Microbiology G235 Lecture 18 PDF

Microbiology G235 Pathogenesis Pathogenicity & Virulence Pathogenicity and virulence are terms that refer to an organism's ability to cause disease. Technically, pathogenicity is used with respect to differences between microbial species whereas virulence denotes d...

Microbiology G235 Pathogenesis Pathogenicity & Virulence Pathogenicity and virulence are terms that refer to an organism's ability to cause disease. Technically, pathogenicity is used with respect to differences between microbial species whereas virulence denotes differences between strains of the same species. In practice they are often used interchangeably. To cause disease, an organism must: 1. maintain a reservoir before and after infection (humans, animals, environment, etc.), 2. leave the reservoir and gain access to the new host, 3. colonize the body, and 4. harm the body. Influences Anything the bacterium does to aid in the above will influence its ability to cause disease they are able to do these things primarily as a result of their structures and their metabolic products. We must keep in mind, however, that whether or not a person actually contracts an infectious disease after exposure to a particular potentially pathogenic bacterium depends not only on the microorganism but also on the number of bacteria that enter the body and the quality of the person's innate and adaptive immune defenses. Virulence factors We are going to look at bacterial virulence factors that can influence its ability to cause infectious disease. These virulence factors will be divided into two categories: 1. virulence factors that promote bacterial colonization of the host, and 2. virulence factors that damage the host. Virulence Factors that Promote Bacterial Colonization of the Host Virulence factors that promote bacterial colonization of the host include the ability to: 1. contact host cells; 2. adhere to host cells and resist physical removal; 3. invade host cells; 4. compete for iron and other nutrients; 5. resist innate immune defenses such as phagocytosis; and 6. evade adaptive immune defenses. The Ability to Contact Host Cells The mucosal surfaces of the bladder and the intestines constantly flush bacteria away in order to prevent colonization. Motile bacteria that can swim chemotactically toward mucosal surfaces may have a better chance to make contact with the mucous membranes, attach, and colonize. The Ability to Contact Host Cells Because of their thinness, their internal flagella (axial filaments), and their motility, spirochetes are more readily able to penetrate host mucous membranes, skin abrasions, etc., and enter the body. Motility and penetration may also enable the spirochetes to penetrate deeper in tissue and enter the lymphatics and bloodstream and disseminate to other body sites. Ulcers : H. pylori Helicobacter pylori, by means of its flagella, is able to swim through the mucus layer of the stomach and adhere to the epithelial cells of the mucous membranes. Here the pH is near neutral. To also help protect the bacterium from the acid, H. pylori produces an acid-inhibitory protein that blocks acid secretion by surrounding parietal cells in the stomach. The bacterium then releases toxins that lead to damage of the gastric mucosa. In turn, results a massive inflammatory response. Leukocytes leave the capillaries, accumulate at the area of infection, and discharge their lysosomes for extracellular killing. This not only kills the bacteria, it also destroys the mucus-secreting mucous membranes of the stomach. Without this protective layer, gastric acid causes ulceration of the stomach. This, in turn, leads to either gastritis or gastric and duodenal ulcers. Streptococcus pyogenes Along a different line, Streptococcus pyogenes produces streptokinase that lyses the fibrin clots produced by the body in order to localize the infection. It also produces DNase that degrades cell- free DNA found in pus and reduces the viscosity of the pus. Both of these enzymes facilitate spread of the bacterium from the localized site to new tissue. The Ability to Adhere to Host Cells and Resist Physical Removal One of the body's innate defenses is the ability to physically remove bacteria from the body through such means as the constant shedding of surface epithelial cells from the skin and mucous membranes, the removal of bacteria by such means as coughing, sneezing, vomiting, and diarrhea, and bacterial removal by bodily fluids such as saliva, blood, mucous, and urine. Bacteria may resist this physical removal producing pili, cell wall adhesin proteins, and/or biofilm-producing capsules. Signal Transduction In addition, the physical attachment of bacteria to host cells can also serve as a signal for the activation of genes involved in bacterial virulence. This process is known as signal transduction.  Bacterial Adherence with Pili By genetically altering the adhesive tips of their pili, certain bacteria are able to: 1) adhere to and colonize different cell types with different receptors, and 2) evade antibodies made against the previous pili. Adhesins Adhesins are proteins found in the cell wall of various bacteria that bind to specific receptor molecules on the surface of host cells and enable the bacterium to adhere intimately to that cell in order to colonize and resist physical removal. Many, if not most bacteria probably use one or more adhesins to colonize host cells. Bordetella pertussis produces several adhesins Capsules and biofilms Many normal flora bacteria produce a capsular polysaccharide matrix or glycocalyx to form a biofilm on host tissue. A biofilm consists layers of bacterial populations adhering to host cells and embedded in a common capsular mass. Dental plaque Inner ear infections Encapsulated Brucella and Scanning Electron Micrograph of Dental Plaque showing "Corncob" Structure The Ability to Invade Host Cells Some bacteria produce adhesin molecules called invasins that activate the host cell's cytoskeletal machinery enabling bacterial entry into the cell by phagocytosis. By entering the cytoplasm of the host cell, it has a ready supply of nutrients and is able to protect the bacteria from complement, antibodies, and certain other body defenses. examples Streptococcus pneumoniae produces phosphorycholine, an invasin that enables the bacterium to enter host cells where it can resist phagocytosis. The phosphorylcholine is also thought to aid the bacterium in entering the blood and the meninges. F protein and M-protein of Streptococcus pyogenes (Group A beta streptococci) enables the bacterium to invade epithelial cells. This is thought to help maintain persistent streptococcal infections and enable the bacterium to spread to deeper tissues. The Ability to Compete for Iron and Other Nutrients Often the ability to be pathogenic is directly related to the bacterium's ability to compete successfully with host tissue and normal flora for limited nutrients. One reason the generation time of bacteria growing in the body is substantially slower than in lab culture is because essential nutrients are limited. In fact this is a major reason why the overwhelming majority of bacteria found in nature are not harmful to humans. Nutritional competition To be pathogenic, a bacterium must be able to multiply in host tissue. The more rapid the rate of replication, the more likely an infection may be established. Pathogens, therefore, are able to compete successfully for limited nutrients in the body. Generally bacteria compete for nutrients by synthesizing specific transport systems or cell wall components capable of binding limiting substrates and transporting them into the cell. A good example of this is the ability of bacteria to compete for iron. As we will see later during nutritional immunity, the body makes considerable metabolic adjustment during infection to deprive microorganisms of iron. Iron is essential for both bacterial growth and human cell growth.  Siderophores Bacteria synthesize iron chelators (compounds capable of binding iron) called siderophores. Many siderophores are excreted by the bacterium into the environment, bind iron, and then re-enter the cell. Others are found on the cell wall where they bind iron and transport it into the bacterium. Meanwhile, the body produces iron chelators of its own (transferrin, lactoferrin, ferritin, and hemin) so the concentration of free iron is very low. The ability of bacterial iron chelators to compete successfully with the body's iron chelators as well as those of normal flora may be essential to pathogenic bacteria. Otherways Some bacteria produce in addition to their own siderophore, receptors for siderophores of other bacteria in this way take iron from other bacteria. a number of pathogenic bacteria are able to bind human transferrin, lactoferrin, ferritin, and hemin and use that as their iron source. a number of bacteria are able to produce exotoxins that kill host cells only when iron concentrations are low. Perhaps in this way the bacteria can gain access to the iron that was in those cells. Resist innate immune defenses An Overview of Phagocytosis First the surface of the microbe must be attached to the cytoplasmic membrane of the phagocyte. Attachment of microorganisms is necessary for ingestion and may be unenhanced or enhanced. Unenhanced attachment Unenhanced attachment is a general recognition of what are called pathogen-associated molecular patterns - common in microbial cell walls but not found on human cells - by means of glycoprotein known as endocytic pattern- recognition receptors on the surface of the phagocytes Enhanced Attachment Enhanced attachment is the attachment of microbes to phagocytes by way of an antibody molecule called IgG or two proteins produced during the complement pathways called C3b and C4b. Molecules such as IgG, C3b, and C4b that promote enhanced attachment are called opsonins and the process is called opsonization. Enhanced attachment is much more specific and efficient than unenhanced. Enhanced Attachment of Bacteria to Phagocytes Phagocytosis continued Following attachment, polymerization and then depolymerization of actin filaments send pseudopods out to engulf the microbe and place it in a vesicle called a phagosome. Finally, lysosomes, containing digestive enzymes and microbicidal chemicals, fuse with the phagosome containing the ingested microbe and the microbe is destroyed Complement pathways An Overview of the Body's Complement Pathways Some bacteria are able to interfere with the body's complement pathways. The complement pathways will be discussed in detail later, but a brief summary is relevant here. There are three complement pathways: the classical complement pathway, the alternative complement pathway, and the lectin pathway. While the three pathways differ in the way they are activated, once activated they all produce the same beneficial complement proteins. Complement pathways Basically the complement proteins are a series of serum proteins that when activated participate in four important body defense functions. These include: Inflammation Phagocyte chemotaxis Opsonization Lysis of biological (bacterial) menbranes Capsules Capsules enable many organisms to resist phagocytic engulfment. The capsules of some bacteria interfere with the body's complement pathways. Capsules can interfere with the complement pathways in a number of ways Capsule stain of Enterobacter aerogenes Note colorless capsules surrounding purple bacilli. Capsules Blocking the Unenhanced Attachment of Bacteria to Phagocytes Other ways to resist phagocytosis Coagulase, causes fibrin clots to form around the organism that help enable it to resist phagocytosis. Pathogenic Yersinia, such as the one that causes plague, contact phagocytes and, by means of a type III secretion system, deliver proteins that depolymerize the actin microfilaments needed for phagocytic engulfment into the phagocytes  The Type III Secretion System in Bacteria the bacterium produces pore-forming proteins that create a pore spanning not only the bacterium's cytoplasmic membrane and outer membrane, but also the plasma membrane of the host cell. This allows the bacterium to deliver proteins directly from its cytoplasm into the cytoplasm of the host cell. Bacteria resist phagocytic destruction and serum lysis by a variety of means, some include: escape from the phagosome into the cytoplasm prior to the phagosome fusing with a lysosome Prevent phagosome from fusing with lysosome by inserting Por proteins Produce enzymes that kill phagosome The Ability to Evade Adaptive Immune Defenses One of the major defenses against bacteria is the immune defenses' production of antibody molecules against the organism. The "tips" of the antibody have shapes that have a complementary shape to portions of bacterial proteins and polysaccharides called epitopes. Avoiding Antibodies One way certain bacteria can evade antibodies is by changing the adhesive tips of their pili Bacteria can also vary other surface proteins so that antibodies already made will no longer "fit.“ antibodies are not made against some capsules. Bacteria are able to coat themselves with host proteins such as fibronectin, lactferrin, or transferrin and in this way avoid antibodies. Virulence Factors that Damage the Host: An Overview Virulence factors that damage the host include: 1. The ability to produce cell wall components (pathogen-associated molecular patterns or PAMPS) that bind to host cells causing them to synthesize and secrete inflammatory cytokines and chemokines; 2. The ability to produce harmful exotoxins. 3. The ability to induce autoimmune responses. Autoimmunity and Exotoxins Autoimmunity is when the body's immune defenses mistakenly attack the body and sometimes certain bacteria can serve as a trigger for this response. Exotoxins are protein toxins usually secreted from a living bacterium but also released upon bacterial lysis. There are three main types of exotoxins: 1. superantigens (Type I toxins), 2. A-B toxins and other toxin that interfere with host cell function (TypeIII toxins), and 3. exotoxins that damage host cell membranes (Type II toxins).