Antibiotic-resistant bacteria gets meaner, nastier and harder to kill when exposed to cigarette smoke, according to a study published in the journal Infection and Immunity.

"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 author Dr. Laura E. Crotty Alexander, a pulmonologist and an assistant clinical professor of medicine at University of California, San Diego, according to EurekAlert.

Smokers already have a weakened immune system, making them more susceptible to infections. The more smoke they inhale, the stronger the bacterium seems to be. According to the study, cigarette smoke helped MRSA survive longer and kill more mice with pneumonia.

"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, according to EurekAlert.

The latest antibiotic-resistant superbug spreading from overseas to the United States is a stomach bug called Shigella sonnei.

The study is titled "Analysis of the Effects of Cigarette Smoke on Staphylococcal Virulence Phenotypes." Study co-authors include Elisa K. McEachern, John H. Hwang, Katherine M. Sladewski (UC San Diego and Veterans Affairs San Diego Healthcare System), Shari Nicatia (UC San Diego, Veterans Affairs San Diego Healthcare System and Utrecht University), Carola Dewitz (UC San Diego, Veterans Affairs San Diego Healthcare System and University of Veterinary Medicine, Hannover, Germany), Denzil P. Mathew (Veterans Affairs San Diego Healthcare System) and Victor Nizet (UC San Diego).

This research was funded partially by the U.S. Department of Veterans Affairs.