Bacteria play a central role in our lives, shaping much of the world around us, as well as the world within us.

We’ve long regarded bacteria as something to be feared - agents of illness and death - but, increasingly, it’s understood that many bacteria are, in fact, harmless and even beneficial.

Bacteria in our gut, for example, help us produce vitamins, such as folic acid, while others help us to absorb important metals, such as calcium, from our food. In a fascinating new book, Adam Hart, a biologist, broadcaster and professor of science communication at the University of Gloucestershire, helps to untangle the good from the bad - with sometimes surprising revelations... Dirty hands are not the only route for disease-causing bacteria to infect you: don’t forget the ‘killer’ toothbrush in your bathroom. It’s well-known that flushing the loo creates thousands of tiny water droplets containing bacteria - and that some can end up on nearby objects, such as your toothbrush.

The NHS therefore suggests that we close the lid before flushing and store toothbrushes upright, more than two metres from the bowl. Yet while we are right to be concerned, there is a big difference between bacteria on the bristles and getting gastroenteritis. Just because we can culture E. coli from a toothbrush, that doesn’t necessarily mean we’re going to be ill after brushing. Most E. coli are harmless - and, in fact, you’ve been brushing your teeth with E. coli-contaminated brushes for most of your life.

Indeed, most of the bacteria in and around our loos probably won’t do us much harm. What’s more, the chance of us ingesting sufficient numbers to cause any problems is extremely low. Similarly, the chances of there being harmful bacteria on door handles is really very small.

Not only would an infectious person have to wash their hands incorrectly, but the bacteria would have to live long enough on the dry, hard metal surface to infect you (in practice, many species don’t thrive on such surfaces). Then, you’d have to touch the right part of the handle - ideally with wet hands - pick up the bacteria and transfer enough to your mouth to become infected.