Fungi, Protists, Pathogens, Antibiotics and Vaccines

Fungi:

• Eukaryotic, with cell walls made of chitin (not cellulose).
• Heterotrophic: most are saprotrophs, which secrete enzymes onto dead matter and absorb the products. They store carbohydrate as glycogen, not starch.
• Moulds and mushrooms are made of threads called hyphae, which form a mycelium; they reproduce with spores.
• Yeasts are single-celled fungi that reproduce by budding; without oxygen they ferment sugar to ethanol and carbon dioxide (bread, beer, wine).
• Penicillium is the mould in which Fleming found penicillin (1928).

Protists: eukaryotes, mostly single-celled. Some are animal-like (protozoa, such as Amoeba, Paramecium, Plasmodium), some are plant-like (algae, which photosynthesise).

• A pathogen is an organism or agent that causes disease: bacteria, viruses, fungi, protists and also prions (misfolded proteins with no nucleic acid, the cause of CJD and BSE).
• Communicable diseases pass from one host to another: by droplets in the air, contaminated water or food, body fluids, direct contact, or a vector.
• A vector is an organism that carries a pathogen between hosts, such as the mosquito for malaria. The vector is not the cause of the disease.
• Many bacteria cause disease with toxins: cholera toxin makes the gut lose water and salts, causing severe diarrhoea.

• An antibiotic kills bacteria (bactericidal) or stops them growing (bacteriostatic). Many were first found in fungi or other bacteria.
• It works by selective toxicity: it hits a bacterial target with no exact human equivalent.
• Antibiotics do not work against viruses, which have no wall and use the host's ribosomes.

How resistance spreads:

• A random mutation gives a few bacteria a resistance allele, before any drug is present.
• The antibiotic kills the sensitive bacteria; the resistant ones survive and multiply (selection).
• Resistance genes on plasmids spread to other bacteria, even other species, by conjugation, transformation and transduction.
• Mechanisms: an enzyme that destroys the drug (β-lactamase breaks down penicillin), pumps that push it out, or a changed target.
• Overuse speeds this up: antibiotics for viral infections, unfinished courses, routine use in farm animals. MRSA (methicillin-resistant Staphylococcus aureus) is a well-known result.

• A vaccine contains antigens (or instructions to make them) that start a primary immune response and leave memory B and T cells.
• On real infection the secondary response is faster, stronger and longer. Boosters raise the level of memory again.
• Active immunity: your own body makes antibodies and memory cells. Natural (after infection) or artificial (after a vaccine). Slow to start, long-lasting.
• Passive immunity: ready-made antibodies are received. Natural (across the placenta, in breast milk) or artificial (an injection of antibodies, such as antivenom). Immediate but short-lived, with no memory cells.
• Herd immunity: when enough people are immune, the pathogen cannot spread, which protects those who cannot be vaccinated.
• Jenner (1796) used cowpox to protect against smallpox; smallpox was declared eradicated in 1980.