Infectious Diseases: Bacteria, Viruses, and Prions

Overview of Microbes and Pathogens

Microbes are microscopic organisms that are widely distributed throughout the environment. They can be found on inanimate objects as well as on and within living things. This category includes a variety of organisms such as bacteria, viruses, and protists. It is important to note that many microbes are highly beneficial to human life. For example, bacteria are essential in the production of various food items, including yogurt, cheese, bread, beer, wine, and pickled foods. Furthermore, certain drugs are produced using bacteria. From an ecological perspective, the biosphere would cease to exist without the presence of decomposers, which are often microbial in nature.

Pathogens are classified specifically as disease-causing agents. This group includes bacteria, viruses, and other infectious entities. To protect against these threats, the human body employs several layers of defense. The primary defense consists of physical and chemical barriers designed to prevent pathogens from entering the body; examples include the skin and mucous membranes. If a pathogen successfully bypasses these initial barriers, phagocytic white blood cells (WBCs) act to fight the infection. Additionally, the body utilizes acquired defenses which are capable of killing specific pathogens and providing protection against the development of cancer.

Bacterial Classification and Morphology

Bacteria are single-celled prokaryotic organisms, meaning they lack a nucleus and membrane-bound organelles. They are primarily classified according to differences in their cell walls, which are identified using a technique known as the Gram stain. Bacteria with cell walls containing a thick layer of peptidoglycan retain a purple color when stained and are termed Gram-positive bacteria. Conversely, if a bacteria lacks this thick peptidoglycan layer, it stains pink and is categorized as Gram-negative. Gram-negative bacteria possess an outer membrane containing lipopolysaccharides. These substances are released when the bacterial cells are destroyed by the immune system, often stimulating symptoms such as fever and inflammation.

There are three common morphological shapes observed in bacteria. The first is the coccus, which is spherical in shape. The second is the bacillus, which is rod-shaped. The third is the spirillum, which appears as a curved or spiral shape.

Basic Structure of Bacteria

All bacterial cells are enclosed by a plasma membrane, which is a lipid bilayer similar in structure to the plasma membranes found in plant and animal cells. Most bacteria are further protected by a rigid cell wall composed of a unique molecule called peptidoglycan. Peptidoglycan is chemically defined as a disaccharide with an attached amino group. In addition to the cell wall, some bacterial species are surrounded by a capsule, which has a thick, gelatinous consistency providing further protection.

The genetic material of bacteria is packaged into a single, circular chromosome. Many bacteria also contain smaller, independent circular pieces of DNA known as plasmids. Genes that provide bacteria with the ability to resist antibiotics are frequently located within these plasmids. For movement, motile bacteria typically use long, thin appendages called flagella (singular: flagellum). Some bacteria also have fimbriae, which are stiff fibers that allow the cell to adhere to surfaces, such as host cells. Furthermore, bacteria may possess a pilus, an elongated and hollow appendage used for the transfer of DNA between cells. This process, known as conjugation, allows genes—including those for antibiotic resistance—to be passed from one bacterial cell to another.

Bacterial Toxins and Reproduction

Certain bacteria produce and release molecules called toxins, which function by inhibiting the cellular metabolism of the host. A notable example is Clostridium tetani, the causative agent of tetanus. This bacterium produces a toxin that prevents muscle relaxation, leading to the condition colloquially known as lockjaw. This is why receiving a tetanus shot is medically critical.

Bacteria reproduce through an asexual process called binary fission. During this process, the single circular chromosome, which is attached to the plasma membrane, is copied. As the cell enlarges, the two chromosomes are separated, and the cell eventually divides into two identical daughter cells. Under favorable environmental conditions, some bacteria are capable of doubling their population size every $20$ minutes. Common bacterial diseases include strep throat, tuberculosis, gangrene, gonorrhea, and syphilis.

Characteristics and Structure of Viruses

Viruses are distinct from bacteria in that they are acellular, meaning they are not composed of cells, and they lack their own metabolic machinery. They are considered obligate intracellular parasites because they must hijack a host cell's machinery in order to reproduce. Pathogenic viruses are significantly smaller than bacteria, measuring approximately $\frac{1}{100}$ the size of a typical eukaryotic cell.

A virus always consists of at least two basic parts. The first is an outer capsid, which is a protective shell composed of protein subunits. The second part is an inner core containing nucleic acids, which can be either DNA or RNA. This genetic core contains the instructions necessary for viral reproduction.

The Viral Life Cycle: Lytic and Lysogenic

The life cycle of a virus, such as a bacteriophage, typically follows a sequence of five steps in the lytic cycle. The first step is Attachment, where the viral capsid combines with a specific receptor on the surface of the host cell. The second step is Penetration, where a viral enzyme may digest a portion of the cell wall to inject viral genetic material, or the entire virus may enter via endocytosis. The third step is Biosynthesis, during which the host cell's machinery is forced to replicate the viral genetic material and produce copies of the capsid protein subunits. The fourth step is Maturation (also known as Assembly), where the new genetic material and capsids combine to form several new viruses. Finally, the fifth step is Release, where new viruses exit the host cell through exocytosis or by disrupting (lysing) the cell membrane or wall.

In some instances, viruses enter a lysogenic cycle where the infected cell does not immediately produce new viruses. Instead, the virus becomes latent. Following attachment and penetration, Integration occurs, where the viral DNA is incorporated into the host cell's DNA. This viral information is سپس replicated along with the host's DNA, meaning all subsequent descendant cells, called lysogenic cells, carry the viral sequence. Certain environmental triggers can later induce these cells to enter the lytic cycle (biosynthesis, maturation, and release).

Prions and Neurodegenerative Diseases

Prions are unique infectious particles because they are composed entirely of proteins and contain no nucleic acids. They are responsible for a group of degenerative diseases of the nervous system, which are often referred to as wasting diseases. These diseases are characterized by the progressive destruction of brain and nerve tissue.

Specific examples of prion-related diseases include Creutzfeldt–Jakob disease (CJD) in humans, Scrapie in sheep, and Bovine spongiform encephalopathy (BSE), which is commonly known as Mad cow disease, in cattle. These infections are typically transmitted to humans or other animals through the ingestion of brain and nerve tissues from infected animals.