The Royal Marsden Hospital Manual of Clinical Nursing Procedures - Lisa Dougherty [83]
This section describes the different types of organisms that may be encountered in a healthcare environment as well as the differences between and within the different types (Goering et al. 2007, Wilson 2000).
Bacteria
Bacteria are probably the most important group of micro-organisms in terms of infection prevention and control because they are responsible for many opportunistic infections in healthcare. A healthy human being will typically be host to one quadrillion (1000 trillion, or 1015) bacteria – around ten times as many organisms as there are cells in the human body. In normal circumstances the relationship between these bacteria and their host is commensal (i.e. their presence does not cause the host any problems) and may be mutually beneficial. For example, E. coli present in the gut can be an aid to digestion. However, when circumstances change, these commensal organisms can cause infections. If the E. coli in the example above are transferred from the gut to the urinary tract, a urinary tract infection can result.
Whether or not any particular situation will result in an infection depends on a wide range of factors and is not always predictable. What is certain is that bacterial infections cannot occur when bacteria are not present, hence the importance of measures designed to minimize the risk of transmission. However, the presence of bacteria does not necessarily indicate an infection – as noted above, many millions of bacteria live on and in the human body without causing harm – so the diagnosis of a bacterial infection and any decision about treatment must be made by considering a combination of the patient’s symptoms and laboratory results that may indicate the presence of any particular bacteria (Wilson 2000).
Bacteria are what are known as prokaryotes, as opposed to eukaryotes, the term used for more complex organisms such as humans. This means that bacterial cells are much smaller and simpler than human cells, typically about the size of some of the structures such as mitochondria that exist within a mammalian cell. This small size means that bacteria do not have separate structures (such as a nucleus) within their cells. What bacteria do have and mammalian cells do not is a cell wall that contains the rest of the cell and gives it a distinctive shape (Goering et al. 2007). Some of these shapes are illustrated in Table 3.1. In terms of healthcare-associated infections, the most important bacteria are generally rod shaped or spherical.
Table 3.1 Bacterial shapes and arrangements
The structure of the cell wall determines another important distinction in medically significant bacteria: whether they are Gram positive or Gram negative. The ‘Gram’ in these terms refers to Gram staining, named after its Dutch inventor, Hans Christian Gram (1853–1938), who devised the stain in 1884. Put simply, the structure of the cell wall determines whether or not the bacteria are able to retain a particular stain in the presence of an organic solvent such as acetone. The structure of the cell wall determines other characteristics of the bacteria, including their susceptibility to particular antibiotics, so knowing whether the cause of a bacterial infection is Gram positive or negative can help to determine appropriate treatment (Goering et al. 2007). The structure of the two different types of cell wall is shown in Figure 3.1.
Figure 3.1 Gram-positive (a) and Gram-negative (b) bacterial cell walls. Used with permission from Elliot (2007).