Respiratory and Invasive Bacterial Pathogens: Structure, Pathogenesis, Epidemiology, and Prevention

Introduction to Respiratory and Invasive Bacterial Pathogens

Overview

This module provides a comprehensive overview of major bacterial pathogens affecting the human respiratory system and those capable of causing invasive disease. It covers terminology, host defences, epidemiology, transmission, pathogenesis, clinical outcomes, and prevention strategies, with a focus on Streptococcus pneumoniae, Neisseria meningitidis, Mycobacterium tuberculosis, Haemophilus influenzae, and Bordetella pertussis.

Respiratory System Terminology and Defences

Anatomy of the Respiratory System

The respiratory system is divided into upper and lower tracts, each with specialized structures and defences against microbial invasion.

• Nasal cavity: Lined with hairs and ciliated mucous membranes to filter, warm, and humidify air, trapping particles and microbes.

• Pharynx: Shared pathway for air and food, lined with ciliated mucosa to move contaminants toward the digestive tract.

• Tonsils: Lymphoid tissue aggregations that detect and respond to pathogens.

• Trachea, bronchi, bronchioles: Conduct air to the lungs; lined with ciliated epithelium and mucus to trap and remove debris.

• Alveoli: Tiny air sacs where gas exchange occurs.

Upper Respiratory Tract Defences

• Mucus and cilia: Trap and move particles and microbes toward the pharynx for swallowing.

• Antimicrobial secretions: Mucus contains lysozyme and defensins that neutralize pathogens.

• Resident flora: Includes Haemophilus influenzae and Staphylococcus aureus, which help prevent colonization by harmful microbes.

Lower Respiratory Tract Defences

• Mucociliary escalator: Cilia drive mucus upward to the pharynx for removal.

• Alveolar macrophages: Phagocytize small particles and pathogens that reach the alveoli.

Risk Factors for Bacterial Respiratory Infections

• Viral infections (damage epithelium)

• Smoking (impairs cilia)

• Immunocompromised state

• Age extremes (children, elderly)

• Environmental exposure (damp, crowded settings)

• Chronic diseases (asthma, diabetes, obesity)

Streptococcus pneumoniae

Biology and Virulence

• Gram-positive diplococci

• Over 90 serotypes, distinguished by polysaccharide capsule

• Capsule is the major virulence factor, protecting against phagocytosis

• Antimicrobial resistance is increasing

Clinical Syndromes and Epidemiology

• Causes pneumonia, bacteremia, meningitis, and otitis media

• Responsible for 30% of community-acquired and up to 50% of hospital-acquired pneumonias

• Common complication of influenza and measles

• Case fatality rate for invasive disease: 5–7%, higher in elderly

Transmission and Colonization

• Humans are the only reservoir

• Colonizes nasopharynx, especially in children (asymptomatic carriage common)

• Spread via respiratory droplets and autoinoculation

• More common in winter/early spring

Potential Infection Outcomes

• Bacteremia (most common in children)

• Meningitis (leading cause in children under 5)

• Acute otitis media (major cause in children)

Pneumococcal Disease Outbreaks and Otitis Media

• Outbreaks rare, usually in crowded settings

• Otitis media: most common reason for antibiotic prescription in children

• Linked to nasopharyngeal colonization

Pneumococcal Vaccines

• Conjugate vaccines (PCVs) are now standard; polysaccharide vaccines phased out

• PCVs induce T-cell dependent immunity and immunologic memory

• Highly effective in infants and young children

• Included in National Immunisation Programs

Neisseria meningitidis

Biology and Pathogenesis

• Gram-negative, non-motile diplococci

• Encapsulated; capsule is a major virulence factor

• Asymptomatic carriage in nasopharynx is common

• Causes invasive disease: meningitis, sepsis, pneumonia, arthritis

Clinical Features and Epidemiology

• Rapid progression; can be fatal within hours if untreated

• Highest risk: infants, young children, adolescents, elderly

• Long-term sequelae in survivors: amputations, brain damage, hearing loss

• Sporadic outbreaks, especially in winter/early spring

Pathogenesis

• Entry via inhalation of droplets; adheres to non-ciliated epithelial cells using pili and adhesins

• Internalized, evades immune detection, crosses mucosal barrier into bloodstream

• Capsule protects against phagocytosis and complement

• Outer membrane vesicles (OMV) with LOS trigger massive cytokine response, vascular damage, DIC

Serogroups and Vaccines

• 12 serogroups; A, B, C, W, Y cause >95% of disease

• MenB is a leading cause in adolescents/young adults

• Vaccines: conjugate for A, C, W, Y; protein-based for B

Epidemiology and Outbreaks

• Meningitis Belt in Africa: hyperendemic, periodic epidemics

• Serogroup replacement observed after vaccination campaigns

• Recent outbreaks in Australia and New Zealand (MenB)

Mycobacterium tuberculosis

Biology and Pathogenesis

• Acid-fast, aerobic, non-motile bacilli

• Cell wall rich in mycolic acids (waxy, hydrophobic, resistant to disinfectants and antibiotics)

• Slow-growing; generation time >20 hours

Transmission and Infection

• Spread via inhalation of respiratory droplets

• Optimal transmission in crowded, poorly ventilated settings

• After inhalation, bacteria reach alveoli, are engulfed by macrophages, and may form granulomas

• Latent TB: bacteria dormant, not contagious; risk of reactivation if immunity wanes

Clinical Features and Epidemiology

• 25% of global population infected (latent TB)

• 5–10% develop active TB in lifetime

• Leading cause of death from a single infectious agent

• High burden in Sub-Saharan Africa, Southeast Asia

Treatment and Prevention

• Standard regimen: Isoniazid, Rifampicin, Pyrazinamide, Ethambutol (2 months), then Isoniazid and Rifampicin (4 months)

• Directly Observed Therapy (DOT) recommended

• MDR-TB and XDR-TB require longer, more complex regimens

• BCG vaccine: partial protection in children, limited in adults

Haemophilus influenzae

Biology and Pathogenesis

• Gram-negative, pleomorphic coccobacillus

• Non-motile, facultative anaerobe

• Requires X (hemin) and V (NAD+) factors for growth

• Capsule (especially type b, Hib) resists phagocytosis

Clinical Features and Epidemiology

• Serious infections: meningitis, pneumonia, bacteremia, cellulitis, arthritis

• Mild infections: otitis media, sinusitis, conjunctivitis, bronchitis

• High-risk: infants <1 year, adults ≥65 years

Prevention and Treatment

• Antibiotics for treatment; resistance increasing

• Hib conjugate vaccine highly effective; incidence dropped by 99% in children under 5

• Non-typeable strains now cause most invasive disease in vaccinated populations

Bordetella pertussis

Biology and Virulence Factors

• Gram-negative, pleomorphic coccobacillus

• Obligate aerobe, fastidious

• Virulence factors: capsule, pertussis toxin (PT), adenylate cyclase toxin (ACT), tracheal cytotoxin (TCT), filamentous hemagglutinin (FHA)

Clinical Features and Epidemiology

• Causes pertussis (whooping cough); highly contagious

• Serious in infants (<6 months): apnea, respiratory failure, pneumonia, seizures, encephalopathy

• Mild in adults/adolescents: prolonged cough

• Diagnosis: PCR preferred, culture gold standard, serology in late stages

Prevention and Treatment

• Antibiotics: macrolides first-line; resistance rare but emerging

• Vaccination: acellular pertussis vaccine (aP) standard; waning immunity a concern

• Maternal vaccination protects infants

• Booster recommended every 10 years

Summary Table: Key Features of Major Respiratory and Invasive Bacterial Pathogens

Pathogen	Gram Stain & Morphology	Main Diseases	Major Virulence Factor(s)	Prevention
Streptococcus pneumoniae	Gram-positive diplococci	Pneumonia, meningitis, otitis media, bacteremia	Polysaccharide capsule	Conjugate vaccine (PCV)
Neisseria meningitidis	Gram-negative diplococci	Meningitis, sepsis, pneumonia	Capsule, LOS, OMV	Conjugate & protein-based vaccines
Mycobacterium tuberculosis	Acid-fast bacilli	Tuberculosis (pulmonary & extrapulmonary)	Mycolic acid-rich cell wall	BCG vaccine (partial)
Haemophilus influenzae	Gram-negative coccobacillus	Meningitis, pneumonia, otitis media	Capsule (Hib)	Hib conjugate vaccine
Bordetella pertussis	Gram-negative coccobacillus	Pertussis (whooping cough)	Pertussis toxin, ACT, TCT	Acellular pertussis vaccine (aP)

Additional info: This summary integrates and expands upon the provided lecture content, ensuring coverage of all major pathogens, their clinical relevance, and public health implications. Images were included only when directly relevant to the adjacent text and explanation.