14.1F: Innate Resistance

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Several barriers protect organisms from infection including mechanical, chemical, and biological barriers.

Learning Objectives

Discuss the various innate barriers within humans that provide protection from infection

Key Points

The flushing action of tears and urine also mechanically expels pathogens, while mucus secreted by the respiratory and gastrointestinal tract serves to trap and entangle microorganisms.
The human microbiome (or human microbiota ) is the aggregate of microorganisms that reside on the surface and in deep layers of skin, in the saliva and oral mucosa, in the conjunctiva, and in the gastrointestinal tracts.
Some of these organisms perform tasks that are useful for the human host, but the majority have no known beneficial or harmful effect.

Key Terms

lysozyme: A bacteriolytic (or antibiotic) enzyme found in many animal secretions and in egg white.
microbiota: The microbial flora harbored by normal, healthy individuals.
flora: the microorganisms that inhabit some part of the body, such as intestinal flora

Several barriers protect organisms from infection, including mechanical, chemical, and biological barriers. However, as organisms cannot be completely sealed against their environments, other systems act to protect body openings such as the lungs, intestines, and the genitourinary tract. In the lungs, coughing and sneezing mechanically eject pathogens and other irritants from the respiratory tract. The flushing action of tears and urine also mechanically expels pathogens, while mucus secreted by the respiratory and gastrointestinal tract serves to trap and entangle microorganisms.

Chemical barriers also protect against infection. The skin and respiratory tract secrete antimicrobial peptides such as the β-defensins. Enzymes such as lysozyme and phospholipase A2 in saliva, tears, and breast milk are also antibacterials. Vaginal secretions serve as a chemical barrier following menarche, when they become slightly acidic, while semen contains defensins and zinc to kill pathogens. In the stomach, gastric acid and proteases serve as powerful chemical defenses against ingested pathogens. Within the genitourinary and gastrointestinal tracts, commensal flora serve as biological barriers by competing with pathogenic bacteria for food and space and, in some cases, by changing the conditions in their environment, such as pH or available iron. This reduces the probability that pathogens will reach sufficient numbers to cause illness. However, since most antibiotics non-specifically target bacteria and do not affect fungi, oral antibiotics can lead to an “overgrowth” of fungi and cause conditions such as a vaginal candidiasis (a yeast infection). There is good evidence that re-introduction of probiotic flora, such as pure cultures of the lactobacilli normally found in unpasteurized yogurt, helps restore a healthy balance of microbial populations in intestinal infections in children and encouraging preliminary data in studies on bacterial gastroenteritis and inflammatory bowel diseases. Inflammation is one of the first responses of the immune system to infection.

The human microbiome (or human microbiota) is the aggregate of microorganisms that reside on the surface and in deep layers of skin, in the saliva and oral mucosa, in the conjunctiva, and in the gastrointestinal tracts. They include bacteria, fungi, and archaea. Some of these organisms perform tasks that are useful for the human host. However, the majority have no known beneficial or harmful effect. Those that are expected to be present, and that under normal circumstances do not cause disease, but instead participate in maintaining health, are deemed members of the normal flora.

Populations of microbes (such as bacteria and yeasts) inhabit the skin and mucosa. Their role forms part of normal, healthy human physiology. However, if microbe numbers grow beyond their typical ranges (often due to a compromised immune system) or if microbes populate atypical areas of the body (such as through poor hygiene or injury), disease can result.

Many of the bacteria in the digestive tract are collectively referred to as the gut flora. In this context, gut is synonymous with intestinal, and flora with microbiota and microflora, the word microbiome is also in use. They are able to break down certain nutrients such as carbohydrates that humans otherwise could not digest. The majority of these commensal bacteria are anaerobes, meaning they survive in an environment with no oxygen. Normal flora bacteria can act as opportunistic pathogens at times of lowered immunity.

Figure: **Gut Flora**: Gut flora consists of microorganisms such as Escherichia Coli that live in the digestive tracts of animals. It is the largest reservoir of human flora. In this context, gut is synonymous with intestinal, and flora with microbiota and microflora. The word microbiome is also in use.

Archaea are present in the human gut, but, in contrast to the enormous variety of bacteria in this organ, the numbers of archaeal species are much more limited. Fungi, in particular yeasts, are present in the human gut. The best-studied of these are Candida species. This is because of their ability to become pathogenic in immune compromised hosts. Yeasts are also present on the skin, particularly Malassezia species, where they consume oils secreted from the sebaceous glands.

A small number of bacteria are normally present in the conjunctiva. Staphylococcus epidermidis and certain coryneforms such as Propionibacterium acnes are dominant. The lachrymal glands continuously secrete, keeping the conjunctiva moist, while intermittent blinking lubricates the conjunctiva and washes away foreign material. Tears contain bactericides such as lysozyme, so that microorganisms have difficulty in surviving the lysozyme and settling on the epithelial surfaces.

The gut flora is the human flora of microorganisms that normally live in the digestive tract and can perform a number of useful functions for their hosts. Though people can survive with no gut flora, the microorganisms perform a host of useful functions such as fermenting unused energy substrates, training the immune system, preventing growth of harmful species, regulating the development of the gut, producing vitamins for the host (such as biotin and vitamin K), and producing hormones to direct the host to store fats. However, in certain conditions, some species are thought to be capable of causing disease by causing infection or increasing cancer risk for the host.