MRSA bacteria, an antibiotic-resistant superbug, become even more resistant to killing by the immune system if they are exposed to cigarette smoke, warns a new study.
Methicillin-resistant Staphylococcus aureus (MRSA) can cause life threatening skin, bloodstream and surgical site infections or pneumonia.
"We already know that smoking cigarettes harms human respiratory and immune cells.
"And now we've shown that, on the flipside, smoke can also stress out invasive bacteria and make them more aggressive," said senior study author Laura E. Crotty Alexander from the University of California, San Diego.
For the study, published in Infection and Immunity, Crotty Alexander and her team infected macrophages -- immune cells that engulf pathogens -- with MRSA.
Some of the bacteria were grown normally and some were grown with cigarette smoke extract.
They found that while the macrophages were equally able to take up the two bacterial populations, they had a harder time killing the MRSA that had been exposed to cigarette smoke extract.
Once inside macrophages, smoke-exposed MRSA were more resistant to killing by reactive oxygen species, the chemical burst that macrophages use to destroy their microbial meals.
The team also discovered that smoke-exposed MRSA were more resistant to killing by anti-microbial peptides -- small protein pieces the immune system uses to poke holes in bacterial cells and trigger inflammation.
The effect was dose-dependent, meaning that the more smoke extract they used, the more resistant the MRSA became.
In a mouse model, MRSA exposed to cigarette smoke survived better and caused pneumonia with a higher mortality rate.
The findings suggest that cigarette smoke strengthens MRSA bacteria by altering their cell walls in such a way that they are better able to repel anti-microbial peptides and other charged particles.
"Cigarette smokers are known to be more susceptible to infectious diseases.
"Now we have evidence that cigarette smoke-induced resistance in MRSA may be an additional contributing factor," Crotty Alexander said.