Bacteria and virusesEdexcel A-Level Biology A: Revision notes

Bacteria are prokaryotic cells, typically 1–5 µm long. A bacterial cell has:

• a cell wall of peptidoglycan (murein)
• a cell surface membrane and cytoplasm
• circular DNA free in the cytoplasm (no nucleus), and often small circular plasmids
• 70S ribosomes
• sometimes a capsule or slime layer, flagella and pili

There are no membrane-bound organelles. Bacteria carry out their own metabolism and reproduce by binary fission.

Viruses are acellular and much smaller than bacteria, typically 20–400 nm. A virus consists of:

• genetic material, either DNA or RNA, which may be single or double stranded
• a protein coat, the capsid
• in some viruses a lipid envelope taken from the host cell membrane, carrying attachment proteins
• sometimes enzymes, such as reverse transcriptase in HIV

Viruses have no cytoplasm, ribosomes or metabolism of their own, so they can replicate only inside a host cell. Antibiotics do not work on viruses because there is no cell wall or metabolism for them to target.

Similarities: both contain nucleic acid and cause disease.

Differences:

• Size: bacteria about 1–5 µm; viruses about 20–400 nm, so roughly 10 to 100 times smaller.
• Cell structure: bacteria are cells with a wall, membrane, cytoplasm and 70S ribosomes; viruses have a capsid and sometimes an envelope.
• Genetic material: bacteria have circular DNA (and plasmids); viruses have DNA or RNA, never both.
• Reproduction: bacteria divide independently; viruses are replicated inside host cells.
• Treatment: antibiotics act on bacteria but not on viruses.

Mycobacterium tuberculosis is a bacterium spread by airborne droplets from the coughs of infected people. It infects the lungs.

• Bacteria reach the alveoli and are engulfed by macrophages, but the thick, waxy cell wall resists digestion so they survive and multiply.
• The immune system walls off the infection in tubercles; the bacteria can lie dormant for years.
• If immunity weakens, the bacteria multiply and tissue is destroyed.
• Symptoms: persistent cough, coughing blood, fever, night sweats and weight loss.
• Untreated, lung tissue is destroyed, gas exchange fails and the patient may die of respiratory failure.

Treatment needs a long course of several antibiotics; incomplete courses lead to antibiotic-resistant strains.

HIV is a retrovirus: an enveloped virus with two RNA strands and reverse transcriptase. It infects helper T cells.

1. Attachment proteins bind to CD4 receptors on the helper T cell.
2. The envelope fuses with the cell surface membrane and the capsid, RNA and enzymes enter the cell.
3. Reverse transcriptase makes DNA from the RNA, and the DNA is inserted into the host's DNA.
4. The host cell makes viral proteins and RNA; new viruses assemble and bud off, destroying the cell.

Symptoms: a flu-like illness at first, then often years with few symptoms while helper T cell numbers fall. Once the immune system is weak the person has AIDS and suffers opportunistic infections (such as TB and pneumonia) and some cancers. Death is usually from these, not directly from HIV.