The human body hosts thousands of microbes, from bacteria to fungi and viruses, that have made our skin, mucous membranes and even our intestines their homes. And even though harmful bacteria can make us sick, we need the beneficial kinds to keep our digestive and immune systems in tip-top shape.

All in all, each of us carries around 1.5 kilograms (3.3 pounds) of microorganisms in our bodies, said pharmacist Ursula Sellerberg, based in Germany, as reported by Deutsche-Presse Agentur (dpa).

What types of microbes these are and how they live varies from one person to the next, and they constantly change. Our hygiene, physical activity, medicine intake, infections and diet all influence our microbiome and our intestinal flora. Basically, the more diverse microorganisms the intestine is populated with, especially through probiotics, the better it is for digestion and for the entire immune system, nutritionist Astrid Donalies said.

You can promote this diversity by eating a varied high-fiber diet, mostly composed of plants, and regularly eating fermented foods – things like tempeh, sauerkraut, kimchi, kefir or kombucha.

In terms of her emphasis on plants, you should eat cereals, grain products or potatoes, three servings of vegetables and two servings of fruit every day, including nuts and legumes.

By fermented foods, Donalies is talking about dairy products such as yogurt or kefir made by lactic acid fermentation. The benefits of consuming homemade yogurt are plenty, which is why doctors recommend eating it every day. Lactobacillus, the good bacteria in yogurt, is believed to restore the yeast balance in the vagina and kill off candida. The lactic acid and probiotics in yogurt have also been found to have a suppressing effect on helicobacter pylori, which causes peptic ulcers and in rare cases can lead to stomach cancer. Yogurt is also an excellent source of vitamin B12, which is essential for a properly functioning brain, mood and high energy levels.

Fermented dairy products contain particularly high levels of probiotics, so do foods such as sauerkraut or kimchi, which are also preserved by fermentation and are non-dairy options for those that are lactose-intolerant.

However, lots of sugar or red meat can throw your microbiome out of balance, as can some medications, such as antibiotics, anti-inflammatories or antidepressants, Sellerberg said. That can lead to flatulence, constipation or diarrhea, as well as fatigue or susceptibility to infection.

Studies suggest that a suboptimal composition of intestinal flora can affect your other organs and lead to disease. There is a link between gut bacteria and mental illnesses such as depression, for example, as well as allergies and diabetes, Donalies said.

There has not yet been enough research to ascertain which microbes have beneficial or unfavorable effects, and this may also vary on an individual basis.

In addition to foods that are probiotic anyway, you can also find products where probiotics have been added. Sometimes, these can help, such as with diarrhea caused by taking antibiotics, or with irritable bowel syndrome.

But Sellerberg said to be careful and advises against taking probiotics in capsule form or as a supplement without medical supervision, saying studies suggest that long-term use can be suboptimal when it comes to the kind of gut bacteria growing in your intestines.