Gram positive Bacilli

How are Gram-positive bacilli broadly classified?

Gram-positive bacilli are broadly classified into:

1. Spore-forming bacilli

   • Mainly Bacillus and Clostridium

2. Non-spore-forming bacilli

   • Can have regular or irregular shapes and staining properties.

   • Examples include Corynebacterium, Listeria, Erysipelothrix, Propionibacterium, Mycobacterium, Nocardia and Actinomyces.

Which Gram-positive bacilli are important spore-forming bacteria?

The important spore-forming Gram-positive bacilli are:

• Bacillus species — generally aerobic or facultative anaerobic

• Clostridium species — anaerobic

Spores allow these bacteria to survive under unfavourable environmental conditions.

How do Bacillus, Clostridium, Corynebacterium, and Listeria differ according to the slide?

Genus	Anaerobic growth	Spore formation	Exotoxins
Bacillus	-	+	+
Clostridium	+	+	+
Corynebacterium	-	-	−
Listeria	-	-

Which Bacillus species are highlighted in the slides?

The slides focus mainly on:

• Bacillus anthracis — causes anthrax

• Bacillus cereus — causes two types of food poisoning

What are the general characteristics of Bacillus species?

Bacillus species generally have the following characteristics:

• Large Gram-positive bacilli

• Often occur in long chains

• Catalase positive

• Found in soil

• Capsulated (inside the host) MacFadyean Rxn

• Can break down glucose by both:

  • Oxidative reactions

  • Fermentative reactions

• Motile except B.anthracis

• Colonies on BA show Medusa head

What is the McFadyean reaction?

The McFadyean reaction is associated with the demonstration of the capsule of Bacillus anthracis.

The slide associates it with:

• Polychrome methylene blue stain

• Encapsulated B. anthracis found inside the host

It is therefore useful in demonstrating the characteristic capsule of the organism.

What are the characteristic colony appearances of Bacillus anthracis?

Colonies of Bacillus anthracis on blood agar may show a characteristic Medusa head appearance. These cultural characteristics help in laboratory identification.

What is anthrax?

Anthrax is an acute infectious disease caused by Bacillus anthracis.

According to the slides, it occurs in:

• Humans

• Animals

It is also:

• A zoonotic disease

• An occupational disease

Direct person-to-person spread is extremely unlikely.

Why is anthrax considered a zoonotic and occupational disease?

Anthrax is considered a zoonotic disease because it occurs in animals and can infect humans.

It is considered an occupational disease because certain occupations may increase exposure to infected animals or animal products.

The slides also emphasise that direct person-to-person transmission is extremely unlikely.

What are the major virulence factors of Bacillus anthracis?

The two major virulence factors are:

1. Capsule

   • Made of poly-D-glutamyl (D-glutamate)

   • Mediates the invasive stage of infection

Anthrax toxin

• Mediates the toxigenic stage of infection

• 
  

Both factors contribute significantly to the ability of B. anthracis to cause disease.

What are the three components of anthrax toxin?

Anthrax toxin consists of three components:

1. Edema Factor (EF)

   • Necessary for edema production

2. Lethal Factor (LF)

   • Inhibits cell growth

3. Protective Antigen (PA)

   • Induces protective antitoxic antibodies

These three components are important in the toxigenic stage of anthrax.

What are the major types of anthrax?

The major types of anthrax are:

1. Cutaneous anthrax

2. Pneumonic/Inhalational anthrax

3. Intestinal/Gastrointestinal anthrax

Each form depends mainly on the route through which B. anthracis enters the body.

What are the clinical features of cutaneous anthrax?

Cutaneous anthrax is also associated with the term malignant pustule.

It typically manifests as:

• A painless ulcer

• A characteristic black eschar

The lesion may develop through the following stages:

Red macule → Papule → Vesicle → Ulcer → Black eschar

There is also brawny edema around the lesion.

What is important about the prognosis and frequency of cutaneous anthrax?

According to the slides:

• Cutaneous anthrax is the commonest type

• It accounts for approximately 95% of human anthrax

• It commonly occurs on the:

  • Face

  • Neck

  • Arm

  • Hand

• It has a good prognosis

• Approximately 80–90% recover

The lesion eventually heals in approximately 3 weeks.

What is inhalational anthrax and why is it also called Woolsorter’s disease?

Inhalational anthrax is caused by the inhalation of Bacillus anthracis spores.

The slides refer to it as Woolsorter’s disease.

Important pathological features include:

• Involvement of mediastinal lymph nodes

• Hemorrhagic mediastinitis

What are the early clinical features of inhalational anthrax?

Early inhalational anthrax may resemble a flu-like illness.

Symptoms include:

• Fever

• Malaise

• Headache

• Cough

• Shortness of breath

• Chest pain

The initial stage may therefore appear relatively nonspecific.

Describe the pathogenesis of inhalational anthrax.

The sequence is:

1. Spores are inhaled

2. Incubation is approximately 2–3 days

   • Range may extend up to 60 days

3. Spores are engulfed by macrophages

4. Macrophages transport the spores to:

   • Mediastinal lymph nodes

   • Peribronchial lymph nodes

5. Initial symptoms develop gradually:

   • Malaise

   • Low-grade fever

   • Nonproductive cough

6. There is abrupt development of:

   • Respiratory distress

   • Hemorrhagic mediastinitis

7. Hematogenous spread may occur.

What happens in the later stages of inhalational anthrax?

After the initial flu-like phase, inhalational anthrax can progress rapidly.

Later features include:

• Abrupt respiratory distress

• Hemorrhagic mediastinitis

• Fever

• Shock

• Hematogenous spread

This makes inhalational anthrax a severe and potentially fatal form of disease.

How does gastrointestinal anthrax occur and what are its clinical features?

Gastrointestinal anthrax occurs after ingestion of contaminated meat.

Clinical features include:

• Abdominal pain

• Diarrhea

• Bloody diarrhea

• Fever

• Mouth sores

• Nausea

• Vomiting

• Vomiting containing blood

• Acute inflammation of the intestinal tract

• Loss of appetite

The slides describe it as bloody fulminating gastroenteritis.

What are the two types of food poisoning caused by Bacillus cereus?

Bacillus cereus causes two types of food poisoning:

1. Emetic form

   • Short incubation period

   • Mainly vomiting

2. Diarrheal form

   • Longer incubation period

   • Mainly diarrhea and abdominal cramps

The two forms differ in their toxins and incubation periods.

Describe the emetic form of Bacillus cereus food poisoning.

The emetic form:

• Has a short incubation period

• Incubation: 1–6 hours

• Is caused by a heat-stable enterotoxin

• Causes:

  • Nausea

  • Vomiting

  • Abdominal cramps

• Resembles Staphylococcus aureus food poisoning

Describe the diarrheal form of Bacillus cereus food poisoning.

The diarrheal form:

• Has a longer incubation period

• Incubation: 8–16 hours

• Is caused by a heat-labile enterotoxin

• Causes:

  • Abdominal cramps

  • Diarrhea

It resembles food poisoning caused by Clostridium perfringens.

How can Bacillus anthracis and Bacillus cereus be differentiated?

Characteristic	B. anthracis	B. cereus
Hemolysis	No hemolysis	β-hemolysis
Motility	Non-motile	Motile

Describe the cultural, staining, and biochemical identification of Bacillus species.

Growth on blood agar

• Bacillus species grow well on blood agar

• Some show a double zone of hemolysis

• B. anthracis has no hemolytic effect

3. Spore staining

Using:

• Malachite green as the primary stain

• Safranin as the counterstain

Results:

• Green endospores and free spores

• Red vegetative cells and sporangia

4. Biochemical testing

• All Bacillus species are catalase positive

• API diagnostic systems may also be used for identification