Smoking cigarettes has long been known to cause several debilitating illnesses, but a recent study suggests that the problem is more serious than most people think.

Researchers from the University of California, San Diego discovered that the antibiotic-resistant superbug, Methicillin-resistant Staphylococcus aureus (MRSA) bacteria, becomes more resistant to the defenses deployed by the immune system after exposure to cigarette smoke. MRSA causes pneumonia and severe skin and circulatory infections.

"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," explained Dr. Laura E. Crotty Alexander, senior author of the study.

Crotty Alexander and her team infected macrophages – the immune cells that absorb pathogens in the bloodstream – with MRSA. They exposed some of the bacteria to cigarette smoke while the rest were grown normally.

The researchers experimented on MRSA's susceptibility to the mechanisms used by the macrophages to destroy bacteria. They found that while the two bacterial populations were absorbed well, the macrophages had a difficult time in killing the MRSA exposed to cigarette smoke. The bacteria were more impervious to the reactive oxygen species that macrophages use to break down their microbial meals.

Crotty Alexander's team also found that the smoke-infused MRSA proved to be more resistant to antimicrobial peptides, a protein-based substance used by the immune system to create holes in the cellular walls of bacteria in order to trigger inflammation. The effect was dose-dependent; the more smoke extract the MRSA was exposed to, the more resistant it became.

In an experiment conducted with a mouse model, researchers observed that the smoke-treated MRSA became more adaptable to invading human cells. The bacteria survived better and caused pneumonia resulting in severe mortality rates.

The findings of the study offer a better understanding on how cigarette smoke bolsters the effects of MRSA by altering the bacteria's cell walls, allowing it to become resilient against antimicrobial peptides and other charged particles.

Crotty Alexander added that as cigarette smokers are known to be more vulnerable to infectious diseases, the results of the study provide evidence that cigarette smoke-induced resistance in MRSA could be a possible contributing factor.

This study was published in the journal Infection and Immunity.