Host Microbe Interactions

Causative Agent

The ancients thought that epidemics and diseases were divine punishment of the people for their sins. By the time of Moses, however, the Egyptians and Hebrews had come to believe that leprosy could be transmitted by contact with lepers. In Europe, around 430 B.C. Thucydides had concluded that some plagues were contagious. By the Middle Ages, many believed this, and they fled cities to escape the diseases. Fracastorius, in 1546, first proposed a theory that communicable diseases were caused by living agents, passed from one person or animal to another. He had no way to test this theory, however.

With the discovery of microorganisms in the late seventeenth century by Leeuwenhoek, people began to suspect that the microorganisms might cause disease, but the techniques of the times could not prove this. It was not until 1876 that Robert Koch offered convincing proof of the "germ theory" of disease when he showed that Bacillus anthracis is the cause of anthrax, an often fatal disease of humans, sheep, and other animals. With his microscope, he observed B. anthracis cells in the blood and spleen of dead sheep. He then inoculated mice with the infected sheep blood and was able to recover B. anthracis from the blood of the mice. In addition, he grew the bacteria in pure culture and showed that they caused anthrax when injected into healthy mice. From these experiments and later work with Mycobacterium tuberculosis, Koch formalized a group of criteria for establishing the cause of an infectious disease, known as Koch's Postulates.

—A Glimpse of History

EVERY DAY WE HAVE INTIMATE CONTACT WITH AN enormous number and variety of microorganisms. Every breath introduces some to our upper respiratory system, others are ingested with each bite of food or sip of a drink, and still more adhere to our skin whenever we touch an object or surface. The vast majority of these microbes generate no ill effects whatsoever. Some may colonize the body surfaces, taking up residence with the variety of other organisms that live there without causing harm, while others are sloughed off with dead skin cells. Most of those swallowed are eventually eliminated in feces.

Relatively few microbes are able to inflict any noticeable damage, invading tissues or producing toxic substances. Those organisms that can, and do, are called pathogens, and have distinct patterns of interaction with the host that enable them to have the upper hand in the relationship and elude at least some of the body's defenses. The mechanisms pathogens use to counter our innate and adaptive defenses, and subsequently cause damage, are the focus of a great deal of research. These studies are unraveling an impressive array of ploys that certain microorganisms and viruses have acquired for subverting and circumventing our sophisticated defense systems. Advances in genomics, immunology, and other areas over the last two decades have given new insight into the pathogenic strategies, fueling hope that therapies targeted to specific relationships between disease-causing microbes and the human host can be developed.

This chapter will explore some of the ways in which microbes colonize the human host, living either as members of the normal flora in harmony with the host or subverting the host defenses and causing disease. We will also discuss how disease-causing microorganisms and viruses are able to at least temporarily evade or overcome the host responses and cause damage to the host. Terms used in the study of infectious diseases are defined in table 19.1.

Chapter 19 Host-Microbe Interactions

Table 19.1 Terms Used in the Study of Infectious Diseases




Bacteria circulating in the bloodstream


Establishment and growth of a microorganism on a body surface


Noticeable impairment of body function


A host with weaknesses or defects in the innate or adaptive defenses

Inapparent infection

Infection with no obvious symptoms

Infectious disease

Disease caused by an infecting microorganism or virus

Latent infection

Infection in which the infectious agent is present but not active

Opportunistic pathogens

Organisms that cause disease only when introduced into an unusual location or into an immunocompromised host


An organism that benefits at the expense of another organism, the host


Any disease-causing microorganism or virus



Primary infection

Infection in a previously healthy person

Secondary infection

An additional infection that occurs as a result of a primary infection and that occurs during or immediately following the primary infection


Acute illness caused by infectious agents or their products circulating in the bloodstream

Systemic infection

Widespread infection through blood or lymph


Toxin circulating in the bloodstream


Viruses circulating in the bloodstream

Virulence determinants

Attributes of a microorganism or virus that promote pathogenicity


Many people think of microorganisms as "germs" that should routinely be avoided, failing to recognize that the microbial cells on the body actually outnumber human cells. Nearly all of these microbes co-exist peacefully with the human body, often providing beneficial aspects. The organisms that routinely reside on the body's surfaces are called the normal flora. It is a delicate balancing act, however; the normal flora, as well as microbes that make incidental contact with humans, are quite able to exploit body fluids and tissues as a source of nutrients should the opportunity arise.

Pathogens have developed the ability to make products such as damaging toxins or molecules to subvert defense systems, causing a disruption in the delicate balance. Weaknesses or defects in the innate or adaptive defenses can leave people vulnerable to invasion even by members of their own normal flora; the individual is said to be immunologically compromised, or immunocompromised. Factors that can lead to an individual becoming immunocompromised include malnutrition, cancer,

AIDS or other diseases, surgery, wounds, genetic defects, alcohol or drug abuse, and immunosuppressive therapy that accompanies procedures such as organ transplants.

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