruses can contaminate buildings -- from just a single doorknob

Using tracer viruses, researchers found that contamination of just a single doorknob or table top results in the spread of viruses throughout office buildings, hotels, and health care facilities. Within 2 to 4 hours, the virus could be detected on 40 to 60 percent of workers and visitors in the facilities and commonly touched objects.

There is a simple solution, though, says Charles Gerba of the University of Arizona, Tucson, who presented the study. "Using disinfecting wipes containing quaternary ammonium compounds (QUATS) registered by EPA as effective against viruses like norovirus and flu, along with hand hygiene, reduced virus spread by 80 to 99 percent," he says.

Norovirus is the most common cause of acute gastroenteritis in the United States, according to the Centers for Disease Control and Prevention (CDC). Each year, it causes an estimated 19-21 million illnesses and contributes to 56,000-71,000 hospitalizations and 570-800 deaths. Touching surfaces or objects contaminated with norovirus then putting your fingers in your mouth is a common source of infection.

In the study, Gerba and his colleagues used bacteriophage MS-2 as a surrogate for the human norovirus, as it is similar in shape, size and resistance to disinfectants. The phage was placed on 1 to 2 commonly touched surfaces (door knob or table top) at the beginning of the day in office buildings, conference room and a health care facility. After various periods of time (2 to 8 hours) they sampled 60 to 100 fomites, surfaces capable of carrying infectious organisms (light switches, bed rails, table tops, countertops, push buttons, coffee pots handles, sink tap handles, door knobs, phones and computer equipment), for the phages.

"Within 2 to 4 hours between 40 to 60% of the fomites sampled were contaminated with virus," says Gerba.

In the intervention phase cleaning personal and employees were provided with QUATS disinfectant containing wipes and instructed on proper use (use of at least once daily). The number of fomites on which virus was detected was reduced by 80% or greater and the concentration of virus reduced by 99% or more.

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Another interesting study that makes you think about our microbiome. From NPR:

Stinky T-Shirt? Bacteria Love Polyester In A Special Way

Anyone with a drawerful of T-shirts knows that the synthetic ones can get sour after just a brief jog, while old-school cotton T-shirts remain relatively stink-free all day. And now science explains why. The bacteria that flourish on a sweaty polyester T-shirt are different from those that grow on cotton, researchers at the University of Ghent in Belgium found. Polyester makes a happy home for Micrococcus bacteria, while Staphylococcus, a common armpit denizen, was found on both poly and cotton.

Microbes love the cozy warmth of the human armpit; it's like a trip to the tropics without ever having to leave home. And it's crowded in there. Those microbes eat compounds in sweat and generate odors, which support a flourishing deodorant industry. 

The scientists asked 26 volunteers to take a spinning class while wearing shirts made of cotton, poly or blends. The shirts were then incubated for a day, and the microbes extracted and DNA fingerprinted. Volunteers also had their armpits swabbed. 

It turns out the bugs on the shirts are different from the bugs in the pits. While Corynebacterium is thought to be the main cause of armpit body odor, there was no Corynebacterium on the clothes. Instead, Staphylococcus flourished on cotton and poly, and Micrococcus, bacteria also known for making malodor, loved polyester.

He's also trying to help people with excessive body odor by giving them armpit bacteria transplants. "We have done transplants with about 15 people, and most of them have been successful," Callewaert, a Ph.D. student in applied biological sciences at the University of Ghent, tells Shots. "All have had an effect short term, but the bad odor comes back after a few months for some people."

Manufacturers have tried to make polyester fabric less hospitable to bacteria by impregnating it with antimicrobials like silver nanoparticles or triclosan. Both products have been criticized as having potentially negative impacts on the environment, and there are few data on how they might affect the wearer. Callewaert thinks the ultimate solution will be something more organic — supplant bad bugs with good ones. 

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Interesting, but does it really mean anything? Just change to a new toothbrush monthly.From Science Daily:

Up to 3,000 times the bacterial growth on hollow-head toothbrushes

Solid-head power toothbrushes retain less bacteria compared to hollow-head toothbrushes, according to new research.

Lead author and professor at the UTHealth School of Dentistry, Donna Warren Morris, R.D.H., M.Ed., notes that microbial counts were lower in the solid-head toothbrush group than in the two hollow-head toothbrush groups in 9 out of 10 comparisons. "Toothbrushes can transmit microorganisms that cause disease and infections. A solid-head design allows for less growth of bacteria and bristles should be soft and made of nylon," Morris said. 

The study was conducted over a three-week period where participants brushed twice daily with one out of three randomly assigned power toothbrushes. Participants used non-antimicrobial toothpaste and continued their flossing routine throughout the study, but refrained from using other dental products like mouthwash.

During the study the brush heads were exposed to five categories of oral microorganisms: anaerobes and facultative microorganisms, yeast and mold, oral streptococci and oral enterococci anaerobes, Porphyromonas gingivalis andFusobacterium species.

The article also states that there is no present or published study that has demonstrated that bacterial growth on toothbrushes can lead to systematic health effects, but as Morris stated, several microorganisms have been associated with systemic diseases.

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Same research study is discussed as in the last post (Your Bacteria All Over Your Home), but different write-up with more and different details. From Washington Post:

Hotel rooms aren’t yucky – you colonize them with your own personal bacteria within hours

When you move from one house to another, you take all your bacteria with you. In fact, your family's microbiome (or your eco-system of inner and outer bacteria) lays claim to hotel rooms with hours. Our bacterial signatures are so persistent and so unique, a new study published Thursday in Science reports, that they could even be used in forensic investigations — and eventually become more useful to police than an old-fashioned fingerprint. And the same research that could track down a serial killer could also help you raise healthier kids.

In studying seven families as they moved from one house to another, the microbiologists had one major takeaway: Bacteria move from your body to your living space at incredible speed.

"Everyone thinks hotels are icky," said Jack Gilbert,, corresponding author of the study and environmental microbiologist at Argonne National Laboratory, "but when one young couple we studied moved into a hotel, it was microbiologically identical to their home within 24 hours." And unpublished further research reveals that the time frame is even swifter than that. "No matter what you do to clean a hotel room," Gilbert said, "your microbial signal has wiped out basically every trace of the previous resident within hours."

What's more, the researchers were able to determine how much individuals in a family interacted, what rooms they used, and even when they'd last been to one part of the house or another. This has obvious applications in forensic science. "We could go all J. Edgar Hoover on this and make a database of microbial fingerprints of people all over the world," Gilbert said, "and it's far more sophisticated than a standard fingerprint, which is just a presence or absence indication. We can see who they are, where they're from, the diet they're eating, when they left, who they may have been interacting with. It gets pretty crazy."

Gilbert and his colleagues are already working with police in Hawaii, hoping to look at the microbiome left on dead bodies. "If someone is, shall we say, recently and inappropriately deceased," Gilbert said, "we can look at their bacterial colonies and try to identify who the last person to come into contact with them was, and when." Based on some promising animal studies, he said, it could be possible. "An actual fingerprint is rarely left on a body," Gilbert said, "but a microbial fingerprint certainly is."

The Home Microbiome Study has more immediate applications, too. Gilbert, a father of two, hopes that fellow parents will use these and future findings to raise their offspring in healthier microbiomes. Before the age of two, the human microbiome remains in flux. Different species of bacteria compete to gain permanent spots — and once the race is run, you're basically stuck with the winners. Research in animals has shown that bacterial exposure in youth can impact physical and mental development and health for the rest of an organism's life.

"Let's say a kid grows up in an apartment block, without going outside much," Gilbert said. "They're just getting this same human bacteria fed back to them, day after day." More exposure is most certainly the better route.

For starters, get a dog. Partway through the study, Gilbert did just that. "We saw dogs acting as a super-charged conduit," he said, "transferring bacteria between one human and another, and bringing in outdoor bacteria. They just run around distributing microbes all willy-nilly." Sure enough, his family saw their home's microbiome benefit from the new addition.

We now know that most bacteria are beneficial to us — and that some can even prevent allergies."Imagine if we could engineer our home environments, optimize our carpeting and air conditioning systems, to bring in the really good bacteria," he said. 

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Exciting research! From Science Daily:

Home is where the microbes are

A person's home is their castle, and they populate it with their own subjects: millions and millions of bacteria. Scientists have detailed the microbes that live in houses and apartments. The study was conducted by researchers from the U.S. Department of Energy's Argonne National Laboratory and the University of Chicago. 

The results shed light on the complicated interaction between humans and the microbes that live on and around us. Mounting evidence suggests that these microscopic, teeming communities play a role in human health and disease treatment and transmission.

"We know that certain bacteria can make it easier for mice to put on weight, for example, and that others influence brain development in young mice," said Argonne microbiologist Jack Gilbert, who led the study. "We want to know where these bacteria come from, and as people spend more and more time indoors, we wanted to map out the microbes that live in our homes and the likelihood that they will settle on us.

The Home Microbiome Project followed seven families, which included eighteen people, three dogs and one cat, over the course of six weeks. The participants in the study swabbed their hands, feet and noses daily to collect a sample of the microbial populations living in and on them. They also sampled surfaces in the house, including doorknobs, light switches, floors and countertops. Then the samples came to Argonne, where researchers performed DNA analysis to characterize the different species of microbes in each sample.

They found that people substantially affected the microbial communities in a house -- when three of the families moved, it took less than a day for the new house to look just like the old one, microbially speaking.

Regular physical contact between individuals also mattered -- in one home where two of the three occupants were in a relationship with one another, the couple shared many more microbes. Married couples and their young children also shared most of their microbial community.

Within a household, hands were the most likely to have similar microbes, while noses showed more individual variation. Adding pets changed the makeup as well, Gilbert said -- they found more plant and soil bacteria in houses with indoor-outdoor dogs or cats.

In at least one case, the researchers tracked a potentially pathogenic strain of bacteria called Enterobacter, which first appeared on one person's hands, then the kitchen counter, and then another person's hands. "It's also quite possible that we are routinely exposed to harmful bacteria -- living on us and in our environment -- but it only causes disease when our immune systems are otherwise disrupted."

Home microbiome studies also could potentially serve as a forensic tool, Gilbert said. Given an unidentified sample from a floor in this study, he said, "we could easily predict which family it came from."

The research also suggests that when a person (and their microbes) leaves a house, the microbial community shifts noticeably in a matter of days."You could theoretically predict whether a person has lived in this location, and how recently, with very good accuracy," he said.

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If taking Clostridia as a probiotic for food allergies works, it would be amazing for food allergy sufferers. Very exciting research. From Time:

The Bacteria That May One Day Cure Food Allergies

Every round of antibiotics a person takes will wipe out strains of bacteria inside the body, some of which are eliminated forever. Considering how early and how often antibiotics are administered to kids—coupled with our increasingly antimicrobial lifestyles—we’ve become more prone to allergies and other ailments, the hygiene hypothesis goes. There’s no cure for food allergies, just lifestyle adjustments and abstention. But Nagler and her team may have the germ of an idea for treatment using gut bacteria, according to a new mice study published in the Proceedings of the National Academy of Sciences.

The team dosed two groups of mice with peanut allergens. One group of mice had been bred to be entirely without gut germs; the other group had sparsely populated gut bacteria due to treatment with antibiotics. Both groups of mice had higher levels of the allergen in their bloodstream compared to mice with healthy gut-bacteria populations.

After giving those same mice a mix that contained the bacteria strain Clostridia, their allergen levels plummeted. Infusing the mice with another group of intestinal bacteria, Bacteroides, didn’t help—so the researchers think the effect is special to Clostridia. “These bacteria are very abundant and they reside very close to the epithelial lining, so they’re in intimate contact with the immune system,” Nagler says.

Next, they’ll transfer gut bacteria from food-allergic infants and healthy infants into germ-free mice, Nagler says. “If we give back Clostridia to a mouse that has the bacteria of an allergic child, can we now reverse susceptibility in that mouse?”

This is a more in-depth article of the research. From Science Daily:

Gut bacteria that protect against food allergies identified

The presence of Clostridia, a common class of gut bacteria, protects against food allergies, a new study in mice finds. The discovery points toward probiotic therapies for this so-far untreatable condition. Food allergies affect 15 million Americans, including one in 13 children, who live with this potentially life-threatening disease that currently has no cure, researchers note.

Although the causes of food allergy -- a sometimes deadly immune response to certain foods -- are unknown, studies have hinted that modern hygienic or dietary practices may play a role by disturbing the body's natural bacterial composition. In recent years, food allergy rates among children have risen sharply -- increasing approximately 50 percent between 1997 and 2011 -- and studies have shown a correlation to antibiotic and antimicrobial use.

"Environmental stimuli such as antibiotic overuse, high fat diets, caesarean birth, removal of common pathogens and even formula feeding have affected the microbiota with which we've co-evolved," said study senior author Cathryn Nagler, PhD, Bunning Food Allergy Professor at the University of Chicago. "Our results suggest this could contribute to the increasing susceptibility to food allergies."

To test how gut bacteria affect food allergies, Nagler and her team investigated the response to food allergens in mice. ...This sensitization to food allergens could be reversed, however, by reintroducing a mix of Clostridia bacteria back into the mice. Reintroduction of another major group of intestinal bacteria, Bacteroides, failed to alleviate sensitization, indicating that Clostridia have a unique, protective role against food allergens.

To identify this protective mechanism, Nagler and her team studied cellular and molecular immune responses to bacteria in the gut. Genetic analysis revealed that Clostridia caused innate immune cells to produce high levels of interleukin-22 (IL-22), a signaling molecule known to decrease the permeability of the intestinal lining.

While complex and largely undetermined factors such as genetics greatly affect whether individuals develop food allergies and how they manifest, the identification of a bacteria-induced barrier-protective response represents a new paradigm for preventing sensitization to food. Clostridia bacteria are common in humans and represent a clear target for potential therapeutics that prevent or treat food allergies. 

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An interesting small study of the human armpit bacterial community. From Real Clear Science:

Antiperspirants Alter Your Armpit Bacteria and Could Actually Make You Smell Worse

In modern society, antiperspirants are widely hailed as a godsend, dispelling the inconvenient odors wafting from armpits everywhere. But a new study casts doubts on their vaunted position. As it turns out, antiperspirants may actually make you smell worse in the long run.

For 90% of all Americans, slathering on deodorants and antiperspirants is a daily occurrence, a precautionary measure against foul odors and unsightly sweat stains. The odors arise when bacteria living in our armpits break down lipids and amino acids excreted in sweat into more smelly substances. Deodorants employ antimicrobial agents that kill off bacteria, as well as chemicals that replace noxious odors with pleasant aromas. Deodorants that double as antiperspirants, like Degree, Old Spice, and Dove, take the process one step further by physically plugging sweat glands with aluminum-based compounds.

While most of us might only concern ourselves with the dry, aromatic benefits of antiperspirants and deodorants, researchers at the Laboratory of Microbial Ecology and Technology at the University of Ghent in Belgium are more interested in the effects on bacteria. Billions of bacteria dwell in the "rain forests" under our arms, and the substances we don are mucking with their habitats!

To uncover how deodorants and antiperspirants affect armpit bacteria, Chris Callewaert, a Ph.D student specializing in microbial ecology, and a team of researchers recruited eight subjects for a task a great many people (and especially their friends) might deem unbearable: Six males and two females pledged not to use deodorant or antiperspirant for an entire month. Specifically, four subjects stopped using their deodorants and another four stopped using their antiperspirant deodorant. (Most antiperspirants are also deodorants. See image below for an example.) Another control subject who did not regularly use either was asked to use deodorant for a month. The duration was chosen because it takes approximately 28 days for a new layer of skin cells to form.

The researchers analyzed the diversity and abundance of subjects' armpit bacteria at various timepoints before they stopped using antiperspirant, during the period of abstaining from antiperspirant, and for a few weeks after resuming the use of antiperspirant. Switching hygiene habits plainly altered the armpit bacterial communities of every subject. Since no two armpits and their resident bacteria are identical, it was difficult to pinpoint precise changes brought about by deodorants or antiperspirants, but one clear trend did materialize: antiperspirants resulted in a clear increase of Actinobacteria.

You might not recognize the name of Actinobacteria, but chances are, you've smelled them. Dominated by Corynebacterium, they are the major instigators of noxious armpit odor. Other microbes that inhabit the armpit, like Firmicutes and Staphylococcus, don't produce odors as quickly, nor are those odors nearly as pungent.

Callewaert believes the aluminum compounds in antiperspirants may be to blame, killing off "good," less smelly bacteria and allowing "bad" bacteria to dominate. His study found that deodorants which lack these sweat-blocking antiperspirant compounds are actually linked to a slight decrease of stinky Actinobacteria.

Though antiperspirants and deodorants are widely used, they are only a temporary fix."The measures we utilize today do not take away the initial source: the odor causing bacteria," Callewaert told RealClearScience. "Deodorants only mask unpleasant odors. We can do better than that. The follow up of this research is finding better solutions."

And Callewaert is already working on one: "armpit bacterial transplantation"."We take away the bad bacteria from the armpit of somebody with a body odor, and replace it with the good bacteria of a relative who doesn't have a body odor," he explained."So far we have helped over 15 people. For most subjects it brings immediate improvements. Most of them on a permanent time scale, although there are also people who suffer again from a body odor after some months."

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The bottom line is to read the ingredients list on products, and avoid all products labeled "antimicrobial" or "antibacterial" (because those are the ones typically containing triclosan and triclorocarban). Over 2000 products contain antibacterial compounds. I've even seen them in pillows, pillow protectors, mattress pads, dish racks, toys, and blankets! As we know from the latest microbiology research, we need to cultivate a healthy microbiome, and not throw it out of whack by continuously trying to kill off all bacteria. From The Atlantic:

It's Probably Best to Avoid Antibacterial Soaps

Antimicrobial chemicals are so ubiquitous that a recent study found them in pregnant mothers' urine and newborns' cord blood. Research shows that their risks may outweigh their benefits.

Antimicrobial chemicals, intended to kill bacteria and other microorganisms, are commonly found in not just soaps, but all kinds of products—toothpaste, cosmetics, and plastics among them. There is evidence that the chemicals aren’t always effective, and may even be harmful, and their ubiquity means people are often continually exposed to them. One such chemical, triclosan, has previously been found in many human bodily fluids. New research found traces of triclosan, triclocarban, and butyl paraben in the urine of pregnant women, and the cord blood of newborn infants. 

The research looked at the same population of 180 expectant mothers living in Brooklyn, New York, most of Puerto Rican descent. In a study published last week in Environmental Science and Technology, researchers from Arizona State University and State University of New York’s Downstate School of Public Health found triclosan in 100 percent of the women’s urine samples, and triclocarban in 87 percent of the samples. Of the 33 cord blood samples they looked at, 46 percent contained triclosan and 23 percent contained triclocarban.

In another, still-unpublished study, the researchers found that all of the cord blood samples contained “at least one paraben,” according to Dr. Rolf Halden, director of ASU’s Center for Environmental Security. 

Triclosan and triclocarban are endocrine disruptors, Halden explains. The risk there is that the chemicals can mimic thyroid hormones, potentially disrupting the metabolism and causing weight gain or weight loss. Previous research has also shown a connection between higher levels of triclosan in urine, and allergy diagnoses in children.

In the study looking at butyl paraben, the researchers found an association between higher exposure to the chemical, and a smaller head circumference and length of babies after they were born. Butyl paraben is used as a preservative, so it’s found in a wider breadth of products, according to Halden.

From Science News: Pregnant women, fetuses exposed to antibacterial compounds face potential health risks 


As the Food and Drug Administration mulls over whether to rein in the use of common antibacterial compounds that are causing growing concern among environmental health experts, scientists are reporting that many pregnant women and their fetuses are being exposed to these substances. The compounds are used in more than 2,000 everyday products marketed as antimicrobial, including toothpastes, soaps, detergents, carpets, paints, school supplies and toys, the researchers say.

The problem with this, explains Pycke, a research scientist at Arizona State University (ASU), is that there is a growing body of evidence showing that the compounds can lead to developmental and reproductive problems in animals and potentially in humans. Also, some research suggests that the additives could contribute to antibiotic resistance, a growing public health problem.

Although the human body is efficient at flushing out triclosan and triclocarban, a person's exposure to them can potentially be constant. "If you cut off the source of exposure, eventually triclosan and triclocarban would quickly be diluted out, but the truth is that we have universal use of these chemicals, and therefore also universal exposure," says Rolf Halden, Ph.D., the lead investigator of the study at ASU.

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Treating tumors with bacteria is very exciting and new.From Medical Express:

Injected bacteria shrink tumors in rats, dogs and humans

A modified version of the Clostridium novyi (C. noyvi-NT) bacterium can produce a strong and precisely targeted anti-tumor response in rats, dogs and now humans, according to a new report from Johns Hopkins Kimmel Cancer Center researchers.

In its natural form, C. novyi is found in the soil and, in certain cases, can cause tissue-damaging infection in cattle, sheep and humans. The microbe thrives only in oxygen-poor environments, which makes it a targeted means of destroying oxygen-starved cells in tumors that are difficult to treat with chemotherapy and radiation. The Johns Hopkins team removed one of the bacteria's toxin-producing genes to make it safer for therapeutic use.

For the study, the researchers tested direct-tumor injection of the C. noyvi-NT spores in 16 pet dogs that were being treated for naturally occurring tumors. Six of the dogs had an anti-tumor response 21 days after their first treatment. Three of the six showed complete eradication of their tumors, and the length of the longest diameter of the tumor shrunk by at least 30 percent in the three other dogs.

In a Phase I clinical trial of C. noyvi-NT spores conducted at MD Anderson Cancer Center, a patient with an advanced soft tissue tumor in the abdomen received the spore injection directly into a metastatic tumor in her arm. The treatment significantly reduced the tumor in and around the bone. "She had a very vigorous inflammatory response and abscess formation," according to Nicholas Roberts, Vet.M.B., Ph.D. "But at the moment, we haven't treated enough people to be sure if the spectrum of responses that we see in dogs will truly recapitulate what we see in people."

"One advantage of using bacteria to treat cancer is that you can modify these bacteria relatively easily, to equip them with other therapeutic agents, or make them less toxic as we have done here, " said Shibin Zhou, M.D., Ph.D., associate professor of oncology at the Cancer Center.  He and colleagues at Johns Hopkins began exploring C. novyi's cancer-fighting potential more than a decade ago after studying hundred-year old accounts of an early immunotherapy called Coley toxins, which grew out of the observation that some cancer patients who contracted serious bacterial infections showed cancer remission.

The researchers focused on soft tissue tumors because "these tumors are often locally advanced, and they have spread into normal tissue," said Roberts, a Ludwig Center and Department of Pathology researcher. The bacteria cannot germinate in normal tissues and will only attack the oxygen-starved or hypoxic cells in the tumor and spare healthy tissue around the cancer.

Verena Staedtke, M.D., Ph.D., a Johns Hopkins neuro-oncology fellow, first tested the spore injection in rats with implanted brain tumors called gliomas. Microscopic evaluation of the tumors showed that the treatment killed tumor cells but spared healthy cells just a few micrometers away. The treatment also prolonged the rats' survival, with treated rats surviving an average of 33 days after the tumor was implanted, compared with an average of 18 days in rats that did not receive the C. noyvi-NT spore injection.

Zhou said that study of the C. noyvi-NT spore injection in humans is ongoing, but the final results of their treatment are not yet available. "We expect that some patients will have a stronger response than others, but that's true of other therapies as well. Now, we want to know how well the patients can tolerate this kind of therapy."

It may be possible to combine traditional treatments like chemotherapy with the C. noyvi-NT therapy, said Zhou, who added that the researchers have already studied these combinations in mice. "Another good thing about using bacteria as a therapeutic agent is that once they're infecting the tumor, they can induce a strong immune response against tumor cells themselves," Zhou said.

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I've posted on whether probiotics can be used to treat mental disorders (see Probiotics and Psychobiotics- Part 1 and 2). But this article poses the interesting reverse question of whether the microbes are engaging in "microbial manipulations"? From NY Times:

Our Microbiome May Be Looking Out for Itself

Your body is home to about 100 trillion bacteria and other microbes, collectively known as your microbiome. We’ve come to appreciate how beneficial our microbes are — breaking down our food, fighting off infections and nurturing our immune system. 

But in the journal Bioessays, a team of scientists has raised a creepier possibility. Perhaps our menagerie of germs is also influencing our behavior in order to advance its own evolutionary success — giving us cravings for certain foods, for example. Maybe the microbiome is our puppet master.

The idea that a simple organism could control a complex animal may sound like science fiction. In fact, there are many well-documented examples of parasites controlling their hosts. How parasites control their hosts remains mysterious. But it looks as if they release molecules that directly or indirectly can influence their brains.

Our microbiome has the biochemical potential to do the same thing. In our guts, bacteria make some of the same chemicals that our neurons use to communicate with one another, such as dopamine and serotonin. And the microbes can deliver these neurological molecules to the dense web of nerve endings that line the gastrointestinal tract.

A number of recent studies have shown that gut bacteria can use these signals to alter the biochemistry of the brain.Compared with ordinary mice, those raised free of germs behave differently in a number of ways. They are more anxious, for example, and have impaired memory.Adding certain species of bacteria to a normal mouse’s microbiome can reveal other ways in which they can influence behavior. Some bacteria lower stress levels in the mouse. When scientists sever the nerve relaying signals from the gut to the brain, this stress-reducing effect disappears.

Some experiments suggest that bacteria also can influence the way their hosts eat. Germ-free mice develop more receptors for sweet flavors in their intestines, for example. They also prefer to drink sweeter drinks than normal mice do. Scientists have also found that bacteria can alter levels of hormones that govern appetite in mice.

Different species of microbes thrive on different kinds of food. If they can prompt us to eat more of the food they depend on, they can multiply. Microbial manipulations might fill in some of the puzzling holes in our understandings about food cravings, Dr. Maley said. Scientists have tried to explain food cravings as the body’s way to build up a supply of nutrients after deprivation, or as addictions, much like those for drugs like tobacco and cocaine. But both explanations fall short.

Take chocolate: Many people crave it fiercely, but it isn’t an essential nutrient. And chocolate doesn’t drive people to increase their dose to get the same high. Perhaps, he suggests, the certain kinds of bacteria that thrive on chocolate are coaxing us to feed them.

John F. Cryan, a neuroscientist at University College Cork in Ireland who was not involved in the new study, suggested that microbes might also manipulate us in ways that benefited both them and us. “It’s probably not a simple parasitic scenario,” he said.

Research by Dr. Cryan and others suggests that a healthy microbiome helps mammals develop socially. Germ-free mice, for example, tend to avoid contact with other mice. That social bonding is good for the mammals. But it may also be good for the bacteria. “When mammals are in social groups, they’re more likely to pass on microbes from one to the other,” Dr. Cryan said.

If microbes do in fact manipulate us, Dr. Knight said, we might be able to manipulate them for our own benefit — for example, by eating yogurt laced with bacteria that would make use crave healthy foods. The most important thing to do now, Dr. Knight and other scientists said, was to run experiments to see if microbes really are manipulating us.