In this chapter, we give an overview of the different kinds of pathogens, as well as those microorganisms that colonize our body without causing trouble. We then discuss the cell biology of infection—the molecular interactions between pathogens and their host. In Chapter 24, we consider how our innate and adaptive immune systems collaborate to defend us against pathogens.
We normally think of pathogens as hostile invaders, but a pathogen, like any other organism, is simply exploiting an available niche in which to live and procreate. Living on or in a host organism is a very effective strategy, and it is possible that every organism on Earth is subject to some type of infection (Figure 23–1). A human host is a nutrient-rich, warm, and moist environment, which remains at a uniform temperature and constantly renews itself. It is not surprising that many microorganisms have evolved the ability to survive and reproduce in this desirable niche. In this section, we discuss some of the common features that microorganisms must have in order to colonize the human body or cause disease, and we explore the wide variety of organisms that are known to cause disease.
The Human Microbiota Is a Complex Ecological System That Is Important for Our Development and Health
The human body contains about 1013 human cells, as well as a microbiota consisting of approximately 1014 bacterial, fungal, and protozoan cells, which represent thousands of microbial species—the so-called normal flora. The combined genomes of the various species of the human microbiota, called the microbiome, contain more than 5 × 106 genes—more than 100 times greater than the number of genes in the human genome itself. A consequence of this genomic diversity is that the microbiota expands the range of biochemical and metabolic activities available to the humans.
The microbiota is usually confined to the skin, mouth, digestive tract, and vagina. With the exception of microbes colonizing the skin, it consists primarily of anaerobic bacteria, with distinct communities of species inhabiting each body part. These communities vary considerably between individual humans, even between close relatives or identical twins. Although the microbiota of an individual is generally consistent over time, it is influenced by a variety of factors, including age, diet, health status, and antibiotic use.
There are various ecological relationships that these microbes have with their host. In mutualism, both the microbe and host benefit. The anaerobic bacteria that inhabit our intestines, for example, gain shelter and a nutrient supply but also contribute to the digestion of our food, produce important nutrients for us, and are essential for the normal development of our gastrointestinal tract and innate and adaptive immune systems. In commensalism, the microbe benefits but offers no benefit and causes no harm: for example, we are infected with many viruses that have no noticeable effect on our health. In parasitism, the microbe benefits to the detriment of the host, as is often the case for pathogens.
Many infectious diseases are caused by a single pathogen. There is increasing evidence, however, that an imbalance in the community of microbes that constitute the microbiota can contribute to some diseases, including autoimmune and allergic diseases, obesity, inflammatory bowel disease, and diabetes. Remarkably, in such cases of microbiota imbalance (referred to as dysbiosis), the transfer of the microbiota from a healthy individual to someone suffering from the disease can be beneficial and sometimes curative, as in the case of Clostridium difficile colitis caused by overgrowth of the bacterium.
Pathogens Interact with Their Hosts in Different Ways
If it is normal for us to live with a community of microbes, why are some of them capable of causing us illness or death? Although the ability of a particular
Figure 23–1 Parasitism at many levels.
Most animals harbor parasites, an example being the blacklegged tick or deer tick (Ixodes scapularis), shown here on a human finger. Although ticks of this species thrive on white-tailed deer and other wild mammals, they can also live on humans.