Picture: Filled Less than 100 species of bacteria cause infections in humans.
This book may not be for you if you’re squeamish, or use sterile wipes every time you push a supermarket trolley.

But our world is made up of microbes (from the Greek mikros - little, and bios - life) and they are everywhere, from the skin of our forearms to deep ocean trenches, from our mouths to Antarctic ice. And there are countless millions of them: there are more bacteria in our guts than stars in our galaxy.

We can’t see them, but they affect our lives and our health. We particularly notice them when they have negative effects, like the cramp of an inflamed bowel, or a sneeze. We tend to assume microbes are germs, what British science journalist Ed Yong describes as “unwanted bringers of pestilence”.

Yet most microbes are not pathogens - less than 100 species of bacteria cause infections in humans. They help us digest our food, produce vitamins and minerals missing in our diet, break down toxins and chemicals, and - importantly - protect us from disease by crowding out more dangerous microbes, or killing them with antimicrobial chemicals.

Killing beneficial microbes with the injudicious use of antibiotics can clear the way for your bowel to be colonised by something nasty, like Clostridium difficile, which causes severe diarrhoea and is hard to cure.

And we all know that antibiotics are over-prescribed: Yong says one estimate is between 1% and 3% of the developed world takes an antibiotic every day.

Without microbes, grazing mammals like cows, sheep and antelope would starve since they depend on their gut microbes to break down the tough fibres in their food. Termites, which also depend on microbes for their digestion, would disappear, as would the larger animals that feed on them and in whose mounds they shelter. In the deepest part of the sea, many life forms rely on bacteria for all their energy, and without them, they and the creatures that feed on them would die. Without algae and bacteria, corals would bleach and erode, and the life they support would suffer.

Humans, says Yong, would be fine for a while. But waste would pile up, because microbes deal with decay. Livestock would die, so we’d be short of food. And plants without the services of nitrogen-fixing microbes would die too.

This is a hefty book and Yong goes into a lot of generally absorbing detail relating not only to humans but to all life forms, from nematode worms to the glowing Hawaiian bobtail squid, from the braconid wasp that uses domesticated viruses to suppress its victims’ immune systems to the desert woodrat which uses microbes in its gut to neutralise the poison of the creosote bush.

But I found the sections on the effects of microbes on humans the most fascinating. Microbes can affect how well we respond to vaccines, how much nourishment children get from food, and how cancer patients respond to treatmen “Many conditions, including obesity, asthma, colon cancer, diabetes and autism are accompanied by changes in the microbiome, suggesting that these microbes are at the very least a sign of illness, and at most a cause of it.”

We’ve all heard of the hypothesis that children who are kept too clean are more likely to develop allergies, and it appears to be true. Children in developed countries no longer face the myriad infectious diseases they used to, but now they grow up with what Yong calls “inexperienced, jumpy immune systems”.

Smaller families, a move from rural areas to more sterile urban homes, chlorinated water and sanitised food, along with increasing distance from livestock and pets, all reduce the range of microbes we are exposed to. As a result there is a higher incidence of allergic and inflammatory diseases.

But a dog can help. Yong says when a scientist used a vacuum cleaner to collect dust from 16 homes, she found those without furry pets were “microbial deserts”. Those with cats were better, and those with dogs better still.

But even better than pets are our mothers. Naturally born newborns emerge coated in - and protected by - their mother’s mother’s vaginal microbes. Caesar babies are more likely to develop allergies, asthma, coeliac disease and obesity later in life, and with an increasing trend towards Caesarean births, this incidence is likely to accelerate.

Then there are the helpful microbes in breast milk which colonise a newborn’s gut. The benefits of breastfeeding may neutralise the differences caused by a Caesarean birth, “but if you go for a C-section and bottle-feeding, I’d say (your baby) is on a different trajectory”, says California-based microbiologist David Mills.

And I haven’t even got on to the amazing results that are being reported among patients with chronic bowel disease who have had bowel-microbe transplants from healthy people. (And, yes, the transplants involve poo.)

Yong is enthralled by microbes and what they do for all life forms. Sometimes yukky, to be sure, but in his words, they are “mostly not to be feared or destroyed, but to be cherished, admired and studied”.