Definition of Pathogen and research into the main features of bacteria, protozoa, fungi and viruses.
Pathogens, or infectious agents, are microorganisms that cause disease or illness to their host. There are divided into four groups; bacteria, protozoa, fungi and viruses. The term pathogen most often describes an organism that disrupts the normal physiology of a plant or animal. In identifying a pathogen it is important to determine the properties that ‘contribute to its infectious capacity—a characteristic known as virulence. The more virulent a pathogen, the fewer the number needed to establish an infection.’1 Non-infectious causes of disease such as chemicals can also be referred to as pathogens.
Bacteria are microscopic single celled organisms that are the most numerous and diverse living creatures on earth. They are not categorized as either plant or animal but as prokaryotes, a single cell that lacks a membrane bound nucleus and other internal structures. Bacteria can use almost any organic compound as a food source allowing them to thrive in almost any environment. They absorb molecules through their membrane; the molecules are then broken down by the enzyme rich cytoplasm. Bacteria reproduce in one of three main ways, binary fission, budding and fragmentation. Binary fission (diagram 1) is the division of the bacteria into two identical bacteria, known as daughter cells. Budding (diagram 2) is when specialised areas of the cell expand outwards on stalks creating new cells at the ends. Fragmentation (diagram 3) is where new bacteria grow from fragments of the parental cell. Bacteria can be grouped in many ways, one of which is shape. These groups are rod-shaped, bacillus; round, coccus; spiral, spirillum and incomplete spirals, vibrios. There are three types of symbiotic relationship between the host and bacteria; commensalism, this is when the relationship is of benefit to the bacteria but has no positive or negative impact on the host organism. Most bacteria in humans are commesalisitc. Mutualism is when both the host organism and the bacteria benefit. There are many types of bacteria that live within the body; in the intestines, the mouth, nose and throat. They are provided with a constant food source and an environment in which they thrive and the host is provided protection from other microbes. A parasitic relationship is when the bacteria benefit to the detriment of the host. Pathogenic parasites are able to resist the body’s immune system and then thrive at the expense of the host. These bacteria produce exotoxins and endotoxins that can cause the symptoms of illness. Many bacteria have whip like limbs that are used for motion, these are called flagella. Other bacteria types move their whole structure to gain propulsion whilst some are unable to move under their own power. Some bacteria have tiny projections on their surface called pili and fimbriae; these are used as tethers with which to attach themselves to other surfaces. The bacteria within the body are more beneficial than they are detrimental and allow for a continuous chemical exchange between host and the environment. The term ‘indigenous microbiota’ is used to describe bacteria that are regularly found in one anatomical area. The variation in shape, reproduction method, lifespan and location of bacteria within the human body is huge, so much so that ‘there are no absolute rules about bacterial composition or structure, and there are many exceptions to any general statement.’2
Diagram 1. Binary fission.
Diagram 2. Budding.
Diagram 3. Fragmentation.
Protozoa are single celled eukaryotes, that is they have a nucleus and intracellular organelles. They are not classified as animals and are categorised on the basis of locomotion. Sarcodinians use pseudopodia, meaning false feet. They are a temporary extension of the cytoplasm and are...
References: 3 Barron, S. (editor, 1996)
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