Doctors in the US have for the first time tracked microbes colonising a brand new hospital, revealing information that could help the fight against superbugs.
Researchers collected more than 10,000 samples from the Center for Care and Discovery in Chicago over a period of a year, starting two months before the hospital officially opened on February 23 2013.
Swabs were taken from patients' hands, nostrils and armpits, as well as multiple surfaces including bed rails, sink taps, floors and air filters.
Samples were also gathered from nurses' hands, gloves, shoes, pagers, phones, chairs and computers.
An immediate change was seen as soon as the first patients were admitted to the hospital.
Bacteria present during construction and pre-opening preparations, such as Acinetobacter and Pseudomonas, were quickly replaced by microbes that thrive on human skin. These included Corynebacterium, Staphyloccus and Streptococcus, all of which can be harmful.
Samples from the rooms of 92 patients kept in the hospital for months showed that over an extensive period of time potentially harmful bacteria acquired genes that can boost antibiotic resistance and promote infection.
Surprisingly, antibiotic treatment and chemotherapy generally had little impact on patients' skin bacteria.
Study leader Professor Jack Gilbert, from the University of Chicago, said: "The Hospital Microbiome Project is the single biggest microbiome analysis of a hospital performed, and one of the largest microbiome studies ever.
"We've created a detailed map, highly relevant to clinical practice, of microbial exchange and interaction in a large hospital environment.
"This describes the ecology of a building, a thriving microbial ecosystem that regularly interacts with patients in a seemingly benign way, at least most people don't appear to be negatively affected.
"It gives us a framework, something we can build on, showing how micro-organisms enter and colonise a hospital environment."
Every patient admitted to the hospital had an effect on the hospital's population of microbes, the study reported in the journal Science Translational Medicine showed.
On the first day in hospital, bugs tended to move from surfaces in the patient's room to the patient. But by the next day, and from then on, most of the movement was in the other direction. Bacteria steadily migrated from the patient to the room, adding to its microbial diversity.
The potential infection risk posed by long-stay patients was concerning, researchers said.
Prof Gilbert added: "This requires further study, but if it proves to be true then these genetic changes could affect the bacteria's ability to invade tissue or to escape standard treatments."