Infections of the respiratory system Flashcards
Describe commensals of URT
- Middle ear and mastoid are normally sterile or have minimal bacteria/fungi
- Nose, sinuses, nasopharynx, oropharynx, and laryngopharynx are colonized with bacteria and yeast
- Common commensals of the nose include Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus species, Corynebacterium species, Haemophilus species ^[vary in frequency between people]
- Common commensals of the pharynx include Staphylococcus. spp, Moraxella spp., Corynebacterium species ^[diphtheriae, not part of normal URT commensal flora, most other species benign], Haemophilus species, Neisseria spp. e.g. meningitidis, Streptococcus viridans group, S. pneumoniae
Describe the development and composition of commensal flora
- Age, season, social factors, and mode of transmission influence commensal flora prevalence.
- age: Prevalence of S. pneumoniae, H. influenzae, and M. catarrhalis decreases with age; N. meningitidis peaks in teenagers-adolescents
- season: prevalence of many pathogens increases in winter
- social factors: Prevalence of S. pneumoniae and H. influenzae is highest in low socioeconomic classes; Children with siblings have increased carriage of S. pneumoniae, H. influenzae, and M. catarrhalis
- mode of transmission: carriage of pathogens requires close proximity; droplets are created by coughing, sneezing; and transmitted by person-person contact or via fomites
Describe RT defence mechanisms
- Physical defenses include nasal hairs, irregular nasal chambers e.g. sinuses and turbinates - channel air, increase surface area to trap, mucus, ciliated epithelium (nasal cavity, sinuses, bronchi and trachea), cough reflex and epiglottic reflex, and mucociliary escalator.
- Chemical defenses include mucus secretions (phagocytes and lysozyme ^[antibacterial]), alveolar fluid(surfactant)
- Immunological defenses include alveolar macrophages and secreted antibodies (IgA).
Describe the role of the mucociliary escalator
- Aids in expelling pathogens and maintaining commensal balance.
- Prevents overgrowth of upper respiratory tract commensals.
- Helps keep middle ear, mastoids, and lungs sterile.
- mucus:
- A viscoelastic gel containing water, carbohydrates, proteins, and lipids - salty and sticky
- Secreted by goblet cells of the respiratory surface epithelium and the submucosal glands
- Traps inhaled particles and microorganisms
- motile cilia:
- Hair-like projections that cycle synchronously, continually
- Move trapped particles and microorganisms in mucus toward pharynx where they’re swallowed
Note: the mucociliary escalator can be inhibited
- Viruses can disrupt the mucociliary escalator through:
– Direct or indirect ciliary impairment, e.g. direct damage to the ciliary system or by inducing excess mucus formation
– Secretion of enzymes that breakdown mucus
- Allows microorganisms to migrate to sterile regions - secondary bacterial infection
- Physical injury, smoking, alcohol and diabetes ^[hgih blood sugar impacts neutrophil activity] can also disrupt the mucociliary escalator
Describe examples of infections of the URT and LRT
- Upper respiratory tract infections (URTI) include otitis media ^[bacterial or viral], mastoiditis ^[bacterial mostly], sinusitis ^[both but viral mostly], pharyngitis, laryngitis, and epiglottitis.
- Lower respiratory tract infections (LRTI) include tracheitis, bronchitis, bronchiolitis ^[usual viral, common in paeds], pneumonia, lung abscess, and empyema ^[infection of pleural cavity; fills with pus].
Note that not all symptoms will be due to infection e.g. allergic sinusitis is very common, smokers and non-infective symptoms of bronchitis
Compare and contrast the respiratory tract in the context of infection
- Upper respiratory tract is home to many commensals and is exposed to both exogenous (viruses) and endogenous infections (URT major portal for entry).
- Lower respiratory tract is essentially sterile; detection of organisms here suggests infection may be due to pathogens from the upper respiratory tract, either exogenous or endogenous.
- identification of causative agent is difficult?
Characterise respiratory tract infections
Viruses - **very common
- Upper Respiratory Tract (URT)
- Pharyngitis- Rhinovirus/coronaviruses
- URT & LRT
- Influenza A&B
- Respiratory syncytial virus
- Adenovirus
- Human metapneumovirus
- Parainfluenza viruses
- Coronaviruses
- Fungi - uncommon
- Bacteria - variable incidence
- Pharyngitis
– Neisseria gonorrhoeae (uncommon) (Yr 1)
– Corynebacterium diphtheriae (uncommon)
- Pharyngitis
- GAS!!!
- LRT
– Bordetella pertussis (whooping cough)
(Yr 3)
– Mycobacterium tuberculosis (Yr 3)
– “Atypical” pneumonia - Mycoplasma pneumoniae
- Chlamydophilia pneumoniae/psittaci
- Legionella species
Characterise endogenous respiratory tract infections
Pharyngitis
- Streptococcus pyogenes ^[can be a/symptomatic] and other beta-hemolytic Streptococci
LRT
- Haemophilus influenzae
- Moraxella catarrhalis
- Streptococcus pneumoniae
Sinusitis/Mastoiditis/Otitis Media
- Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, Staphylococcus aureus (20-30% population, asymptomatic carriers)
In hospitalised patients, commensal flora changes:
- E. coli and other Gram negatives
- Pseudomonas aeruginosa
- Staphylococcus aureus
- All of the above may colonize the URT and then cause LRTI
Detail host factors increasing risk of infection with endogenous flora
- Problems with drainage
- Blocked sinuses or auditory tube due to viral infection or allergies
- Problems with normal physical motion
- Poor cough, aspiration, intubation, paralysis
- Problems with mucociliary escalator: Poor cough, aspiration, intubation, paralysis
- specific immunocompromise or lack of immunity
Describe streptococcus features and as a pathogen
Streptococcus species are commensals and common causes of respiratory tract infections. (lobar pneumo if microaspiration into LRT)
Streptococci: General Characteristics
- Gram-positive cocci in chains
- Streptococcus pneumoniae classically in pairs (diplococci)
- **Facultative anaerobes
Haemolysis and Lancefield Classifications
- β-hemolytic Streptococci
- S. pyogenes (Group A Streptococcus/GAS)
- S. agalactiae (Group B Streptococcus/GBS)
- S. dysgalactiae (Group C and G Streptococcus)
- β-, α- or γ-hemolytic
- S. milleri group (Group A, C, F, G or untypable)
- α- and/or γ-hemolytic
- S. pneumoniae (α-hemolytic; untypeable)
- S. viridans group (α- or γ-hemolytic; untypeable)
S. pneumoniae as an Endogenous Pathogen
- A commensal of the URT of healthy people
- More common in children (40 %) than adults (10 %)
- Children initially colonized ~ 6 months of age
- Highest concentration of organisms usually in nasopharynx
- Children are transiently colonized by different serotypes (sometimes simultaneously)
- 91 known capsular serotypes
- Endogenous pathogen of the URT, can cause otitis media, mastoiditis, sinusitis
- Can disseminate into the LRT or other parts of the body, causing pneumonia, bacteremia, meningitis
- Note: Risk of developing these infections appears highest immediately after colonization *because patients have not yet produced specific antibodies to the organism
Describe typical pneumonia or pneumococcal pneumonia and list host risk factors
- Abrupt onset
- High fever +/- rigors
- Productive cough with usu. purulent sputum
- Shortness of breath
- Pain on breathing (pleuritic)
- Lobar consolidation (or anat segments of lobe) on CXR
Host Risk Factors for Invasive Pneumococcal Infection
- Age: Young and old
- Asplenic/hyposplenic
- HIV infection
- CLL
- **Impaired or reduced antibody production
- Hypogammaglobulinemia
- Multiple myeloma
Describe the capsule as a virulence factor
S. pneumoniae Capsule - Virulence Factor
enables evasion
- Prevents entrapment in mucus, allowing access to epithelial surfaces
- Protects against phagocytosis and complement-mediated lysis: bacteria persist and multiply
- Anti-S. pneumoniae capsule antibodies (generated through acquired immune response) are protective, but typically not cross-protective
- Vaccine contains purified capsular polysaccharide antigen from many different S. pneumoniae serotypes ^[hence why acquiring one does not necessarily confer immunity against another]
Describe features of haemophilus
Haemophilus influenzae
- Gram-negative coccobacillus ^[may look like a bacillus or a short bacillus which is almost coccus]
- Unencapsulated - less invasive and less virulent
- Capsulated - a, b, c, d, e, f
- H. influenzae B (Hib) most virulent
Haemophilus influenzae Non-Invasive Infection
i.e. not on mucosal surfaces
- Sinusitis
- Otitis media
- Conjunctivitis
- Pneumonia
Invasive Infection (Hib) ^[very rare since vaccination introduced]
- Epiglottis
- Bacteremia
- Meningitis
- Septic arthritis
Describe features of Neisseria
- Gram-negative diplococcus
- Unencapsulated - not often associated with infection - 10-25% of young people carry in pharynx
- Capsulated - A, B, C, W, Y
- Invasive disease
- Vaccination: Previously only C but now also combined tetravalentA, C, W, Y and standalone B
Neisseria meningitidis Invasive Disease
- Risk factors for invasive disease
- Age <5 years and 15-25 years ^[living in close quarters]
- Asplenia/hyposplenia
- Deficiency or impairment of complement membrane attack complex (C5-C9)
- Invasive disease:
- Bacteremia (meningococcemia)
- Meningitis
Describe features of Moraxella
- Gram-negative diplococcus
- Diseases
- Sinusitis
- Otitis media
- Pneumonia
- Infective exacerbations of chronic obstructive pulmonary disease
