C h a p t e r 1 4
Mechanisms of Infectious Disease
311
Diagnosis of Infectious
Diseases
The diagnosis of an infectious disease requires two cri-
teria: the recovery of a probable pathogen or evidence
of its presence from the infected sites of a diseased host
and accurate documentation of clinical signs and symp-
toms (symptomatology) compatible with an infectious
process. In the laboratory, the diagnosis of an infectious
agent is accomplished using three basic techniques:
culture, serology, or the detection of characteristic anti-
gens, genomic sequences, or metabolites produced by
the pathogen.
Culture
Culture
refers to the propagation of a microorganism
outside of the body, usually on or in artificial growth
media such as agar plates or broth (Fig. 14-10). The
specimen from the host is inoculated into broth or onto
the surface of an agar plate, and the culture is placed
in a controlled environment such as an incubator
until the growth of microorganisms becomes detect-
able. In the case of a bacterial pathogen, identification
is based on microscopic appearance and Gram stain
reaction, shape, texture, and color (i.e., morphology)
of the colonies, by a panel of biochemical reactions
that fingerprint salient biochemical characteristics of
the organism, and by the organism’s protein profile.
Certain bacteria such as
Mycobacterium leprae
, the
agent of leprosy, and
T. pallidum
, the syphilis spi-
rochete, do not grow on artificial media and require
additional methods of identification. Fungi and myco-
plasmas are cultured in much the same way as bacteria,
but with more reliance on microscopic and colonial
morphology for identification.
Chlamydiaceae, Rickettsiaceae
, and all human viruses
are obligate intracellular pathogens. As a result, the
propagation of these agents in the laboratory requires
the inoculation of eukaryotic cells grown in culture (cell
cultures). Cell culture is becoming more infrequent in
modern clinical laboratories due to the extended time
required for growth of viruses and reported poor sen-
sitivity of culture. Diagnosis of viruses,
Chlamydiaceae
,
and
Rickettsiaceae
now relies primarily on serology and
nucleic acid detection, which will be discussed later in
this chapter.
Although culture media have been developed for
the growth of certain human-infecting protozoa and
helminths in the laboratory, the diagnosis of parasitic
infectious diseases has traditionally relied on micro-
scopic or, in the case of worms, visible identification of
organisms, cysts, or ova directly from infected patient
specimens.
■■
Virulence factors, which are substances or
products generated by infectious agents that
enhance their ability to cause disease, include
toxins (endotoxins and exotoxins), adhesion
factors, evasive factors, and invasive factors.
■■
The natural history of an infectious disease
includes its incubation period, as well as its
prodromal, acute, convalescent, and resolution
stages.
FIGURE 14-10.
Use of agar culture for propagation and identification of microorganisms.
(A)
Photograph showing numerous Legionella species colonies that have been cultivated on an agar
culture plate and illuminated using ultraviolet light.
(B)
Quantitative differences in hemolytic
reactivity seen in trypticase soy agar culture plate containing 5% sheep’s blood growing group
D Streptococci (left wedge), group B Streptococci (middle wedge), and group A Streptococci
(right wedge) bacteria.The plate was grown under normal atmospheric conditions, at 35°C,
for a period of 18 hours. It is important to note that the group B Streptococci (GBS) in the
center wedge produced a hemolytic reaction that is less than the reaction produced by group
A Streptococci (GAS) in the right wedge of bacterial growth. Hemolysis, which results from
destruction of red blood cells, is one of the traits used to help identify the bacteria. (From the
Centers for Disease Control and Prevention Public Health Images Library. Nos. 7925, 10861. A
courtesy James Gathany; B courtesy Richard Facklam.)
A
B
