CAPTURA DE DIOXIDO DE CARBONO POR ESPECIES

From infancy until death, people are in continuous contact with various micro­ organisms (Moore and Moore, 1994). However, in spite of this, each person tends to maintain his or her own unique periodontal, skin and intestinal floras (Moore et al., 1984).

The earliest microbiota to colonise the mouth of the new-born infant is derived from the mother’s genital tract, oral cavity and skin. As there are no teeth in the new­ born baby the first microbial colonisers are those bacteria which are able to adhere to the available surfaces. Streptocoœus salivarius is such an organism (Long and Swenson, 1976), which becomes established in the mouth within one day of birth (Carlsson ef a/., 1975).

As the teeth erupt, the dominant species which colonise the tooth surfaces are

Lactobacillus easel, Streptococcus sanguis and Streptococcus mutans (Carlsson et al., 1975; Kononen et al., 1992a; 1992b). Although most infants harbour a predominantly Gram-positive, facultative microbiota, anaerobes can also be recovered, particularly after tooth emergence. As many as 61 % of children aged 5-7 years harbour black-pigmented Gram-negative anaerobes (Mackler and Crawford, 1973; Friskin et al., 1990) as well as spirochaetes (Mackler and Crawford, 1973). The proportion of these organisms and other strict anaerobes increases in adolescence and adulthood (Wojcicki et al., 1987) but may show a great deal of site-to-site variability (Bowden et al., 1975), with local factors such as pH, Eh and assorted bacterial interactions playing an important role (Midtvedt, 1990). Some variability is also due to such factors as the state of health of the dentition, the periodontal status (Asikainen et ai, 1991a; 1991b) the medical status of the patients and their racial origin. Although the suspected periodontal pathogens such as the Gram-negative anaerobic rods of the Porphyromonas, Fusobacterium, Prevotella and \Nolinella genera and the Gram-negative rods of the genera Capnocytophaga, Eikenella and Actinobacillus are members of the normal oral flora, there remains some confusion as to how these bacteria cause periodontal disease. This gives rise to the specific, non-specific and the ecological plaque hypotheses.

1.2.1 Plaque and oral diseases

The specific plaque hypothesis states that, out of the diverse collection of organisms comprising the resident plaque microflora, only a limited number of species are involved in disease. In contrast, the non-specific plaque hypothesis considers that any of a heterogeneous mixture of micro-organisms can play a role in disease aetiology (Marsh and Martin, 1992). The ecological plaque hypothesis concentrates on the bacterial environment, where for example lactic acid production by oral Streptocoœus mitis inhibits the growth of oral Capnocytophaga

(Mashimo, 1985a). In contrast the rise in lactic acid production however, would be favourable for the increased growth of Veillonella spp.

1.2.2 Formation of supraglngival dental plaque

After a tooth surface is freshly cleaned, an acquired pellicle is formed, composed primarily of salivary proteins (Listgarten, 1976). The earliest colonisers are streptococci, which are found during the first 4 h after professional tooth cleaning (Nyvad and Kilian, 1987). However, within 12 h the population diversifies to include actinomyces, capnocytophaga, haemophili, prevotellae, propionibacteria, and veillonellae (Kolenbrander eta!., 1993).

1.2.3 Formation o f subgingival dental plaque

Within a few days of undisturbed plaque formation, the gingival margin begins to show typical inflammatory changes, including redness and swelling. This then results in the creation of a deepened gingival sulcus (pseudopocket), which provides a relatively anaerobic environment for the development of an anaerobic microbiota. Anaerobic bacteria that colonise this subgingival region include motile rods and spirochaetes. They are able to increase their mass hence contributing to the deepening of the sulcus, thereby increasing the volume of their habitat. Because many of the subgingival micro-organisms are motile, the structural organisation of this microbial population is quite different from that seen supragingivally. Rods and filaments tend to be arranged in a palisading pattern, with the long axes of the cells perpendicular to the tooth surface. The bulk of the subgingival microbiota consists of a complex mixture of predominantly anaerobic bacteria that surround and cover the bacterial aggregates.

The bottom of the sulcus or pocket is formed by the coronal, desquamative surface of the junctional epithelium, which is attached to the tooth surface on one side and to the gingival connective tissue on the other. This portion of the junctional epithelium is subject to bacterial as well as mechanical injuries, which may result in enlarged intercellular spaces and vertical tears in the epithelium. These alterations in the integrity of the junctional epithelium allow a gradual apical colonisation of the tooth surface by coccoid cells and rods (Vrahopoulos et al., 1992a; 1992b; 1995). Irregularities in the root surface, such as those caused by localised root resorption may shelter plaque micro-organisms and contribute to their retention at such sites. The tendency for bacteria to colonise tooth surfaces freshly exposed, because of disruptions in the junctional epithelium, leads to a gradual deepening of the sulcus or pocket. Thus, a distinctive subgingival microbiota, predominantly composed of Gram-negative anaerobic bacteria, including a number of motile species, becomes established in the gingival sulcus between 3-12 weeks after the beginning of supragingival plaque formation. Each of these microbial populations appears to facilitate the colonisation of this region by the next wave of bacterial settlers, with the ultimate establishment in the subgingival region of a predominantly anaerobic, Gram-negative microbiota. In this protected environment they are in an excellent position to participate in the destruction of the periodontal tissues, with the resulting maintenance and expansion of their subgingival habitat.