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Bacillus
Aerobic Gram-Positive Bacilli
Bacillus
• Large, G(+) rods, square
cut ends
• Saprophyte in nature; soil,
dust
• Aerobic, facultative
anaerobic
• Spore former, survive in
environment
• Lab presumptive ID by
spore stain
• Most motile
• B. anthracis = non-motile
Bacillus: Genera
• B. anthracis - “charcoal” carbuncle;
major pathogen, anthrax disease
• B. cereus – “waxen”, gastroenteritis
(rice, meat, vegetable), opportunistic
infections
• B. subtilis – isolated as contaminant
in clinical specimens, opportunistic
infections (catheter, prosthesis)
Bacillus: Lab Culture
• Grows well on
ordinary lab media,
CBA
• Large granular
colonies, irregular
edge; coarse, dull or
frosted-glass
texture
• B. cereus = hemolytic
• B. anthracis = nonhemolytic
Bacillus anthracis: Virulence
Factors
• Capsule resist phagocytosis; lab
presumptive ID by stain or DFA (direct
fluorescent antibody) test
• Exotoxins complex, coded by plasmid;
three genes (three proteins, each alone
not toxic)
–
–
–
–
Protective antigen (PA) - bind
Lethal factor (LF) - active
Edema factor (EF) - active
PA combine with LF or EF, binds to host cell
receptor, entry into host cell; LF & EF toxic
Bacillus anthracis: Two Toxins
• Lethal toxin: PA + LF; protease,
causes cell death
• Edema toxin: PA + EF; increase in
cAMP, results in edema
• Both toxins:
– increase vascular permeability
– interferes with phagocytosis
Bacillus anthracis: Anthrax
• Mainly disease of animals - acquire MO by
ingestion or inhalation of spores
• Spores extremely resistant, source of
infection in soil 2-3 years
• Humans acquire anthrax:
– Contact animal products (hides, fur, wool, hair)
– Less commonly, work in agricultural setting
with infected animals
• Disease depends on mode of transmission
– Skin (cut, abrasion, wound)
– Ingestion
– Inhalation
Cutaneous Anthrax
• Most common (95%
infections)
• Spores enter exposed skin,
germinate, multiply
• Exotoxin released, rapid
development of pustule
• Occasionally, MO
disseminate - septicemia,
death in few days
• Vascular injury - edema,
hemorrhage, thrombosis
• Death - respiratory failure,
anoxia by toxin on CNS
• Mortality ~20% if untreated
Gastrointestinal Anthrax
• Ingest spores, inoculated into lesions of
mucosa
• Ulcers in mouth, esophagus, intestine
• GI tract symptoms – nausea, vomiting,
malaise
• Lead to lymphadenopathy, edema, sepsis
• Rapidly progress to systemic disease
• High mortality, as anthrax not suspected
Inhalation Anthrax
(Woolsorter‘s Disease)
• Inhale spores by aerosols into RT
• Germinate in lungs, multiply, spread to
cause fatal septicemia or meningitis
• Most serious form of disease
• High mortality, as anthrax not suspected
• Use as biologic weapon – need to break up
spore clumping, aerosolize, so can reach
airway
• CDC category A Select Biological Agent
– Weaponized spores inhaled, easily disseminated
– Inhalation anthrax fatal unless treated
immediately
Bacillus anthracis:
Treatment and Prevention
• Sensitive to penicillin, but some
strains now showing resistance
• Ciprofloxacin drug of choice,
• Control infection in animals:
– Vaccine useful to prevent infection
– Bury diseased animals
• Short-term PA vaccine available for
at high risk individuals, less useful
Bacillus cerus: Opportunistic
Infections
• Ubiquitous, infecton via contaminated soil
• Gastroenteritis – two different
enterotoxins (emetic, diarrheal)
• Cytotoxin - cerolysin, phospholipase)
• Eye infection – traumatic injury
• Endocarditis – commonly IV drug abuser
• Pneumonitis, bacteremia, meningitis immunocompromised patient, neonate, IVcatheter, surgery patient
• Resistant to penicillin – combination
treatment of clindamycin + gentamycin
Food Poisoning: Emetic Form
• Contaminated fried or boiled rice, spores
survive cooking
• Rice not refrigerated, spores germinate,
MO release enterotoxin
• Intoxication – ingest toxin in rice; toxin
not destroyed by reheating rice
• Short incubation <6 hr.- vomiting, nausea,
abdominal cramps, ±diarrhea (33%)
• Last <24 hr.
Food Poisoning: Diarrheal Form
• Infection – ingest contaminated soup,
meat, vegetable, sauce, pudding
• Longer incubation >6 hr., MO multiplies,
release toxin; diarrhea, nausea, abdominal
pain, ±vomiting (25%)
• Last >24 hr.
• Both forms of food poisoning short,
uncomplicated
• Symptomatic treatment adequate
• Prevent by proper food handling
Class Assignment
• Textbook Reading: Chapter 16
Aerobic Gram-Positive Bacilli
– Bacillus
• Key Terms
• Learning Assessment Questions
Case Study 6 - Bacillus
• A 56-year-old female postal worker sought
medical care for fever, diarrhea, and
vomiting.
• She was offered symptomatic treatment
and discharged from the community
hospital emergency department.
• Five days later she returned to the
hospital with complaints of chills, dry
cough, and pleuritic chest pain.
• A chest radiograph showed a small right
infiltrate and bilateral effusions but no
evidence of a widened mediastinum.
Case Study 6 - Bacillus
• She was admitted to the hospital, and the
next day her respiratory status and
pleural effusions worsened.
• A computerized tomographic (CT) scan of
her chest revealed enlarged mediastinial
and cervical lymph nodes.
• Pleural fluid and blood was collected for
culture and was positive within 10 hours
for gram-positive rods in long chains.
Case Study - Questions
• 1. The clinical impression is that this
woman has inhalation anthrax. What tests
should be performed to confirm the
identification of the isolate?
• 2. What are the three primary virulence
factors found in B. anthracis?
• 3. Describe the mechanisms of action of
the toxins produced by B. anthracis.
• 4. Describe the two forms of B. cereus
food poisoning. What toxin is responsible
for each form? Why is the clinical
presentation of these two diseases
different?