Epithelial surfaces form a physical barrier that is highly impermeable to most infectious agents.
Skin serves as the first line of defense against invading organisms.
Desquamation (shedding) of skin epithelium also aids in removing bacteria and other infectious agents adhering to the epithelial surfaces.
Movement of cilia or peristalsis helps maintain air passages and the gastrointestinal tract free from microorganisms.
Flushing action of tears and saliva helps prevent infections of the eyes and mouth.
Mucus traps in the respiratory and gastrointestinal tracts provide protection for lungs and digestive systems against infection.
2. Chemical Factors
Fatty acids in sweat inhibit bacterial growth.
Lysozyme and phospholipase found in tears, saliva, and nasal secretions break down bacterial cell walls and destabilize membranes.
Low pH of sweat and gastric secretions prevents bacterial growth.
Defensins, low molecular weight proteins found in the lungs and gastrointestinal tract, exhibit antimicrobial activity.
Surfactants in the lungs function as opsonins which promote phagocytosis by phagocytic cells.
3. Biological Factors
Normal flora of the skin and gastrointestinal tract prevent colonization of pathogenic bacteria by secreting toxic substances and competing for nutrients or attachment to cell surfaces.
Lung Function
On average, a person breathes nearly 25,000 times daily, inhaling large volumes of air.
Airborne pollutants, such as bacteria, viruses, tobacco smoke, car exhaust, can result in lung disease and breathing difficulties.
Avoidance of pollution and effective clearance mechanisms are essential for healthy lung function.
Normal vs. Smokers’ Lungs
Difference in lung condition is visualized through comparative images of healthy lungs and smokers' lungs.
Anatomy of the Lungs
Main Structures:
Right main stem bronchus
Right lobes
Left main stem bronchus
Left lobes
Trachea
Bronchi
Bronchioles
Pleura
Pleural fluid
Diaphragm
Alveoli
Approximately 300 million alveoli are present, with a total surface area of about 140 m².
Clearance of Particles from Lungs
The lung has effective mechanisms for removing deposited particles.
Important particle clearance mechanisms include:
The mucociliary escalator
Coughing
Phagocytosis by alveolar macrophages.
Particle clearance is categorized as biphasic:
Fast clearance with a half-life of minutes to hours from the tracheobronchial area (mucociliary clearance).
Slow clearance with a half-life from days to thousands of days in the alveolar regions.
Therefore, the location of particle deposition affects both the mechanism and rate of clearance.
Mucociliary Function in Cystic Fibrosis (CF)
In non-CF airways, normal periciliary fluid depth allows for mucus islands to be propelled by ciliary movement toward the mouth.
In CF airways, mucus is poorly hydrated and hypoxic; compacted mucus inhibits ciliary movement.
Lung Tissue Comparisons
Lung tissue from cystic fibrosis patients shows extensive destruction due to obstruction and infection, contrasting with normal lung tissue.
Microbial Infections in Cystic Fibrosis
Shortly after birth, mucus in the bronchial tree of CF patients becomes stagnant, leading to early infections predominantly with:
Staphylococcus aureus
Haemophilus influenzae
Later infections (before 10 years of age) can include:
Pseudomonas aeruginosa
Co-infection with Burkholderia cepacia.
Pseudomonas aeruginosa
Characteristics:
Aerobic, motile, Gram-negative rod.
Capable of thriving in diverse environments, primarily found in water, soil, and vegetation.
Known for environmental versatility and ability to cause disease, along with resistance to antibiotics.
Serious complications in CF patients include respiratory tract infections and its role as an opportunistic pathogen in immunocompromised individuals (e.g., cancer and burn patients).
Known to cause various infections including ventilator-associated pneumonia, urinary infections, peritoneal dialysis catheter infections, bacterial keratitis, otitis externa, and burn wound infections.
Virulence Factors of Pseudomonas aeruginosa
Produces several toxic proteins regulated under quorum sensing (QS) that can cause extensive tissue damage and disrupt human immune defenses.
Key factors include:
Potent toxins that kill host cells at the infection site.
Degradative enzymes that disrupt cell membranes and connective tissue.
High alginate production contributes to biofilm formation in the lung, leading to chronic inflammatory responses.
Biofilms and Protection Mechanisms
Biofilms are composed of complex exopolysaccharides (slime) that provide adhesion and protection from macrophages, biocides, and antibiotics.
Intracellular Pathogens
These are pathogens that live and replicate within endosomal compartments or the cytosol of diverse host cells, including macrophages, dendritic cells, neutrophils, fibroblasts, epithelial cells, and erythrocytes.
All viruses are obligate intracellular pathogens; many bacterial pathogens also fall under this category, as do certain protozoa and fungi.
Macrophages and Clearance Mechanisms
Lung macrophages serve as a primitive defense system lacking mucociliary clearance.
Particles deposited in the lungs are cleared primarily by alveolar macrophages through phagocytosis.
Legionnaires’ Disease
Associated with symptoms linked to environmental exposure and underlying illnesses.
American Legionnaires Outbreak (1976)
Acute pneumonia outbreak among American veterans during a conference resulted in 221 pneumonia cases, with 34 deaths.
The causative agent, Legionella pneumophila, was identified.
Legionella-Associated Diseases
Legionnaires’ Disease
Acute fulminating pneumonia with a low attack rate and >12% fatality.
Notable outbreaks include those in Philadelphia (1976) and Stafford District Hospital, UK (1985).
Pontiac Fever
A mild, non-pneumonic febrile infection with a high attack rate and notable outbreaks such as in Pontiac County (1968, 1981).
Legionella Species Overview
The genus Legionellaceae has over 48 species, with the majority of cases (85%) attributed to L. pneumophila serogroup 1 (50%) and serogroup 6 (10%).
Other L. pneumophila serogroups account for 20%, while L. micdadei accounts for approximately 5% of cases.
Factors in Transmission
Legionella infection often involves inhalation of aerosolized droplets contaminated with bacteria from man-made water distribution systems.
Potential settings for outbreaks include warm storage tanks, air-conditioning systems, showers, whirlpool spas, and decorative fountains.
Certain outbreaks have also been connected to contaminated potting compost (e.g., L. longbeachae).
Survival Mechanisms of Legionella
Amoebal Symbiosis:
Legionella thrives within amoebae, providing protection against biocide treatments.
Improved survival in man-made water systems due to its resilience against high temperatures, low pH, and efficient reproduction in environments with nutrient sources like algae and bacteria.
Pathogenicity of Legionella pneumophila
Invades macrophages by triggering phagocytosis and establishes a replication-permissive vacuole that inhibits lysosome fusion.
Survival within the host is facilitated by the icm/dot genes responsible for intracellular multiplication and secretion.
Infection and Growth Dynamics
After macrophage engulfment, L. pneumophila remains in isolated phagosomes that merge with the endoplasmic reticulum rather than lysosomes, allowing replication while suppressing virulence traits.
Nutrient-rich environments within the phagosome enable the bacteria to thrive, with regulatory mechanisms dictating transitions for stationary phases and virulence trait expression for secondary infection transmission.
Mycobacterium tuberculosis
Identified as a weakly Gram-negative, strongly acid-fast aerobic rod with a slow doubling time (24-30 hours) characterized by a multi-lobate colony morphology.
Mycobacterial cell walls contain unique lipids such as mycolic acids that contribute to their resistance to antibiotics and biocides.
Pathogenicity and Spread of TB
Initial infection occurs through inhalation, with macrophages engulfing bacilli, leading to intracellular multiplication and granuloma formation.
Key characteristics of the bacterial lifecycle include the elicitation of immune responses (T-cell recruitment) and eventual granuloma formation that attempts to isolate the infection.
Factors regulating infection and immune System interplay fundamentally influence the course and outcome of TB, allowing M. tuberculosis to effectively evade host defenses.
Treatment and Control of Tuberculosis
TB control prioritizes the identification and treatment of active and latent infections.
Roughly 30% of the global population is estimated to be infected, primarily affecting the lungs.
Effective treatment regimens, including directly observed treatment short-course (DOTS) with drugs such as isoniazid and rifampin, tend to render patients non-infectious within two weeks of commencing treatment.
Comprehensive treatment regimens for active TB generally span six months.
Summary
The lungs face constant airborne challenges, but their defense mechanisms generally suffice for combating infections.
Healthy lung structure and function reduce susceptibility to both extracellular and intracellular pathogens, emphasizing the impact of lifestyle choices, such as smoking cessation, on respiratory health.
The evolution of pathogens to exploit immune evasion tactics (such as biofilm formation or intracellular predation) poses ongoing risks for lung infection.