 Infection and Disease Cellular and Non-cellular Pathogens

Disease  A disease is any change that impairs the function of an individual in some way.  Diseases may be inherited or caused by many other factors.  An infectious disease is one caused by a pathogen.

Pathogens  A pathogen may be a:  Multicellular organism, eg. fungus and worm  Single-celled organism, eg. eukaryotes such as protozoan or prokaryotes such as bacterium  Non-cellular agent, eg. virus and prion

Transmission of Pathogens  Some diseases can be transmitted from person to person because the pathogenic organism or agent causing the disease can be transmitted.  Infective agents can be transmitted from one host to another by:  Direct contact  Droplets in air breathed or sneezed out by an infected person  Sexual contact  Contaminated food or water  A carrying vector, eg. rats carrying fleas  An injecting vector, eg. mosquitoes carrying malarial parasites  Injection with infected needles and syringes

Bacteria  Are prokaryotic organisms lacking a nuclear membrane.  Have cell walls made of carbohydrates, lipids, amino acids and sometimes other substances.  Contain a single circular molecule of DNA.  Can be classified by shape, eg:  coccus (round)  bacillus (rod shaped)  spirochaete (spiral)  This often relates to their naming – eg. Staphylococcus aureus are a round-shaped bacteria

Bacteria  Can have special features, including a flagellum, capsule or spore.  Gram positive bacteria have a thick peptidoglycan layer  Gram negative bacteria have a thin peptidoglycan layer and a lipopolysaccharide wall, which makes them more virulent ☢ Can be classified by their metabolic characteristics ☢ Reproduce exponentially and quickly (as little as 20 minutes) in favourable conditions.

Bacteria  Treatment of bacteria diseases:  Chemotherapy (term used when a disease is treated with chemicals).  Antibiotics/antimicrobials  Some drugs are:  narrow-spectrum and act against a limited variety of micro- organisms;  broad-spectrum and act against many different kinds of pathogens.  Treatment must be carefully administered as bacteria can become antibiotic resistant. Eg. Golden Staph

Fungi, Yeast & Protozoa  Fungi, yeasts and protozoa are all eukaryotic organisms that can cause disease.  Fungi  Decompose dead organic matter  Spread readily as spores  Bacterial antibiotics do not act against fungi. Antifungal ointments and oral preparations are used to treat fungal infections.

Fungi, Yeast & Protozoa  Yeasts are unicelluar fungi.  Protozoa are single-celled organisms.  Examples include Plasmodium, Giardia and Amoeba

Arthropods and worms  Arthropods  Are generally parasitic but are not necessarily pathogenic  Include head lice and body lice.  Worms  May be parasitic but are not necessarily pathogenic  Include Echinococcus granulosis, a pathogenic hydatid tapeworm, which normally lives in dog intestines but which can spend part of its life cycle in human host tissues.  Include parasitic nematode worms that live in plant tissue.

Viruses  Viruses :  Are extremely small  Have a protein coat  Have a core of either DNA or RNA  Reproduce only after entering a host cell and using its components  Are parasitic and pathogenic  Can infect humans, other animals, plants and bacteria

Viruses  How do viruses affect a host cell?  Viruses must enter a host cell to reproduce.  They use components of the host cell as a source of energy and material to make new viruses and ultimately destroy the host cell.  Different kinds of viruses may reproduce in slightly different ways.

Types of viruses Adenovirus  DNA virus  Utilises host cell ribosomes to replicate  Does not possess a viral envelope Retrovirus  RNA virus  Uses reverse transcriptase to create DNA which is incorporated into the host DNA  Has a viral envelope

Viruses  Antiviral drugs  Because viruses enter host cells, it is difficult to inhibit the reproduction of a virus without interfering with the metabolism of the host cell.  Some antiviral drugs have been developed that specifically act on enzymes important to the virus without damage to the host cell.

Viroids and Prions  Viroids :  Contain very short pieces of naked RNA  Are important as plant pathogens  Are known to infect plants including potatoes, tomatoes and grapevines.  Completely dependent on the host cell for replication.  Prions :  Do not contain genetic material  Are an abnormal and infectious form of protein  Cause a range of brain diseases including spongiform encephalopathy.  No treatment is available for individuals infected with abnormal prions.