VOL: 98, ISSUE: 38, PAGE NO: 48
Dinah Gould, PhD, BSc, RGN, is professor of nursing, South Bank University, LondonApproximately 10% of all inpatients in the UK will develop an infection that they did not have on admission to hospital. Traditionally such infections have been described as hospital-acquired infections. However, they are now increasingly described as health-related infections (HRI) because they can develop in people who have received some, or indeed, all of their care in community settings.
Approximately 10% of all inpatients in the UK will develop an infection that they did not have on admission to hospital. Traditionally such infections have been described as hospital-acquired infections. However, they are now increasingly described as health-related infections (HRI) because they can develop in people who have received some, or indeed, all of their care in community settings.
Infection is a major cause of continuing ill health among people who have received health care and, in severe cases, can result in death. Those who develop HRIs take longer to return to their normal activities than those who have not developed infection. They require more medical and nursing care and are much more expensive to treat.
Controlling HRIs is therefore critically important to the provision and management of health care services. Infection control polices and procedures are designed to protect all patients and the staff responsible for their care from infection, irrespective of the setting in which that care is delivered. Nurses have professional and legal responsibility to integrate effective infection control practices into the care they provide.
Causes of HRI
HRI is caused mainly by bacteria and also by some viruses and fungi. Bacteria are present everywhere, and most live harmlessly in the environment. Only a few species of bacteria are pathogenic. Virulence, the ability to generate infection, is a complex phenomenon related to the physiology of both pathogen and host.
Some bacteria are always highly virulent. However, others - particularly those causing HRI - are of low pathogenicity. They cause infection only in people whose immune status is compromised by illness, drugs or the invasive procedures they have undergone (for example, surgery, intubation or insertion of intravenous lines). They do not attack healthy tissues and these bacteria are called opportunists. Examples include Pseudomonas, Klebsiella, and Proteus.
Other bacteria live harmlessly in or on one particular part of the body. These make up the normal flora and are called commensals. They receive shelter and benefit the host by keeping potentially dangerous micro-organisms at bay. However, if they gain access to a different anatomical location they can generate infection. For instance, Escherichia coli - normally present in the bowel - can cause urinary infection if it gains access to the bladder. This is an example of endogenous (self-) infection, occurring when the organisms responsible originate from the same individual. Exogenous (cross) infection occurs when micro-organisms originate from another source, another patient, staff or the environment.
Infection and colonisation
Infection occurs when pathogens gain access to host tissues and stimulate a response: infection in a wound is indicated by the appearance of inflammation and pus.
The patient may become pyrexial, and a swab will indicate the presence of large numbers of the causative organism.
However, response to a pathogen may be slight or absent, a situation described as colonisation. The skin, including any wounds that may be present, is colonised by a population of commensal bacteria that do no harm in health.
Before infection is possible, a susceptible patient must encounter a virulent micro-organism. The pathogen must gain access to host tissues, move to a favourable site, multiply successfully despite defence mechanisms mustered by the host and reproduce so that new pathogens can escape to be disseminated, completing the life cycle.
Gaining access; portals of entry
Invasion occurs by inhalation, ingestion, via the urogenital tract, by inoculation and by vertical transmission (Table 1).
The ability to establish infection depends on the virulence of the micro-organism. Several factors contribute:
- The size of the inoculating dose - except in the case of very virulent pathogens, large numbers of microorganisms are more likely to cause infection;
- The ability to invade host tissues depends on the structure of the bacteria and the production of enzymes and toxins. Fine hair-like processes called fimbriae on the cell surfaces of some bacteria enable them to attach to the cells of the host before they invade. Strains without fimbriae are unable to attach and cannot therefore operate as pathogens;
- The ability to damage host tissues is closely related to ability to invade. Damage may be structural (tissues may be destroyed physicaIly) or physiological (normal function becomes disturbed). In most cases both types occur: Staphylococcus aureus, for exampIe, destroys tissue because infection causes abscess formation. These bacteria are sometimes described as coagulase-positive because they release an enzyme called coagulase that is responsible for tissue degradation. Staphylococcus epidermidis, which does not release the enzyme, cannot cause the same type of damage. It is described as coagulase-negative.
Escape from the body
In many cases bacteria leave the body via the entry route, but there are exceptions. Those causing gastroenteritis gain access via the mouth and leave in faeces. They are said to be disseminated by the faecal-oral route.
Host resistance and infection
The condition of the host is an important factor in helping to determine whether the individual exposed to pathogenic micro-organisms will go on to develop a clinical infection. Although colonised by bacteria that have the potential to operate as pathogens, some people do not themselves become infected and remain symptom-free. Host resistance to infection depends on the body's general defence mechanisms: innate and acquired immunity.
Non-specific immunity helps the body to resist invasion by micro-organisms. Invasion is prevented by the anatomical arrangement of the tissues, secretion of fluids which wash foreign materials from the body, and the presence of normal flora covering the skin and lining the gut. Intact skin and mucous membranes are the body's most important non-specific defence.
The gastrointestinal system is protected by its powerful acid and alkaline secretions. For example, the pH of gastric acid is too low for most bacteria to survive.
The respiratory system is protected by the coughing and sneezing reflexes. Inside the nose the turbinate bones trap smaller particles as inspired air travels over them. Lymphoid tissue in the pharygeal and nasopharygeal tonsils traps remaining pathogens. The entire respiratory tree, except for the alveoli, is lined with specialised epithelium. Mucus traps foreign substances and is then carried upwards to the pharynx by the action of cilia.
The vagina contains a population of lactobacilli. These bacteria metabolise glycogen, a chemical present in cervical secretions, forming lactic acid. The pH of the healthy adult vagina is approximately 4.5, inhibiting the growth of other organisms. The bladder has little protection against pathogens, and urinary infections are common. Lysozyme, present in many body fluids, destroys bacteria.
If bacterial invasion occurs, inflammation and the activity of phagocytic cells in the blood and tissues limit spread. Inflammation is the response of tissues to trauma, whether injury involves cuts, chemical damage or pathogenic invasion. The hallmarks of inflammation are:
- Erythema (redness);
- Loss of function (depending on the extent of injury).
Phagocytosis occurs when a neutrophil or macrophage engulfs a pathogen. The first step is opsonisation, the attachment of a ligand (group of molecules) to the surface of the phagocytic cell, stimulating the action of the contractile proteins myosin and actin present within the cytoplasm.
Endocytosis follows: the pathogen is engulfed, entering a vacuole created by the phagocytic membrane. Lysozomes in the cytoplasm fuse with the vacuole, emptying strongly acidic enzymes (catalase, myeloperoxidase) on to the pathogen, destroying it.
Acquired immunity is active when an individual develops their own antibodies in response to an antigen. This can occur after immunisation or an infection.
Some patients are much more vulnerable to infection than others. Very sick people are described as immunocompromised. They are more likely to undergo invasive procedures and treatments which bypass the body's normal barriers to micro-organisms. These people are therefore much more susceptible to infections.
Micro-organisms are spread by direct or indirect contact. Dissemination is also possible via the airborne route and in contaminated food and water. In a few cases spread is by insect vectors - for example, malaria is spread by mosquitoes. Most HRIs are disseminated by contact, chiefly on the hands of health professionals (Gould and Brooker, 2000).
The transfer of micro-organisms from one person to the next is described as cross-infection or exogenous infection. It is the main cause of HRIs (Gould et al, 2000). The source is frequently another patient, a member of staff or objects in the immediate patient environment. When the source is environmental, transfer generally occurs via hands, which can become contaminated after handling patients, washing things (soaps, flannels, damp towels), after handling patients' clothing, after bed-making and drawing bedside curtains. Endogenous infection results when micro-organisms are transferred from one site on the patient where they do no harm to a vulnerable site. For example, bacteria carried harmlessly on intact skin can cause infection if transferred via the hands of a health professional or the patient to an open wound.
Large numbers of bacteria are present deep in the crevices of the stratum corneum of the skin and the ducts of sweat glands (the resident or normal flora). These are difficult to dislodge and, as a result, they are not likely to be implicated in the spread of infection. However, they have the potential to cause infection if they are carried into the underlying tissues during invasive procedures, such as surgery.
The bacteria that cause cross-infection are carried loosely in the upper layers of the stratum corneum (the transient flora). They become detached readily when any object in the environment is handled. However, they can be removed easily by decontamination, and this immediately reduces the risk of cross-infection. Skin that is damaged or moist carries more bacteria than healthy skin. This is an important factor contributing to cross-infection. Wearing a ring or wrist watch tends to increase the moisture of the skin beneath, thus increasing the risk of cross infection.
Hand decontamination and reduction in infection rates
Ignaz Semmelweis demonstrated the relationship between hand decontamination and reducing infection rates in the 1840s. By persuading doctors to decontaminate their hands before attending women in labour, he was able to demonstrate a dramatic reduction in the rate of puerperal sepsis over a period of only a few weeks.
The value of hand decontamination in reducing the transmission of infection has since been supported by studies in the UK and many other countries (Larson and Kretzer, 1995).
It has been established that micro-organisms carried on the hands of health professionals are the same strains as those on the skin but not usually the same as those found in the environment distant from the patient - for instance on the floor, in sinks and down drains. When strict regimes of hand decontamination have been introduced the rate of infection falls but inevitably increases again as lapses creep back into practice.
Bacteria causing health-related infection
Bacteria are classified as Gram positive or Gram negative on the basis of a laboratory test that stains them differently to aid identification during diagnosis. Both can be spread by cross-infection and must be effectively controlled. Gram staining is valuable because it distinguishes structural differences between Gram positive and negative bacteria and provides an indication of their behaviour.
Gram positive bacteria
Gram positive bacteria tend to be more resistant to desiccation and to tolerate dry conditions better than Gram negative species. Streptococci and staphylococci are examples of Gram positive bacteria that have been implicated in health related infections.
The maternal deaths, which Semmelweis investigated and prevented, were caused by Streptococcus pyogenes. Streptococci were originally the bacteria most frequently implicated in cross-infection in health care settings. Between 1930 and 1950 - the time when cross-infection first began to be considered as a serious concern by medical microbiologists - they were notorious for causing outbreaks of infection. Over the years, however, staphylococci have become the dominant pathogens associated with HRIs because of their ability to develop resistance to a wide range of antibiotics. In contrast, most strains of streptococci have remained sensitive to penicillin and its derivatives.
Staphylococcus aureus is carried by 30-50% of the general population. Carriers are not clinically infected, but the bacteria can be transmitted from a healthy person to someone whose natural resistance is low and may cause infection in this vulnerable new host. The bacteria can also be transferred from their usual site of carriage to a more vulnerable site on the same person - for example, from the nose to an open wound. Once established, they multiply, setting up clinical infection. In some cases this can be very damaging - for example, in hospital Staphylococcus aureus is responsible for serious infections, including osteomyelitis, endocarditis and wound infections. In the community troublesome infections such as conjunctivitis, boils, abscesses and pneumonia are caused by it. Staphylococcus aureus first became a significant cause of infection in hospitals in the 1940s and has been responsible for many outbreaks, leading to ward closures, disrupted services and considerable morbidity and mortality. By the 1950s a penicillin-resistant strain had evolved and today a very high proportion of staphylococcal infections remain resistant to this antibiotic.
Initially two factors contributed to successful control in hospital: research to prevent wound infections and the development of new synthetic penicillins. However, careless prescribing and lack of attention to infection control policies and procedures throughout the 1960s and 1970s promoted the emergence of methicillin resistant Staphylococcus aureus.
Skin lesions often become heavily colonised and are thus likely to operate as sources of infection to others. The bacteria are most likely to spread throughout a population if patients are taking a number of different antibiotics and infection control precautions are indifferent, especially hand hygiene.
Gram negative bacteria
Gram negative bacteria thrive in damp situations and have greater natural resistance to antibiotics than Gram positive bacteria, mainly because of the different chemical composition of their cell walls. These bacteria were once regarded as harmless. However, the initial conquest of staphylococci in the 1950s was followed by increasing levels of colonisation and infection by Gram negative species.
It is suggested that Pseudomonas aeruginosa is spread by direct contact in critical care units. Items directly touching patients (washbowls, soapdishes, nail-brushes) can become contaminated. Nurses' hands often became colonised, supporting the view that people and objects in close proximity to the patient operate as the source. Prevention of cross infection is vital, as Gram negative bacteria continue to cause a high proportion of HRIs, especially among critically ill patients.
The significance of hand hygiene in preventing HRI
Hand hygiene is the most effective means of controlling HRI, regardless of the clinical setting in which care is delivered (Gould et al, 2000). Many Gram positive bacteria, particularly staphylococci, have undergone mutation and are resistant to most antibiotics in common use, while Gram negative bacteria tend to have natural immunity. Meanwhile, elderly people and the very sick who are naturally susceptible to infection are receiving invasive interventions that place them at high risk. In contrast, there has been little indication of bacteria developing resistance to antiseptics, and this has led to increased interest in the use of traditional measures, such as hand decontamination, to help break the chain of infection.
- Part 2: the importance of hand hygiene appears in NTplus Infection Control on November 12.