Fundamentals of Microbiology

Lesson 3. VIROLOGY AND MEDICAL MICROBIOLOGY

Module 1. History and scope of microbiology

Lesson 3

VIROLOGY AND MEDICAL MICROBIOLOGY

3.1 Virology

Viruses are very small, infectious, obligate intracellular molecular parasites, which do not respire, move or grow. The virus genome is composed either of DNA or RNA and directs the viral replication by the synthesis of virion components within an appropriate host cell.

3.1.1 History

One of the first written records of a virus infection consists of a heiroglyph from Memphis, the capital of ancient Egypt, drawn in approximately 1400 B.C, which depicts Siptah. Judging from his mummy, he died at about 20 years of age. The body’s deformed left leg suggests that Siptah suffered from a neuromuscular disease (poliomyelitis). The generally recognized beginning of virology is a paper presented to the St. Petersburg Academy of Science on the 12th February, 1892 by Dmitri Iwanowski (1864-1920), a Russian botanist. He showed that extracts from diseased tobacco plants could transmit disease to other plants after passage through ceramic filters fine enough to retain the smallest known bacteria. Six years later in Holland, Martinus Beijernick (1851-1931) confirmed Iwanowski's results on tobacco mosaic virus. He gave the term ‘contagium vivum fluidum’ ('soluble living germ') as the first idea of virus. Agents that pass through filters that retain bacteria came to be called ultra-filterable viruses, appropriating the term virus from the Latin for ‘poison’. During the same time, the German scientists Friedrich Loeffler (1852-1915) and Paul Frosch, both former students and assistants of Robert Koch (1843-1910), observed that a similar agent was responsible for foot and mouth disease. In spite of these findings, there was resistance to the idea that these mysterious agents might have anything to do with human diseases. Bacterial viruses were first described by Frederick Twort (1915) and Felix d'Hérelle (1917). D'Hérelle named them bacteriophages because of their ability to lyse bacteria on the surface of agar plates. Following this, many scientists utilized these viruses as model systems to investigate many aspects of virology, including virus structure, genetics, and replication.

3.1.2 Virus structure

The capsid (coat) protein (Fig. 3.1) is the basic unit of structure; functions that may be fulfilled by the capsid protein are to:

• Protect viral nucleic acid
• Interact specifically with the viral nucleic acid for packaging
• Interact with vector for specific transmission
• Interact with host receptors for entry to cell
• Allow for release of nucleic acid upon entry into new cell
• Assist in processes of viral and/or host gene regulation

Nucleoprotein has two basic structure types

1. Helical: Rod shaped, varying widths and specific architectures; no theoretical limit to the amount of nucleic acid that can be packaged.
2. Cubic (Icosahedral): Spherical, amount of nucleic acid that can be packaged is limited by the particle.

Fig. 3.1 Structure of Viruses

3.2 Medical Microbiology

Medical microbiology is the study of parasites, fungi, bacteria, and viruses that are the agents of infectious disease in humans. Modern medicine relies on the control of microorganisms to maintain human health and quality of life. The divisions of medical microbiology include bacteriology, the study of bacteria that inhabit and/or colonize the human body and cause disease; mycology, the study of fungi as causative agents of human disease; parasitology, the formal study of the human parasitic organisms (protozoans, helminths, nematodes, trematodes and arthropods); and virology, the study of viruses that cause infectious syndromes in humans. Sizes for the pathogens considered include the smallest, viruses (50-100 nm), bacteria that range from 0.1 μm (Chlamydiae) to 10μm (Bacillus rods), fungi ranging from ~8 μm (yeasts) up to 10 mm in size (filamentous fungi) and metazoan parasites that are visible to the naked eye. Medical microbiology as a discipline requires a working knowledge of human anatomy and histology, and a comprehension of the pathologies associated with the infectious disease process. The human immune response to pathogens is key to the consideration of infectious disease. Understanding the relationship between pathogens and antimicrobial pharmacology is essential as well. Microbiology places information about pathogenic organisms and their specific characteristics within the context of host disease. Developing connections between microbiology and immunology will make learning more effective in both disciplines.

3.2.1 Epidemiology

It is the study of disease patterns and trends, of the occurrence, distribution and control of disease in populations. It also deals with disease tracking and prevention. Disease transmission is the movement of the infectious agent from one host to another. The risk of infection is dependent not just upon an individual patient's susceptibility, but other factors such as on the level of disease within the population, the extent of population mixing and ‘herd immunity’, the specific features of disease spread (such as communicable period, route and ease of transmission).

3.2.2 Modes of transmission

Numerous modes of transmission both direct and indirect contribute to the spread of human disease. For an infectious agent to persist within a population a cycle of transmission must be established leading from a contaminated source to a susceptible host and further propagating through the population.

Direct infection is the movement of a pathogen from human to human. Examples of organisms causing direct infection are Salmonella typhi and Shigella species, both of which infect humans only.

Indirect transmission involves human to soil, water or surface dissemination of pathogens and infection of another human through contact with contaminated materials. This idea encompasses vehicle-borne transmission, whether through infected everyday objects or surgical instruments. Vibrio cholera is an example of a pathogen that is transmitted through contaminated water, especially in brakish coastal waters.

Zoonotic transmission occurs through exposure to a nonhuman animal source of infection. The arthropod-borne agents like Borrelia and Rickettsia are included here as well as Spirillum, Brucella and Bacillus anthracis, that are contracted through animal bites or contact with animal products.
Elimination is a potential outcome for a healthy host, whereby the parasite is eradicated at the end of infection cycle.

3.2.3 Routes of transmission

The direct route means physical contact between humans or between a human and an animal to cause disease. Portals of entry include the gastrointestinal tract, respiratory mucosa, genital mucosa, and direct inoculation through the skin. Mucous membranes are especially important (STDs are transmitted in this way). The airborne route or respiratory droplet transmission is very important for viral pathogens and respiratory tract infections (aerosols). Fomites are inanimate objects contaminated with microorganisms, like drinking cups, towels and computer keyboards. The water and food borne route is an especially important for enteric disease and it’s an important route for fecal-oral transmission (ingestion). Vector borne transmission is critical for some viral (arbovirus) and zoonotic infections (arthropod borne parasites).

A list of pathogens and related diseases is provided in Table 3.1.

Table 3.1 Pathogens and related disease