Chapter 1: Organisms that cause disease

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Introduction to pathogens and micro-organisms

Pathogens are organisms that cause disease. They are frequently, but not necessarily micro-organisms. Whether an organism is pathogenic depends on the species with which it is in contact. The organism that causes sleeping sickness, Trypanosoma sp., lives in reptiles, cattle and wildebeest in Central Africa without any apparent ill effects to these animals, but when a human becomes infected a potentially lethal disease develops.

Micro-organisms are organisms that are too small to be seen with the naked eye. They include the bacteria, yeast and single celled algae.

Not all micro-organisms are unicellular, and not all micro-organisms cause disease. Many micro-organisms reside in or on animals or plants without seemingly causing any harm. Indeed, in many cases the union is advantageous for both (symbiosis), for example, the cellulose digesting bacteria in the rumen of cattle.

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Many different types of organisms may cause disease, but they fall into four main groups. The Table which follows summarises the characteristics of each of these groups of pathogenic organisms.

VIRUSES 20-100 x 10-9M Electron microscope to view. Protein coat with DNA or RNA inside. Maybe membrane outer layer. Unable to live outside other cells.May infect prokaryotes and/or eukaryotes.Replicates inside host cell by coding (with viral nucleic acid) for new viral synthesis there. Bacteriophages, Plant mosaic viruses, HIV, Herpes, Influenza, Hepatitis Good personal hygiene. Some immunisation(eg 'flu, polio) NOT antibiotics. Immune system fights viruses. Recently, some antiviral drugs developed.
BACTERIA 0.1-1.0 x 10-6M Light microscope (Stains aid identification) Prokaryotes - Revise structure! (Kingdom Monera) Ubiquitous. In almost all environmental niches. Most are non pathogens.Pathogenic bacteria:Cause disease in eukaryotes. Classified by shape (spheres, rods, spirals). Classified by chemistry (eg Gram +/-) Classified by structures (cilia, flagella) Some produce destructive toxins. Streptococcus, Salmonella, Staphylococcus, Escherischia coli (E. coli) Mycobacterium tuberculosis, Clostridium tetani Good personal hygiene. Immunisation (eg TB), Public sanitation, Surgical aseptic techniques. Various antibiotics.(1928 Fleming discovered penicillin, 1938 Florey developed it for human use.) Anti-toxins (eg tetanus)
FUNGI Most are microscopic (L.M.) (eg yeast) Some are macroscopic Yeasts - Unicellular, divide by binary fission or budding. Can exist as spores.Moulds - Filamentous mat of thread-like hyphae produces a mycelium. Fruiting bodies produce spores.(Some pathogenic fungi can exist as either of the above forms, depending on the environment.) Candida (-> thrush) Trichophyton (-> tinea) Aspergillus (-> pneumonia or asthma) Good personal hygiene. Various anti-fungal drugs.
PARASITES Some microscopic. (eg Plasmodium vivax -> malaria, Entamoeba histolytica -> dysentery)Some macroscopic (eg fleas, ticks, tapeworm, hydatids) ALL ARE EUKARYOTES Either ectoparasites (outer surfaces of host) or endoparasites (inside host's body). Many have complex life cycles which include a period away from humans and a time in or on humans. Many have specialised structures for attachment to humans either to prevent dislodgment or obtain nutrients or both. VECTOR = living transmitter of disease (eg mosquito -> malaria) RESERVOIR = source of parasite in abiotic environment (eg contaminated soil or water). Malaria, Dysentery, Liver fluke, Intestinal worms, Schistoma, Fleas, Ticks, Lice Good personal hygiene.Public sanitation.Break life-cycle.Cook meat before eating. Some insecticides. Some drugs.Surgery to remove cysts.Topical insecticides.

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The nature of disease caused by pathogenic organisms.

Pathogens can cause disease in a number of different ways.

Viral infections

occur when viral nucleic acid enters a host cell and the cell begins the process of replication and assembly of new viral particles. The steps below show how viral replication occurs.

Virions stick to host cell membrane at specific receptor sites.
Virions enter cytoplasm and cell proteases uncoat the particles releasing the nucleic acid
Nucleic acid is transported to nucleus where needed enzymes are located
Biochemical steps for nucleic acid replication occur (steps vary depending on whether it is a double- or single-stranded DNA or RNA virus)
Viral mRNA synthesis begins and redirects the ribosomes to assemble only viral proteins
Eventually ribosomes make nothing but viral proteins
Mature virions are assembled from the nucleic acid and proteins
Cells die and lyse, releasing a new batch of infective viruses.

The cell lysis releases factors which can cause fevers and/or inflammation due to non-specific reaction to cell contents released in response to the infection.
This is the usual result of infection with the virus of the common cold.

In some cases (such as infection with the poliovirus), the loss of cell function (neurones) causes serious physiological consequences. Other viruses cause damage to normal immunological mechanisms (HIV virus causing AIDS), or in some cases, begin the process of tumor formation that may lead to either benign (eg warts) or malignant tumours (carcinomas).

Many relatively common diseases are caused by viruses (eg the common cold, hepatitis, measles, mumps, glandular fever and many more), and normally healthy individuals recover completely in a relatively short time.

Apart from the infected person's own immune system, there are few pharmacological agents that can stop the viral reproduction process. Antibiotics DO NOT work on viruses. For this reason, doctors rarely prescribe anything other than Panadol for pain and fever relief and rest for viral infections.

Pathogenic Bacteria

The effects of infection by pathogenic bacteria are also variable and can include
Some of the diseases caused by bacteria include tetanus, whooping cough, pneumonia, gonorrhoea, meningitis and some forms of tonsillitis.

Bacterial toxins

are soluble substances that alter the normal metabolism of host cells with harmful effects on the host. They are the reason for some of the signs and symptoms of many bacterial diseases.

It is likely that the disease processes caused by some fungi, protozoa and worms are also due to the effects of toxins.

Toxins are often classified as either exotoxins or endotoxins.

Exotoxins are usually secreted by bacteria into the surrounding environment, endotoxins are only found on the surface membrane of a few types of bacteria and are only released in special circumstances.

Bacterial toxins vary in their specificity. Some act on certain cell types only, others can affect a wide variety of cells and tissues. Bacterial toxins work at extremely low levels, and they include the strongest poisons known (eg 1 gram of tetanus toxin is enough to kill about 10 million people).

Toxins cause damage in various ways, they may


have been discovered as natural anti-bacterial agents (for example penicillin was first isolated from a fungus), and other similar, chemically synthesised compounds have also been found to kill bacteria without harming the infected host.

Most of these chemicals rely on the subtle differences between prokaryotic (bacterial) cells and the eukaryotic host's cells. Therefore, a doctor who suspects that a patient has a bacterial infection will prescribe an antibiotic and may send a sample of infected tissue to a lab. where tests can be done to determine exactly which of the dozens of antibiotics is best at killing that particular strain of bacteria.

A parasite

is an organism that completes some part of its life cycle in another organism. It does not necessarily cause disease whilst infecting in the host, but those parasites that do cause damage whilst in the host are said to be pathogenic.

Parasites usually belong to the Kingdom Protista, although there are some significant worms and insects (Animals) which cause disease in animals and significant destruction of plant tissue.

The organism in which a parasite completes the adult stage of its life cycle is known as the primary host. For most human parasitic infections, the human is the primary host (eg tapeworm), but the organism leaves the human to enter another animal (secondary host) or the environment (reservoir) in which it completes the other stages of the life cycle. In some cases, the organism is carried between hosts by another animal (a vector), without any perceptible damage to the vector. The diagram below illustrates this idea.


Parasite Lige cycle diagram
Details of each stage will vary with different parasites.

If parasite's laval stage is in human and parasite's adult stage in an extra human (other than a human) reservoir, the human is a secondary host.

Pathogenic parasites cause disease in humans when they invade cells and tissues, disrupting the normal functions of those cells. Some of the intestinal parasites, such as worms, cause damage by 'robbing' the host of nutrients as the products of digestion diffuse readily into the thin worms. These intestinal worms have evolved some interesting structures for attachment to the intestine's wall, preventing them from being passed out of the body.

You should study the life cycles of a variety of parasites from your textbooks and activity manuals. For each life cycle, make sure that you can identify the primary and secondary hosts, any environmental reservoirs and any vectors. Note also any special structures of the parasite that enable it to be successful either within a human or in surviving outside the human.

Controlling parasitic infections inevitably requires breaking the life cycle of the parasite. Since many of these infections occur in underdeveloped areas of the world, where rivers and streams act as sources of drinking and washing water as well as the sewage system, it is hardly surprising that parasites that find their way into such waterways quickly find a new human host. Improved sanitation is, therefore, an obvious means of controlling water borne parasites.

In some cases, immunisation is possible against some of these parasitic infections.

Similarly, control of insect vectors or drug treatment to kill the parasites within a human are also suitable points of attack. If all of these are ineffective or impossible, isolation of infected individuals and careful nursing practise to prevent infection from contaminated bedding and body secretions are likely to limit the spread of the infection.

For the parasites you considered above, identify as many ways of controlling them as you can.

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Last update :8 April 97