Some oral organisms such as the motile rods and spirochetes are quite fastidious and need particular nutrients. These nutrients may only be produced by other organisms in the gingival sulcus.
It is of clinical importance to be able to measure the amount of plaque on the teeth as it is an indication of the effectiveness of oral hygiene. While plaque is visible as a white coating on the teeth, it is more readily seen if a stain is used. There are several methods of measuring plaque, but most use an index which records a score of how many surfaces are covered with plaque. Of perhaps even greater importance than the plaque index (amount of plaque) is the rate at which it forms. The rate of plaque formation is surprisingly not influenced by the plaque index but by the position of the tooth in relation to the lips; the rate increases with distance from the lips and is highest on the buccal surfaces of maxillary molars and lowest on the mandibular incisors.
4.3.6 From Symbiont to Pathobiont
A biofilm may form on suitable surface of a host, which provides moisture, food, and shelter, without doing the host any harm and may even contribute to the welfare of the host. The organisms of such a biofilm may be described as symbionts. This symbiosis has been established over many millions of years in the guts of a large variety of animal species from termites to man. How does this symbiont biofilm remain, for the most part benign, but yet clearly able to switch into a pathogenic mode?
Our commensal gut organisms not only keep out a more pathogenic flora, but contribute to the full development of our immune system and provide us with some nutrients such as vitamin K2. However, if there are even slight changes in the environment, which includes changes in the health status of the host, the balance between species may be disturbed. Certain species may become more dominant than others, and no longer be as harmless to the host. From being symbionts, they become pathobionts.
A distinction needs to be made between pathogens and pathobionts. Pathogenic organisms are understood to be capable of causing disease when introduced into a susceptible host. Pathobionts which have shifted from a symbiotic relationship to a less benign one become damaging to the host, but only when part of a consortium of other organisms have made the same shift. For example, Treponema denticola is a symbiont in the healthy gingival sulcus. It may become a pathobiont with other species causing gingivitis, but if introduced on its own to a susceptible host such as germ-free mice, gingivitis does not occur. It will only occur in the mice if the whole consortium is introduced. Pathobionts, unlike pathogens, may return to a symbiotic relationship if the balance of the ecosystem is restored to a state of health.
If broad-spectrum antibiotics are taken for any length of time, one of the side effects is disruption of the balance of resident gut organisms, which are not the target of the antibiotic. This disruption causes a shift in dominance of organisms such as Clostridium difficile, which cause diarrhea. Doctors advise patients who are taking antibiotics to eat yoghurt, which helps rebalance the intestinal flora. A more dramatic treatment for patients whose gut flora is unbalanced (e.g., Crohn’s disease) is to administer fecal transplants containing a less pathogenic spectrum of gut bacteria. It is considered by some microbiologists that most chronic, noncommunicable diseases are linked to a disordered microbiome. The term dysbiosis is used to describe the inbalance of a microbiome which leads to a shift to pathobiotic bacteria.
The most common oral diseases include dental caries, gingivitis, periodontal disease, alveolar abscess, pulpitis, periapical infection, peri-implantitis, and denture stomatitis. They are all caused by bacteria endogenous to the oral cavity, which have transformed from symbionts to pathobionts. In view of the origins of the causative organism, these oral infections should be described as noncommunicable. From this perspective, it may be useful to revisit the role of cross-infection control in dentistry.
4.3.7 Cross-Infection Control
It is instructive to consider the surgical classification of wounds according to the level of contamination. This classification ranges through four progressive levels of contamination, starting with clean and increasing through intermediary levels, clean/contaminated, contaminated/dirty, to dirty. According to these criteria, oral surgery wounds would be classified as clean/contaminated. The endogenous nature of most oral diseases therefore suggest that cross-infection would be most effective when directed toward intraoral control of potential pathobionts. For example, the importance of using a rubber dam barrier, to exclude salivary organisms during endodontic treatment, has been well established and is considered a best practice by endodontists. The use of a barrier rubber dam has also been recommended for restorative procedures when attempts are made by the operator to remove all infected caries, leaving a sterile cavity to restore.
There are other measures which have been recommended, which reduced the burden of endogenous contamination. For many years, it has been considered as the best practice to remove the plaque around the root surface of a tooth before extracting it. The purpose of this measure is to reduce the amount of dental plaque forced into the socket during application of the extraction forceps. In view of the endogenous origins of bacteria which cause an infected (dry) socket, a pre-extraction prophylaxis is a sensible precaution (see Chapter 5.3.2 Healing of a Tooth Socket). A similar preparation of a surgical site has been a recommended practice by many implant surgeons. An incision into the gingival sulcus, when creating a surgical flap for the placement of an implant fixture, releases orders of magnitude more pathobionts into the surgical site than might enter on the blade of the scalpel on which some airborne organisms may have alighted. For this reason, prophylaxis of teeth adjacent to a planned surgical site should proceed any surgery so as to reduce the unintentioned spread of plaque organisms.
The term cross-infection may deserve an expanded interpretation, as the non-communicable diseases of the oral cavity are not caused by foreign organisms transmitted from other individuals, or indeed by any single species of organism. Viral infections such as herpes, Epstein-Barr and AIDS are communicable via the oral cavity, though in quite specific conditions of transmission.
The trend toward central sterilization in an effort to reduce cross-contamination in the operating area should be seen in context with the threat to infection by endogenous pathobionts. It might be well to recall the Roman writer Cicero's warning that “The enemy is within the gates… it is our own criminality that we have to contend with.”
4.3.8 Single Organism or Consortium
The traditional theory of organisms causing diseases was defined by Koch in his isolation of Mycobacterium tuberculosis as the cause of tuberculosis. This theory stresses the vital role of the single organism and makes the isolation of pure cultures essential for proper diagnosis. However, diseases are also caused by communities of organisms which are for the most part endogenous or common symbionts of the body. In these conditions, it is necessary to culture at least a consortium of likely organisms as no single organism produces the disease. The diversity of organisms in the collective consortium is essential for the system to exert its effect. Marsh was the first microbiologist to culture a consortium of organisms in the investigation of dental caries and periodontal disease. This break with tradition led to the development of variations in artificial mouth simulations which have led to great advances in understanding the complexity of these diseases. We now know that we should not be looking for a single organism responsible for either caries or periodontal disease, but a group whose interdependence may be crucial to their success as pathobionts.
4.3.9 Dental Caries
A shift in the oral environment, such as an increase in dietary sugar, causes an imbalance in the dominance of some key organism in