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Spore-Forming Gram-Positive Bacilli:
Bacillus & Clostridium Species
BACILLUS SPECIES
includes:
- Large aerobic, gram-positive rods arranged in long chains.
- are saprophytic organisms prevalent in soil, water, and air and on
vegetation.
- spores are located in the center of the nonmotile bacilli.The spores
are resistant to environmental changes, and certain chemical
disinfectants, and persist for years in dry earth.
BACILLUS ANTHRACIS
Pathogenesis
- Anthrax is a disease of herbivores—goats, sheep, cattle, etc.
Humans become infected by contact with infected animals or their
products.
-
In animals, the portal of entry is the mouth.
- In humans, entry of spores through injured skin (cutaneous
anthrax) or mucous membranes (gastrointestinal anthrax), or by
inhalation of spores into the lung (inhalation anthrax).
- The spores germinate in the tissue at the site of entry, and results in
formation of a gelatinous edema and congestion. Bacilli spread via
lymphatics to the bloodstream, and they multiply in the blood and
tissues shortly before and after the animal's death.
- B anthracis that does not produce a capsule is not virulent. The
poly-D-glutamic acid capsule is antiphagocytic.
- Anthrax toxin is made up of three proteins: protective antigen
(PA), edema factor (EF), and lethal factor (LF). PA binds to
specific cell receptors, and forms a membrane channel that
mediates entry of EF and LF into the cell. EF is an adenylyl
cyclase; with PA it forms a toxin known as edema toxin. LF plus
PA form lethal toxin, which is a major virulence factor and cause
of death.
- In inhalation anthrax ("woolsorter's disease"), the spores from the
dust of wool, hair, or hides are inhaled, phagocytosed in the lungs,
and transported by the lymphatic drainage to the mediastinal lymph
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nodes, where germination occurs. This is followed by toxin
production and the development of hemorrhagic mediastinitis and
sepsis, which are fatal.
Pathology
- In susceptible animals, the organisms proliferate at the site of
entry. The capsules remain intact, and the organisms are
surrounded byproteinaceous fluid containing few leukocytes from
which they rapidly and reach the bloodstream.
- In resistant animals, the organisms proliferate, there is massive
accumulation of leukocytes. The capsules disintegrate and
disappear. The organisms remain localized.
Clinical Findings
- In humans, 95% of cases are cutaneous anthrax and 5% are
inhalation. Gastrointestinal anthrax is rare.
- The bioterrorism events in the fall of 2001 resulted in 22 cases of
anthrax: 11 inhalation and 11 cutaneous.
- Cutaneous anthrax occurs on arms or hands, face and neck. A
pruritic papule develops 1–7 days after entry of the organisms or
spores. The papule changes into a vesicle, and a necrotic ulcer
develops. The lesions have a characteristic central black eschar.
Lymphangitis and lymphadenopathy and systemic signs and
symptoms of edema, fever, malaise, and headache also occur.
- After 7–10 days the eschar is dries, and separates; healing is by
granulation and leaves a scar.
- In 20% of patients, cutaneous anthrax leads to sepsis,
meningitisand death.
- The incubation period in inhalation anthrax 6 weeks. Clinical
manifestations are hemorrhagic necrosis and edema of the
mediastinum. Substernal pain. Hemorrhagic pleural; cough is
secondary to the effects on the trachea. Sepsis occurs, and there
may bebowel ulceration, or hemorrhagic meningitis. The fatality
rate is high.
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Diagnostic Laboratory Tests
- Specimens are fluid or pus from a local lesion, blood, and sputum.
Stained smears from dead animals often show chains of large
gram-positive rods.
- Immunofluorescence staining techniques.
- When grown on blood agar plates, the organisms produce
nonhemolytic gray to white colonies with a rough texture and a
ground-glass
appearance.
Comma-shaped
outgrowths
(Medusahead) may project from the colony.
- An enzyme-linked immunoassay (ELISA) has been developed to
measure antibodies against edema and lethal toxins.
Resistance & Immunity
- Immunization is based on the experiments of Louis Pasteur. In
1881 he proved that cultures grown in broth at 42–52 °C for
several months lost of their virulence and could be injected live
into sheep and cattle without causing disease; such animals proved
to be immune.
- Active immunity to anthrax can be induced by vaccination with
live attenuated bacilli, with spore suspensions, or with protective
antigens.
- Four countries produce vaccines for anthrax. Russia and China use
attenuated spore-based vaccineadministered by scarification. The
US and Great Britain use a bacteria-free filtrate of cultures
adsorbed to aluminum hydroxide. The current US Food and Drug
Administration approved vaccine contains cell-free filtrates of a
toxigenic nonencapsulatednonvirulent strain of B anthracis. The
dose schedule is 0, 2, and 4 weeks, then 6, 12, and 18 months,
followed by annual boosters.
Treatment
-
In humans, treatment must be started early. Ciprofloxacin is
recommended; penicillin G, along with gentamicin or
streptomycin, has previously been used to treat anthrax.
- In the setting of potential exposure to B anthracis , prophylaxis
with ciprofloxacin or doxycycline should be continued for 4
weeks while three doses of vaccine are being given, or for 8
weeks if no vaccine is administered.
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- Some other gram-positive bacilli, such as B cereus, are resistant to
penicillin. Doxycycline, erythromycin, or ciprofloxacin may be
effective alternatives to penicillin.
Epidemiology, Prevention, & Control
- Soil is contaminated with anthrax spores from dead animals. These
spores remain viable for decades. Contact with infected animals or
with their hides, hair, and bristles is the source of infection in
humans.
- Control measures include:- (1) disposal of animal carcasses by burning or by deep burial in
lime pits,
- (2) decontamination by autoclaving of animal products
-
(3) protective clothing and gloves for handling potentially
infected materials
- (4) active immunization of domestic animals. Persons with high
occupational risk should be immunized.
BACILLUS CEREUS
1. Food poisoning caused by Bacillus cereus has two forms:
- The emetic type, associated with fried rice, is manifested by
nausea, vomiting, abdominal cramps, and diarrhea and is selflimiting, with recovery occurring within 24 hours. It begins 1–5
hours after ingestion of rice and pasta dishes. When large amounts
of rice are cooked and allowed to cool slowly, the B cereus spores
germinate and the vegetative cells produce the toxin.
- The diarrheal type, associated with meat dishes and sauces. Has
an incubation period of 1–24 hours and is manifested by:
diarrhea with abdominal pain and cramps. The enterotoxin may be
performed in the food or produced in the intestine.
2. eye infections, severe keratitis, endophthalmitis, and
panophthalmitis. The organisms are introduced into the eye by
foreign bodies associated with trauma. B cereus has also been
associated with localized infections and with systemic infections,
including endocarditis, meningitis, osteomyelitis, and
pneumonia; the presence of a medical device or intravenous drug
use predisposes to these infections.