Older mattresses are more dangerous
Bacteria linked to sudden infant death syndrome (SIDS) seem to thrive in vomit-soaked polyurethane foam, especially if the babies drink formula, rather than breast milk, it is revealed.
The situation is worse if the mattress is old or has been used previously by another baby.
Experiments at De Montfort University in Leicester also found that if mattresses are disturbed to mimic a baby's movements, enough bacteria can rise into the air to cause life-threatening throat infections.
Once in the throat, the bacteria colonise the respiratory tract where they warm up and produce "supertoxins" that trigger the immune system into abnormally severe assaults.
When a baby's developing immune system overreacts, the infant can die of anaphylactic shock, according to an article in New Scientist.
Dr Richard Sherburn, who carried out the study, found the problem occurred in mattresses which were two-thirds covered by PVC, but the top section, supporting the baby's head, was covered by a plastic mesh, making it porous.
After use, these mattresses can be heavily contaminated with bugs, the dominant species of which is Staphylococcus aureus, a bug often found in the throats of SIDS babies.
The bacterial levels were 100-fold higher if the baby had vomited formula, rather than breast milk.
The abundance of S. aureus rose with the number of babies that had used the mattress.
He said: "My gut instinct is that these bacteria last longer than other bacteria and have a head start when a second child comes along and uses the mattress.
"I would say it's safer to get a new mattress for each child, but if you had one that's completely covered with PVC and it can be cleaned, I would assume it could be used again."
PVC protection
Mimicking a baby's movement also created clouds of bugs in the air above a mattress.
Dr Sherburn said: "Four hours after throwing up, there could be 10,000 bacteria in the air, enough to cause infection."
Ironically, the "breathable" part-PVC covered mattresses were introduced in the 1990s to overcome perceived dangers of mattresses fully covered with PVC.
The theory, now largely rejected, was that fungi inside the PVC covering broke down flame retardant chemicals in the plastic and released toxic vapours that poisoned babies.
However, Dr Sherburn's work is adding weight to other studies suggesting breathable mattresses actually pose the highest risks.
The Foundation for the Study of Infant Deaths (FSID), has welcomed the study findings.
An FSID spokeswoman said: "This research supports our advice to keep cot mattresses well-aired and clean
"A mattress with a PVC cover or a removable, washable cover is easiest to keep clean."
However she stressed there is no single established cause of cot death and more research needs to be carried out.
Seven babies die every week in the UK from cot death.
Dr Sherburn's study was funded by the Scottish Cot Death Trust.