The spirochetes are slender, coiled organisms with a longitudinal axial filament that gives them motility. Seen in action, they are
long, flexible, and always actively spinning or undulating. Their cell walls are extremely thin and not readily stainable. In unstained
wet mounts they are too transparent to be seen by direct condenser light, but become quite visible by “dark-field” condenser illumination.
A special condenser lens is used to block the passage of direct light through the mount and to permit only the most
oblique rays to enter, at an angle that is nearly parallel to the slide. When viewed in such minimal light, the background of the
mount is very dark, almost black, but any particles in suspension are brightly illuminated because they catch and reflect light upward
through the objective lens. To stain spirochetes in fixed smears, stains containing metallic precipitates are used. Silver, for
example, can be precipitated out of solution and will coat spirochetes on the slide, giving them a black color when viewed by ordinary
light microscopy (see colorplate 7
There are three major genera of spirochetes: Treponema, Borrelia,
, each containing species associated with
human disease. Many of these organisms are obligate parasites that grow only in human or animal hosts, and others are difficult
to cultivate on artificial culture media. The leptospires are an exception, for they will grow in a special serum-enriched medium
or in embryonated eggs. The laboratory diagnosis of spirochetal diseases is made by microscopic demonstration of the organisms
in appropriate clinical specimens, when possible; in special cultures in the case of leptospirosis; or, most frequently, by serological
methods for detecting antibodies in the patient’s serum.
The most important member of this genus is Treponema pallidum
, the agent of syphilis. The organism can be
demonstrated by dark-field examination of material from the primary lesion of the disease, called a chancre
. Diagnostic serological
tests for syphilis are numerous. They are particularly valuable because syphilis can be a latent, silent infection, with few or no
obvious symptoms in its early stages. It is a chronic, progressive disease, however, and if unrecognized and untreated it can have
very serious consequences. Also, it is a sexually transmitted disease, highly communicable in its primary stage. Laboratory diagnosis
of syphilis is, therefore, essential in its recognition, treatment, and control.
Nonpathogenic species of Treponema
are frequent members of the normal flora of the mouth and gums, and sometimes
of the genital mucous membranes.
species are pathogenic for humans and a wide variety of animals including rodents, birds, and cattle. They are
transmitted by the bites of arthropods. Two important species for humans are Borrelia recurrentis
, the agent of relapsing fever, and Borrelia burgdorferi
, the agent of Lyme disease.
fever is now primarily a tropical disease, and is transmitted by lice. As the name implies, the infection is characterized
by repeated episodes of fever with afebrile intervals in between. Diagnosis is made primarily by seeing the organisms in
the patient’s blood either in an unstained preparation viewed by dark-field microscopy or in a smear stained with routine dyes
used in the hematology laboratory (e.g., Giemsa stain).
, transmitted by ticks, occurs in a number of countries. In the United States, it was first recognized in children
living in Lyme, Connecticut. Although once thought to be confined to the eastern part of the United States, this disease is
a growing problem in many parts of the country and throughout the world. The first sign of infection is a circular, rashlike lesion
(called erythema migrans) that begins at the site of the tick bite. This lesion may remain localized or spread to other body areas.
The rash may be accompanied by flulike or meningitis-like symptoms and, if untreated, many patients develop arthritis, chronic
skin lesions, and nervous system abnormalities after many weeks or even years. Because the signs and symptoms mimic those of
other infections, the correct diagnosis is often not suspected. Currently, diagnosis is best made by detecting antibodies against the
spirochete in the patient’s serum but a history of tick bite provides an important clue.
This genus has been classified as having only one species, Leptospira interrogans
, but molecular studies show several
species are included in this group. Several different serological strains are pathogenic for animals (dogs, rodents) and one is associated
with a human disease called icterohemorrhagia
, or, more simply, leptospirosis. (It is also sometimes called Weil’s disease.) The
spirochetes infect the liver and kidney, producing local hemorrhage and jaundice, hence the clinical term icterohemorrhagia. It
can also cause meningitis.
The laboratory diagnosis of leptospirosis can sometimes be made by demonstrating the organism in dark-field preparations
of blood or urine specimens (this spirochete is very tightly coiled, its ends are characteristically hooked, and it has a rapid,
lashing motility). Culture of such specimens in serum medium (Fletcher’s) or animal inoculation can also lead to recovery of the
spirochete. Serological diagnosis can be made by testing the patient’s serum for leptospiral antibodies.
In this experiment you will see the morphology of some spirochetes in prepared slides and demonstration material.
||Demonstration of important spirochetes
||Prepared stained slides
Projection slides, if available
Examine the prepared material. From your observations and/or reading, illustrate the microscopic morphology of each spirochetal