Photo: Getty Images
Some smokers get lung cancer and others do not. The reason for this may involve bacterial infection, according to a review by Dr. Seyed Javad Moghaddam of the University of Texas MD Anderson Cancer Center, Houston, Texas, and colleagues at three other research institutions. Nontypeable Haemophilus influenzae (NTHi) is the main culprit, they reported. This pathogen is similar to Haemophilus influenzae type b (Hib), which is a common cause of meningitis, pneumonia, and ear infections in children who have not been vaccinated against it. The nontypeable variety is missing the polysaccharide capsule present in Hib and other, less common varieties.
The NTHi bacteria colonize the upper respiratory tract of 75 percent of healthy individuals, and generally cause no problems. Lower respiratory tract infections, however, are common in adults with chronic obstructive pulmonary disease (COPD), and contribute to airway inflammation. COPD is strongly associated with smoking and lung cancer: five out of six COPD patients have a history of smoking. However, only 25 percent of smokers develop this condition. Those who do face an increased risk of lung cancer, which ranges from three-fold for mild COPD cases to ten-fold for severe COPD.
“The variable susceptibility to COPD most likely reflects genetic variations in the inflammatory and structural responses to inhaled smoke and to microorganisms colonizing the airways of smokers,” Moghaddam reported. In one study, NTHi was found in the bronchial tissues of 87 percent of COPD patients experiencing exacerbations, and 33 percent of those with stable COPD.
Another report on the link between NTHi, cigarette smoke, and lung damage was provided by Dr. Pau Marti-Lliteras and colleagues at seven research institutions in Spain and United Kingdom. These authors reported that smoke impairs the ability of the immune system to clear bacteria from the lungs. Thus, some smokers develop recurrent bronchial infections which lead to progressive airway obstruction and lung cancer. Unfortunately, the glucocorticoid drugs which reduce inflammation also impair the immune system's ability to fight infection.