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The Chain of Infection and Main Groups of Micro-organisms Causing Infection

The Scope of Microbiology

Microbiology is the scientific study of organisms too small to be seen with the naked eye. They are ubiquitous and many perform essential ecological functions, for example breaking down the molecules of dead animals and plants which then re-enter the ecosystem. Some micro-organisms tolerate extreme conditions where other organisms would not survive (e.g. high temperatures) while others cause disease and are medically important.

Chain of Infection

The Chain of Infection is an epidemiological model applicable to all pathogens (micro-organisms able to cause disease). It comprises a series of events that must occur before pathogenic micro-organisms can spread and describes interactions between the pathogen, its host and the environment. Links in the chain are shown in Figure 1.1. Breaking a link in the chain can prevent infection.

Reservoir

The reservoir is where the pathogen lives and multiplies. Possible reservoirs include people (e.g. patients, nursing home residents, health workers and those visiting healthcare premises). They may show signs and symptoms of infection or be asymptomatic because they are mildly infected, incubating the infection or recovering from it. Two people affected by the same organism may present differently and many organisms are carried asymptomatically. For the classic communicable diseases of childhood (e.g. measles, mumps, rubella) and influenza, other people are the reservoir. Inanimate surfaces and objects (e.g. clinical equipment, clothing) can operate as reservoirs and are sometimes described as fomites (Box 1.1).

Figure 1.1 The chain of infection.

The environment operates as a reservoir for infections in premises where healthcare is delivered (Box 1.2). It is also the source of infections acquired in the community (e.g. tetanus, legionnaire's disease). Animals are reservoirs for a number of infections including rabies, Ebola disease, Lyme disease and exotic emerging infections such as monkeypox (MPX virus) and zika virus.

Box 1.1 Fomites

Fomites are defined as inanimate objects that can operate as vehicles for the transmission of an infectious agent. In healthcare settings, they include patient care items (e.g. bedclothes, bedpans, urinals) and environmental surfaces. They often give rise to outbreaks of infection because disinfection has not been undertaken or undertaken poorly.

Source: Adapted from Kanamori et al. (2017).

Box 1.2 Environmental Reservoirs for Healthcare-Associated Infection

In premises where healthcare is delivered, drains and sinks can become heavily contaminated with micro-organisms and have been identified as reservoirs when outbreaks occur. Problems are compounded when sinks in clinical areas are used for non-clinical purposes (e.g. to empty washbowls). Innovations to reduce risks include sinks which self-disinfect with chemicals or heat and 'waterless wards' where conventional sinks and plumbing are removed from patient care areas.

Portal of Exit

The portal of exit is the path taken by pathogens to escape from the reservoir. Respiratory pathogens (e.g. colds, influenza) are released in coughs, sneezes and spluttered speech. Enteric pathogens causing food-poisoning escape in vomit and faeces. Skin scales and dust can provide a portal of exit for some bacteria, including those frequently causing healthcare-associated infection (e.g. Staphylococcus aureus).

Mode of Transmission

The mode of transmission describes how the pathogen spreads (Table 1.1).

Transmission can occur by direct contact between surfaces such as hands and fomites and via contaminated food and water (e.g. cholera, typhoid). Houseflies (Musca domestica) breed in faecal material. Their feet become contaminated with micro-organisms which can be transferred to open wounds. This type of zoonosis is called mechanical transmission. It has been documented as a means of spread for Clostridioides difficile, methicillin-resistant S. aureus (MRSA), Esherichia coli and Salmonella spp. (Davies et al. 2016). Biological zoonotic transmission occurs when the pathogen lives and multiplies inside a vector. The infectious agent causing malaria (Plasmodium spp.) lives inside female mosquitoes (Anopheles spp.). Infection is transmitted when the mosquito bites a human host.

The air-borne route is an important mode of transmission for respiratory pathogens and the classic communicable diseases. The 2020 COVID-19 pandemic stimulated renewed interest in air-borne transmission. Conventionally, it was thought to occur via two distinct routes: droplet and air-borne transmission (Table 1.2).

Droplet transmission was proposed according to the findings of research dating from the 1930s that described the theoretical behaviour of particles according to their size (Wells 1934). From this work it was concluded that respiratory secretions are spread in two distinct ways according to their dimensions. According to this school of thought, droplets are thought unlikely to remain air-borne for long periods. Instead, they fall through gravity because of their relatively large size and their period of infectivity is correspondingly brief.

Table 1.1 Modes of microbial transmission.

Table 1.2 Respiratory transmission.

Aerosols were thought to remain suspended for much longer because of their minute size, depending on environmental conditions (e.g. humidity, turbulence, ventilation) (Tang et al. 2021). It has now been suggested that the distinction between air-borne, aerosol/droplet nuclei and large droplet transmission should be replaced by a unique non-contact air-borne transmission mode (Drossinos et al. 2021).

For many pathogens, there is more than one mode of transmission. Norovirus is spread by droplets released when vomiting occurs, by direct and indirect contact and by eating contaminated shellfish (Hassard et al. 2017). Many viruses responsible for respiratory and gastrointestinal infections can probably also be spread by fomites contaminated with body fluids (Boone and Gerba 2007).

Portal of Entry

The portal of entry is the route taken by pathogens to gain access to the tissues of the new host. The micro-organisms responsible for influenza and the classic communicable diseases are inhaled. Enteric pathogens gain access by ingestion. The urogenital tract is the portal of entry for urinary and sexually transmitted pathogens. Inoculation via skin and mucous membranes is the mode of entry for pathogens causing surgical site infection. Needlestick injury allows access for pathogens causing blood-borne infection: the viruses responsible for human immunodeficiency disease (HIV) and hepatitis B and C. Vertical transmission from mother to foetus occurs by two possible mechanisms: from the maternal to foetal circulation via the placenta (e.g. congenital syphilis) and via the contaminated birth canal (e.g. congenital gonorrhoea). Some pathogens are transmitted vertically via both routes (Box 1.3).

Box 1.3 Vertical Transmission: Group B Streptococcal Infections in Neonates

One in four women carry Group B Streptococcus vaginally without symptoms. It can infect the amniotic fluid before delivery or be acquired during passage down the birth canal, causing neonatal meningitis, pneumonia and septicaemia. Very low-weight babies are at greatest risk of developing severe infection and mortality can be as high as 30%. Infection is either early onset (during the first week of life) or late onset occurring when the...