Lung cancer is the most common cancer worldwide and is a leading causes of mortality worldwide . Many recent studies have underscored the etiologic role of chronic pulmonary infection in lung carcinogenesis, concluding that inflammation increases the risk for incident lung cancer [2–5]. Numerous studies on lung cancer have pointed out the appearance of Mycoplasma strains in patients and suggest association of infection with tumorigenesis; it has been postulated that mycoplasma-infected cells have a higher ability to metastasize in vivo than non-mycoplasma-infected cells . Candida species have been isolated from patients with lower respiratory tract infection [7, 20–22]. Haemophilus influenza [6, 7, 19–21], Staphylococcus epidermidis [6, 7, 17–19], Streptococcus species [6, 7, 17, 19, 25], Legionella pneymophila , as well as strains of Bacillus , Listeria  and Streptococcus [6, 7, 17, 19, 25] have been also identified in patients with different pulmonary diseases. Very similarly, the bacterium Chlamydia pneumoniae, a common cause of community-acquired pneumonia, has been implicated in lung carcinogenesis [11–16]. A recent meta-analysis by Zhan et al.  of 12 studies involving 2595 lung cancer cases and 2585 controls from four prospective studies and eight retrospective studies, was conducted to analyze the association between C. pneumoniae infection and risk of lung cancer. Overall, people exposed to C. pneumoniae infection had an odds ratio (OR) of 1.48 (95% confidence interval (CI), 1.32-1.67) for lung cancer risk, relative to those not exposed. Of interest, a higher titre was an even better risk prognosticator (OR for IgA ≥64 cutoff group, 2.35; 95% CI, 1.88-2.93; OR for IgA ≥16 cutoff group, 1.22; 95% CI, 1.06-1.41).
These data strongly support the idea that lung cancer is a biofilm associated chronic infection. Biofilms are microorganism populations organized in a form of colonies using self-produced extracellular matrix that works as infrastructure material. The vast majority of the micro-"colonists" establish biofilms on any inert or diseased biological surface. They adhere to each other, divide, cooperate, and, progressively, their bio-mass grows, matures and finally disperses. It resembles malignant behavior (tumors composed by cancer cells and by stroma cells-monocytes, lymphocytes, microvessels, can metastasize). Therefore, many researchers imply that lung malignancies are communities of diverse pathogens resistant to antibiotics.
One of the major limitations in most of the previous studies was the use of serologic characterization to identify chronic bacterial or fungal infections . This has resulted in conflicting results and great variability in relative risk estimations among seropositive individuals [14, 15, 26–29]. This wide variability could also reflect the retrospective nature of most of the studies, the small sample sizes, or inadequate adjustment for confounding factors . New techniques, such as PCR-RFLPs, Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) and microcolony methods allow examination and analysis of microbial communities [30, 31]. Analyzing the constituents of microbial biofilms responsible for lung disease may help us discover novel strategies to control malignancies.
To the best of our knowledge, the prevalence of bacterial and/or fungal infection in surgically extracted samples of patients with lung cancer has not been previously reported. Therefore, the main purpose of the present study was to determine the presence of bacterial and fungal microflora in surgically removed tissue samples of patients with lung cancer, by using PCR methods and special primers. In this study, specific primers were designed in order to amplify as many different strains of microorganisms. Pairs of primers that were designed were capable of amplifying Treponema, Neisseria, Legionella, Borrelia, Listeria, Helicobacter, Staphylococcys, Haemophilus, Bacillus, Leptospira, Streptococcus, Mycoplasma, Candida and Brachyspira species. It is worth noting that Mycoplasma species were observed in all samples. Staphylococcus epidermidis and Streptococcus mitis were almost seen in one quarter of patients. Neither Treponema strains nor Leptospira, Helicobacter, and Staphylococcus aureus strains were observed in this study.