This section will introduce three major groups of pathogens(disease-causing microbes).They are:bacteria,virus,protozoa.Each of them has their individual chararcterisitcs which limits the way they cause disease.
Bacteria are important organisms in disease and our ecosystem.Bacteria are very small, measuring about 0.1 to 20 micrometres. There are two distinct groups f bacteria:the Gram-positive and the Gram-negative. They differ in their cell walll composition and the way they cause disease.The Gram-negative bacteria normallly secrete toxins,poisonous substances which destroy cells. Bacteria can also be grouped according to their shape:straight rods, round coccus or curved spirochetes or comma-shaped vibrios. Most bacteria have a cell wall, but unlike other cells, their genetic material is not enclosed by a nucleus. Bacteria can be helpful to us by fixing nitrogen and breaking down dead plants and animals back into the ecosystem. Some bacteria can survive in adverse enviroments by forming spores.Some bacteria have a tail known as flagella which beats around to move the bacteria forward .Below are the different shapes of bacteria and also a basic structure of bacteria:
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Picture of a basic bacterial cell
Viruses are not considered living things. They consists of DNA or RNA enclosed by a protein coat. They only reproduce when they can infect cells.Disease results from the destruction of cells.All viruses are pathogenic and harmful ad they destroy animals and plants.However, some viruses which infect bacteria have been used as medicine to treat disease.Viruses can survive for a long time as their protein coats can form crystals.As their genetic material is limited, viruses conserve their genes by using only one type of proteins and building their coat from sub-units of proteins known as capsomeres.Viruses are very small, ranging from 20-250 nanometres and can only be seen by powerful electron microscopes.Below is a picture of a virus:
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Picture
of a virus
Protozoa are animal,unicellular organisms.They can live freely or in a host as a parasite.All of the parasites are pathogenic.The protozoa has many life stages but most have a cyst stage where the organism is prevented from harm by a membrane forming the cyst.Protozoa can move using hairs on the outside of their cells known as cilia which beat to propel the protozoa.Some like the amoeba, move by extending part of their cell forward and then fill it with material from the back of the cell, thus moving the cell forward.Their sizes range from 2-100 micro metres.