Porth's Essentials of Pathophysiology, 4e

298

Infection and Immunity

U N I T 4

with a brief discussion of the terminology used to char- acterize interactions between humans and microbes. Any organism capable of supporting the nutri- tional and physical growth requirements of another is called a host. Throughout this chapter, the term host most often refers to humans supporting the growth of microorganisms. Occasionally, infection and coloniza- tion are used interchangeably. However, the term infec- tion describes the presence and multiplication within another living organism, with subsequent injury to the host, whereas colonization describes the act of estab- lishing a presence, a step required in the multifaceted process of infection. One common misconception should be dispelled from the start: not all interactions between microor- ganisms and humans are detrimental—in fact, most are beneficial. The internal and external exposed surfaces of the human body are normally and harmlessly inhab- ited by a multitude of bacteria, collectively referred to as the normal microflora. Although the colonizing bacteria acquire nutritional support from the host, the host is not adversely affected by the relationship. An interaction such as this is called commensalism , and the colonizing microorganisms are sometimes referred to as commensal flora. The term mutualism is applied to an interaction in which the microorganism and the host both derive ben- efits from the interaction. For example, certain inhabit- ants of the human intestinal tract extract nutrients from the host and secrete essential vitamin by-products of metabolism (e.g., vitamin K) that are absorbed and used by the host. A parasitic relationship is one in which only the infecting organism benefits from the relationship and the host either gains nothing from the relationship or sustains injury from the interaction. An infectious disease occurs if the host sustains injury in a parasitic relationship. The severity of an infectious disease can range from mild to life-threatening depending on many variables, including the health of the host at the time of infection and the virulence (disease-producing potential) of the microorganism. A select group of microorganisms called

pathogens are so virulent that they are rarely found in the absence of disease. Fortunately, there are few human pathogens in the microbial world. Most microorganisms are harmless saprophytes , free-living organisms obtain- ing their growth from dead or decaying organic material in the environment. However, all microorganisms, even saprophytes and members of the normal flora, can be opportunistic pathogens capable of producing an infec- tious disease when the health and immunity of the host have been severely weakened by illness, malnutrition, or medical therapy. Agents of Infectious Disease The agents of infectious disease include prions, viruses, bacteria, Rickettsiaceae and Chlamydiaceae , fungi, and parasites. A summary of the salient characteristics of these human microorganisms is provided in Table 14-1. Prions Can a protein alone cause a transmissible infectious dis- ease? Prior to the discovery of prions, scientists assumed that all infectious agents must possess a genetic mas- ter plan (a genome of either ribonucleic acid [RNA] or deoxyribonucleic acid [DNA]) that codes for the production of the essential proteins and enzymes nec- essary for survival and reproduction. Prions, protein particles that lack any kind of a demonstrable genome, appear to be an exception to this rule. A number of prion-associated diseases have been identified, includ- ing Creutzfeldt-Jakob disease in humans, chronic wast- ing disease in deer and elk, scrapie in sheep, and bovine spongiform encephalopathy (BSE or mad cow disease) in cattle. The various prion-associated diseases produce very similar pathologic processes and symptoms in the hosts and are collectively called transmissible neurode- generative diseases. All are characterized by a slowly progressive, noninflammatory neuronal degeneration, leading to loss of coordination (ataxia), dementia, and death over a period ranging from months to years.

TABLE 14-1 Comparison of Characteristics of Human Microbial Pathogens

Intracellular or Extracellular

Organism Defined Nucleus Genomic Material

Size*

Motility

Prions Viruses Bacteria

No No No No No No No Yes Yes

Unknown

55 kDa

E

– – ± – + – – – –

DNA or RNA

0.02–0.3 0.5–15 0.2–0.3

I

DNA DNA DNA DNA DNA DNA DNA

I/E

Mycoplasmas Spirochetes Rickettsiaceae Chlamydiaceae

E E

6–15

0.2–2 0.3–1

I I

Yeasts Molds

2–60 2–15

I/E

E

(hyphal width)

Protozoans Helminths

Yes Yes

DNA DNA

1–60

I/E

+ +

2 mm to >1 m

E

*Micrometers unless indicated.