16.4: Pathogens

An infection is the invasion of an organism's body tissues by disease-causing agents, their multiplication, and the reaction of host tissues to the infectious agents and the toxins they produce. An infectious disease, also known as transmissible disease or communicable disease, is an illness resulting from an infection.

Infections can be caused by a wide range of pathogens, most prominently bacteria and viruses. Hosts can fight infections using their immune systems. Mammalian hosts react to infections with an innate response, often involving inflammation, followed by an adaptive response.

Pathophysiology

There is a general chain of events that applies to infections, sometimes called the chain of infection (Figure \(\PageIndex{1}\)). The chain of events involves several steps – which include the infectious agent, reservoir, entering a susceptible host, exit, and transmission to new hosts. Each of the links must be present in chronological order for an infection to develop. Understanding these steps helps healthcare workers target the infection and prevent it from occurring in the first place.

Figure \(\PageIndex{1}\): Chain of infection; the chain of events that lead to infection

Colonization

Infection begins when an organism successfully enters the body, grows, and multiplies. This is referred to as colonization. Individuals with compromised or weakened immune systems have an increased susceptibility to chronic or persistent infections. Individuals who have a suppressed immune system are particularly susceptible to opportunistic infections. Entrance to the host at host-pathogen interface generally occurs through the mucosa in orifices like the oral cavity, nose, eyes, genitalia, anus, or the microbe can enter through open wounds. While a few organisms can grow at the initial site of entry, many migrate and cause systemic infection in different organs. Some pathogens grow within the host cells (intracellular) whereas others grow freely in bodily fluids (Figure \(\PageIndex{2}\)).

Wound colonization refers to non-replicating microorganisms within the wound, while in infected wounds, replicating organisms exist and tissue is injured. All multicellular organisms are colonized to some degree by extrinsic organisms, and the vast majority of these exist in either a mutualistic or commensal relationship with the host. An example of the former is the anaerobic bacteria species, which colonizes the mammalian colon, and an example of the latter are the various species of staphylococcus that exist on human skin. Neither of these colonizations are considered infections.

The difference between an infection and a colonization is often only a matter of circumstance. Non-pathogenic organisms can become pathogenic given specific conditions, and even the most virulent organism requires certain circumstances to cause a compromising infection. Some colonizing bacteria, such as Corynebacteria sp. and viridans streptococci, prevent the adhesion and colonization of pathogenic bacteria and thus have a symbiotic relationship with the host, preventing infection and speeding wound healing.

Figure \(\PageIndex{2}\): This image depicts the steps of pathogenic infection.

The variables involved in the outcome of a host becoming inoculated by a pathogen and the ultimate outcome include:

• the route of entry of the pathogen and the access to host regions that it gains

• the intrinsic virulence of the particular organism

• the quantity or load of the initial inoculant

• the immune status of the host being colonized

As an example, several staphylococcal species remain harmless on the skin, but, when present in a normally sterile space, such as in the capsule of a joint or the peritoneum, multiply without resistance and cause harm.

Disease

Disease can arise if the host's protective immune mechanisms are compromised and the organism inflicts damage on the host. Microorganisms can cause tissue damage by releasing a variety of toxins or destructive enzymes. For example, Clostridium tetani releases a toxin that paralyzes muscles, and staphylococcus releases toxins that produce shock and sepsis. Not all infectious agents cause disease in all hosts. For example, less than 5% of individuals infected with polio develop disease. On the other hand, some infectious agents are highly virulent. The prion-causing mad cow disease and Creutzfeldt–Jakob disease invariably kill all animals and people that are infected.

Persistent infections occur because the body is unable to clear the organism after the initial infection. Persistent infections are characterized by the continual presence of the infectious organism, often as latent infection with occasional recurrent relapses of active infection. There are some viruses that can maintain a persistent infection by infecting different cells of the body. Some viruses once acquired never leave the body. A typical example is the herpes virus, which tends to hide in nerves and become reactivated when specific circumstances arise.

Transmission

For infecting organisms to survive and repeat the infection cycle in other hosts, they (or their progeny) must leave an existing reservoir and cause infection elsewhere. The relationship between virulence versus transmissibility is complex; if a disease is rapidly fatal, the host may die before the microbe can be passed along to another host. Infection transmission can take place via many potential routes:

• Droplet contact, also known as the respiratory route, and the resultant infection can be termed airborne disease. If an infected individual coughs or sneezes on another individual the microorganisms, suspended in warm, moist droplets, may enter the body through the nose, mouth or eye surfaces.

• Fecal-oral transmission, wherein food or water become contaminated and individuals who eat and drink them become infected. Common fecal-oral transmitted pathogens include Vibrio cholerae, Giardia species, rotaviruses, Entameba histolytica, Escherichia coli, and tapeworms. Most of these pathogens cause gastroenteritis.

• Sexual transmission, with the resulting disease being called sexually transmitted disease

• Oral transmission, diseases that are transmitted primarily by oral means may be caught through direct or indirect oral contact between individuals.

• Transmission by direct contact, Some diseases that are transmissible by direct contact include athlete's foot, impetigo and warts.

• Vehicle transmission, transmission by an inanimate reservoir (food, water, soil)

• Vertical transmission, directly from the mother to an embryo, fetus or offspring during pregnancy or birth. It can occur as a result of a pre-existing infection or one acquired during pregnancy.

• Latrogenic transmission, due to medical procedures such as injection or transplantation of infected material.

• Vector-borne transmission, transmitted by a vector, which is an organism that does not cause disease itself but that transmits infection by conveying pathogens from one host to another (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): A southern house mosquito (Culex quinquefasciatus) is a vector that transmits the pathogens that cause West Nile fever and avian malaria among others.