Definition: Periodontal disease is inflammatory disease of the supporting tissues of the teeth (the periodontium) caused mainly by bacterial plaque, leading to progressive destruction of the gingiva, periodontal ligament, and alveolar bone, which can eventually result in tooth mobility and tooth loss if untreated.
Clinical Case:
You are a final year dental student in clinic. A new patient is seated in your chair. They complain that their “gums bleed all the time” and a couple of back teeth “feel a bit loose”. You pick up your probe and start a periodontal chart.
As you work around the mouth you notice heavy plaque and calculus on the molars, rolled, red gingival margins and bleeding on probing at almost every site. In several posterior areas you record pockets of 6–7 mm, and when you later check the radiographs you see clear bone loss.
This is the clinical surface of periodontal disease. The aim of this chapter is to connect what you see today with what has been happening over weeks, months and years at the microscopic level: in the biofilm, in the host response, and in the periodontal tissues. We will keep that clinic patient in mind throughout, so the pathology reads as a continuous story rather than a list of disconnected facts.
Rewinding the story: from clean tooth to mature biofilm
Before the bleeding, the pockets and the bone loss, there was a completely normal tooth.
Imagine, for a moment, one lower incisor immediately after a meticulous scale and polish. For a short time, the enamel or restoration is essentially clean. However, the mouth is far from sterile, and the surface does not remain bare for long. Within minutes, salivary proteins and glycoproteins adsorb to the tooth, forming a thin acellular layer known as the acquired pellicle. This pellicle is rich in mucins and other salivary molecules that present specific receptors to which bacteria can bind.
As soon as this pellicle is in place, early colonising bacteria begin to attach. These are mostly Gram-positive, facultative organisms such as streptococci and Actinomyces species. Using adhesins on their surfaces, they bind to complementary receptors in the pellicle, and once attached, they start to divide. As they multiply, they secrete extracellular polysaccharides and other polymers that form a sticky matrix, helping them adhere more firmly and providing a scaffold for other organisms.
Even at this early stage, what is forming is not a random smear of bacteria but a biofilm: an organised community of microbes embedded in a self-produced matrix. Within this matrix, bacteria communicate, exchange nutrients, and gradually create microenvironments with different oxygen tensions and nutrient levels. These changes make them more resilient and less susceptible to antibiotics and host defences than free-floating bacteria.
If your patient were to disrupt this early plaque consistently with good brushing and interdental cleaning, the biofilm would remain in this relatively simple, early form. But in your clinic periodontal patient, this has not happened, especially in posterior interdental areas and along the gingival margins.
Maturation and ecological change
In areas that are not properly cleaned, the biofilm thickens. Nutrients from saliva and the diet feed the supragingival plaque, and oxygen is consumed rapidly in the outer layers. Deeper within the plaque mass, oxygen levels fall and the environment becomes increasingly anaerobic. This shift allows new, more fastidious organisms to join the community.
Many of these newcomers are Gram-negative, often obligate anaerobes, which cannot adhere directly to the pellicle. Instead, they co-adhere to the Gram-positive pioneer species that now form a scaffold within the plaque. As time goes on, the flora becomes more complex and more heterogeneous; the balance moves from predominantly Gram-positive, facultative bacteria to a community in which Gram-negative, anaerobic species and motile rods are much more prominent.
The process can be visualised as a rough timeline:
Time since effective cleaning | Dominant features | Representative organisms |
|---|---|---|
0–24 hours | Pellicle; early colonisers | Streptococci (Gram+ cocci) |
1–3 days | Increasing plaque; rods, filaments | Actinomyces spp. (Gram+ rods/filaments) |
3–7 days | Thicker biofilm; ↓ oxygen in deeper layers | Mixed flora; early Gram– species appear |
>7 days (undisturbed) | Complex, anaerobic, heterogeneous | Gram– anaerobes, motile rods, spirochaetes |
On smooth, easily accessible surfaces this process is repeatedly interrupted by brushing, and the biofilm rarely reaches the fully mature, anaerobic stage. However, in stagnation areas (like the distal of molars, interdental spaces, furcations, and around overhanging restorations) plaque is frequently left undisturbed long enough to mature. By the time you see the patient in clinic, these are exactly the places where you are recording bleeding and deep pockets.
Host–microbial balance: from health to gingivitis
Periodontal disease is not simply “bacteria causing infection”. It is the result of a dynamic interaction between the biofilm and the host. The biofilm provides the microbial challenge; the host provides the reactive tissue and the immune responses that aim to contain that challenge. Disease occurs when this balance is disrupted.
On the microbial side, changes in plaque quantity, composition or virulence may increase its pathogenic potential. On the host side, innate and adaptive immunity, genetic predisposition, systemic conditions and environmental factors such as smoking all influence how the tissues respond.
Your patient’s plaque has been left undisturbed around the gingival margin. Bacterial products such as lipopolysaccharide (LPS), peptidoglycan, and a variety of enzymes and toxins diffuse through the junctional epithelium. The host does not ignore this; an inflammatory response begins almost immediately.
The initial lesion: invisible inflammation
Within 24 to f48 hours of plaque accumulation at a previously clean site, changes in the tissues can already be identified histologically. The gingival capillaries become dilated and more permeable, and there is an increase in gingival crevicular fluid. Neutrophils (also known as polymorphonuclear leukocytes, PMNs) migrate from the vasculature, pass through the junctional epithelium and emerge into the sulcus, where they form a moving barrier between the plaque and the underlying tissues.
This is referred to as the initial lesion. It is an acute inflammatory response driven primarily by the innate immune system. Clinically, at this stage, the gingiva may still appear entirely healthy. You would not detect it with your probe. Nonetheless, every site that currently shows visible gingivitis and bleeding in your patient’s mouth will have passed through this invisible stage at some point.
Early and established lesions: the gingivitis you can see
If plaque is not removed, the response becomes more complex. After about four to seven days, the lesion progresses to what is termed the early lesion. Neutrophils are still present in the junctional epithelium and sulcus, but now T lymphocytes accumulate in the adjacent connective tissue. Around the inflammatory infiltrate, collagen is lost, and the junctional and sulcular epithelium may begin to proliferate and thicken in response to the ongoing challenge.
At this point, the changes become clinically evident. The marginal gingiva look slightly red and swollen, and gentle probing produces bleeding. This is early gingivitis, the point at which you first see something is wrong.
With continued plaque accumulation over weeks, a chronic or established lesion develops. In this stage, the inflammatory infiltrate in the gingival connective tissue is dominated by plasma cells, indicating a well-developed adaptive immune response. There is more extensive collagen destruction, and locally produced antibodies (mainly IgG and IgA, with some IgM) are present within the tissues and in the gingival crevicular fluid. There is increased vascularity and persistent exudation.