Lessons learned from Florence Nightingale could prevent the spread of harmful bugs by allowing "friendly" bacteria into hospitals, an expert has claimed.

Sterile conditions in wards and operating theatres may be doing more harm than good by wiping out organisms that keep dangerous microbes at bay, Dr Jack Gilbert believes. Florence Nightingale, the 19th-century "mother" of modern nursing, advocated cleanliness, but also plentiful fresh air for patients.

Dr Gilbert, who heads an international project to construct a bacterial "field guide" of all the world's known bugs, thinks she was right, despite knowing nothing about microbial diversity.

He points to emerging evidence that what happens in buildings mirrors what occurs in the gut. It is well known that beneficial bacteria, or "flora" in the intestinal tract help to ward off infection by out-competing potentially harmful organisms.

Dr Gilbert, who is British-born but based at the Argonne National Laboratory near Chicago, said: "There's a good bacterial community living in hospitals and if you try to wipe out that good bacterial community with sterilisation agents and excessive antibiotic use you actually lay waste to this green field of protective layer. Then these bad bacteria can just jump in and start causing hospital borne infections.

"This is going back to Florence Nightingale. Florence Nightingale said if you have an open window where air from the environment is coming in.. you'll have less illness. You open up these windows, you keep it nice and airy, and you'll see less sick soldiers in this hospital theatre.

"You let in all these bacteria from outside, and you will either dilute out the pathogens (harmful infectious agents) or you don't allow the pathogens to establish themselves, because there is too much competition for the nutrients and energy that the bacteria need to survive."

Speaking at the annual meeting of the American Association for the Advancement of Science (AAAS) in Vancouver, Canada, he cited a study published just last month conducted by Dr Jessica Green, from the University of Oregon.

She carried out an experiment in which bacterial samples were taken from clinic rooms that either had their windows open or closed. The rooms with open windows had a more diverse range of microbial types. The sealed and artificially ventilated rooms had less variety of bugs, and more of the kinds of bacteria that could potentially be harmful.

Microbiologist Professor Mark Enright from the University of Bath said: "I do think that opening windows is a good thing. Air flow is a good thing in hospitals, you don't want pockets where organisms can pool and swarm and pass on." But he described the idea that hospitals were too clean as "quite an extreme view".