Bacteria, viruses and infectionEdexcel International A Level Biology: Revision notes

Bacteria are prokaryotic cells, about 1 to 5 µm long. A typical bacterium has a cell wall of peptidoglycan, a plasma membrane, cytoplasm, 70S ribosomes, and a circular chromosome of DNA, often with small circular plasmids. They have no nucleus or membrane-bound organelles, can carry out metabolism, and reproduce independently by binary fission.

Viruses are acellular and much smaller (about 20 to 300 nm). A virus has a nucleic acid genome, which is DNA or RNA and is single- or double-stranded, surrounded by a protein coat called a capsid, made of subunits called capsomeres. Some viruses also have an envelope, a lipid bilayer taken from the host cell membrane, with viral glycoproteins. Viruses have no cytoplasm, ribosomes or metabolism, so they can only reproduce inside host cells, which they use to make new viruses.

• Ebola virus: an enveloped, filamentous virus with a single strand of RNA enclosed in a helical capsid. Glycoproteins in the envelope allow it to bind to host cells.
• Tobacco mosaic virus (TMV): a rod-shaped virus with a single strand of RNA and a helical capsid of identical protein subunits. It has no envelope and infects plants.
• HIV: an enveloped virus, roughly spherical, with two copies of single-stranded RNA and the enzyme reverse transcriptase inside a cone-shaped capsid. Glycoproteins in the envelope bind to CD4 receptors.
• Lambda phage: a bacteriophage, a virus that infects bacteria. It has a head (capsid) containing double-stranded DNA, and a tail with tail fibres that attach to the bacterial surface. It has no envelope.

Viruses replicate in a host cell. In the lytic cycle (shown by lambda phage), the phage attaches by its tail fibres to receptors on the bacterial surface and injects its DNA. The host cell's enzymes and ribosomes replicate the viral DNA and make new capsid proteins, new phages assemble, and the cell bursts (lysis), releasing them.

In latency the viral genome is inserted into the host's DNA and replicated with it when the host cell divides, while no new viruses are made. The inserted DNA is a prophage in a bacterium and a provirus in HIV. Because no viral proteins are made, the infected cell is not recognised by the immune system. A change in conditions, such as ultraviolet light, can trigger the DNA to leave the chromosome and start the lytic cycle.

A glycoprotein in the HIV envelope binds to the CD4 receptor on a helper T cell (and on macrophages). The envelope fuses with the cell membrane and the capsid enters and releases the RNA. Reverse transcriptase makes a DNA copy of the RNA, which is inserted into the host's DNA as a provirus and may remain latent for years. When it is active, the host cell makes viral RNA and proteins, and new viruses assemble and leave by budding, taking a section of the cell membrane as their envelope.

HIV destroys helper T cells, so their number falls and the immune response (B cells, cytotoxic T cells, macrophages) weakens. After a long latent period the patient has AIDS: immune deficiency and the symptoms of opportunistic infections and some cancers, such as pneumonia and tuberculosis. HIV itself causes few symptoms.

Mycobacterium tuberculosis is spread in droplets that are inhaled. In the alveoli the bacteria are engulfed by macrophages. Their waxy cell wall, rich in mycolic acid, resists digestion by lysosomal enzymes, so they survive and multiply inside the macrophages. The immune system responds by surrounding the infected cells with more macrophages and lymphocytes, forming a tubercle that walls off the bacteria. In many people the bacteria stay dormant (latent) in the tubercles.

If the immune system is weakened (for example by HIV), the tubercles break down, the bacteria spread, and lung tissue is destroyed. Symptoms include a persistent cough with blood in the sputum, weight loss, fever and night sweats, from the damaged tissue and the continuing immune response. HIV and TB make each other worse because HIV destroys helper T cells.