Pathogens and Defence


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Ben Whitten


What are bacteria?

Bacteria are considered cellular pathogens, and are prokaryotic. They are considered the most abundant and diverse group of organisms. It's important to note that not all bacteria are pathogenic, and most are actually beneficial!

What are the structural features of bacteria?

Bacteria are unicellular and microscopic. Similarly to all cells, bacteria have a plasma membrane that encloses the cytoplasm. They lack membrane-bound organelles as they are prokayotic, but they do possess ribosomes and a single circular strand of DNA (called a chromosome). Alongside the chromosome, bacteria contain plasmids; small circular pieces of DNA, which can be transferred out of the bacteria without having any effect on its function. 

Most bacteria have a cell wall made out of peptidoglycan, found surrounding the plasma membrane. Some bacteria have a flagellum, a tail-like structure used for increased movement and motility. Some species of bacteria have a bacterial capsule, which can be used to stick to different surfaces. The capsule usually increases the virulence of the bacteria.

Many bacteria are capable of creating tough structures termed endospores, which are resistant to extreme temperature, drying out and chemicals; this helps bacteria to survive or resist unfavourable conditions.

Topic Menu
Introduction to Infectious Disease
Transmission of Pathogens
External Defences
Protective Reflexes
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How do bacteria reproduce?

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Bacteria reproduce asexually, through binary fission. This is where one cell splits into two identical daughter cells; the bacterial DNA doubles in quantity, divides into two, and the bacterial cell elongates and splits into two identical daughter cells.

Some bacteria can also reproduce by budding off spores, which allow bacteria to reproduce very rapidly in favourable conditions.

How can bacteria be classified?

There are four main shapes of bacteria which you need to be aware of.

  • Coccus, which is spherical

  • Bacillus, which is rod-shaped

  • Spirillium, which is spiral shaped

  • Vibrio, which is shaped like a comma

How do bacteria cause disease?

Bacteria divide rapidly once inside a host. Some may damage tissues in the host directly, however some may produce toxins which disrupt cell function, either in close proximity or far away. Bacteria are relatively large in comparison to viruses, and some bacteria can enter host cells via phagocytosis, as opposed to endocytosis for viruses. Phagocytosis is performed by macrophages which ingest and destroy microbes.