Bacteria are unicellular organisms that evolved millions of years ago. They are visible under the high magnification of an ordinary light microscope using an appropriate stain. Surrounding the bacterial cell is a membrane made up of proteins and phospholipids and surrounding the membrane is a hard cell wall, which gives the organism its shape.

Bacteria are most commonly classified by their shape (Fig. 5.1) and their response to a laboratory reaction when treated with a dye called Gram’s stain. The response is determined by a chemical present in the bacteria’s cell wall. Bacteria are termed gram positive if they stain blue/purple and gram negative if they fail to take up the stain and remain the red colour of the counterstain.

Wilson (2006) states there are four main groups of bacteria: gram positive cocci and bacilli and gram negative cocci and bacilli, with other important groups including acid-fast bacilli, spirochaetes and atypical bacteria (see Box 5.1).

MRSA is an example of a gram positive coccus, and as with all bacteria has the potential to cause a whole range of infections including those of the lungs, wounds and urinary tract. More detail regarding specific bacteria, including the increasing problem of antibiotic resistance and methods used to prevent the spread of infection, are included in the Care Delivery section of this chapter.

Some bacteria have the ability to form spores to allow them to survive for longer when environmental conditions are not suitable for their cells to multiply. An example of this is Clostridium difficile, responsible for antibiotic-associated infections of the gastrointestinal tract.

Viruses are not cells but minute particles consisting of genetic material and protein. Each virus is a piece of nucleic acid that is protected by a protein coat or lipid membrane. Viruses are not usually defined as a living organism, and instead of growing and dividing like cells, they infect cells. For example, the human immunodeficiency virus (HIV) infects human T cells of the immune system; while viruses that cause the common cold attach themselves to the epithelial cell membrane and invade the cell, releasing new virus particles and destroying the host cell. Viruses are extremely small and can only be seen with a high-powered electron microscope. Unlike bacteria, most viruses are very fragile and cannot survive outside a living cell for very long. Viruses are fairly resistant to disinfectants, such as chlorhexidine, which are unable to penetrate the protein coat or lipid membrane.

There are many viruses (see Box 5.1), of which a number, primarily those transmitted by blood and body fluid, are of particular importance for nurses and other healthcare workers. They are referred to in some detail within the Care Delivery section of this chapter.

Prions – virus-like agents – are abnormal proteins that contain no nucleic acid. They are unique among microbes and appear to cause disease by replacing normal proteins on the surface of host cells and then gradually compromising their function. The main prion disease in humans in the UK is Creutzfeldt–Jakob disease (CJD), although scrapie, a prion disease in sheep, has been known for centuries. CJD is associated with the destruction of brain tissue. In 1996 a new form of CJD was found called variant CJD (vCJD) which generally affects young people under the age of 30. These cause a range of unusual neurological symptoms and it is likely that such cases are linked to exposure to bovine spongiform encephalitis (BSE) (Wilson 2006). Prion proteins are highly resistant to conventional methods of decontamination, including heat and many chemical disinfectants.

Fungi have a more complicated structure than bacteria and contain a nucleus. They are either branch shaped (e.g. mushrooms) or form buds (e.g. yeasts). There are over 700 000 species but few are pathogenic (see Box 5.1).

Candida, the yeast that is responsible for causing thrush, destroys the normal bacteria of the area it infects, namely the mouth, large bowel and vagina. The most common, C. albicans, causes vaginal thrush. Tinea is a fungus that produces a superficial infection. It is responsible for athlete’s foot, causing a painful and irritant rash in the folds of skin, most commonly between the toes. Deeper fungal infections are more common in hot climates.

Protozoa are microscopic single-celled animals (see Box 5.1), and although unusual in the UK, they are very common in other parts of the world. Associated with poor sanitation, UK cases are often acquired from abroad. Two pathogens found in the UK are Cryptosporidium, a water borne pathogen which is a common cause of gastroenteritis in children under the age of 5 years, and Trichomonas vaginalis, a sexually transmitted infection which causes a foul-smelling, green–yellow vaginal discharge (Wilson 2006). Malaria, caused by the protozoon Plasmodium falciparum, remains an endemic disease in many parts of the African, Indian and Asian continents. Although malaria does not occur in the UK, nearly 2000 cases are reported in travellers returning to the UK each year (Health Protection Agency 2005); it can be easily avoided by taking antimalarial medication and other precautionary measures. Neither Trichomonas nor malaria poses any risk of cross-infection in hospitals. A third protozoon, Toxoplasma gondii, is a parasite that lives in the intestine of cats, cysts of which are released in their faeces. These parasites can remain alive in soil and it is possible for humans to become infected by either handling cat faeces or contaminated soil. People at particular risk are women in early pregnancy when the infection can cause foetal death or brain damage.

The patient or host is the final link in the chain (see Fig. 5.2) and is the most common reservoir or source of microorganisms in hospital departments, particularly their body secretions, excretions and skin lesions. A person who acquires microorganisms does not necessarily develop an infection; they may simply act as a source but be a risk to other susceptible patients. These patients are sometimes called ‘carriers’ as they carry infections around and transfer them to other patients.

The hepatitis B virus and the meningitis bacterium (Neisseria meningitidis) are two examples of microorganisms present in carriers. There are between 2% and 10% of those infected who fail to completely eliminate the virus and continue to carry it in their blood (Wilson 2006). Similarly many people are unknowingly carriers of N. meningitidis bacteria in their respiratory tract; they show no symptoms but can pass the bacteria on to other susceptible people.

Prior to the introduction of antibiotics death from sepsis was common and until recently many infections could be successfully treated despite the increasing problems of resistance to antibiotics. While few microorganisms show resistance to all antibiotics, there are key pathogens which pose significant problems in healthcare settings in the UK. These include MRSA and multidrug-resistant Mycobacterium tuberculosis (Wilson 2006). A number of factors are considered to play a key role in the increasing resistance of antimicrobials, including the unnecessary and inappropriate use of antibiotics (Wilson 2006).

Staphylococcus aureus, a gram positive coccus, is an important cause of infection in both hospitals and the community, and remains the commonest cause of wound infection, after either accidental injury or surgery (Department of Health 2005b). Today, owing to this bacterium’s remarkable ability to adapt to the presence of antibiotics, approximately 90% of hospital strains and 50% of community strains are resistant to penicillin. Today MRSA occurs worldwide with strains usually resistant to two or more antibiotics (see Evolve 5.1 for more detailed information).

Resistance to drugs originally used to treat tuberculosis has occurred since streptomycin was first used in the 1940s. Treatment regimens were then devised using a second drug to destroy resistance to the first drug. Today treatment now requires a combination of three or four drugs for at least 6 months. A World Health Organization report has indicated resistance to the four first-line drugs in 35 countries (Lawrence & May 2003). In England, Wales and Northern Ireland, a report covering the years 2002–5 showed 8.7% had a resistance to one or more of the first-line drugs (isoniazid and rifampicin), and 0.9% were multidrug-resistant (Health Protection Agency 2006a). Of particular concern worldwide is HIV related drug-resistant tuberculosis, with infection more likely to become active or a latent infection more likely to reactivate. The poor absorption of drugs and interactions with other drugs makes the treatment of tuberculosis in HIV-positive patients much more complicated.

The body’s immune system resists the invasion of microorganisms and protects against foreign material. The first line of defence is the non-specific or passive immune response. Body systems play unique roles providing anatomical and physical barriers against the invasion of pathogens (Table 5.2).

The body’s specific or active immune response involves innate (non-specific) immunity and acquired immunity. Innate immunity is provided by genetic and cellular factors and can be influenced by age, nutritional status and any underlying disease. Active immunity results when the host develops antibodies following immunization. The active immune system is made up of two types of white blood cells:

Lymphocytes have memory cells and if they encounter a particular antigen – a response that may trigger an immune response – they become active and respond by producing large numbers of specific lymphocytes and create an antibody to fight the invading microorganisms.

Decontamination is the collective term used to describe the three important physical processes of cleaning, disinfection and sterilization. Inadequate decontamination has been cited as being responsible for outbreaks of infection in hospital (Wilson 2006).

The choice of decontamination method depends on the level of risk the item poses as a source of infection and the toleration of the method of decontamination (Lawrence & May 2003). The emergence of infections such as HIV, CJD and hepatitis C has seen an increased focus on decontamination procedures and policies, with particular attention paid to the potential of medical equipment to transmit such infections. An EU Directive (93/42EEC) prevents the reuse of single-use items by healthcare staff, and national standards for the provision of decontamination processes (NHS Estates 2003) ensures that all reusable medical devices are decontaminated to an acceptable standard.

Cleaning – the physical removal of microorganisms and organic matter on which they thrive – is considered appropriate for equipment or practices that are considered low risk (Box 5.4). After cleaning, any article should have fewer microorganisms on it, but the dry method might simply redistribute the microorganisms into the air, while the wet method might distribute and increase the microorganisms through the use of contaminated articles such as mop heads and cloths or contaminated water. Approximately 80% of microorganisms are removed during the cleaning procedure, with drying equally important to prevent any remaining bacteria from multiplying (Wilson 2006). While cleaning alone may be an adequate method of decontamination, it is also an essential preparation for items requiring disinfection or sterilization.

Disinfection – the destruction of vegetative microorganisms to a level unlikely to cause infection – reduces the number of viable microorganisms but may not inactivate some bacterial spores (Lawrence & May 2003). It is associated with equipment that may come in close contact with mucous membranes but is not used for invasive procedures (see Box 5.4). Pathogens remaining after disinfection may pose an infection risk to particularly susceptible patients, for example those receiving cytotoxic (chemotherapy) therapy.

The two main methods of disinfection are heat and chemicals (see Box 5.4). Heat is the preferred method for disinfecting articles (e.g. surgical instruments) as it is more penetrative and easier to control than chemicals. Chemical disinfection may be required if heat is unsuitable, for example skin disinfection and heat sensitive items such as fibreoptic endoscopes. The choice is complex and requires a working knowledge of the disinfectants available and the make-up of the article that requires disinfecting (see Box 5.4).

Sterilization is the complete destruction or removal of all living microorganisms including bacterial spores (Lawrence & May 2003) and involves the use of heat, gas, chemicals or irradiation (see Box 5.4). Items requiring sterilization are described as being high risk to patients. Sterilization is recommended for all instruments and equipment used during invasive procedures (for example intravenous (IV) cannulas, surgical instruments, urinary catheters). As with disinfection, the choice of method used depends upon the item being sterilized (see Box 5.4).

Individual susceptibility varies enormously and can be caused when a person’s immunity is impaired. Particular groups of patients are known to be at a greater risk (Table 5.3). The Third Prevalence Survey of HCAI in acute hospitals showed that in 2006 one in seven (7.6%) of patients in UK and Ireland had an infection or were being treated for an infection that they did not have on admission to hospital (Hospital Infection Society 2006). The cost to the National Health Service has previously been approximately £1000 million per year (National Patient Safety Agency 2004). While it is acknowledged that not all HCAI are preventable, the 2005 Lowbury lecture stated that globally 10–70% are preventable (Hambraeus 2006).