105. Microbiological Aspects of Respiratory Tract Infections

Covers respiratory tract infections, splitting them into upper and lower tract conditions, then the normal commensal flora and the organisms responsible at each site. It walks through the stages of pathogenesis, including immune evasion mechanisms, followed by clinical features and diagnostic specimens and techniques such as Gram stain, culture, PCR and Ziehl–Neelsen staining. It closes with vaccines, infection control and antibiotic stewardship. Four summary tables list the causative agents by condition and the diagnostic methods.

Inside: 8 sections · 2 labelled pictures · 4 tables · 15 flashcards · 3 min

Topic 105 of 113 in the MU Sofia syllabus for Microbiology (Medicine, Year 3). The free AceMed note covers it in 8 sections, about 193 words (3 min of reading), with 15 flashcards.

I. Overview

Respiratory tract infections (RTIs) are among the most common infectious diseases worldwide.
Divided into:
Upper Respiratory Tract Infections (URTIs): nasal cavity, pharynx, larynx
Lower Respiratory Tract Infections (LRTIs): trachea, bronchi, lungs

II. Normal Flora of the Respiratory Tract

URT contains commensal flora:
Streptococcus viridans, Neisseria spp., Staphylococcus epidermidis, Corynebacterium spp.
These prevent colonization by pathogens through competition and immune priming.

III. Etiological Agents by Site

A. Upper Respiratory Tract Infections (URTIs)

B. Lower Respiratory Tract Infections (LRTIs)

IV. Pathogenesis

1. Entry via inhalation or contact with contaminated droplets
2. Colonization of mucosa (URT) or alveoli (LRT)
3. Evasion of host defenses:

Capsule (S. pneumoniae, H. influenzae)
Adhesins (Mycoplasma pneumoniae)
Toxins (B. pertussis, influenza)
Intracellular survival (M. tuberculosis)

4. Host response causes inflammation, mucus production, and in severe cases, alveolar damage and hypoxia

V. Clinical Features (general)

URTI: nasal congestion, sore throat, cough, fever, hoarseness
LRTI: productive cough, dyspnea, chest pain, abnormal auscultation, high-grade fever
In TB: weight loss, night sweats, hemoptysis

VI. Microbiological Diagnosis

A. Specimen Collection

URT: throat swab, nasal swab, nasopharyngeal aspirate
LRT: sputum, bronchoalveolar lavage (BAL), tracheal aspirate, pleural flui