Cigarettes contain hundreds of different strains of bacteria, including many human pathogens that may play a role in lung diseases and respiratory infections, new research shows.

Most health research has focused on the impact of chemicals in cigarettes and the particulates that are produced when tobacco is burned. But a new study, published in the journal Environmental Health Perspectives, paints the most complete picture to date of the bacteria in tobacco, suggesting that the germs could be another potential source of infection and disease.

The research--which shows that smokers are inhaling live bacteria--is the first time cigarettes have been implicated as a source of potentially pathogenic microbes.

"We thought it was a crazy idea to look at commercially available cigarettes to understand bacterial diversity," said Amy Sapkota, an epidemiologist at the University of Maryland, who led the research. "We were surprised to find that within a broad array of bacterial species that we found human pathogens were present as well."

Researchers are grappling with the public health implications of the findings.

Smoking cigarettes harms almost every organ system in the human body. The chemicals and heavy metals found in cigarettes--there are nearly 3,000 of them—and the particulates get most of the blame for the harmful effects of cigarettes, such as lung cancer and chronic obstructive pulmonary disease (COPD). However, emerging research points to smoking as a risk factor for respiratory illnesses such as the common cold, influenza, asthma, bacterial pneumonia and interstitial lung disease.

The discovery of pathogens “makes this story very exciting because it's a new mechanistic explanation to help us understand the variety of different diseases from cigarette smoking," said John Pauly, a cancer research scientist at the Roswell Park Cancer Institute in Buffalo, New York, who studies tobacco bacteria but was not involved in the new study.

With 1.2 billion smokers over the age of 15 worldwide, studying the bacterial diversity of tobacco is critical, according to scientists.

Bacteria have been known to be present in tobacco leaves, but only a few studies have looked into the contamination, and no research until now has attempted to catalog the bacteria diversity of cigarettes in order to investigate how these bugs may affect smokers or people exposed to second-hand smoke.

Sapkota became curious about bacteria living on tobacco while studying antibiotic-resistant genes in genetically modified tobacco plants. She was troubleshooting contamination problems when she realized the plants were full of bacteria. She remembers thinking that "if fresh tobacco leaves are loaded themselves, what is happening when the tobacco is harvested and produced to make cigarettes?"

The answer is that tobacco, too, was contaminated with bacteria.

In the study, more than 700 bacterial species were found in well-known cigarette brands including Camels, Kool Filter Kings, Lucky Strike Original Red, and Marlboro Red. Sapkota suspects their screen actually underestimates the number of bacteria present, and thinks it could be closer to the thousands of chemicals found in cigarettes. The researchers noted that no one brand had significantly different levels of bacteria.

To create cigarettes, green tobacco, which has relatively few bacterial species, are fermented under conditions that are perfect for dense bacterial growth. Instead of removing the bacteria during process, the fermentation concentrates them to as much as 1 million bacteria per cigarette, Pauly said. And the bacteria are alive and capable of reproducing. A single tobacco flake from a cigarette, when placed on a dish with nutrients, will lead to the growth of live bacterial colonies, he said.

"It’s amazing how hearty these little critters are," said Sapkota. "We’re finding out more each day how they can survive."

The research team found 15 different classes of bacteria and a number of potentially pathogenic organisms. The most notorious organisms present were Acinetobacter, Bacillus, Burkholderia, Clostridium, Klebsiella, Pseudomonas aeruginosa, and Serratia. These bacteria were found in more than 90 percent of all cigarette samples tested. Also found in the samples were the pathogens Campylobacter, Enterococcus, Proteus, and Staphylococcus.

Sapkota said it's troubling that so many human pathogens are present in cigarette tobacco. Microbes are known to cause acute infectious illnesses and also are risk factors for chronic diseases such as cancers and neurological disorders. However, future studies are necessary to determine whether or not the bacteria in cigarettes actually play a role in those diseases.

What researchers do know is that smokers are inhaling living bacteria into their lungs, which are sterile in healthy individuals, and this could lead to certain types of lung disease such as COPD and inflammation that is associated with solid tumors.

According to Pauly, even dead bacteria produce endotoxins that can activate cells that cause inflammation. He says there is some concern that the chemicals and bacteria might work together to speed up the malignancy of cancer cells.

However, at this stage in the research, it is too early to indict the pathogens found in cigarettes for causing disease in humans. The goal of the study was simply to evaluate cigarettes’ bacterial metagenome, or all the bacteria present.

"The organisms are there. Now the question is, what are they doing?" said Sapkota.

This article originally ran at Environmental Health News, a news source published by Environmental Health Sciences, a nonprofit media company.