The Royal Marsden Hospital Manual of Clinical Nursing Procedures - Lisa Dougherty [84]
Other structures visible outside the cell wall may include pili, which are rigid tubes that help the bacteria attach to host cells (or, in some cases, other bacteria for the exchange of genetic material), flagellae, which are longer, mobile projections that can help bacteria to move around, and capsules, that can provide protection or help the bacteria to adhere to surfaces. These are illustrated in Figure 3.2. The presence or absence of different structures will play a part in determining an organism’s pathogenicity – its ability to cause an infection and the severity of that infection (Goering et al. 2007).
Figure 3.2 Bacterial structures.
A final bacterial structure to consider is the spore. Bacteria normally reproduce by a process called binary fission – they create a copy of their genetic material and split themselves in two, with each ‘daughter’ cell being an almost-exact copy of the parent (there are mechanisms by which bacteria can transfer genetic material between cells and so acquire characteristics such as antibiotic resistance, but they are beyond the scope of this chapter). However, some bacteria, notably Clostridium difficile, have the capacity, in adverse conditions, to surround a copy of their genetic material with a tough coat. Because this structure is created within the bacterial cell, it is sometimes referred to as an endospore, but is more often simply called a spore. The parent cell then dies and disintegrates, leaving the spore to survive until conditions are suitable for it to germinate into a normal, ‘vegetative’ bacterial cell that can then reproduce (Goering et al. 2007). Spores are extremely tough and durable. They are not destroyed by boiling (hence the need for high-temperature steam under pressure in sterilizing autoclaves) or by the alcohol handrubs widely used for hand hygiene – hence the need to physically remove them from the hands by washing with soap and water when caring for a patient with Clostridium difficile infection (DH/HPA 2008).
Some medically significant bacteria are listed in Table 3.2.
Table 3.2 Medically significant bacteria
Spherical Rod-shaped
Gram positive Staphylococcus aureus Streptococcus spp Clostridium difficile Clostridium tetanii Bacillus spp
Gram negative Neisseria meningitides Neisseria gonorrhoeae Pseudomonas aeruginosa Escherichia coli Legionella pneumophila Acinetobacter baumanii Salmonella
A few bacteria do not easily fit into the Gram-positive/negative dichotomy. The most medically significant of these are the Mycobacteria, which are responsible for diseases including tuberculosis and leprosy (Goering et al. 2007).
Viruses
Viruses are much smaller, and even simpler, than bacteria. They are often little more than a protein capsule containing some genetic material. They do not have cells, and some people do not even consider them to be alive. They have genes and will evolve through natural selection, but have no metabolism of their own. The most significant characteristic of viruses is that they can only reproduce within a host cell, by using the cell’s own mechanisms to reproduce the viral genetic material and to manufacture the other elements required to produce more virus particles. This often causes the death of the cell concerned (Goering et al. 2007).
The small size of viruses (poliovirus, for example, is only 30 nanometres (nm) across) means that most are smaller than the wavelengths of visible light. They can only be ‘seen’ with a specialist instrument such as an electron microscope, which will only be available in a very few hospital microbiology laboratories. Diagnosis of viral infections is normally by the patient’s symptoms, with confirmation by laboratory tests designed to detect either the virus itself or antibodies produced by the patient’s immune system as a response to infection (Wilson 2000).
There are viruses that specifically infect humans, or other animals, or plants, or even bacteria. This is one characteristic that can be used in classifying them. However, the main basis for classification is by the type of genetic material