Lower Respiratory Tract Infection, LRTI - Introduction, Introduction, Etiological agents

Introduction to Respiratory Tract

Anatomically, the human respiratory tract can be divided into two major areas:

The upper respiratory tract (URT)

consists of all structures above the larynx i.e. nasal cavity, pharynx (nasopharynx, oropharynx, laryngopharynx)

Lower respiratory tract (LRT)

The air flows through the trachea to the bronchi and bronchioles and then terminates in the alveolar spaces (alveoli), where gas exchange occurs. 300 million alveoli are estimated to be present in the lungs. Alveoli are primary microscopic gas exchange structures of the respiratory tract.

Etiological agents of Lower Respiratory Tract Infection (LRTI)

Pathogenic microorganisms can cause disease in the respiratory tract in a limited no. of ways. The establishment of infection after exposure is an exception rather than a rule. When micro-organisms are successful in establishing an infection, it is not only dependent on pathogenicity but also on the host's ability to prevent the infection.

If micro-organisms have been isolated from normally sterile sites by sterile procedures (that avoid contamination with normal flora) should be identified and reported to the clinician.

Many species of bacteria, viruses, and fungi are considered human pathogens.

Some common pathogens of the respiratory tract, especially Lower Respiratory Tract Infection (LRTI), include:

• Corynebacterium diphtheriae (toxin-producing)

• Mycobacterium tuberculosis

• Mycobacterium pneumoniae

• Chlamydia trachomatis

• C. pneumoniae

• Bordetella pertussis

• Legionella spp.

• Nocardia spp.

• β-hemolytic streptococci

• Coccidioides immitis

• Acinetobacter spp

• Blastomyces dermatitidis

• S. aureus

• Streptococcus pneumoniae

• Neisseria miningitidis

• Mycoplasma spp.

• Haemophilus influenzae

• H. parainfluenzae

• Moraxella catarrhalis

• Actinomyces spp.

• Cryptococcus neoformans

• Histoplasma capsulatum

• C. albicans

• Viruses

  • RSV

  • human metapneumovirus

  • adenovirus

  • HSV (Herpes Simplex Virus)

  • Severe acute respiratory syndrome

  • Enterovirus

  • Hantavirus

  • Influenza and parainfluenza virus

  • Rhinovirus

  • Coronaviridae

Rare/opportunistic pathogens of Lower Respiratory Tract Infection (LRTI) include:

• Bacillus anthracis

• Yersinia pestis

• Coxiella burnetii

• Brucella spp.

• Salmonella spp.

• Pasteurella multocida

• Klebsiella rhinoscleromatis

• Varicella – Zoster virus

• Non-hemolytic Streptococci

• Coagulase-negative Staphylococci (CoNS)

• Staphylococci

• Micrococci

• Corynebacterium spp.

• Neisseria spp. except (N. gonorrhoea and N. meningitidis)

• Lactobacillus spp.

• Veillonella spp.

• Spirochetes

• Campylobacter spp.

• Selenomonas

• Wolinella

• Parasites