Bacteria, viruses and infection

What you’ll learn

• Compare bacterial and viral structures.
• Distinguish lytic infection and latency.
• Explain the biological effects of tuberculosis and HIV.

Understand the idea

Bacteria are cells with a membrane, cytoplasm, ribosomes and DNA; many have a cell wall and plasmids. They can carry out metabolism and reproduce by binary fission. Viruses are acellular: a DNA or RNA genome is enclosed by a protein capsid, sometimes with a lipid envelope and surface attachment proteins. They depend on host-cell machinery for replication. Their small size alone is less informative than these cellular differences.

Tobacco mosaic virus has a helical protein coat around RNA and lacks an envelope. HIV has an RNA genome, capsid and envelope, and uses reverse transcription to make a DNA copy. Ebola virus is an enveloped, filamentous RNA virus. Lambda bacteriophage has a DNA-containing head and a tail that attaches to bacteria. In a lytic cycle, new viral particles are made and released with host-cell destruction; in latency a viral genome persists without immediate production of many particles. Lambda DNA may integrate into the host genome.

Mycobacterium tuberculosis usually enters through inhaled droplets and can persist inside macrophages; the immune response forms granulomas that contain infection but may damage lung tissue. Active disease impairs gas exchange and can spread. HIV targets cells with suitable receptors, including helper T cells; progressive loss of immune coordination increases vulnerability to opportunistic infections. A period with few symptoms does not mean the viral genome is absent.

Worked example

An infectious particle contains RNA, a protein capsid and a lipid envelope but no ribosomes. Explain why it cannot reproduce independently like a bacterium.

1. No ribosomes means the particle cannot translate its genome into proteins using its own machinery.
2. It also lacks the complete cellular metabolic machinery needed to supply and organise replication.
3. It must enter a suitable host cell and redirect host resources to make viral components.

Answer: Its acellular structure makes replication dependent on a living host cell; the described features are consistent with an enveloped RNA virus.