Microbiology: Bacteria & Respiratory Pathogens

Microbiology: Bacteria 1 – Respiratory Pathogens 1

• Lecture objectives include differentiating and describing the roles of epidemiology, microbial virulence factors, and host-defense mechanisms in respiratory infections.
• Objectives also include differentiating bacterial pathogens based on unique microbiologic features, with an emphasis on pneumonia-causing bacteria.
• Objectives further include differentiating tests to identify disease-causing bacteria for clinical diagnosis.

Classification of Pneumonia/Pneumonia-like Syndromes by Bacteria

• Acute/Community-acquired: Bacteria causing person-to-person or autologous pneumonia include Streptococcus pneumoniae, Mycoplasma pneumoniae, Haemophilus influenzae, Streptococcus pyogenes, Staphylococcus aureus, Klebsiella pneumoniae, and Chlamydophila pneumoniae.
• Animal/Environmental Exposure: Somewhat less common causes include Legionella pneumophila (from heating/cooling systems), Francisella tularensis (rabbits/ticks), Coxiella burnetii (farm animals), Chlamydophila psittaci (exotic birds), and Yersinia pestis (the plague – fleas/rats). 
• Acute/Nosocomial: Enteric bacteria (e.g., Klebsiella), Pseudomonas, Acinetobacter, and Staphylococcus aureus are involved in hospital-acquired pneumonia.

Subacute/Chronic Pneumonia

• Atypical Mycobacteria and various fungi are associated with these types.
• Immunocompromised patients may experience infection from atypical mycobacteria (including tuberculosis) and various fungi like Pneumocystis jiroveci (formerly P. carinii).

Natural Defense Mechanisms

• The respiratory tract has natural defenses: vibrissae (filter large particles), epiglottal/cough reflexes, ciliated respiratory epithelium, mucus-producing cells, and anti-microbial substances like lysozyme, lactoferrin, secretory IgA, and complement—along with pulmonary macrophages,

Predisposing Factors to Serious Infection/Pneumonia

• Previous viral respiratory infection
• Allergies (dust, pollen)
• Chronic lung diseases (emphysema)
• Alcoholism and smoking
• Diabetes, cancer, and other illnesses
• Immunosuppression
• Age extremes
• Debility (from development/congenital anomalies, post-surgery, anaesthesia, stroke)

Normal Microbial Flora

• Mouth and Oropharynx: Various streptococcal bacteria (mostly viridans), Streptococcus mutans (viridans group), Streptococcus salivarius (viridans group), Streptococcus pneumoniae, Streptococcus pyogenes(Group A strept), and some yeasts (e.g., Candida albicans).
• Lung: The lung is typically sterile; however, transient colonization with low microbial numbers is possible in healthy people, but this is not a problem until the immune system is compromised.

Types of Upper Respiratory Tract Infections

• Sinusitis (inflamed sinuses from infection)
• Otitis media (inflamed middle ear)
• Otitis externa (inflamed outer ear)
• Pharyngitis (sore throat)
• Epiglottitis (inflamed epiglottis, can be life-threatening, but controlled by vaccine)

Pneumonia

• A severe complication of bronchitis.
• Bronchioles and alveoli fill with pus (dead microbes, inflammatory cells, fluid exudate)—leading to a “shadow” or opaque area visible on X-ray.
• Most common causes are various bacteria; some fungi are especially life-threatening.

Common & Less Common Causes of Bacterial Pneumonia by Age Group

• Young adults: Streptococcus pneumoniae and Mycoplasma pneumoniae (common)
• Adults: Streptococcus pneumoniae and Legionella (common)
• Older adults: Streptococcus pneumoniae, Haemophilus influenzae, and Legionella (common); Tuberculosis and Mycoplasma (less common)

Streptococcus pneumoniae

• Gram-positive cocci in pairs or chains, have a unique lancet shape
• Catalase-negative (all streptococcus bacteria are)
• Capsule: Important virulence factor. 84 known serotypes (basis for pneumococcal vaccine). 
• Also produces an IgA protease and a pneumolysin.
• Grows readily on blood agar, causing alpha-hemolysis.

Pneumococcal Pneumonia

• Caused by Streptococcus pneumoniae in its lobar form.
• Common in young to middle-aged adults, somewhat rare in infants and the elderly, but lethal in people with other conditions (e.g., cancer, alcoholism).
• Symptoms include sudden onset with fever, chest pain, and thick sputum.

Pneumococcal Pneumonia (Diagnosis)

• Diagnosis involves Gram stain and culture of sputum samples to detect alpha-hemolysis and optochin disk sensitivity.
• Additional tests are bile solubility test and urine antigen test.

Pneumococcal Pneumonia (Pathogenesis)

• Aerosolized or colonized bacteria are aspirated into the alveoli.
• Inflammation ensues, filling the alveoli with edema fluid, leading to capillary congestion, massive infiltration of PMNs, and hemorrhage (red hepatization), followed by phagocytosis—leading to grey hepatization.
• Bacteremia may occur, potentially resulting in life-threatening complications depending on the patient's condition.

Other Pneumococcal Diseases

• Common causes of meningitis, otitis media, and sinusitis involving Streptococcus pneumoniae.
• Bacteremia can also lead to meningitis, endocarditis, and pneumonia (if not primary respiratory).

Haemophilus influenzae

• Gram-negative coccobacillus (or short rod).
• Fastidious grower (needs chocolate agar for culturing).
• IgA protease-producing bacteria that may facilitate colonization.
• Outer capsule is a key virulence factor.

Haemophilus influenzae (Continued)

• Six capsular serotypes exist. Serotype b is the most common cause of infection and the basis for Hib vaccine
• Other infections related to H. influenzae include pneumonia, meningitis, epiglottitis, otitis media, sinusitis, conjunctivitis, bacteremia, and cellulitis/arthritis

Mycoplasma pneumoniae

• The cause of "walking pneumonia" or "viral pneumonia."
• Occurs mostly in older children and young adults.
• Has a relatively long incubation period (10–14 days).
• Unique characteristics: smallest free-living bacteria, lack a cell wall, cannot be gram-stained, difficult to grow on agar or in culture media.

Mycoplasma pneumoniae (Continued)

• Pneumonia often mild or asymptomatic to mild.
• Treatment is typically readily treatable—with non-beta-lactam antibiotics.
• Complications can include otitis media, erythema multiforme, hemolytic anemia, myocarditis, pericarditis, and neurological complications—like tracheobronchitis and pharyngitis.
• Diagnosis is done through serological testing (ELISA, cold hemagglutinins)—or molecular techniques (PCR) for detection (e.g., of the presence of cold hemagglutinins).

Bordetella pertussis

• The causative agent of whooping cough.
• A gram-negative, fragile coccobacillus.
• Requires special media (Bordet-Gengou) for culturing.
• Rare due to the DTP vaccine (though possible waning immunity and emerging genetic variants can occasionally cause outbreaks).
• Disease has catarrhal, paroxysmal, and convalescent stages; complications can arise in each.

Klebsiella pneumoniae

• A gram-negative, non-motile bacillus, member of the enteric family.
• Grows on various agars (blood, MacConkey).
• Lactose fermenter
• Possesses endotoxin (LPS) and a capsule, which hinders antibiotic treatment and is often life-threatening.
• Common in pneumonia cases in people with chronic conditions (e.g., alcohol use disorder, diabetes, cancer).

Pseudomonas aeruginosa

• Gram-negative rod, found almost everywhere in moist environments.
• Oxidase positive but lactose negative
• Colonies are colorless on MacConkey agar but produce a bluish-green pigment (pyocyanin).
• A significant pathogen for cystic fibrosis patients.

Clinical Case Summary

• A 59-year-old man with emphysema presents with symptoms such as fever, chills, chest pain, and a cough, with rust-colored sputum.
• He has acute respiratory distress in the lower lobes of his left lung—which includes crackles, rhonchi, and expiratory wheezing.
• Diagnosing his possible infection relies on lab tests and X-ray findings.

Clinical Case Questions

• What is the most likely cause of the patient's illness?
• What would one expect to see in a sputum Gram stain?
• What pathogen is most likely based on the patient's condition?
• What are the unique characteristics of this organism?

Summary

• Microbes can cause various respiratory diseases
• Pneumonia, a severe respiratory illness, is primarily caused by bacteria (e.g., Streptococcus pneumoniae, Haemophilus influenzae, Mycoplasma pneumoniae, Legionella pneumophila).
• Tuberculosis caused by Mycobacterium tuberculosis is a chronic respiratory infection.
• Key virulence factors include outer capsule, LPS, IgA protease, and mycolic acids (specific to certain bacteria or pathogens).