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Topic last updated: 25 Oct 2021
    • Ico Human Biology Human biology
    • 16+
    • 45

Pathogens and the immune system

2  of  10

Bacteria

Bacteriabacteria
 Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus.  are prokaryotes, so the size and makeup of their cells is different to that seen in eukaryoticeukaryotic
 Cells that make up animals, plants, fungi and protista. They are three-dimensional, membrane-bound sacs containing cytoplasm, a nucleus and a range of membrane-bound organelles. cells. They are around 50 times smaller than eukaryotic cells on average, with a diameter of around 0.2- 2 µm and a length of around 0.5-10 µm. They do not have membranemembrane
 A thin, flexible sheet-like structure that acts as a lining or a boundary in an organism. -bound organellesorganelles
 A distinct part of the cell, such as the nucleus, ribosome or mitochondrion, which has structure and function.  such as the nucleus and mitochondriamitochondria
 Organelle(s) within cells that produce ATP, used as a store of chemical energy. Often called the cell's powerhouse  we see in eukaryotic cells.

Typical features of bacterial cells include:

  • Cell walls – the bacterial cell wall prevents the cell swelling and bursting as water moves in by osmosisosmosis
     The movement of water through a partially permeable membrane down a concentration gradient from a dilute solution (where there is a high concentration of water) to a concentrated solution (where there is a relatively low concentration of water).     , maintains the shape of the bacterium, and supports and protects the cell contents. All bacterial cell walls consist of a layer of peptidoglycan made up of many parallel polysaccharide chains with short peptide cross-linkages. This forms enormous molecules with a net-like structure. Some bacteria have a capsule or slime layer formed from starch, gelatin, protein or glycolipid which helps protect the bacterium from phagocytosisphagocytosis
     Energy-requiring process by which a cell takes large particles into membrane-bound vesicles.      by white blood cellswhite blood cells
     Cells specialised in defending the body against disease.     . It also covers the antigenantigen
     A protein, nucleic acid, or other molecule, which can stimulate a response from the immune system.     s that identify the cell, making it easier for a bacterium to be pathogenicpathogenic
     The state of a bacterium, virus, or other microorganism, capable of causing disease.      (cause disease) because it is harder for it to be identified by the immune systemimmune system
     The body's natural defence mechanism against infectious diseases.     .
Pathogens39
  • Pili and flagellaeflagellae
     Extensions of cells used for movement. In bacterial cells they are made of many-stranded helices of the protein flagellin that can rotate at about 100 times per second.      – bacteriabacteria
     Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus.     may have from one to several hundred thread-like protein projections from their surface called the pili (or fimbriae).They seem to be used for attachment to a host cell and for sexual reproduction. Some bacteria can move themselves using flagellae, made of many-stranded helices of the protein flagellin that can rotate at about 100 times per second.
  • Cell surface membranemembrane
     A thin, flexible sheet-like structure that acts as a lining or a boundary in an organism.      - this is similar in both structure and function to the membranes of eukaryoticeukaryotic
     Cells that make up animals, plants, fungi and protista. They are three-dimensional, membrane-bound sacs containing cytoplasm, a nucleus and a range of membrane-bound organelles.     cells    . As bacteria have no mitochondriamitochondria
     Organelle(s) within cells that produce ATP, used as a store of chemical energy. Often called the cell's powerhouse     the cell membranecell membrane
     The membrane which forms the boundary between the cytoplasm of a cell and the medium surrounding it and controls the movement of substances into and out of the cell.      is the site of the respiratory enzymes needed during respiration. In some bacteria the membranes are folded to form a region called the mesosomemesosome
     Region of folded membrane where the respiratory enzymes are found in some types of bacteria.     .
  • Nucleoid – the area of the bacterial cell that contains the genetic material. This is a single length of DNA, often circular, which is folded and coiled to fit into the bacterium. There is no membrane-bound nucleus.
  • Plasmidplasmid
     A piece of genetic material in a cell. Normally a small, circular strand of DNA, which is independent of the main genetic material of the organism.      - one or more small circles of DNA that code for a particular characteristic in addition to the genetic information in the nucleoid (eg toxin production or antibioticantibiotic
     Medicine that is used to treat bacterial infections and works by killing or stopping the growth and reproduction of bacteria. These can be specific to a particular bacteria or act on groups of related bacteria.     resistance).
  • 70S ribosomes – prokaryoticprokaryotic
     A unicellular organism that lacks a membrane bound nucleus or any other membrane bound organelle.      ribosomes for protein synthesis. The ribosomes in bacterial cells are smaller than ribosomes in eukaryotes which are 80S.

Classifying bacteriabacteria
 Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus.

It is important to be able to identify bacteria – it helps us identify the bacteria that keep us healthy, the bacteria that can be useful to us, and the harmful bacteria that act as pathogens. We need to be able to identify specific bacterial pathogens so we know what is causing a disease and how it can be treated.

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Bacteriabacteria
 Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus. l growth

If conditions are right, the fastest growing bacteria can reproduce once every 20 minutes. In reality this doesn’t happen because bacteria are always growing in a system that is closed in one way or another. Their growth is limited by shortages of the nutrients they need, less than optimum temperatures, and/or a build up of waste products.

Bacterial growth – in theory and in reality

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Bacteriabacteria
 Single-celled organism. Has a cell wall, cell membrane, cytoplasm. Its DNA is loosely-coiled in the cytoplasm and there is no distinct nucleus. l diseases

Bacteria can cause the symptoms of disease in a number of ways. Some may invade and destroy the cells, however the majority of bacteria cause disease as a result of the toxins they make.

There are two main types of bacterial toxins

  • Endotoxinsendotoxins
     Lipopolysaccharides that form the outer layer of the cell wall of Gram negative bacteria and act as local toxins to the eukaryotic host cells.      - these are the lipopolysaccharides that form the outer layer of the cell wall of Gram negative bacteria. The lipid part of the molecule acts as a local toxin, affecting the cells directly around the bacterium.
  • Exotoxinsexotoxins
     Toxins secreted by bacteria that can cause damage to the host by destroying cells or altering normal cell activity.      – these are usually soluble proteins that can be produced by both Gram positive and Gram negative bacteria. The toxin is released into the body of the host as the bacteria grow and reproduce. Exotoxins are carried around the body in the blood and can cause widespread effects.

Bacterial diseases can be very mild – most people have suffered from tonsillitis or a septic cut at some stage of their lives. They can also be extremely dangerous and even life-threatening. Bacterial diseases such as septicaemiasepticaemia
 Severe and often fatal blood poisoning. It is caused when an infection spreads to the blood. , pneumoniapneumonia
 Swelling (inflammation) of the tissue in the lungs, often initially caused by a virus and often complicated by a secondary bacterial infection.  and tuberculosistuberculosis
 A communicable disease caused by bacteria. It affects millions of people around the world and can be cured by antibiotics, but increasingly the pathogenic bacteria are becoming resistant to the most widely used antibiotics.  still kill millions of people every year. In theory, antibioticantibiotic
 Medicine that is used to treat bacterial infections and works by killing or stopping the growth and reproduction of bacteria. These can be specific to a particular bacteria or act on groups of related bacteria. s - drugs that can treat bacterial diseases - should have made them a problem of the past. Unfortunately we have overused these drugs, so now many bacteria are resistant to antibiotics and they are no longer effective. Also, many people do not have access to medical care and the drugs that could save their lives.

This inflammation is caused by Streptococcus bacteria.
(Photo credit: CDC/Heinz.F.Eichenwald)

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