Smoking alters the genes of mice involved in metabolic and stress responses, which alters their immune response and makes them more susceptible to lung infections. The scientists found that e-cigarette vapors do not turn on the bacteria's genes, meaning they have no effect on the bacteria's ability to infect.

Researchers from the University of Louisiana at lafayette infected mice with a strain of streptococcus pneumonia that causes most out-of-hospital infections. In the new study, researchers divided mice into four groups.

One group was exposed to strawberry-flavored e-cigarette vapor containing nicotine, the same vapor containing no nicotine, cigarette smoke, or nothing.

Next, they analyzed patterns of gene expression, the degree to which genes are turned on or off like a light switch.

They found that the e-cigarette with nicotine and without nicotine activated only a few genes: 264 and 14, respectively.

By contrast, smoking altered the expression of 982 genes, most of which were involved in metabolic and stress responses, increasing susceptibility to infection. "Interestingly, neither nicotine-containing nor without nicotine e-cigarette smoke altered the ability of pneumococcus to cause lung infections in mice," Dr. Kulkarni said.

The experiment proved that e-cigarette vapors did not turn on the bacteria's genes, meaning they had no effect on the bacteria's ability to infect.