Pathogens and infection (6.5, 6.11, 6.6, 6.10)
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Bacteria are prokaryotes, meaning their cells lack a nucleus and other membrane-bound organelles. They can cause disease by directly damaging cells or by releasing toxins that harm the organism.
Bacteria reproduce rapidly by binary fission, allowing infections to spread quickly.
Examples of bacterial diseases include:
- Tuberculosis (TB) in animals.
- Ringrot in plants such as tomatoes and potatoes.
Viruses are very small, non-living infectious agents. They infect and take over the host’s cells to produce more viruses, often causing cell damage when new viruses are released.
They have a simple structure consisting of either DNA or RNA enclosed in a protein coat.
Examples of viral diseases include:
- HIV which leads to AIDS in humans.
- Influenza (flu) in animals.
- Tobacco mosaic virus (TMV) in plants.
There are some major differences between viruses and bacteria:
Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB), is spread via droplets, released when an infected organism coughs and sneezes. It infects the lungs, where it is engulfed by macrophages but survives and replicates inside them.
The immune system responds by forming tubercles, which are small, firm, rounded nodules of tissue. These tubercles help contain the bacteria, which can remain dormant for many years.
TB can cause symptoms including fever and night sweats. Fever is part of the inflammatory response, caused when immune cells release chemical signals which the hypothalamus responds to by raising the body temperature.
This increased temperature enhances the immune function and phagocytosis. It can also slow down the rate at which bacteria and viruses can reproduce.
TB in dormant cells can be reactivated, the bacteria rapidly multiply and destroy lung tissue, causing coughing, chest pain and weight loss. If left untreated, it can spread to other organs (bones, lymph nodes and even the central nervous system), leading to organ failure and death.
HIV (human immunodeficiency virus) is a retrovirus that infects and destroys helper T cells (also called T helper cells).
This weakens the immune system by reducing the number of helper T cells. If left untreated, HIV infection can progress to AIDS (acquired immune deficiency syndrome). People with AIDS are highly susceptible to opportunistic infections and certain cancers, as their immune system is no longer able to respond effectively.
During the acute phase of HIV infection, the amount of HIV in the blood increases rapidly and the number of helper T cells decreases as the virus infects and destroys them.
At this stage, some people experience flu-like symptoms, including fever, headache, sore throat, swollen lymph nodes and rash, while others may have few or no symptoms.
During the chronic phase of HIV infection, the virus continues to reproduce, but the immune system helps keep it under control. The number of helper T cells gradually decreases over time.
During this phase, people may have few or no symptoms, although they may experience recurrent infections that take longer to clear. Latent infections, such as tuberculosis (TB) or shingles, may reactivate as the immune system becomes progressively weaker. This stage can last for many years.
During the disease phase of HIV infection, the amount of HIV in the body is high and the number of T helper cells is very low.
As a result, the immune system is severely weakened, and people can develop opportunistic infections (e.g., pneumonia) and unusual cancers (e.g., Kaposi’s sarcoma). These diseases are responsible for most of the symptoms and deaths associated with AIDS.
Post-transcriptional modification occurs after transcription and before translation.
During RNA splicing, introns are removed from pre-mRNA and exons are joined together. Alternative splicing allows different combinations of exons to be included in the mature mRNA.
As a result, a single gene can produce different mRNA molecules and therefore different proteins. This means that the diversity of proteins produced can be increased without increasing the number of genes.
There are several major routes by which pathogens can enter the body:
- Inhalation: pathogens are breathed in via infected droplets / aerosols.
- Ingestion: pathogens are swallowed in contaminated food or water.
- Breaks in the skin (inoculation): cuts, wounds, insect bites, needles or scratches give direct access to tissues or the bloodstream.
- Direct contact/mucous membranes: pathogens enter through the eyes, nose, mouth or reproductive tract.
- Vectors: Organisms such as mosquitoes, ticks or flies transmit pathogens.
In animals, the skin acts as a physical barrier covering the entire body and helping to prevent pathogen entry.
The outer layer consists of dead cells that are continually shed. This layer is waterproof, making it hard for pathogens to penetrate into an organism’s body.
Skin also produces antimicrobial secretions (e.g., sweat, sebum) that kill or inhibit microbes on the skin’s surface.
The body has several barriers, in addition to the skin, that help protect against pathogens:
- Stomach acid: low kills pathogens.
- Gut flora: compete with pathogens for nutrients and space.
- Skin flora: prevents colonisation by pathogens.