3: Microbiology and Pathology

3 Microbiology and Pathology

JEAN. YANG

OUTLINE

1. IMMUNOLOGY AND IMMUNOPATHOLOGY
2. GENERAL MICROBIOLOGY
3. MICROBIOLOGY AND PATHOLOGY OF SPECIFIC INFECTIOUS DISEASES
4. SYSTEMIC PATHOLOGY
5. GROWTH DISTURBANCES

1.0 IMMUNOLOGY AND IMMUNOPATHOLOGY

The human body’s immune system is an elegant and elaborate process involving two types of immunity. In general, the body’s first defense mechanism against infection includes anatomic (e.g., skin, mucosal membranes) or physiologic (e.g., temperature, low pH) barriers. When a pathogen invades this physical barrier, the body has two different types of immune responses: the innate (nonspecific) immune response and the acquired (specific/adaptive) immune response. Innate immunity is the body’s early defense to any kind of bodily injury, including trauma or infection. Since this nonspecific immune response has a limited ability to recognize specific antigens, it generally reacts to most pathogens in the same manner. When the innate immune response fails to effectively combat invading pathogens, the body mounts an acquired immune response. Acquired immunity, however, involves a highly sophisticated recognition of foreign structures. This ability to learn to identify specific structures allows the body’s defenses to act quickly and efficiently in killing specific pathogens.

1.1 Host Defense Mechanisms

A. Anatomical barriers

1. Skin

a. Acts as a physical barrier to external pathogens.
b. Langerhans cells—dendritic cells that are found in the stratum spinosum layer and are important in their ability to initiate the immune system.
c. Acidic environment of pH 3 to 5 retards growth of microbes.

2. Mucosal membranes

a. Line the gastrointestinal, respiratory, and urogenital tracts.
b. There are a variety of protective mechanisms. For instance, in the stomach and vaginal tract the pH remains low, discouraging microbial growth. The pulmonary mucosa contains cilia and secretes mucus, clearing the air that enters the respiratory tract.
c. Lysozyme is an important antimicrobial enzyme that is secreted in saliva and tears.

B. Physiologic barriers

1. Temperature—raised body temperature (i.e., fever) inhibits growth of some pathogens.
2. Low pH—acidic pH of the stomach kills most ingested microorganisms.

C. Innate immune response

1. Acute inflammation

a. Early immune response to injury or infection includes:

(1) Edema—caused by the increased vascular permeability of endothelial cells. Initially, the arterioles briefly vasoconstrict. This is followed by vasodilation and increased permeability of the endothelium. Fluid then moves from the circulation into the interstitial tissue, resulting in hyperemia and local edema. These actions are described by the four classic signs of inflammation: tumor (swelling), rubor (redness), calor (heat), and dolor (pain).
(2) Complement activation (discussed later in the complement system section).
(3) Release of inflammatory mediators by polymorphonuclear leukocytes or neutrophils, basophils, and mast cells.
(4) Activation of natural killer cells, which resemble cytotoxic T cells. They act to destroy cells primarily infected with viruses or tumors.

b. Possible outcomes of acute inflammation:

(1) Complete resolution.
(2) Scarring.
(3) Abscess formation. Clinically, if the abscess spreads into the soft tissues, a cellulitis develops. Two dental-significant formations of cellulitis include Ludwig’s angina and cavernous sinus thrombosis.

(a) Clinically, if the abscess spreads into the soft tissues, a cellulitis develops.
(b) Two dental-significant formations of cellulitis include Ludwig’s angina and cavernous sinus thrombosis.

2. Chronic inflammation

a. Usually a more moderate inflammatory response that persists.
b. Mediators of chronic inflammation include mononuclear leukocytes or macrophages, plasma cells, and lymphocytes.
c. Example in dentistry: a periapical lesion that persists at the root apex for many years due to continual stimulation of pathogens from inside the tooth.

3. Granulomatous inflammation

a. A form of chronic inflammation, characterized by the formation of granulomas.
b. Granulomas are areas that the immune system “walls off” if phagocytes fail to destroy foreign particles or microbes present in it.
c. Inflammatory mediators include a dense concentration of macrophages, fibroblasts, lymphocytes, and (less frequently) plasma cells. The continuous activation of macrophages induces them to attach to one another, assuming an epithelium-like (epithelioid) appearance; they may also fuse with each other to form multinucleated giant cells.
d. Two types of granulomas:

(1) Granulomas associated with infectious diseases. Examples include:

(a) Tuberculosis (Mycobacterium) infections—granulomas found in infected areas are called tubercles and often display central caseous necrosis.
(b) Fungal infections (Cryptococcus, Histoplasma, Coccidioides).
(c) Syphilis (Treponema pallidum)—granulomas seen during syphilis infections are called gumma.
(d) Cat scratch disease.

(2) Granulomas associated with foreign bodies

(a) Examples: foreign particles (glass, metals), surgical sutures, and so forth.

D. Acquired immune response

1. Immunologic attributes of the acquired immune response:

a. Specificity (antibody recognition can be as specific as a single amino acid).
b. Diversity (antibodies can recognize billions of unique structures).
c. Memory (i.e., memory B cells).
d. Self/non-self recognition (i.e., major histocompatibility complex).

2. There are two types of specific immune responses: cell–mediated immunity, which is mediated by T cells, and humoral immunity, which is mediated by antibodies that are produced by B cells.
3. Cell-mediated immunity

a. Is mediated by T cells.
b. Since T cells cannot directly recognize antigens, antigens must be processed and presented to them by antigen-presenting cells (APCs), such as macrophages, dendritic cells, B cells, and endothelial cells. APCs digest the antigens into small peptides and present them with a class II MHC molecule to the CD4 receptor on the helper T cell. This activates the helper T cells to release cytokines that cause inflammation and the activation of other T cells, B cells, and macrophages (Figure 3-1).

4. Humoral immunity (antibody-mediated immunity)

a. This branch of the specific immune response serves two purposes:

(1) To target antigens—the aggregation of antibodies produced by B cells on an antigen specifically identifies, neutralizes, and opsonizes the antigen, making it easier to phagocytize.
(2) Serves as a memory function—allows the immune system to more efficiently recognize and destroy microbes from previous exposure or infection.

b. Antigens are first recognized by macrophages or directly by B cells. Like cell-mediated immunity, the macrophage will process and present the antigens to helper T cells. This causes them to secrete cytokines, interleukin-4 (IL-4), and interleukin-5 (IL-5), which results in B-cell stimulation and growth, thus resulting in the production of specific antibodies against the antigen presented (Figure 3-2).

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Figure 3-1 Overview of cell-mediated immunity.

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Figure 3-2 Overview of humoral immunity.

1.1.1 Wound healing

A. Primary wound healing

1. Describes the repair of two well-apposed edges, such as surgical incisions.
2. Timeline of cellular events:

a. First 24 hours—formation of a fibrin clot. Fibronectin, an adhesive molecule found in plasma and on cell surfaces, forms crosslinks to stabilize the clot. PMNs or neutrophils migrate to the injured site.
b. Day 1 to 2—basal cells begin to proliferate (i.e., undergo mitosis) to close the epidermal defect.
c. Day 3—proliferation of basal cells continues. Granulation tissue forms at the injured site. Macrophages replace PMNs. Fibroblasts appear and secrete proteoglycans and type III collagen. Neovascularization begins.
d. End of the first week—formation of a scar. Fibroblasts and microphages continue to clean up and remodel the area. Fibroblasts secrete type I collagen. The vascularization and cellular infiltrate largely disappear.

3. Secondary wound healing

a. Describes repair of a larger wound with margins that are not well-apposed.
b. Cellular events are similar to primary wound healing except that more granulation tissue is formed and there is a larger inflammatory response. Also, the duration of healing is longer.

1.1.2 Immune effector cells

A. T cells

1. Important regulators of the immune response.
2. Involved in both cell-mediated and humoral immunity.
3. Play a role in type IV delayed hypersensitivity reactions.
4. Can be categorized by their function, namely, regulatory or effector. The regulatory function is primarily mediated by helper T cells, which have CD4 surface receptors. The effector function is primarily mediated by cytotoxic and suppressor T cells, which contain CD8 surface receptors:

a. CD4—helper T cells

(1) Recognize class II MHC molecules on antigen-presenting cells.
(2) Mainly for the recognition of bacterial antigens.
(3) Regulatory functions include:

(a) Stimulate B cells to develop into antibody-producing plasma cells.
(b) Stimulate CD8 cells to become activated cytotoxic T cells.
(c) Stimulate the actions of macrophages during delayed hypersensitivity reactions.

b. CD8—cytotoxic and suppressor T cells

(1) Recognize class I MHC molecules on antigen-presenting cells.
(2) Function mainly in recognition of viral or neoplastic antigens.
(3) Cytotoxic functions are carried out by:

(a) Releasing perforans—destroy cell membranes.
(b) Inducing apoptosis—programmed cell death.

c. T cell maturation occurs in the thymus. There are three developmental stages:

(1) Immature: CD4−CD8−, a double-negative cell that expresses neither CD4 nor CD8.
(2) Less mature: CD4+CD8+, a double-positive cell that expresses both CD4 and CD8.
(3) Mature: CD4+CD8− or CD4−CD8+, a single-positive cell that expresses either CD4 or CD8.

B. B lymphocytes

1. Involved in the humoral immune response.
2. Perform two important functions:

a. Responsible for the production and secretion of antibodies.
b. Are antigen-presenting cells. Antigens are recognized by surface immunoglobulins.

3. Activated B cells differentiate into:

a. Plasma cells—produce and release monoclonal antibodies.
b. Memory B cells—circulate for long periods of time and are able to respond quickly to re-exposures.

C. Mononuclear cells—monocytes (in blood) and macrophages (in tissues)

1. Produced in the bone marrow.
2. Important functions include:

a. Phagocytosis—defense against microbes, removal of cellular debris or breakdown products.
b. Cytokine production.
c. Act as antigen-presenting cells.

3. When monocytes enter tissues, they mature into macrophages.
4. The body has an extensive network of macrophages known as the reticuloendothelial system. These macrophages are fixed and serve different functions, depending on their tissue location. Examples include:

a. Dust cells and heart–failure cells in the lungs.
b. Kupffer cells in the liver.
c. Mesangial cells in the kidney.
d. Macrophages in the lymph nodes.
e. Splenocytes in the spleen.
f. Microglia in the CNS.
g. Histocytes in connective tissues.

5. The presence of macrophages indicates chronic inflammation.
6. Phagocytosis

a. Occurs in three stages:

(1) Migration and attachment to site of injury/infection.
(2) Ingestion—formation of a phagosome.
(3) Killing—release of enzymes to destroy ingested microbe.

b. Phagocytosis is a process that initiates with the formation of a membrane-bound vacuole (phagosome) around the foreign particle or microbe. The phagosome then fuses with a lysosome, which contains many degradative enzymes, resulting in the formation of a phagolysosome. This process results in the destruction of the microbe.

D. Mast cells

1. Contain surface antigen receptors, including IgE. IgE plays an important role in type I immediate hypersensitivity reactions.
2. Contain dense granules with inflammatory mediators, including:

a. Histamine. There are two receptors that bind histamine:

(1) H1 receptors—cause increased permeability and vasodilation in capillaries. Also cause bronchoconstriction in the lungs.
(2) H2 receptors—play a role in gastric acid and pepsin secretion.

b. Slow-reacting substance of anaphylaxis (SRS-A)

(1) A leukotriene.
(2) Actions include bronchoconstriction in the lungs.
(3) May play an important role as a mediator of asthmatic bronchoconstriction.

1.1.3 Cytokines

A. Cytokines are inflammatory mediators that are released by a variety of cells, such as macrophages or lymphocytes, to regulate immune responses. Think of them as the words that immune cells use to communicate with each other.
B. Cytokines secreted from lymphocytes are also called lymphokines.
C. Types of cytokines:

1. Interleukins (ILs)

a. Are mediators that affect lymphocytes.
b. A summary of important interleukins is shown in Table 3-1.

2. Interferons (INFs)

a. Are mediators that are important for antiviral immunity.
b. A summary of important interferons is shown in Table 3-2.

D. Prostaglandins (PGs) and leukotrienes (LTs)—mediators of the inflammatory response. PGs and LTs are metabolites of arachidonic acid. They are produced by the cyclo-oxygenase and the lipoxygenase pathway, respectively (Figure 3-3). General actions of these cytokines include vasoconstriction, bronchoconstriction, and increased inflammatory activity.

TABLE 3-1 IMPORTANT INTERLEUKINS AND THEIR ACTIONS

CYTOKINE SOURCE ACTIONS
IL-1 Macrophages Stimulate cell activity or production of mediators in a variety of cells, including lymphocytes, macrophages, and endothelial cells. They also cause fever.
IL-2 Helper T cells Activate helper and cytotoxic T cells.
IL-3 Activated T cells Stimulate production of RBCs in bone marrow.
IL-4 Helper T cells Stimulate B cell growth and production of IgE and IgG.
IL-5 Helper T cells Stimulate B cell differentiation into plasma cells, activity of eosinophils, and production of IgA.

TABLE 3-2 IMPORTANT INTERFERONS AND THEIR ACTIONS

CYTOKINE SOURCE ACTIONS
INF-α Leukocytes Inhibit viral growth
INF-β Fibroblasts Inhibit viral growth
INF-γ Helper T cell Strong activator of macrophages, important in cell-mediated immunity
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Figure 3-3 Cyclo-oxygenase and lipoxygenase pathways.

1.1.4 Complement System

A. Consists of a group of nearly 30 preformed serum and membrane proteins found in blood serum or on cell surfaces. They become activated through protease activity in a cascading order.
B. Main function is to generate reaction products that enhance antigen clearance and stimulate an inflammatory response. It achieves this by the following actions:

1. Lysis of foreign body by formation of membrane attack complexes (C5–C9).
2. Opsonization—specific complement proteins (C3b), or opsonins, bind to the surfaces of microbes or foreign particles to encourage their phagocytosis.
3. Chemotaxis of inflammatory mediators—C5a acts to recruit inflammatory cells (i.e., neutrophils) to the site of injury/infection.
4. Production of anaphylatoxins (C3a, C4a, and C5a)—act to stimulate mast cells in releasing mediators (histamine), resulting in increased vascular permeability (edema), vasodilation, and bronchoconstriction.

C. The complement cascade: two pathways (Figure 3-4):

1. The alternative pathway—complement becomes activated by direct contact with the microbe or injured site. C3 binds directly to the surface antigen, activating the complement system.
2. The classical pathway—antibodies first bind to the surface antigen present on the microbe or injured site. Complement (C1) will then bind onto the antibody, activating the complement cascade.
3. The end product of both pathways is the same—the formation of a membrane attack complex. This complex forms directly on the surface of the microbe, “punches a hole in it,” and results in cell or microbe lysis.
image

Figure 3-4 Complete cascade.

1.1.5 Immunoglobulins (Antibodies)

A. Antibody effector functions include:

1. Neutralization—by binding directly to the antigen, the antibody neutralizes the antigen by preventing it from further interaction with other cells.
2. Opsonization—when antibodies bind to an antigen, they act as opsonins to help facilitate with host phagocytosis or complement activation.
3. Activation of the complement system.

B. Antibodies can also cause agglutination of antigens (i.e., antigens clump together). The classic example of this is blood typing, where blood type is determined by agglutination of RBCs to specific agglutins.
C. Immunoglobulin classes

1. IgG

a. 75% of antibodies.
b. Associated with the secondary immune response. (Note: IgM responds first.)
c. Important for its actions against microbes.
d. Major actions:

(1) Activates complement.
(2) Acts as an opsonin to encourage phagocytosis of bacteria.
(3) The only antibody that can cross the placenta and is present in newborns.

e. IgG’s ability to activate complement suggests that it likely plays a role in the pathogenesis of adult periodontitis.

2. IgA

a. 15% of antibodies.
b. Important for the immunity of mucous membranes.
c. Found in mucosal secretions of the genitourinary, intestinal, and respiratory tracts. It is also found in tears, saliva, and colostrum.
d. Major action—prevents adhesion of microbes to mucous membranes by binding surface antigens.

3. IgM

a. 9% of antibodies.
b. Associated with the body’s primary immune response. It is the first antibody to arrive at the site of injury/infection.
c. Important for its actions against microbes.
d. Major actions:

(1) The most efficient Ig activator of the complement system.
(2) Acts as an antigen receptor for B cells.

4. IgD

a. 0.2% of antibodies.
b. Major actions unknown.
c. May act as an antigen receptor for B cells and induce the activation of B cells by the antigen.

5. IgE

a. 0.004% of antibodies.
b. Important in its role in type I hypersensitivity reactions.
c. Acts as antigen receptors for granulocytes, including basophils and mast cells, and stimulates their degranulation.

1.2 Hypersensitivity

Hypersensitivity reactions are characterized by exaggerated immune responses that result in injury. There are four types of hypersensitivity reactions.

A. Type I immediate hypersensitivity

1. Antibody mediator: IgE.
2. A hyperresponse of the immune system caused primarily by the production and accumulation of IgE antibody.
3. The accumulation of IgE requires prior sensitization or exposure to a specific allergen. Reexposure of the allergen causes it to bind to IgE receptors on mast cells, resulting in large releases of inflammatory mediators.
4. Two subtypes:

a. Atopic allergies

(1) Are limited to specific target tissue or organ.
(2) Family members display a strong hereditary predisposition to allergy sensitization, which may result from a genetic defect affecting the regulation of the IgE response.
(3) Important examples:

(a) Asthma

(i) Primary inflammatory mediator: SRS-A.
(ii) Symptoms include bronchoconstriction and edema in the lower respiratory tract.

(b) Hay fever (allergic reaction)

(i) Primary inflammatory mediator: histamine.
(ii) Symptoms include rhinitis, teary eyes, and respiratory congestion.

b. Anaphylaxis

(1) Primary inflammatory mediator: histamine.
(2) Allergen binding of IgE causes a very large release of histamine, which can result in anaphylactic shock, a life-threatening condition that includes severe bronchoconstriction and low blood pressure.
(3) Treatment: epinephrine (vasoconstrictor, bronchodilator), antihistamines.

B. Type II antibody-dependent cytotoxic hypersensitivity

1. Antibody mediators: IgG and IgM.
2. The binding of antibodies to cell membrane antigens activates the complement system, resulting in the formation of a membrane attack complex and cell death. Antibodies can also mediate cell destruction by antibody-dependent, cell-mediated cytotoxicity.
3. Important examples:

a. Erythroblastosis fetalis.
b. Blood transfusion reactions.

C. Type III immune-complex mediated hypersensitivity

1. Antibody mediator: IgG.
2. Antigen–antibody interactions result in the formation of immune complexes. These complexes become trapped along the vascular walls, activating complement and the immune response. Damage to the blood vessel walls occurs primarily from phagocytosis of the immune complex by the cells of the reticuloendothelial system.
3. Important examples:

a. Arthus reaction

(1) A localized type III reaction.
(2) Injection of an antigen into a patient who already has a high level of circulating antibody (IgG), usually resulting from repeated exposure, leads to formation of localized immune complexes.
(3) Symptoms include severe swelling and hemorrhaging within a few hours after exposure.

b. Serum sickness

(1) A generalized type III reaction.
(2) Systemic injection of drug serum (i.e., specific antigens) causes the formation of immune complexes throughout the microvasculature.
(3) Symptoms typically develop within days or weeks after exposure and can include fever, hives, lymphadenopathy, and arthritis.

c. Other examples of generalized type III reactions:

(1) Rheumatoid arthritis—an autoimmune disease with chronic inflammation of the joints.
(2) Systemic lupus erythematosus.
(3) Although the specific antigen is unknown, immune complexes have been found to be deposited in inflamed areas.

D. Type IV delayed (cell-mediated) hypersensitivity

1. There are no antibody mediators. Helper T cells are the main mediators in delayed hypersensitivity.
2. Antigens are presented by antigen-presenting cells to helper T cells. The activated helper T cells release cytokines to activate the immune response, including stimulating macrophages to increase their release of lytic enzymes. Note: the leakage of these enzymes may result in tissue damage.
3. Symptoms may appear days after exposure (i.e., delayed).
4. Important examples:

a. Contact dermatitis.

(1) Mediated by haptens. Haptens are small proteins that have antigenic properties They are unable to initiate an immune response by themselves, unless they can join with a larger body protein.
(2) Examples include poison ivy or oak, reactions to cosmetics, drugs (penicillin), and so forth.

b. Tuberculin (PPD) test—skin test, used to test for M. tuberculosis exposure.
c. Rejection of skin graft.

1.3 Immunopathology

A. Bruton’s agammaglobulinemia

1. Characterized by a deficiency or very low levels of antibodies.
2. Results from a failure of B cells to differentiate, leading to a decreased concentration of plasma cells and antibody production.
3. Cell-mediated immunity is unaffected.
4. Recurrent bacteria-related (pyogenic) infections are common in these patients.
5. Treatment: administration of IgG.

B. Transient hypogammaglobulinemia

1. Seen in infants.
2. There is a decreased amount of antibodies present due to slow antibody production.
3. Treatment: administration of IgG after the first 6 months.

C. DiGeorge’s syndrome

1. Characterized by a deficiency of T cells.
2. Results from a failure of the third and fourth brachial (pharyngeal) pouches to develop normally, leading to a lack of thymus and parathyroid development. Mandibular development is also affected.
3. Symptoms include:

a. Tetany—caused by hypoparathyroidism hypocalcemia.
b. Recurrent viral and fungal infections.

D. Severe combined immunodeficiency (SCID)

1. Characterized by a deficiency in both T cells and B cells.
2. Severe, recurrent, opportunistic infections are common in infants.
3. Treatment: bone marrow transplantation or gene therapy.

E. Angioedema

1. Acquired angioedema is caused by a deficiency of the complement inhibitor, C1-INH, which inhibits the conversion of C1 to C1 esterase. Note: angioedema can also be caused by allergic reactions and may be seen in patients taking certain medications, such as angiotensin-converting enzyme (ACE) inhibitors.
2. Symptoms include diffuse, soft tissue swellings in the face, extremities, and pelvic area.
3. Can be life-threatening.

F. Chronic granulomatous disease

1. Results from neutrophils with a defective NAPDH oxidase system. This affects the ability of neutrophils to kill microorganisms, since they are unable to produce superoxide radicals.
2. Chronic bacterial infections and the formation of granulomas are common.
3. Genetic transmission: X-linked.

G. Autoimmune diseases

1. Systemic lupus erythematosus (SLE or lupus)

a. Exact cause is unknown.
b. More common in female patients.
c. Characterized by the presence of antinuclear antibodies (ANAs). Common ANA findings include anti-DNA, anti-RNA, and anti-Sm antigen. ANAs can form immune complexes in the microvasculature of many organ systems, including:

(1) Skin—causing characteristic butterfly rash along the malar and nose area.
(2) Joints—causing arthritis-like symptoms.
(3) Kidney—most significant problem because it can lead to kidney failure.
(4) Heart—increased risk of endocarditis due to vegetations found on heart valves.

d. Raynaud’s phenomenon may be seen in patients. This describes when a patient’s fingertips or toes become white and blue upon exposure to cold temperatures, due to a decrease in blood flow. Note: Raynaud’s phenomenon is also seen in patients with CREST syndrome (calcinosis, Raynaud’s syndrome, esophageal dysmotility, sclerodactyly, and telangiectasia) or acrosclerosis.

2. Scleroderma (systemic sclerosis)

a. Exact cause is unknown.
b. Abnormal deposition of collagen leads to the development of fibrosis in certain organs, which can result in organ failure.
c. Raynaud’s phenomenon is common in these patients. Other symptoms may include dysphagia, dyspnea, and myocardial fibrosis.
d. Oral findings include microstomia, or decreased opening of the mouth.

2.0 GENERAL MICROBIOLOGY

2.1 Biology of Micro-Organisms

2.1.1 Bacteria

A. Growth of bacteria—can be divided into four phases (Figure 3-5):

1. Phase 1: lag phase—period when bacteria are metabolically active but are not reproducing.
2. Phase 2: log phase—period when bacteria grow exponentially, via rapid cell division (binary fission, when a parent cell divides into two separate cells). This exponential growth results in a very rapid increase of the number of bacteria present. Note: penicillins act on this phase.
3. Phase 3: stationary phase—occurs when the bacteria have reached a steady state (i.e., reproduction rate = death rate) due to nutrient depletion and/or toxic by-products produced.
4. Phase 4: death—as resources decrease and toxic waste increases, bacteria begin to die.
B. Classification of bacteria

1. Classification by oxygen metabolism

a. Oxygen metabolism in bacteria results in the formation of two toxic molecules: hydrogen peroxide (H2O2) and a free radical superoxide (O−2).
b. Since both of these molecules are toxic, bacteria utilizing oxygen for growth require the presence of two enzymes to neutralize their effects. H2O2 is catalyzed by superoxide dismutase; O−2 is catalyzed by catalase (Figure 3-6).

2. Classification by oxygen requirement

a. Aerobes: require oxygen for growth.
b. Facultative anaerobes: do not require oxygen for growth but utilize it when it is present.
c. Anaerobes: cannot grow in the presence of oxygen.
d. Note: supragingival plaque consists of mostly aerobes and facultative anaerobes; subgingival plaque consists mostly of anaerobes.

3. Classification by shape

a. Cocci—round, circular shape. For example:

(1) Diplococci—occur in pairs.
(2) Streptococci—occur in chains.
(3) Staphylococci—occur in clusters.

b. Bacilli—rod-shaped.
c. Spirochetes—spiral-shaped.
d. Pleomorphic—bacteria that appear with different, inconsistent shapes.

C. Classification by gram-staining

1. General characteristics of gram-staining:

a. Used for identification purposes.
b. Divides most bacteria into two groups:

(1) Gram-positive bacteria—will stain a blue color.
(2) Gram-negative bacteria—will stain a red color.

c. May be useful in determining which antibiotic to prescribe.

2. Stain protocol

a. Stain with crystal violet—all bacteria stain blue.
b. Iodine solution—all bacteria retain blue color.
c. Rinse with acetone or ethanol—gram-negative bacteria, with their thin walls, will lose their blue color. Gram-positive bacteria retain the blue color.
d. Stain with safranin—gram-negative cells stain red. Gram-positive remain blue.

D. Classification by acid-fast staining

1. Used mostly for mycobacteria—which cannot be gram-stained.
2. Stain protocol

a. Stain with carbolfuchsin—all bacteria stain red.
b. Wash with acid alcohol.
c. Stain with methylene blue—mycobacteria will retain the red color, while other bacteria will stain blue.

image

Figure 3-5 Bacterial growth curve.

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Figure 3-6 Equations for the neutralization of H2O2 and O–2 in bacteria by superoxide dismutase and catalase.

2.2 Bacterial Cell Walls and Their External Surfaces

All bacteria are surrounded by a cell wall except Mycoplasma. They only have a cell membrane.

A. Cell wall—a multilayered structure located external to the cytoplasmic membrane

1. Cell wall characteristics of gram-positive bacteria:

a. A thick peptidoglycan layer.
b. The peptidoglycan backbone consists of alternating N-acetylmuramic (NAM) and N-acetylglucosamine (NAG) acid.
c. Contains teichoic acid, which is antigenic, outside the peptidoglycan.

2. Cell wall characteristics of gram-negative bacteria:

a. A thin peptidoglycan layer.
b. In the space between the outer and cytoplasmic membranes, certain species contain enzymes called β-lactamases, which degrade penicillins and other β-lactam drugs.
c. Lipopolysaccharide (LPS or endotoxin).

(1) Found in the outer membrane.
(2) Causes an immune response by activating macrophages, B cells, and complement.
(3) The toxicity of LPS is attributed to presence of lipid A.
(4) O antigen—the outer membrane of the LPS. Often used for identification purposes.
(5) Found in dental plaque and gingival inflammation.

3. Cell wall characteristics of acid-fast bacteria:

a. An unusual cell w/>

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Jan 5, 2015 | Posted by in General Dentistry | Comments Off on 3: Microbiology and Pathology

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