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Antibiotic resistance

The evolution of strains of bacteria that are not affected by a particular antibiotic as a result of natural selection.

White blood cells

Defend the body against disease.

Eukaryotic cells

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.


Complex carbohydrates consisting of more than one sugar molecule

Cell 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.


Large molecule, made up of polysaccharide chains with short peptide cross-linkages, found in all bacterial cell walls.


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.



Energy-requiring process by which a cell takes large particles into membrane-bound vesicles.


Organelle(s) within cells that produce ATP, used as a store of chemical energy. Often called the cell's powerhouse


Severe and often fatal blood poisoning. It is caused when an infection spreads to the blood.


A group of single-celled organisms with few organelles and where the genetic material is not contained in a membrane-bound nucleus. They include bacteria and blue-green algae (cyanobacteria).


Medicine that acts against bacterial infections. Penicillin is an example of an antibiotic.


A unicellular organism that lacks a membrane bound nucleus or any other membrane bound organelle.


The biochemical process by which the cells in the body releases energy


Lipopolysaccharides that form the outer layer of the cell wall of Gram negative bacteria and act as local toxins to the eukaryotic host cells


A distinct part of the cell, such as the nucleus, ribosome or mitochondrion, which has structure and function.


Proteins, usually soluble, produced by both Gram positive and negative bacteria. They are released into the host and carried around in the blood and they can cause widespread effects in the body.


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


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


The protein markers found on the surface of a cell that causes the immune system to produce antibodies against it.


Region of folded membrane where the respiratory enzymes are found in some types of bacteria.


A polymer made up of amino acids joined by peptide bonds. The amino acids present and the order in which they occur vary from one protein to another.


Reusable protein molecules which act as biological catalysts, changing the rate of chemical reactions in the body without being affected themselves


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).


The part of a cell that controls the cell function and contains the chromosomes.


A complex carbohydrate made as an energy store plants


Molecules which contain a lot of stored energy built up of fatty acids and glycerol. Lipids include oils and fats


A poisonous or toxic substance produced by pathogens.


Thread-like protein projections from the surface of some bacteria used for attachment to host cells and for sexual reproduction.


Deoxyribonucleic acid. This is the molecule which contains the genetic code. It coils up tightly inside chromosomes. DNA is a double helix made from two strands which are joined together by pairs of bases.


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.

Main structures of a typical bacterium

The main structures of a typical bacterium

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.

Classifying bacteria

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

Bacterial growth – in theory and in reality

Bacterial growth

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 septicaemia, pneumonia 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.

Strep Throat

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