1 3 Bacteria and viruses
Objectives
To know the structure of a typical bacterial cell and a typical virus
To know the requirements for bacterial growth
To know how bacteria reproduce
To understand some of the ways in which bacteria and viruses affect human activities
To understand why viruses do not fit into the five kingdoms of living organisms
Bacterial structure
Bacteria (singular: bacterium) are single-celled organisms that have no true nucleus. Bacterial cells do not contain organelles like those found in typical animal and plant cells (see page 23), but are able to carry out all of their life processes without them. A few can photosynthesise, but most feed off other organisms. They may be parasites, feeding off living organisms, or saprotrophs, feeding off dead organisms.
Bacteria are very small, usually about 1–2 0m in length, and so are only visible using a high-powered microscope. The structure of a typical bacterium is shown in the diagram below.
Bacteria exist in a number of different shapes, some of which are shown below. Shape can be used to classify bacteria.
Requirements of bacteria
Bacteria have certain requirements that their environment must provide. An understanding of these requirements (see page 272) has been important in biotechnology and in the control of disease. If the environment supplies these needs, the
Plasmids are small rings of DNA, carrying only a few genes.
bacteria can multiply rapidly by binary fission (see page 164). In this process each bacterium
divides into two, then each of the two divides again and so on, until very large populations are built up. A bacterial colony can quickly dominate its environment, making great demands on food and oxygen and perhaps producing large quantities of waste materials.
The generation time (time taken for each cell to divide into two) can be as little as 20 minutes under ideal conditions. One E. coli cell in the human gut could theoretically become 272 cells in
24 hours – this number of cells weighs about 8000 kg!
Bacteria have a cell wall but do not have a nucleus or organelles.
The importance of bacteria
Bacteria are important to humans in many ways.
Some are pathogenic – they cause disease (see page 258). All pathogenic bacteria are parasites.
Some are involved in nutrient cycles (see pages 240–3).
Some are exploited by humans in food production and in biotechnology (see page 48).
Bacteria are probably the organisms that carry out the largest number of different activities, and are the most numerous organisms on Earth. There may be as many as 5000 undiscovered bacterial species in 1 m3 of woodland soil, and more bacteria live on or in your body than all of the humans that ever existed!
Viruses
When the five-kingdom system of classification was devised, no one was able to find a place for the group of organisms called the viruses. This is because viruses do not show the typical features of living things – respiration, nutrition and reproduction, for example – unless they are inside the cells of another living organism. In other words, all viruses are parasites and therefore cause harm to their host. Some taxonomists have suggested that viruses belong in a sixth kingdom. There is great variation in the structure of viruses, but they all have certain common features. The structure of a typical virus is shown below.
Most viruses cause disease – they may infect humans, domestic animals or plants.
Genetic material – may be DNA or RNA, but always a single strand.
Protein coat – protects the single strand of nucleic acid Made of many subunits.
Spikes – may contact and recognise the cell to be infected.
A typical virus has genetic material and a protein coat, but cannot carry out its life processes. It has no cytoplasm.