Microbiology: Bacillus and Clostridium

Bacillus and Clostridium

• Bacillus is an aerobic, spore-forming bacterium, while Clostridium is an anaerobic, spore-forming bacterium
• Corynebacterium is a non-spore-forming bacterium, and Listeria is a non-spore-forming bacterium

Clostridium Species

• Clostridium species are spore-forming, anaerobic, Gram-positive, motile rods with peritrichous flagella
• Many Clostridium species form toxins
• Their natural habitat is soil, marine sediments, sewage, or the intestinal tract of animals and humans
• There are four medically important Clostridium species: Clostridium botulinum, Clostridium tetani, Clostridium perfringens, and Clostridium difficile

Clostridium Botulinum

• C. botulinum causes botulism
• Spores of C. botulinum are widespread in soil and can contaminate vegetables and meats
• When these foods are canned or vacuum-packed without adequate sterilization, spores survive and germinate in the anaerobic environment
• Toxin is produced within the canned food and ingested preformed
• The toxin is relatively heat-labile and is inactivated by boiling for several minutes
• Disease can be prevented by sufficient cooking
• Botulinum toxin (BT) blocks the release of acetylcholine, causing flaccid paralysis
• There are eight immunologic types of toxin, with types A, B, and E being the most common in human illness

Clinical Findings of Botulism

• Descending weakness and paralysis, including diplopia, dysphagia, flaccid paralysis, and respiratory muscle failure
• No fever is present
• Special clinical forms occur:
  • Food-borne botulism: consumption of contaminated canned food
  • Wound botulism: spores contaminate a wound, germinate, and produce toxin at the site (e.g., in injection drug users)
  • Infant botulism: ingestion of contaminated food (e.g., honey), with affected infants developing weakness or paralysis and may need respiratory support

Laboratory Diagnosis of Botulism

• The organism is usually not cultured
• Gram staining of smears from suspected food or stool samples

Treatment and Prevention of Botulism

• Trivalent antitoxin (types A, B, and E) is given immediately without waiting for laboratory results, along with respiratory support
• Proper sterilization of all canned and vacuum-packed foods is essential
• Food must be adequately cooked to inactivate the toxin
• Swollen cans must be discarded (clostridial proteolytic enzymes form gas, which swells cans)

Clostridium Tetani

• C. tetani causes tetanus
• Spores are widespread in soil, and the portal of entry is usually a wound site (e.g., where a nail penetrates the foot)
• Tetanus toxin (tetanospasmin) is an exotoxin produced by vegetative cells at the wound site
• The toxin prevents the presynaptic release of inhibitory neurotransmitters (GABA), leading to spastic muscle contraction
• Clinical findings:
  • Strong muscle spasms or contraction (spastic paralysis, tetany)
  • Rigid contraction of the jaw muscles, which prevents the mouth from opening (lockjaw, trismus)
  • Pronounced arching of the back due to spasm of the strong extensor muscles of the back (Opisthotonos)
  • Respiratory failure often follows, and a high mortality rate is associated with this disease

Laboratory Diagnosis of Tetanus

• No microbiologic or serologic diagnosis
• Organisms are rarely isolated from the wound site
• C. tetani produces a terminal spore (i.e., a spore at the end of the rod), giving the organism a characteristic appearance of a "tennis racket"

Treatment and Prevention of Tetanus

• Tetanus immunoglobulin (tetanus antitoxin) is used to neutralize the toxin
• Metronidazole or Penicillin G can be given, but the role of antibiotics is uncertain
• An adequate airway must be maintained, and respiratory support given
• Tetanus is prevented by immunization with tetanus toxoid (formaldehyde-treated toxin) in childhood and every 10 years
• Tetanus toxoid is usually given to children in combination with diphtheria toxoid and the acellular pertussis vaccine (DTaP)

Clostridium Perfringens

• C. perfringens causes two distinct diseases, depending on the route of entry into the body:
  • Gas gangrene
  • Food poisoning
• Spores are located in the soil; gas gangrene is associated with war wounds, automobile and motorcycle accidents
• Organisms grow in traumatized tissue (especially muscle) and produce a variety of toxins
• The most important toxin is lecithinase, which damages cell membranes, including those of erythrocytes, resulting in hemolysis
• Degradative enzymes produce gas in tissues

Clinical Findings of Gas Gangrene

• Pain
• Edema
• Cellulitis
• Gangrene (necrosis) occurs in the wound area
• Crepitation indicates the presence of gas in tissues
• Shock and death can ensue, with high mortality rates

Laboratory Diagnosis of Gas Gangrene

• Specimen: necrotic tissues, muscle fragments from deeper part of the wound
• Smears of tissue and exudate samples show thick, stubby, large Gram-positive rods
• C. perfringens colonies exhibit a double zone of hemolysis on blood agar (double zone hemolysis)

Treatment of Gas Gangrene

• Early surgical debridement
• Combination of Penicillin and clindamycin are recommended for 10-14 days

Food Poisoning by C. Perfringens

• Spores are located in soil and can contaminate food
• The heat-resistant spores survive cooking and germinate
• The organisms grow to large numbers in reheated foods, especially meat dishes
• The disease has an 8- to 16-hour incubation period and is characterized by watery diarrhea with cramps and little vomiting
• It resolves in 24 hours

Laboratory Diagnosis of Food Poisoning

• This is not usually done
• There is no assay for the toxin
• Large numbers of the organisms can be isolated from uneaten food

Treatment and Prevention of Food Poisoning

• Symptomatic treatment is given; no antimicrobial drugs are administered
• Food should be adequately cooked to kill the organism
• There are no specific preventive measures

Clostridium Difficile

• C. difficile causes antibiotic-associated pseudomembranous colitis
• C. difficile is the most common nosocomial (healthcare-associated infection - /hospital-acquired) cause of diarrhea
• The organism is carried in the gastrointestinal tract in approximately 3% of the general population and up to 30% of hospitalized patients
• Most people are not colonized, which explains why most people who take antibiotics do not get pseudomembranous colitis
• The hands of hospital personnel are important intermediaries
• Antibiotics suppress drug-sensitive members of the normal flora, allowing C. difficile to multiply and produce exotoxins A and B
• The main effect of exotoxin in particular is to cause depolymerization of actin, resulting in a loss of integrity, apoptosis, and death of the enterocytes

Clinical Findings of Pseudomembranous Colitis

• Diarrhea associated with pseudomembranes (yellow-white plaques) on the colonic mucosa
• The diarrhea is usually not bloody
• Fever and abdominal cramping often occur
• Surgical resection of the colon may be necessary

Laboratory Diagnosis of Pseudomembranous Colitis

• The presence of exotoxins in the filtrate of a patient's stool specimen is the basis of the laboratory diagnosis
• Antigen detection: various methods such as rapid test in stool specimen
• PCR assay for the presence of the toxin gene DNA is also used

Treatment and Prevention of Pseudomembranous Colitis

• The causative antibiotic should be withdrawn
• Oral metronidazole or vancomycin should be given, and fluids replaced
• Also, in life-threatening cases, surgical removal of the colon may be required