LOWER RESPIRATORY TRACT INFECTIONS: PSEUDOMONAS, MORAXELLA, AND BACILLUS ANTHRACIS

Morphology and Structure

1. Pseudomonas

• Morphology: Pseudomonas species, particularly Pseudomonas aeruginosa, are Gram-negative, rod-shaped bacteria. They possess a single polar flagellum that aids in motility.
• Structure: The outer membrane contains lipopolysaccharides (LPS), which contribute to its virulence by triggering strong immune responses. It also has pili and fimbriae for adhesion to host cells.
• Relation to Virulence: The structural components such as LPS, flagella, and biofilm-forming ability enhance its survival in hostile environments and resistance to phagocytosis.
• Antibiotic Resistance: Pseudomonas is intrinsically resistant to many antibiotics due to efflux pumps, low outer membrane permeability, and the production of beta-lactamases.
• Pathogenesis: It produces toxins like exotoxin A (inhibits protein synthesis) and enzymes like elastase that degrade host tissues.
• Clinical Presentation: Causes pneumonia, especially in immunocompromised patients or those with cystic fibrosis. Symptoms include fever, productive cough with green sputum (due to pyocyanin pigment), dyspnea, and chest pain.
• Laboratory Diagnosis: Identified via culture on MacConkey agar (non-lactose fermenter) or blood agar. Produces a characteristic grape-like odor and blue-green pigments (pyocyanin).

2. Moraxella

• Morphology: Moraxella catarrhalis is a Gram-negative diplococcus resembling Neisseria species.
• Structure: Its outer membrane contains proteins that help evade complement-mediated killing. It forms biofilms in respiratory tracts.
• Relation to Virulence: Adhesion factors such as UspA proteins allow colonization of mucosal surfaces. Lipooligosaccharides (LOS) contribute to immune evasion.
• Antibiotic Resistance: Frequently produces beta-lactamases that confer resistance to penicillin derivatives.
• Pathogenesis: Commonly causes exacerbations of chronic obstructive pulmonary disease (COPD) and bronchitis by inducing inflammation through LOS-mediated pathways.
• Clinical Presentation: Presents with symptoms like wheezing, increased sputum production, fever, and dyspnea in COPD patients or otitis media in children.
• Laboratory Diagnosis: Cultured on blood or chocolate agar; colonies are smooth and opaque.

3. Bacillus anthracis

• Morphology: Bacillus anthracis is a Gram-positive rod that forms spores under adverse conditions. Spores are oval-shaped and highly resistant to environmental stressors.
• Structure: Encapsulated with a poly-D-glutamic acid capsule that prevents phagocytosis. Produces anthrax toxin composed of protective antigen (PA), lethal factor (LF), and edema factor (EF).
• Relation to Virulence: The capsule aids immune evasion while the toxins disrupt cellular signaling pathways leading to cell death or edema.
• Antibiotic Resistance: Generally susceptible to penicillin but resistance can emerge through plasmid acquisition; ciprofloxacin is often used as first-line therapy due to its efficacy against both vegetative cells and spores.
• Pathogenesis: Spores germinate in the lungs after inhalation, releasing toxins that cause hemorrhagic mediastinitis and systemic infection.
• Clinical Presentation: Inhalational anthrax presents with flu-like symptoms progressing rapidly to severe respiratory distress, cyanosis, shock, and death if untreated.
• Laboratory Diagnosis: Identified by Gram staining showing large rods in chains; cultured on blood agar producing nonhemolytic colonies with a “ground-glass” appearance.

Growth Characteristics

Pseudomonas

1. Aerobic growth; thrives at 37°C but can grow between 4–42°C.
2. Forms biofilms on surfaces like medical devices or tissues.

Moraxella

1. Grows well on enriched media like chocolate agar at 35–37°C under aerobic conditions.

Bacillus anthracis

1. Facultatively anaerobic; grows optimally at 37°C on nutrient-rich media forming large colonies.

Classification

1. Pseudomonas belongs to the family Pseudomonadaceae within Gammaproteobacteria class.
2. Moraxella belongs to the family Moraxellaceae within Gammaproteobacteria class.
3. Bacillus anthracis belongs to the family Bacillaceae within Firmicutes phylum.

Toxins & Extracellular Products

1. Pseudomonas: Exotoxin A inhibits elongation factor 2; elastase degrades connective tissue; pyocyanin generates reactive oxygen species damaging host cells.
2. Moraxella: Produces LOS triggering inflammation.
3. Bacillus anthracis: Anthrax toxin disrupts immune signaling causing edema or necrosis.

Pathogenesis & Immunity

1. Pseudomonas: Adheres via pili/fimbriae → secretes toxins → induces inflammation → damages lung parenchyma.
2. Moraxella: Colonizes mucosa → evades complement → triggers neutrophilic inflammation.
3. Bacillus anthracis: Spores germinate → release toxins → impair macrophages → systemic spread.

Immunity:

1. Innate immunity plays a critical role initially for all three pathogens; adaptive immunity involves antibody production against specific antigens like PA for B. anthracis.

Mode of Transmission & Epidemiology

1. Pseudomonas: Spread via contaminated water/medical equipment; common nosocomial pathogen.
2. Moraxella: Transmitted via respiratory droplets; prevalent in COPD exacerbations.
3. Bacillus anthracis: Acquired through inhalation of spores from animal products or bioterrorism events.

Laboratory Diagnosis

1. Culture characteristics as described above aid identification alongside biochemical tests like oxidase positivity (Pseudomonas, Moraxella) or spore staining (Bacillus).
2. PCR assays detect specific virulence genes for rapid diagnosis.

Treatment & Prevention

Treatment:

1. Pseudomonas: Combination therapy using ceftazidime + aminoglycosides or carbapenems for multidrug-resistant strains.
2. Moraxella: Amoxicillin-clavulanate or macrolides are effective options.
3. Bacillus anthracis: Ciprofloxacin combined with antitoxin therapy for inhalational cases.

Prevention:

1. Infection control measures reduce nosocomial transmission (Pseudomonas, Moraxella).
2. Vaccination with an acellular vaccine protects against B. anthracis exposure.