Viruses

General Properties and Structure of Viruses

Viruses are acellular infectious agents composed of nucleic acid (DNA or RNA) enclosed in a protein coat called a capsid. Some viruses also possess an outer lipid envelope derived from the host cell membrane.

• Viral Examples: Viruses vary in shape (helical, icosahedral, complex) and size. Examples: Influenza (orthomyxoviruses), HIV (retrovirus), Herpesvirus (enveloped DNA virus).

• Structure: All viruses contain a nucleic acid genome and a protein capsid; some have an envelope with glycoprotein spikes.

Viral Replication Strategies

Viruses replicate by infecting host cells and hijacking their machinery to produce new viral particles. The main steps include:

• Attachment

• Penetration

• Uncoating

• Replication (genome and protein production)

• Assembly

• Release (by cell budding or lysis)

Viral Taxonomy and Classification

Viruses are classified based on their genetic type (DNA or RNA, single or double-stranded), capsid configuration, and envelope presence.

• Example: Table 5-3 (not shown) provides a summary of viral families and their characteristics.

Multiplication Cycle of Animal Viruses

Animal viruses must fuse with the endocytosis or membrane fusion. The multiplication cycle includes:

• Attachment to specific host receptors

• Entry and uncoating

• Assembly of new virions

• Release by budding (enveloped viruses) or lysis (non-enveloped viruses)

Potential Effects of Viral Infection

Viral infections can cause cell death, persistent infection, or transformation (leading to cancer). The cytopathic effect (CPE) results in visible changes in host cells due to viral infection.

• Examples of CPE: Cell rounding, detachment, syncytia formation, inclusion bodies.

Viruses and Cancer

Some viruses are oncogenic, meaning they can cause cancer by integrating their genome into host DNA and disrupting normal cell regulation.

• Examples: Human papillomavirus (HPV) and cervical cancer; Epstein-Barr virus (EBV) and Burkitt's lymphoma.

Viral Diseases: Classification and Examples

Viruses are grouped into families based on genome and structural similarities. Key families include:

• Herpesviridae: Includes HSV-1, HSV-2, HSV-3 (varicella-zoster virus).

• Retroviridae: Includes HIV.

• Paramyxoviridae: Measles, mumps, RSV.

Coronaviruses

Coronaviruses are enveloped, positive-sense single-stranded RNA viruses. They cause respiratory and systemic diseases and are responsible for severe illnesses like SARS, MERS, and COVID-19.

• Transmission: Primarily via respiratory droplets.

• Structure: Spike (S) glycoproteins, envelope (E), membrane (M), nucleocapsid (N).

Microbial Growth

Elements Required for Microbial Growth

Microbes require various elements for growth, including carbon, nitrogen, sulfur, phosphorus, energy, and trace elements.

• Autotroph: Uses CO2 as a carbon source.

• Heterotroph: Uses organic carbon.

• Phototroph: Uses light as an energy source.

• Chemotroph: Uses chemical compounds for energy.

Oxygen Requirements

Microbes are classified by their oxygen requirements:

• Obligate aerobes: Require oxygen.

• Obligate anaerobes: Cannot survive oxygen.

• Facultative anaerobes: Can grow with or without oxygen.

• Microaerophiles: Require low oxygen levels.

• Aerotolerant anaerobes: Tolerate but do not use oxygen.

Temperature Requirements

Microbes are also classified by their temperature preference:

• Psychrophiles: Cold-loving.

• Mesophiles: Moderate temperatures.

• Thermophiles: High temperatures.

• Extreme thermophiles: Very high temperatures.

Growth Curve and Bacterial Reproduction

Bacteria reproduce by binary fission. The bacterial growth curve includes:

• Lag phase

• Log (exponential) phase

• Stationary phase

• Death phase

Bacterial numbers can be measured by turbidity, direct counts (hemocytometer), and plating for colony-forming units (CFU).

Control of Microbial Growth

Outcomes and Methods

Microbial control can result in sterilization, disinfection, antisepsis, or sanitization. Methods include physical (heat, filtration, radiation) and chemical (disinfectants, antiseptics) approaches.

Physical Methods

• Heat: Moist heat (autoclaving, boiling) and dry heat (oven).

• Filtration: Removes microbes from liquids or air.

• Radiation: UV or ionizing radiation damages DNA.

Chemical Methods

• Halogens: Chlorine, iodine.

• Phenolics: Disrupt cell membranes.

• Alcohols: Denature proteins, disrupt membranes.

• Surfactants: Soaps, detergents.

• Heavy metals: Silver, mercury (less common now).

Disinfectants and Antiseptics

Disinfectants are used on inanimate objects; antiseptics are safe for living tissues. Effectiveness depends on concentration, contact time, and presence of organic matter.

Antimicrobial Chemotherapy

Therapeutic Index

The therapeutic index is the ratio of the toxic dose to the effective dose of a drug. A higher therapeutic index indicates a safer drug.

• Formula:

Mechanisms of Action of Antibiotics

Antibiotics target specific bacterial processes:

• Inhibition of cell wall synthesis: Beta-lactams (penicillins, cephalosporins).

• Inhibition of nucleic acid synthesis: Fluoroquinolones, rifampin.

• Inhibition of protein synthesis: Tetracyclines, aminoglycosides.

• Disruption of cell membrane: Polymyxins.

• Inhibition of metabolic pathways: Sulfonamides, trimethoprim.

Antifungal and Antiviral Agents

Antifungal drugs target ergosterol in fungal membranes (e.g., amphotericin B). Antiviral drugs inhibit viral replication (e.g., acyclovir for herpesviruses).

Synergism and Antagonism

Synergism occurs when two drugs work better together than alone; antagonism is when one drug reduces the effect of another. Combinations may be used to prevent resistance or enhance efficacy.

Table: Comparison of Physical and Chemical Methods of Microbial Control

Additional info: Some details, such as specific viral examples and mechanisms, were inferred based on standard microbiology curriculum. Table 5-3 and some figures referenced in the original notes were not provided; content was supplemented with typical textbook information.