Periodontitis, a local chronic inflammation in the supporting tissues of the teeth leading to progressive loss of periodontal ligament and bone, is believed to result from disruption of the homeostatic balance between periodontopathic bacteria and the host response to these microorganisms [1, 2]. In addition, several factors, including smoking, socioeconomic status, behaviour and stress have been identified as potential risk factors for periodontitis [3].

Among these risk factors, smoking is strongly implicated in the development of periodontal disease. There is accumulating evidence for a higher level of periodontal disease among smokers [4, 5]. Greater levels of clinical alveolar bone loss [5–7], tooth mobility [8], probing pocket depth [4, 9] and tooth loss [10–12] have all been reported to be more severe in smokers than in non-smokers. Stoltenberg et al. [13] reported that the odds ratio for having a mean probing depth ≥ 3.5 mm was 5.3 times greater in smokers.

Mechanisms by which smoking affects the development of periodontitis are thought to be both direct and indirect. It has been suggested that modification of the periodontal microflora by smoking is involved in the development of periodontitis. It was shown that in vitro exposure of bacteria to cigarette smoking resulted in a marked decrease in the numbers of viable bacteria [14, 15]. Zambon et al. [16] reported that smokers had significantly higher levels of, and were at greater risk of infection by Tannerella forsythia, in a study on 798 subjects with different smoking histories. Furthermore, they showed that smokers were 2.3 times more likely to harbour this periodontal pathogen than former smokers or non-smokers, after adjusting for disease severity. Umeda et al. [17] reported that current smokers displayed an increased risk (odds ratio, 4.6) for harbouring Treponema denticola in periodontal pockets, and that the presence of Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis, Prevotella intermedia, Eikenella corrodens or Fusobacterium nucleatum and smoking increased the risk of having a mean pocket depth of ≥ 3.5 mm. Likewise, Stoltenberg et al. [13] found that smoking was the strongest risk indicator for increased pocket depth. With regard to early onset periodontitis, Kamma et al [18] reported that smokers harboured higher levels of periodontal pathogens. On the other hand, other studies [13, 19] found that both smokers and non-smokers exhibited similar subgingival microflora, suggesting that smoking has limited influence on the microflora involved in periodontal disease. Given these discrepancies, more information is needed to delineate the relationship between smoking and periodontal microflora. The aim of the present study was to gain insight into the influence of smoking on the microflora of Japanese patients with periodontitis.

In the present study, the detection of T. denticola, C. rectus, T. forsythia or P. intermedia was associated with having a PPD of 4 mm or greater. This finding was in line with the previous reports, which implicate these periodontal pathogens in the etiology of periodontitis [17, 25–27]. Also in our study, the detection of C. rectus or P. intermedia increased at sites with PPD ≥ 4 mm or BOP positive. Takeuchi et al [28] reported that the detection of C. rectus was high in aggressive periodontitis lesions in Japanese. [29]. The detection of C. rectus in plaque samples was significantly correlated with clinical parameters such as PPD, BOP, and Gingival index in Japanese subjects [30]. Increased proportions of P. intermedia and C. rectus have been associated with both the initiation and progression of the disease in other populations as well [31, 32]. Alves et al [33] reported that an increase in probing depth correlated with the number of genotypes of P. intermedia. These reports suggested that P. intermedia and C. rectus are associated with advanced periodontitis.

In this study, the colonization by C. rectus was significantly higher in smokers, while that by A. actinomycetemcomitans was lower. This finding of C. rectus in smokers agrees with that of a previous report by Kamma et al [18]. They reported that P. gingivalis, T. forsythia, Parvimonas micra and C. rectus were predominant in subgingival microflora in smokers, but were not so in non-smokers with early-onset periodontitis. It is important, however, to note that our results might be affected by differences in PPD or BOP between smokers and non-smokers, because of the statistical method we used.

The presence of C. rectus was reported to be correlated with the presence of P. gingivalis, T. denticola or T. forsythia [34]. C. rectus utilizes end products of other microorganisms such as formate from streptococci and H₂ from Prevotella and Porphyromonas [35]. Our finding that the prevalence of C. rectus and P. intermedia increased at sites with PPD ≥ 4 mm or BOP positive, suggested that the change in microflora induced by smoking would provide a certain advantage to colonization by a subset of pathogens including C. rectus. However, further analysis is required. Although C. rectus and A. actinomycetemcomitans are facultative anaerobes, C. rectus is asaccharolytic and negative in many routine biochemical tests, while A. actinomycetemcomitans is saccharolytic [36]. It is possible that these differences in metabolism affect colonization by A. actinomycetemcomitans.

Several reports demonstrated limited influence of smoking on periodontal pathogens [19, 37, 38]. Among them, Bostrom et al [37] found that the detection rate of periodontopathic bacteria was higher in smokers, although not significantly. Haffajee et al [39] reported that species such as T. forsythia, P. gingivalis, T. denticola, P. intermedia, P. micra and P. nigrescens were significantly higher in the current smokers at sites with pocket depth < 4 mm compared with the non-smokers. Recently, Teixeira et al [40] reported that P. gingivalis with type IV fimbriae was associated with disease severity of smokers. Further cohort analysis between colonization of periodontopathic bacteria and smoking is required to clarify the whole situation.

In our study, the risk for having a PPD of 4 mm or greater in smokers was no different from that in non-smokers (odds ratio for smoking; 0.637). It has been reported that PPD in smokers was higher than that in non-smokers [4], and being a moderate/heavy smoker was a risk indicator for PPD ≥ 6 mm (odds ratio = 3.7, 95% confidence intervals: 1.4-10.1) [41]. In this study, the mean cigarette consumption per day was 16.0. Twenty seven of 30 patients in the present study can be categorized as light smokers in Corraini's study [41]. In their study, being a light smoker was not a risk indicator at p < 0.05. It is possible that the level of cigarette consumption affects the odds ratio, although further analysis after adjusting for deposition of plaque and frequency of smoking is required to clarify this discrepancy.

We found that the odds ratio for BOP-positive was significantly higher in smokers, although the risk of having a PPD of 4 mm or greater was not associated with smoking. Several studies have reported that BOP in smokers was at the same level or lower than that in non-smokers [42, 43]. It was also reported that no difference was observed in BOP or gingival bleeding between non-smokers and smokers [44, 45]. Although BOP indicates presence of inflammation, its sensitivity was reported as only 29% [46]. The difference in the relationship between BOP and smoking found in this and earlier studies might be explained by differences in the condition of periodontitis.

The relevant weakness of the present study is the relatively small sample size. Exact age and gender matching of smokers and non-smokers was not possible. Finally we could not obtain clinical attachment level data, mainly because of the limited time allowed for the single-appointment clinical examination. However, our results add to the existing literature by suggesting the salient relationship between the detection of periodontal pathogens and smoking status in a population of periodontitis patients in Japan. Further studies utilizing more comprehensive clinical assessment and quantitative microbiological method such as real-time PCR are necessary.

Within limits, our data suggest that smoking favours colonization by the specific periodontopathic bacteria including C. rectus, and that this contributes to the disease severity in smokers.