Host–Bacterial Interactions During Induction and Resolution of Experimental Gingivitis in Current Smokers Chad R. Matthews,* Vinayak Joshi, † Marko de Jager, ‡ Marcelo Aspiras, ‡ and Purnima S. Kumar* Background: Changes in clinical profiles, microbial succes- sion, and immune mediator fluctuations have all been sepa- rately examined during onset and resolution of experimental gingivitis in smokers. However, because both the bacterial challenge and the host response contribute to periodontal dis- ease, the purpose of this investigation is to simultaneously ex- amine clinical, bacterial, and immune changes that occur during the onset and resolution of disease in smokers. Methods: Experimental gingivitis was induced in 15 smokers for 21 days, followed by treatment with a sonic toothbrush for 21 days. Marginal and subgingival plaque and gingival crevicular fluid samples were collected at baseline; after 7, 14, and 21 days of undisturbed plaque formation; and 21 days after rein- stitution of brushing. 16S cloning and sequencing was used for bacterial quantification, and multiplexed bead-based flow cy- tometry was used to quantify the levels of 27 immune mediators. Results: Onset of clinical gingivitis was preceded by signifi- cant changes in the marginal and subgingival biofilms, with a decrease in the abundance of early colonizers, namely, Streptococcus, Veillonella, and Pseudomonas, and an increase in levels of periodontopathogens, such as Treponema, Sele- nomonas, Parvimonas, Dialister, and Campylobacter. This was accompanied by a decrease in anti-inflammatory, chemokine, and T-helper 2 (Th2) responses and altered Th1/Th2 ratios. Although the bacterial communities continued to shift in the same direction after onset of clinical gingivitis and returned to baseline levels after resolution of disease, the anti-inflammatory, chemokine, and Th2 profiles demonstrated an increase from day 14 that continued even after clinical health was evident. Conclusion: Both marginal and subgingival biofilms in smokers are characterized by early acquisition of pathogenic organisms, which elicit a sustained host response that persists even after removal of the bacterial challenge. J Periodontol 2013;84:32-40. KEY WORDS Bacteria; cytokines; DNA; gingivitis; longitudinal studies; smoking. T obacco smoking significantly in- creases the risk for extensive and severe periodontal disease. 1-3 For several decades, it was suggested that this susceptibility was predominantly attributable to tobacco-smoke-induced changes in the inflammatory and im- mune responses. 4-6 More recently, it has been shown that smoking affects bacterial acquisition and long-term col- onization in the subgingival microenvi- ronment. 7-9 Together, the evidence indicates that the mechanisms by which smoking increases susceptibility to peri- odontal disease are multifactorial. Previous studies have examined the clinical, microbial, or immunologic changes that occur during transitions from gingival health to disease in smokers. 5,7,10-17 However, it is known that the bacterial biofilm exists in a state of dynamic equilibrium with the host and that periodontal disease occurs when this equilibrium is disrupted. Hence, to examine the effect of smoking on sub- gingival host–bacterial interactions, it is important to elucidate the effect of smoking on both the bacterial biofilm and the host response to this microbial challenge simultaneously. The experimental gingivitis model provides a controlled and reversible method of examining the shifts that oc- cur in the subgingival microbiome and the host response in health and disease. * Division of Periodontology, College of Dentistry, The Ohio State University, Columbus, OH. † Currently, Department of Periodontics, Maratha Mandal Dental College, Belgaum, India; previously, Division of Periodontology, College of Dentistry, The Ohio State University. ‡ Philips Oral Healthcare, Bothell, WA. doi: 10.1902/jop.2012.110662 Volume 84 • Number 1 32