Viruses, Viroids, and Prions: Structure, Classification, and Comparison with Bacteria

General Characteristics of Viruses

Obligatory Intracellular Parasites

Viruses are unique infectious agents that require living host cells to multiply. Unlike most other microorganisms, they cannot reproduce or carry out metabolic processes independently.

• Obligatory intracellular parasites: Viruses must invade a host cell to replicate.

• Genetic material: Viruses contain either DNA or RNA, but never both.

• Protein coat: The genetic material is enclosed within a protein coat called a capsid.

• Lack of ribosomes: Viruses do not possess ribosomes and cannot synthesize proteins independently.

• No ATP-generating mechanism: Viruses lack the machinery to generate energy (ATP).

• Note: Some bacteria, such as Chlamydia and Rickettsia, also require living host cells to multiply, but they differ fundamentally from viruses.

Viruses and Bacteria Compared

Key Differences Between Viruses and Bacteria

Viruses and bacteria are both microscopic agents of infection, but they differ significantly in structure, metabolism, and replication.

Feature	Typical Bacteria	Rickettsias/Chlamydias	Viruses
Intracellular Parasite	No	Yes	Yes
Plasma Membrane	Yes	Yes	No
Binary Fission	Yes	Yes	No
Pass through Bacteriological Filters	No	No/Yes	Yes
Possess Both DNA and RNA	Yes	Yes	No
ATP-Generating Metabolism	Yes	Yes/No	No
Ribosomes	Yes	Yes	No
Sensitive to Antibiotics	Yes	Yes	No
Sensitive to Interferon	No	No	Yes

Additional info: Interferons are proteins produced by host cells in response to viral infection and can inhibit viral replication.

Host Range of Viruses

Specificity and Spectrum of Infection

The host range of a virus refers to the spectrum of host cells it can infect. Most viruses are highly specific, infecting only certain cell types within one host species.

• Determined by: Specific host attachment sites (receptors) and cellular factors.

• Examples:

  • HIV infects CD4+ T cells.

  • Hepatitis C infects hepatocytes via LDL receptors.

• Bacteriophages: Viruses that infect bacteria; also called "phages."

• Size range: Viruses range from 20 nm to 1000 nm in length.

Additional info: The specificity of viral infection is a key factor in disease transmission and pathogenesis.