Some examples of bacteria. a Scanning electron micrograph (SEM) of the rod-shaped Gram-negative bacterium Escherichia coli (x 14 500). b Transmission EM of Campylobacter jejunii which causes food poisoning in humans. The bacterium moves by means of two single terminal flagella (x 9000). c Transmission EM of Staphylococcus aureus. The cell has just divided (x 62000). The technique developed by Gram is still com-monly used today. It involves heat-fixing a smear of bacteria to a clean microscope slide and then flooding it with crystal violet. All bacteria take up this stain. The smear is washed with Gram's iodine to fix the stain and then decolourised with alcohol or propanone. Gram-positive bacteria retain the crystal violet/iodine complex appearing purple, but Gram-negative bacteria do not. Finally the smear is counter-stained with a red stain such as safranin or carbol fuchsin. Gram-negative bacteria take up this stain and become red. Gram-positive bacteria stay purple. The different reaction to the stain is due to the structure of the two basic types of cell wall . Gram-positive bacteria have a plasma membrane surrounded by a rigid cell wall about 20-80 nm thick. This rigid layer is made of a pepti-doglycan, (a polymer of sugars and amino acids) murein, which has a complex three-dimensional structure. Gram-negative bacteria also have a rigid layer outside the surface membrane but it is much thinner, only 2-3 nm thick. On the outside of this is an outer membrane which contains lipopolysaccharides instead of phospholipids. This
• Figure 1.3 The structure of bacterial cell walls based on electron micrographs of the cell walls of Bacillus (Gram-positive) and E. coli (Gram-negative). forms an extra physical barrier to substances, such as antibiotics and enzymes like lysozyme, which normally destroy or inhibit bacteria. The crystal violet/iodine complex is a large molecule and it is thought that, during Gram staining, it becomes trapped inside the Gram-positive cell wall, whereas it is more easily washed out of the thinner Gram-negative cell wall.
Shapes of bacteria When viewed with a microscope, bacteria show several distinct shapes and these may be used to help in identification (figure 1.4).
Reproduction Bacteria grow very quickly in favourable condi-tions. The generation time may be as little as 20 minutes, though for many species it is 15-20 hours. Division is usually by binary fission (figure 1.5).
Bacteria
1 The circular bacterial chromosome divides but there is no mitotic spindle. The chromosome attaches itself to the plasma membrane or, in some cases, to the mesosome.
1 The circular bacterial chromosome divides but there is no mitotic spindle. The chromosome attaches itself to the plasma membrane or, in some cases, to the mesosome.
2 A septum starts to be synthesised to divide the cell. This often starts growing where there are mesosomes. 3 The septum grows right across the cell, dividing it into two daughter cells.
Plasmids The cytoplasm of certain bacteria contains one or more small circles of DNA called plasmids which are able to replicate independently of the main circular chromosome. Plasmids are known to carry genes which may help the bacterium to survive in adverse conditions. For example, plasmids known as R-factors cause resistance to antibiotics, virus infection and ultraviolet radiation. Plasmids can be transferred to another bacterium in conjugation, transformation or transduction.
Conjugation, transformation and transduction Some bacteria have 'mating' cells which come together and are joined by their pili in a process known as conjugation (figure 1.6). The donor passes a plasmid called the F-factor, or fertility factor, to the recipient. Genetic information on the F-factor provides the bacterial cell with every-thing needed to be a donor, including the capacity to synthesise the sex pilus. The F-factor may exist as a free element within the cytoplasm, replicating independently of the bacterial chromosome, or it may become incorporated within the bacterial chromosome and be replicated whenever the chromosome replicates. Transformation occurs when one bacterium releases DNA which is absorbed by a second
bacterium. The second cell therefore acquires new characteristics. Transduction is where new genes are inserted into the chromosome of a bacterium by a bacteriophage virus . Conjugation, transformation and transduction are not forms of sexual reproduction since fertilisation does not occur. In each of the above cases, DNA has been transferred from a donor to a recipient.
The economic importance of bacteria Some genera of bacteria contain species which are commercially useful and some which are harmful to humans. For example, most Bacillus spp. live in
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