14.1B: Colonization and Growth

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Infection begins when an organism successfully colonizes a host by entering the host’s body, growing and multiplying from there.

Learning Objectives

Distinguish between colonization and infection

Key Points

Some virulent bacteria produce special proteins that allow them to colonize parts of the host body.
Wound colonization refers to nonreplicating microorganisms within the wound, while in infected wounds replicating organisms exist and tissue is injured.
While a few organisms can grow at the initial site of entry, many migrate and cause systemic infection in different organs.

Key Terms

infection: An uncontrolled growth of harmful microorganisms in a host.

Infection begins when an organism successfully colonizes by entering the body, growing and multiplying from there. Most humans are not easily infected. Those who are weak, sick, malnourished, have cancer or are diabetic possess 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 generally occurs through the mucosa in orifices like the oral cavity, nose, eyes, genitalia, anus, or 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. Some virulent bacteria produce special proteins that allow them to colonize parts of the host body. Helicobacter pylori is able to survive in the acidic environment of the human stomach by producing the enzyme urease. Colonization of the stomach lining by this bacterium can lead to gastric ulcer and cancer. The virulence of various strains of Helicobacter pylori tends to correlate with the level of production of urease.

Wound colonization refers to nonreplicating 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 is various species of staphylococcus that exist on human skin. Neither of these colonizations are considered infections.

Figure: **Staphylococcus aureus**: Staphylococcus is a Gram-positive bacteria which includes several species that can cause a wide variety of infections in humans and other animals through infection or the production of toxins.

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. They thus have a symbiotic relationship with the host, preventing infection and speeding wound healing.

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, and the immune status of the host being colonized. As an example, the Staphylococcus species remains harmless on the skin. But when present in a normally sterile space, such as in the capsule of a joint or the peritoneum the Staphylococcus species multiplies without resistance and creates a burden on the host.