One of the most important recent medical breakthroughs has been the impact of gut flora（肠道菌群）on overall health. This dynamic mix of bacteria in the digestive tract（消化道）may affect everything from obesity to asthma（哮喘）．
The Gut-Mood Link
Anyone who's ever experienced "butterflies" in his or her stomach knows that the gut and the brain are connected. But until recently, scientists hadn't realized that the trillions of bacteria in our digestive tract may be driving the relationship.
One recent experiment suggests that you can control calm. Canadian researchers gave healthy mice a mixture of antibiotics, which change the makeup of GI-tract（胃肠道）bacteria. Over two weeks, some animals became more anxious and some less, depending on which drugs they received. In another study, when gut bacteria from calm mice were transferred to anxious mice, the jittery（紧张的）creatures seemed less nervous.
UCLA professor Kirsten Tillisch, doctor of medicine, recently had healthy women eat a yogurt
rich in certain types of "good" probiotic（益生菌的）bacteria. Two 'Control groups ate yogurt without such bacteria or ate nothing. Brain scans of the probiotic-yogurt eaters indicated changes in regions that could be associated with a less-anxious response in fearful or stressful situations compared with the control groups.
Scientists are even exploring whether gut microbes（微生物）might treat some brain disorders. A new study in Cell found that mice with features of autism（自ICJ症）given a type of bacteria in healthy human GI tracts exhibited less autism-like behavior. In a recent case report, a Boston ps. chiatrist says a course of certain probiotics and antibiotics helped relieve a patient's compulsive disorder.  -We tend to focus on how your mood affects your body from the top down, not the bottom up,"  says Dr. Tillisch. "Now we know that the gut affects how your brain responds to the environment- it's a remarkable change in thinking.”
How Bacteria Talk to the Brain Scientists aren't sure how gut flora affect brain chemistry. One theory is that the bacteria access pathways along the nerve, the main highway in the nervous system that links the brain to the gut.  Another is that the bacteria influence the immune system, releasing chemicals that affect conditions like depression. Finally, researchers believe that bacteria produce or affect the metabolism（新陈代泻'i) of chemicals, which change brain function.
Dr. Tillisch told me that patients always ask which probiotics will lift their mood．Her answer:
"We don't know yet, but we hope to one day." In the future, patients might receive probiotics or transplant（移植） in which bacteria from the gut of a healthy person are transferred to the gut of a sic': one-to prevent or treat mental health issues. But these approaches are years away from primary time．Until then, a diet rich in fruit and veggies helps cultivate（培养）better-balanced microbes than a typical Western diet high in animal and processed products. Avoiding unnecessary antibiotics, particularly in early childhood, may also establish healthier gut bacteria.
According to the passage, gut bacteria might help us in many ways EXCEPT＿_．

A．lightening our mood	B．treating autism
C．relieving depression	D．transplanting gut flora

The missing sentence "Everyday foods may also affect the composition of gut bacteria, and in
turn, brain chemistry." could be put before＿_·

A．Paragraph 2	B．Paragraph 3
C．Paragraph 4	D．Paragraph 5

Which of the following can be the best subtitle for the last paragraph?

A．Cultivating a Happier Gut

B．Growing Bacteria in the Future

C．Balancing Microbes and Diet

D．Living a Happy Healthy Life

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