Question

10. Consider a mutant strain of a pathogen that is unable to produce siderophores. What

likely impact would this have on the mutant pathogen during an infection?

11. Describe some general damaging effects of an infection.

12. Define, compare, and contrast exotoxins and endotoxins.

13. Describe the three classes of exotoxins and their mode of action.

Answer 1

A. 10.

Iron is essential for almost all living organisms, including bacteria. The ability of pathogenic bacteria to acquire iron in hosts is absolutely essential for bacterial growth and infection. In animal hosts, iron is usually bound to proteins such as transferrin and lactoferrin in extracellular fluid and to ferritin, hemoglobin, and heme-containing enzymes in cells. To utilize such complexes as iron sources, bacteria generally possess some sophisticated mechanisms, which include an iron uptake system mediated by high-affinity iron chelators called siderophores. However, mutated pathogens, who have lost the ability to produce sideropohores, would not survive in host due to inability to acquire iron and hence would be unable to produce infection.

A. 11. An infection happens when a foreign organism enters a person's body and causes harm. Different types of bacteria, viruses, fungi, protozoa, parasites, and prions cause infections.

Depending on the infection, the body can be affected minimally or severely. In a viral cold infection, the body may feel general discomfort and illness for a short period of time. The Epstein-Barr viral infection, however, can cause the body to become extremely exhausted, which can last for months in some cases. Other infections can cause fevers, pain, and weight loss.

Pathogenic microbes challenge the immune system in many ways. Viruses make us sick by killing cells or disrupting cell function.Some viruses target skin cells, causing warts. Others target a wider range of cells, leading to various symptoms. A flu virus can cause a runny nose, muscle aches, and an upset stomach.

Sometimes bacteria multiply so rapidly they crowd out host tissues and disrupt normal function. Sometimes they kill cells and tissues outright. Sometimes they make toxins that can paralyze, destroy cells’ metabolic machinery, or precipitate a massive immune reaction that is itself toxic. A person with a bacterial infection will often experience redness and heat, swelling, fever, pain at the site of infection, and swollen lymph glands.

A rash can be an indicator of a fungal infection of the skin.

Common symptoms of prion diseases include brain damage, memory loss, and cognitive difficulties. They can also trigger the buildup of plaque in the brain, causing it to waste away.

A.12

Exotoxins are toxic substances secreted by bacteria and released outside the cell. Whereas Endotoxins are bacterial toxins consisting of lipids that are located within a cell. Following are the few differences between exotoxin and endotoxin.

Characteristics

Exotoxins

Endotoxins

Source

Living gram positive and gram negative bacteria

Lysed gram negative bacteria

Location

Released from the cell

Part of cell

Chemical Composition

Protein

Lipopolysaccaride

Heat Sensitivity

Liable (60-80C)

Stable (250C)

Immune Reactions

Strong

Weak

Conversion to Toxoids

Possible

Not possible

Fever

No

Yes

Enzyme Activity

It has mostly enzymatic activity.

It has no enzymatic activity.

Molecular Weight

Its molecular weight is 10KDa.

Its molecular weight is 50-1000KDa.

Denaturing

On boiling it get denatured.

On boiling it cannot be denatured.

Specificity

Specific to particular bacterial strain

Non specific

Antigencity

High

Poor

Examples

Staphylococcus aureus, Bacillus cereus, Streptococcus pyogenes, Vibrio cholera, Bacillus anthrcis.

E.coli, Salmonella typhi, Shigella.

A. 13.

Type I: cell surface-active

Type I toxins bind to a receptor on the cell surface and stimulate intracellular signaling pathways. Two examples are described below.

Superantigens

Superantigens are produced by several bacteria. The best-characterized superantigens are those produced by the strains of Staphylococcus aureus and Streptococcus pyogenesthat cause toxic shock syndrome. Superantigens bridge the MHC class II protein on antigen-presenting cells with the T cell receptor on the surface of T cells with a particular V? chain. As a consequence, up to 50% of all T cells are activated, leading to massive secretion of proinflammatory cytokines, which produce the symptoms of toxic shock.

Heat-stable enterotoxins

Some strains of E. coli produce heat-stable enterotoxins (ST), which are small peptides that are able to withstand heat treatment at 100 °C. Different STs recognize distinct receptors on the cell surface and thereby affect different intracellular signaling pathways. For example, STa enterotoxins bind and activate membrane-bound guanylate cyclase, which leads to the intracellular accumulation of cyclic GMP and downstream effects on several signaling pathways. These events lead to the loss of electrolytes and water from intestinal cells.

Type II: membrane damaging

Membrane-damaging toxins exhibit hemolysin or cytolysin activity in vitro. However, induction of cell lysis may not be the primary function of the toxins during infection. At low concentrations of toxin, more subtle effects such as modulation of host cell signal transduction may be observed in the absence of cell lysis. Membrane-damaging toxins can be divided into two categories, the channel-forming toxins and toxins that function as enzymes that act on the membrane.

Channel-forming toxins

Most channel-forming toxins, which form pores in the target cell membrane, can be classified into two families: the cholesterol-dependent toxins and the RTX toxins.

Cholesterol-dependent cytolysins

Formation of pores by cholesterol-dependent cytolysins (CDC) requires the presence of cholesterol in the target cell. The size of the pores formed by members of this family is extremely large: 25-30 nm in diameter. All CDCs are secreted by the type II secretion system; the exception is pneumolysin, which is released from the cytoplasm of Streptococcus pneumoniae when the bacteria lyse.

The CDCs Streptococcus pneumoniae Pneumolysin, Clostridium perfringens perfringolysin O, and Listeria monocytogenes listeriolysin O cause specific modifications of histones in the host cell nucleus, resulting in down-regulation of several genes that encode proteins involved in the inflammatory response.Histone modification does not involve the pore-forming activity of the CDCs.

RTX toxins

RTX toxins can be identified by the presence of a specific tandemly repeated nine-amino acid residue sequence in the protein. The prototype member of the RTX toxin family is haemolysin A (HlyA) of E. coli. RTX is also found in Legionella pneumophila.

Enzymatically active toxins

One example is the ? toxin of C. perfringens, which causes gas gangrene; ? toxin has phospholipase activity.

Type III: intracellular

Type III exotoxins can be classified by their mode of entry into the cell, or by their mechanism once inside.

By mode of entry

Intracellular toxins must be able to gain access to the cytoplasm of the target cell to exert their effects.

Some bacteria deliver toxins directly from their cytoplasm to the cytoplasm of the target cell through a needle-like structure. The effector proteins injected by the type III secretionapparatus of Yersinia into target cells are one example.
Another group of intracellular toxins is the AB toxins. The 'B'-subunit (binding) attaches to target regions on cell membranes, the 'A'-subunit (active) enters through the membrane and possesses enzymatic function that affects internal cellular bio-mechanisms. A common example of this A-subunit activity is called ADP-ribosylation in which the A-subunit catalyzes the addition of an ADP-ribose group onto specific residues on a protein. The structure of these toxins allows for the development of specific vaccines and treatments. Certain compounds can be attached to the B unit, which is not, in general, harmful, which the body learns to recognize, and which elicits an immune response. This allows the body to detect the harmful toxin if it is encountered later, and to eliminate it before it can cause harm to the host. Toxins of this type include cholera toxin, pertussis toxin, Shiga toxin and heat-liable enterotoxin from E. coli.

By mechanism

Once in the cell, many of the exotoxins act at the eukaryotic ribosomes (especially 60S), as protein synthesis inhibitors. (Ribosome structure is one of the most important differences between eukaryotes and prokaryotes, and, in a sense, these exotoxins are the bacterial equivalent of antibiotics such as clindamycin.)

Some exotoxins act directly at the ribosome to inhibit protein synthesis. An example is Shiga toxin.
Other toxins act at elongation factor-2. In the case of the diphtheria toxin, EF2 is ADP-ribosylated and becomes unable to participate in protein elongation, and, so, the cell dies. Pseudomonas exotoxin has a similar action.

Other intracellular toxins do not directly inhibit protein synthesis.

For example, Cholera toxin ADP-ribosylates, thereby activating tissue adenylate cyclase to increase the concentration of cAMP, which causes the movement of massive amounts of fluid and electrolytes from the lining of the small intestine and results in life-threatening diarrhea.
Another example is Pertussis toxin.

Extracellular matrix damage

These "toxins" allow the further spread of bacteria and, as a consequence, deeper tissue infections. Examples are hyaluronidase and collagenase. These molecules, however, are enzymes that are secreted by a variety of organisms and are not usually considered toxins. They are often referred to as virulence factors, since they allow the organisms to move deeper into the hosts tissues.