The Disease Process

The Disease Process
How Microbes Cause Disease
Microorganisms act in certain ways that allow them to cause disease. These actions include gaining way access to the host, adhering to and colonizing cell surfaces, invad ing tissues, and producing toxins and other harmful meta bolic products. However, host defense mechanisms tend to thwart the actions of microorganisms. The occur rence of a disease depends on whether the pathogen or the host wins the battle; if it is a draw, a chronic disease may result.

How bacteria cause disease
Bacterial pathogens often have special structures or phys iological characteristics that improve the chances of suc cessful host invasion and infection. Virulence factors are structural or physiological char acteristics that help organisms cause infection and disease. These factors include structures such as pili for adhesion to cells and tis sues, enzymes that either help in evading host defenses or protect the organism from host defenses, and toxins that can directly cause disease.



Direct action of bacteria
Bacteria can enter the body by penetrating the skin or mucous membranes, by sexual transmission, by being ingested with food, by being inhaled in aerosols, or by transmission on contaminated object with an infectious agent. If the bacteria are immediately swept out of the body in urine or feces or by coughing or sneezing, they cannot initiate an infection. A critical point in the production of bacterial disease is the organism’s adherence, or attachment, to a host cell’s surface. The occurrence of certain infections depends in part on the interaction between host plasma membranes and bacterial adhesions: are proteins or glycoproteins found on attachment pili (fimbriae) and capsules . Most adhesins that have been identified permit the pathogen to adhere only to receptors on membranes of certain cells or tissues the capsules and attachment pili are also antiphagocytic structures.
Attachment to a host cell surface is not enough to cause an infection. The microbes must also be able to colonize the cell surface
. Colonization refers to the growth of microorganisms on epithelial surfaces, such as skin or mucous membranes or other host tissues. For colonization to occur after adherence, the pathogens must survive and reproduce despite host defense mechanisms.
example, pathogenic bacteria on the surface of skin must withstand environmental conditions and bacteriostatic skin secretions. The degree of invasiveness of a pathogen—its ability to invade and grow in host tissues—is related to the virulence factors the pathogen possesses and determines the severity of disease ,most pathogen have additional virulence factors like enzymes that enable the pathogen to invade tissues. Among bacteria that release enzymes is Streptococci produce hyaluronidase, or spreading factor. This enzyme digests hyaluronic acid, a gluelike substance that helps hold the cells of certain tissues together .Digestion of hyaluronic acid allows streptococci to pass between epithelial cells and invade deeper tissues. Other example is Staphylococcus aureus that produces coagulase to aid in infection Coagulase is a two-edged sword: It keeps organisms from spreading but also helps wall them off from immune de fenses that might otherwise destroy them the bacterial enzyme streptokinase dissolves blood clots. Pathogens trapped in blood clots free themselves to spread to other tissues by secreting these virulence factors





A
B Enzymatic virulence factors help bacteria invade tissues and evade host defenses. (a) Hyaluronidase dissolves the “cement” that holds together the cells that line the intestinal tract. Bacteria that produce hyaluronidase can then invade deeper cells within the intestinal tissues. (b) Coagulase triggers blood plasma clotting, allowing bacteria protection from immune defenses. Streptokinase dissolves blood clots. Bacteria trapped within a clot can free themselves and spread the infection by producing streptokinase.


BACTERIAL TOXINS.
A toxin is any substance that is poisonous to other organisms. Some bacteria produce toxins, which are synthesized inside bacterial cells and are classified according to how they are released. Exotoxins are soluble substances secreted into host tissues. Endotoxins are part of the cell wall and are released into host tissues—sometimes in large quantities—from Gram negative bacteria, often when the bacteria die or destroyed

endotoxins
are produced by certain Gram-negative bacteria. All endotoxins consist of lipopolysaccharide (LPS) complexes They are relatively stable molecules that do not display affinities
for particular tissues. Bacterial endotoxins have nonspecific effects such as fever They also cause tissue damage in diseases such as typhoid fever and epi demic meningitis
Exotoxins
are more powerful toxins produced by sev eral Gram-positive and a few Gram-negative bacteria. Most are polypeptides, which are denatured by heat, ultra violet light, and chemicals such as formaldehyde. Species of Clostridium, Bacillus, Staphylococcus, Streptococcus, and several other bacteria produce exotoxins

Some exotoxins are enzymes. Hemolysins were first discovered in cultures of bacteria grown on blood agar plates. The action of these exotoxins is to lyse (rupture) red blood cells
Virulence factors called leukocidins are exotoxins produced by many bacteria, including the streptococci and staphylococci. These toxins damage or destroy certain kinds of white blood cells called neutrophils and macrophages
Table-1- types of bacterial toxinsb14.5 Properties of Toxins
property exotoxins endotoxins
Organisms producing Almost all Gram-positive; some Gram-negative Almost all Gram-negative
Location in cell Extracellular, excreted into medium Bound within bacterial cell wall; released upon
death of bacterium
Chemical nature Mostly polypeptides Lipopolysaccharide complex
Stability Unstable; denatured above 60°C and by ultraviolet
light Relatively stable; can withstand several hours
above 60°C
Toxicity Among the most powerful toxins known (some are100 to 1 million times as strong as strychnine) Weak, but can be fatal in relatively large doses
Effect on tissues Highly specific; some act as neurotoxins or cardiac
muscle toxins Nonspecific; ache-all-over systemic effects or
local site reactions
Fever production Little or no fever Rapid rise in temperature to high fever
Antigenicity Strong; stimulates antibody production and
immunity Weak; recovery from disease often does not
produce immunity
Toxoid conversion and use By treatment with heat or chemicals; toxoid used to
immunize against toxin Cannot be converted to toxoid; cannot be used
to immunize

Many exotoxins have a special attraction for particular tissues. Like Neurotoxins that act on tissues of the nervous system to prevent muscle contraction (botulism) Enterotoxins, such as the toxin that causes cholera, are exotoxins that act on tissues of thegut