Chapter 24: Microbial Diseases of the Respiratory System

Upper Respiratory Infections: Overview

Chapter focus: microbial diseases of the respiratory system (Chapter 24). A brief lecture highlighting select pathogens of the respiratory tract.
Baseline reality: upper respiratory tract infections are very common and typically mild; most people recover without medical help.
Daily microbial exposure: the average person ingests about \approx 10{,}000 microbes per day, which helps explain why respiratory infections are so common.
Transmission routes: respiratory pathogens can be transmitted directly (droplet spread) or indirectly via fomites (contaminated objects).
General rule: most URIs are self-limiting and mild, acting merely as nuisances rather than severe illnesses.

Diphtheria: Corynebacterium diphtheriae ( transcript: Corneobacterium diphtheriae )

Disease name: diphtheriae; causative agent commonly referred to as Corneobacterium diphtheriae in the transcript (note: classical naming is Corynebacterium diphtheriae).
Key virulence factor: exotoxin produced by the bacterium.
Pathology: exotoxin contributes to the formation of a membrane (clot) in the throat which can obscure the airway and cause breathing problems.
Clinical consequences if the membrane forms and airway is blocked: potential suffocation; when toxins circulate, can cause heart failure and fever.
Contagious period: highly contagious for an extended period if untreated.
Prevention: vaccines widely used; reduced incidence due to vaccination.
Treatment/management: antibiotic therapy when indicated.
Visual aids: membrane/clot observed in the throat can be seen on slides; may progress if not treated.
Important note: membrane formation is gradual, not an immediate overnight event; significant obstruction occurs only if not treated.

Pertussis: Bordetella pertussis (Whooping Cough)

Causative agent: Bordetella pertussis; described as a coxobacillus (football-shaped) gram-negative bacterium.
Presentation: begins like a common cold, but progresses with copious mucus and a deep, paroxysmal cough (whooping cough).
Complications: intense coughing can cause breathing difficulty, rib fractures, and vomiting during coughing fits; vomiting and poor nutrition are particularly problematic in developing countries where malnutrition is a concern.
Recovery: convalescence can last months, even with treatment.
Public health context: pertussis has shown a comeback in places like New Hampshire and elsewhere.
Treatments and prevention: antibiotics available; vaccines (DPT: Diphtheria, Pertussis, and Tetanus) play a key role in prevention.

Tuberculosis: Mycobacterium tuberculosis

Staining characteristics: acid-fast organism due to high lipid content in the cell wall; acid-fast staining yields a pink color, and standard Gram staining is poor for this organism.
Cell wall properties: high lipid content makes staining difficult with Gram stain; requires acid-fast staining.
Oxygen requirement: obligate aerobe; lungs are favored due to high oxygen levels.
Growth rate: slow-growing relative to many aerobes.
Intracellular lifestyle: capable of infecting and persisting within macrophages; can replicate inside wandering macrophages and spread to other body sites.
Transmission: primarily airborne; remains suspended in the air for hours, facilitating transmission.
Infection dynamics: exposure does not guarantee infection; the infectious dose is related to ID
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Risk and mortality: tuberculosis remains a major global health issue, with about 3\times 10^6 deaths per year.
Environmental resilience: due to lipid-rich cell wall, TB bacilli have greater environmental survivability outside the host.
Treatment and vaccines: attenuated vaccine (BCG) is used in some settings; antibiotic therapy is required for extended periods, often months.
Clinical note: TB can become latent and, if reactivated, cause active disease years after initial infection.

The Common Cold and Rhinovirus: Causative Agents and Challenges

Causative organisms: rhinovirus historically linked to the common cold, but it is not the sole cause; many different viruses can cause cold-like symptoms.
Implication for vaccines: vaccine development for the common cold is impractical due to the enormous number of strains and other viruses capable of causing similar symptoms.
Incubation and duration: incubation is about 3\ \text{days}, and illness typically lasts around 1\ \text{week}.
Summary: rhinovirus is a major contributor to cold symptoms but not the only etiologic agent.

Transmission Dynamics and the Role of Mucus

General transmission theme: respiratory illnesses are highly transmissible from person to person.
Sneezing: expels a large amount of pathogens in droplets; high potential for direct person-to-person transmission.
Fomite environment: if a sneeze is covered with a hand and then that hand contacts surfaces (e.g., a doorknob), mucus plus pathogens can contaminate surfaces and facilitate indirect transmission to others.
Mucus as a protective factor for pathogens: mucus shields pathogens from dehydration, increasing their survivability outside the host and contributing to infection spread.
Practical implication: sitting at a desk after a sneezer could expose the next person to mucus-entrained pathogens on surfaces.

Chapter 25 Preview

Look ahead: transfer of pathogens via the fecal-oral route and the gastrointestinal tract will be covered in Chapter 25.

Connections, Implications, and Key Takeaways

Vaccination and public health impact: diphtheria and pertussis vaccines (e.g., DPT) have significantly reduced disease burden; vaccination is a central theme in preventing severe respiratory infections.
Antibiotic stewardship and treatment duration: some diseases (notably tuberculosis) require prolonged antibiotic therapy spanning months.
Pathogen biology and disease outcome: differences in cell wall structure (acid-fast TB vs non-acid-fast bacteria), environmental survivability, and intracellular lifecycles (TB in macrophages) influence transmission, clinical outcomes, and control strategies.
Ethical and practical considerations: vaccine development challenges for diseases like the common cold highlight the balance between practicality, economic impact, and public health benefit.
Real-world relevance: how travel, outbreaks, and regional resurgence (e.g., pertussis in New Hampshire) influence vigilance, vaccination, and surveillance.