A new study, led by Dr Laura Crotty Alexander of the Veterans Affairs San Diego Healthcare System and the University of California San Diego, shows that methicillin-resistant Staphylococcus aureus exposed to cigarette smoke become even more resistant to killing by the immune system.
Scanning electron micrograph of methicillin-resistant Staphylococcus aureus, yellow. Credit: NIAID.
“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 Dr Crotty Alexander, who is the senior author of the paper published in the journal Infection and Immunity.
In their study, Dr Crotty Alexander and her colleagues from the Netherlands, the United States, and Germany, infected macrophages with methicillin-resistant Staphylococcus aureus (MRSA). Some of the bacteria were grown normally and some were grown with cigarette smoke extract.
The scientists 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.
To better understand why, they tested the MRSA’s susceptibility to individual mechanisms macrophages typically employ to kill bacteria.
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 scientists also discovered that smoke-exposed MRSA were more resistant to killing by antimicrobial 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.
MRSA treated with cigarette smoke extract were also better at sticking to and invading human cells grown in the lab.
In a mouse model, MRSA exposed to cigarette smoke survived better and caused pneumonia with a higher mortality rate.
“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,” Dr Crotty Alexander said.
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 antimicrobial peptides and other charged particles.
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Elisa K. McEachern et al. Analysis of the Effects of Cigarette Smoke on Staphylococcal Virulence Phenotypes. Infection and Immunity, published online March 30, 2015; doi: 10.1128/IAI.00303-15
