Washington, Oct 16 (IANS): Females have been known to be naturally more resistant to respiratory infections than males. Now, scientists have shown that the increased resistance to bacterial pneumonia in female mice is linked to an enzyme that is activated by the release of the female sex hormone oestrogen.

For this, the team from Harvard University's School of Public Health introduced Streptococcus pneumoniae into the lungs of mice to mimic the inhalation of bacteria that occurs naturally as we breathe.

Female mice and male mice that had been treated with oestrogen were able to clear the bacteria from their lungs more rapidly than normal male mice.

Female lung host defence cells were also better at killing other bacteria (Escherichia coli and Staphylococcus aureus) when tested in vitro.

"Ultimately, this work could be especially useful in reducing risk of secondary bacterial pneumoniae during seasonal or pandemic influenza," said senior study author professor Lester Kobzik.

The scientists then took another set of both male and female mice and deleted the gene responsible for the production of the enzyme called nitric oxide synthase 3 (NOS3).

They found that deleting this gene meant that the female mice were no longer more resistant to infection.

The team hopes that this knowledge could be used to enhance resistance to common and serious lung infections.