Damaging Host Cells

Toxins

Microorganisms produce poisonous substances called toxins.

Learning Objectives

Describe the major toxin types (bacterial toxins and mycotoxins) and their mechanisms of action

Key Takeaways

Key Points

  • Microbial toxins may include those produced by the microorganisms bacteria (i.e. bacterial toxins) and fungi (i.e. mycotoxins ).
  • Bacterial toxins can include both endotoxins and exotoxins, which vary in mechanism of action and are species -specific.
  • Exotoxins are immediately released into the surrounding environment whereas endotoxins are not released until the bacteria is killed by the immune system.
  • Mycotoxins can be classified into numerous categories and are not species-specific because the same mycotoxin can be produced by different fungal species.

Key Terms

  • endotoxin: Any toxin secreted by a microorganism and released into the surrounding environment only when it dies.
  • exotoxin: Any toxin secreted by a microorganism into the surrounding environment.
  • cytokines: Regulatory proteins that function in the regulation of the cells involved in immune system function

Toxins are poisonous substances produced within living cells or organisms and can include various classes of small molecules or proteins that cause disease on contact. The severity and type of diseases caused by toxins can range from minor effects to deadly effects. The organisms which are capable of producing toxins include bacteria, fungi, algae, and plants. Some of the major types of toxins include, but are not limited to, environmental, marine, and microbial toxins. Microbial toxins may include those produced by the microorganisms bacteria (i.e. bacterial toxins) and fungi (i.e. mycotoxins).

Bacterial Toxins

Bacterial toxins are typically classified under two major categories: exotoxins or endotoxins. Exotoxins are immediately released into the surrounding environment whereas endotoxins are not released until the bacteria is killed by the immune system. The release of toxins into the surrounding environment, regardless of when released, results in the disruption of metabolic pathways in the host eukaryote. These metabolic pathways include damaging cell membranes, disrupting protein synthesis, inhibiting neurotransmitter release, or activating the host immune system. The mechanisms of action by which toxins disrupt eukaryotic cell processes are dependent on the target. For example, the bacteria Listeria monocytogenes, associated with food-borne illnesses, specifically targets cholesterol by producing a pore-forming toxin protein, listeriolysin O. This exotoxin affects intracellular processes and creates unregulated pores within the cell membranes of the host. Another example of an exotoxin includes an enterotoxin produced by the bacteria Staphlycoccal aureus. S. aureus can producestaphylococcal enterotoxin B (SEB), associated with intestinal illness, which promotes activation of the immune system. Upon activation of the immune system, the release of large amounts of cytokines, inflammatory related molecules, causes significant inflammation. Lastly, an example of an endotoxin, includes the protein lipopolysaccharide (LPS) produced by gram-negative bacteria. The LPS is a component of the bacteria’s outer membrane and promotes structural integrity. Upon destruction of the membrane by an immune response, the LPS is released and functions as a toxin.

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Bacterial Toxin Mechanism of Action: A schematic of various processes utilized by bacterial toxins to damage host cells.

However, bacterial toxins are also currently serving as new sources for potential drug development. Toxins have been shown to exhibit anticancer characteristics and fight again microbial virulence. The investigation of toxins as potential medicinal compounds is currently underway.

Mycotoxins

Mycotoxins are the classes of toxins produced by fungi. Mycotoxins are numerous and production of a specific mycotoxin is not restricted to one specific species. Mycotoxins are secondary metabolites that are toxic to humans and produced by fungi. There are various types of mycotoxins including, but not limited to, aflatoxins, ochratoxins, citrinin, and ergot alkaloids.

Aflatoxins

Aflatoxins are a type of mycotoxin that are produced by certain strains of Aspergillus fungi. The aflatoxins are further broken down into types: AFB1, AFB2, AFG1, and AFG2. These strains are present in a wide range of agricultural commodities associated with tropic and subtropic zones. These commodities include species of peanuts and corn. The most potent toxin is AFB1 and it is associated with carcinogenic effects.

Ochratoxin

Ochratoxin is a type of toxin produced by both Penicillium and Aspergillus species. Ochratoxins are further classified in types A, B and C and differ in structure. Ochratoxins have demonstrated carcinogenic properties and are often found in beverages such as beer and wine, as the fungal species which produce ochratoxins are often found on the plants used to produce these products.

Citrinin

Citrinin is a mycotoxin that has been isolated in numerous species of both Penicillium and Aspergillus. Many of these fungal species are utilized in food processing and are often found in foods including cheese, wheat, rice, corn, and soy sauce. Citrinin is known to function as a nephrotoxin, indicating it has toxic effects on kidney function.

Ergot Alkaloids

Ergot Alkaloids are specific compounds that are produced as toxic alkaloids in Claviceps, a group of fungi associated with grasses, rye, and related plants. The disease caused by ingestion of this fungi is called ergotism. Ergotism is characterized by detrimental effects on the vascular system in particular, including vasoconstriction of blood vessels resulting in gangrene, and eventually, limb loss if left untreated. Additionally, ergotism can present as hallucinations and convulsions as ergot alkaloids target the central nervous system. Due to the vascular system effects of ergot alkaloids, they have been used for medicinal purposes.

Direct Damage

Direct damage to the host is a general mechanism utilized by pathogenic organisms to ensure infection and destruction of the host cell.

Learning Objectives

Describe the different processes used by pathogens to damage the host and ensure infection

Key Takeaways

Key Points

  • Pathogenic organisms must have mechanisms in place to evade attack by the immune system.
  • Pathogens can produce enzymes that disrupt normal tissue and allow for further invasion into the tissues.
  • Pathogens can produce toxins that interfere with protein function deemed necessary by the host cell for proper maintenance.

Key Terms

  • diphtheria: A disease of the upper respiratory tract caused by a toxin secreted by Corynebacterium diphtheriae.
  • phagocytosis: the process by which a cell incorporates foreign particles intracellularly.

Direct damage to the host is a general mechanism utilized by pathogenic organisms to ensure infection and destruction of the host cell. The pathogenic organism typically causes damage due to its own growth process. The promotion of disease is characterized by the ability of a pathogenic organism to enter a host and inflict damage and destruction onto the host cell. The pathogenic organism must exhibit specific characteristics that promote its growth into a host cell including, but not limited to, the ability to invade, colonize, and attach to host cells.

The ability of a pathogen to gain entrance to a host cell is fundamental in the ability of the pathogen to promote and cause disease. The ability to manipulate the process of phagocytosis is a mechanism often utilized by bacteria to ensure they effectively invade a host. Phagocytosis is a process utilized by phagocytes (white blood cells) as a defense mechanism to protect from foreign bodies. The phagocytes engulf invaders and present them to additional factors within the immune system that result in their destruction. However, a successful and destructive pathogen often exhibits the ability to evade phagocytosis.

The mechanism(s) utilized by pathogens to avoid phagocytosis include avoiding both contact and engulfment. Pathogens that exhibit the ability to avoid contact utilize various processes to accomplish this, including: the ability to grow in regions of the body where phagocytes are incapable of reaching; the ability to inhibit the activation of an immune response; inhibiting and interfering with chemotaxis which drives the phagocytes to site of infection; and ‘tricking’ the immune system to identify the bacteria as ‘self. ‘ Additional mechanism(s) by which bacteria can avoid destruction is by avoiding engulfment. This is accomplished by the ability of the bacteria to exhibit produce molecules that interfere with the phagocytes ability to internalize the bacteria. Molecules that interfere with this process include certain types of proteins and sugars that block engulfment.

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Protected from Phagocytosis: Staphylococcus aureus exhibit physical properties, specifically a capsule, that protect the bacteria from phagocytosis.

Once the pathogen has successfully evaded engulfment and destruction by the immune system, it is detrimental because the bacteria then multiply. Often times, bacteria will directly attach themselves to host cells and utilize nutrients from the host cell for their own cellular processes. Upon the use of host nutrients for its own cellular processes, the bacteria may also produce toxins or enzymes that will infiltrate and destroy the host cell. The production of these destructive products results in the direct damage of the host cell. The waste products of the microbes will also damage to the cell. Examples of bacteria that will damage tissue by producing toxins, include, Corynebacterium diphtheriae and Streptococcus pyogenes. Specifically, Corynebacterium diphtheriae causes diphtheria, which isa disease of the upper respiratory tract. It produces a toxin, diphtheria toxin, which alters host protein function. The toxin can then result in damage to additional tissues including the heart, liver, and nerves. Streptococcus pyogenes is associated with strep throat and “flesh-eating disease. ” The bacteria produce enzymes which function in disrupting fibrin clots. Fibrin clots will form at sites of injury, in this case, at the site of foreign invasion. The enzymes, capable of digesting fibrin, will open an area within the epithelial cells and promote invasion of the bacteria into the tissues.

Type III and Type IV Secretion

Type III and IV secretion systems are utilized by pathogenic bacteria to transfer molecules from the bacterial cell to the host cell.

Learning Objectives

Distinguish between Type III and IV secretion systems

Key Takeaways

Key Points

  • Type III secretion system use a process which injects the secretory molecule into the host cell.
  • Type IV secretion systems use a process which is similar to the bacterial conjugation machinery.
  • Type IV secretion systems require attachment to the host cell by direct cell-to-cell contact or via a bridge-like apparatus.
  • Type IV secretion systems can be used to both transport and receive molecules.
  • Type III secretion systems requires a large protein complex to ensure proper transfer of secretory molecules.

Key Terms

  • peptidoglycan: A polymer of glycan and peptides found in bacterial cell walls.
  • effector: a small molecule that effects additional molecules
  • bacterial conjugation: transfer of genetic material between bacterial cells by direct contact

In regards to pathogenecity, secretion in microorganisms such as bacterial species involves the movement of effector molecules from the interior of a pathogenic organism to the exterior. The secretion of specific molecules allows for adaptation to occur, thereby promoting survival. Effector molecules secreted include proteins, enzymes or toxins. The mechanisms by which pathogenic bacteria secrete proteins involve complex and specialized secretion systems. Specifically, Type III and Type IV secretion systems are utilized by gram-negative pathogenic bacteria to transport proteins that function as pathogenic components.

Type III Secretion Systems

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Type III Secretion System: The type III secretion system is characterized by the ability to inject secretory molecules into the host eukaryotic cell.

Type III secretion systems are characterized by the ability to inject a protein directly from the bacterial cell to the eukaryotic cell. It is often compared to the bacterial flagellar basal body which functions as a motor unit and extracellular appendage that is comprised of numerous proteins. The pathogenic bacteria which exhibit this capability contain a critical structural component, considered a protein appendage, that allows the injection of the protein into the host cell. The type III secretion system involves the formation of a complex, roughly ~20 proteins, that reside within the cytoplasmic membrane of the bacterial cell. The process of injecting or transferring the secretory protein from the bacterial cell to the host eukaryotic cell requires a membrane-associated ATPase. Certain species of pathogenic bacteria, including: Salmonella, Shigella, Yersinia and Vibrio exhibit type III secretion systems. The system is regulated by Ca2+ concentrations which regulate the opening and closing of gates present in the membrane by which the type III secretion system complexes can utilize for translocation. For example, in Salmonella, most commonly associated with Enteritis salmonellosis, or food poisoning, the bacteria injects a toxin, AvrA, that inhibits activation of the innate immune system of the host. The mechanism by which AvrA is injected involves exact and proper assembly of proteins which promote invasion of the host cell. Misalignment or improper organization of proteins involved in the type III secretion system prevent injection of secretory substances from the pathogen into the host cell. Another pathogen, Shigella, which utilizes type III secretion systems is able to successfully carry out its infection by evading the immune system. The movement between neighboring cells and evading the immune system, enhances its ability to inject its secretory protein into the host cell.

Type IV Secretion Systems

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Type IV Secretion System: Type IV secretion systems are characterized by the ability to transfer material using machinery similar to the bacterial conjugation machinery.

Type IV secretion systems are characterized by the ability to transfer secretory molecules via a mechanism similar to the bacterial conjugation machinery. The type IV secretion systems can either secrete or receive molecules. The bacterial conjugation machinery allows transfer of genetic material to occur via direct cell-to-cell contact or by a bridge-like apparatus between the two cells. The type IV secretion system utilizes a process similar to this. However, the exact mechanism(s) this process utilizes is unknown but there is a general understanding.

This specific secretion system can transport both DNA and proteins. An example of a pathogenic bacteria that utilizes the type IV secretion system is Helicobacter pylori. H. pylori, most commonly associated with stomach ulcers, attaches itself to epithelial cells within the stomach, then via a type IV secretion system, injects a secretory molecule. The secretory molecule injected into the epithelial cells is an inflammation-inducing agent derived from their own cellular wall. The secretory molecule, peptidoglycan, is recognized by the host system as a foreign substance and activates expression of cytokines which promotes an inflammatory response. This inflammatory response of the stomach is a key characteristic of individuals with ulcers. Peptidoglycan is not the only secretory molecule transferred to the stomach epithelial cells but additional proteins, such as CagA, which function in disruption of host cell cellular activities can be transferred as well.

Plasmids and Lysogeny

Both plasmids and lysogeny are used by bacteria and viruses to ensure transfer of genes and nucleic acids for viral reproduction.

Learning Objectives

Distinguish between plasmids and lysogeny in regards to pathogenecity

Key Takeaways

Key Points

  • Plasmids are double-stranded circular forms of ‘naked DNA ‘.
  • Plasmids are responsible for horizontal gene transfer which promotes the development of antibiotic resistance in bacterium.
  • Lysogeny is a major method of viral reproduction characterized by the integration of viral nucleic acids in the bacterium genome.

Key Terms

  • bacteriophage: A virus that specifically infects bacteria.
  • lysogeny: the process by which a bacteriophage incorporates its nucleic acids into a host bacterium

Plasmids

Plasmids are DNA molecules that are capable of replicating independently from the chromosomal DNA. Plasmids are often characterized by their circular appearance and double-strands; they also vary in size and number. Plasmids are present in the three major domains (Archaea, Bacteria and Eukarya) and are considered to be ‘naked DNA’. ‘Naked DNA’ refers to a specific type of DNA which does not encode for genes promoting the transfer of genetic material to a new host. The plasmids are present within the cells as extra chromosomal genomes and are a common tool used in molecular biology to integrate new DNA into a host. In the field of molecular biology, plasmid DNA is often referred to as ‘ vectors ‘ due to their ability to transfer DNA between organisms. The use of plasmid DNA in molecular biology is considered to be recombinant DNA technology. In addition, plasmid DNA provides a mechanism by which horizontal gene transfer can occur, contributing to antibiotic resistance.

Horizontal gene transfer is a major mechanism promoting bacterial antibiotic resistance, as the plasmid DNA can transfer genes from one species of bacteria to another. The plasmid DNA which is transferred often has developed genes that encode for resistance against antibiotics. The ability to transfer this resistance from one species to another is increasingly becoming an issue in clinics for treatment of bacterial infections. The process of horizontal gene transfer can occur via three mechanisms: transformation, transduction and conjugation. Plasmid DNA transfer is associated with conjugation as the host-to-host transfer requires direct mechanical transfer. The advantages of plasmid DNA transfer allow for survival advantages.

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Horizontal Gene Transfer: There are three mechanisms by which horizontal gene transfer can occur. Specifically, the exchange of plasmid DNA falls under transformation.

Lysogeny

Lysogeny is the process by which a bacteriophageintegrates its nucleic acids into a host bacterium’s genome. Lysogeny is utilized by viruses to ensure the maintenance of viral nucleic acids within the genome of the bacterium host. The virus displays the ability to infect the bacterium host and integrate its own genetic materials into the host bacterium genome. The bacteriophages newly integrated genetic material, called a prophage, is transferred to new bacterial daughter cells upon cell division. The prophage is integrated into the bacterium genome at this point. The lysogenic cycle is key to ensure the transmittance of bacteriophage nucleic acids to host bacterium’s genome. Lysogeny is one of two major methods of viral reproduction utilized by viruses.

Lysogenic cycles are utilized by specific types of viruses to ensure viral reproduction, but they also need the second major method of viral reproduction, the lytic cycle, as well. The lytic cycle, considered the primary method of viral replication, results in the actual destruction of the infected cell. Upon destruction of the infected cell, the new viruses, which have developed after undergoing biosynthesis and maturation, are free to infect other cells. The lytic cycle is characterized by the breakdown of the bacteria cell wall intracellularly. The viruses cause disruption of the bacterial cell by producing enzymes which facilitate this process. An example of a virus which can promote the transformation of bacterium from a nontoxic to toxic strain via lysogeny is the CTXφ virus. Specifically, the bacterium, Vibrio cholerae, is transformed into a toxic strain upon infection with the bacteriophage. This bacterium is then able to produce a cholera toxin, the cause of the disease cholera.

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Lysogenic and lytic cycles: Schematic of lysogenic and lytic cycle utilized by viruses to ensure viral reproduction.

Siderophores

Siderophores produce specific proteins and some siderophores form soluble iron complexes to aid in iron acquisition for survival.

Learning Objectives

Compare and contrast the role of various siderophores in pathogenecity, including: yersiniabactin, enterobactin and ferrichromes

Key Takeaways

Key Points

  • Siderophore – iron complexes are necessary for iron acquisition to various pathogenic organisms for metabolic processes.
  • The types of siderophores produced are species specific and exhibit different properties.
  • The siderophores are necessary to obtain iron by binding to cell surfaces and transporting the siderophore-iron complexes intracellularly.
  • Siderophores are produced in environments that have low iron concentration, such as host tissues and fluids. They are considered advantageous to pathogenic organisms.

Key Terms

  • siderophore: Any medium-sized molecule that has a high specificity for binding or chelating iron; they are employed by microorganisms to obtain iron from the environment
  • chelating agent: A compound that reacts with a metal ion to produce a chelate.

Siderophores are specific types of molecules utilized by microorganisms to obtain iron from the environment. Specifically, in regards to pathogenicity, organisms that exhibit the ability to produce siderophores release these iron-specific molecules and scavenge iron from their hosts organisms. The siderophores are then utilized by the pathogen to obtain iron. Therefore, siderophores are chelating agents that bind the iron ions. The ability of pathogens to obtain iron from the host is essential for survival because the iron is limited in the host environment, in particular, the host tissues and fluids. The iron is used to allow for formation of soluble ferric ion (Fe3+) complexes that are necessary for maintenance of homeostatic mechanisms within the pathogen.

The ability to form water soluble Fe3+ complexes is a key component to the active transport of the Fe-siderophore complex across the cellular membrane. In iron deficient environments, the siderophores are released and allow for the formation of water soluble-Fe3+ complexes to increase iron acquisition. The complexes then generally bind to the cellular membrane using cell specific receptors. They are transported across the membrane utilized for the necessary processes. However, there are differences in the mechanisms employed by various sideorophobes to obtain iron and the specific type of siderophore utilized varies.

Yersiniabactin

The pathogenic bacteria, Yersinia pestis, Yersinia pseduotuberculosis, and Yersinia enterocolitica have the ability to produce a siderophore called yersiniabactin. Pathogenic yersinia is responsible for numerous diseases including the bubonic plague. The ability of pathogenic Yersinia to establish and spread disease is based on its ability to obtain iron for fundamental cellular processes. In areas of low iron, the organism will release yersiniabactin to form Fe3+ complexes. The yersiniabactin-Fe3+ complex will then bind to the outer membrane of the bacteria based on specific receptor recognition. The complex is then translocated through the membrane via membrane-embedded proteins and iron is released from the yersiniabactin. The iron will then be utilized in numerous cellular processes.

Enterobactin

Pathogenic bacteria such as Escherichia coli and Salmonella typhimurium have the ability to produce a siderophore called enterobactin. This specific type of siderophore is the strongest identified siderophore, to date, with an extremely high binding affinity to Fe3+. Upon a decrease in iron, the bacterial cells release enterobactin which forms a complex with Fe3+. The complex is then transported intracellularly via an ATP-binding cassette transporter. Once the enterobactin-Fe3+ complex arrives intracellularly, it is necessary to remove the Fe3+ from the complex. Due to the high-binding affinity of enterobactin, the bacteria require a highly specific enzyme, ferrienterobactin esterase, to cleave the iron from the complex. The iron released from the complex will then be utilized in metabolic processes.

Ferrichrome

Another type of siderophore produced by pathogenic fungi includes a ferrichrome. Fungi that have been shown to produce ferrichromes include those in the genera Aspergillus, Ustilago, and Penicillum. The ferrichrome allows for formation of a ferrichrome-iron complex which can then interact with a protein receptor on the cell surface. The ferrichrome promotes iron transport within the organism to allow metabolic processes to occur.

The discovery and identification of siderophores have allowed for the development of treatments targeting these siderophore-iron complexes. By targeting these complexes, the pathogenic microorganisms can be targeted by inhibiting necessary cellular processes. The production and importance of these siderophores to pathogenic organisms is key to their survival.