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Pathogens and the immune system

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Bacteria are prokaryotes, so the size and makeup of their cells is different to that seen in eukaryotic 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 membrane-bound organelles such as the nucleus and mitochondria 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 osmosis, 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 phagocytosis by white blood cells. It also covers the antigens that identify the cell, making it easier for a bacterium to be pathogenic (cause disease) because it is harder for it to be identified by the immune system.
  • Pili and flagellae – bacteria 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 membrane - this is similar in both structure and function to the membranes of eukaryotic cells. As bacteria have no mitochondria the cell membrane is the site of the respiratory enzymes needed during respiration. In some bacteria the membranes are folded to form a region called the mesosome.
  • 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.
  • Plasmid - 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 antibiotic resistance).
  • 70S ribosomes – prokaryotic ribosomes for protein synthesis. The ribosomes in bacterial cells are smaller than ribosomes in eukaryotes which are 80S.

Classifying bacteria

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.

Bacterial 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

Bacterial 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

  • Endotoxins - 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.
  • Exotoxins – 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 septicaemiapneumonia and tuberculosis still kill millions of people every year. In theory, antibiotics - 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)