298
U N I T 4
Infection and Immunity
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
Organism Defined Nucleus Genomic Material
Size*
Intracellular or
Extracellular
Motility
Prions
No
Unknown
55 kDa
E
–
Viruses
No
DNA or RNA
0.02–0.3
I
–
Bacteria
No
DNA
0.5–15
I/E
±
Mycoplasmas
No
DNA
0.2–0.3
E
–
Spirochetes
No
DNA
6–15
E
+
Rickettsiaceae
No
DNA
0.2–2
I
–
Chlamydiaceae
No
DNA
0.3–1
I
–
Yeasts
Yes
DNA
2–60
I/E
–
Molds
Yes
DNA
2–15
E
–
(hyphal width)
Protozoans
Yes
DNA
1–60
I/E
+
Helminths
Yes
DNA
2 mm to >1 m
E
+
*Micrometers unless indicated.
