Overview of infections of respiratory tract and its pathogens

By: Shaily Sharma (MSIWM041)

  • A brief overview of the infections of the respiratory tract and its pathogens:

The respiratory tract along with the gastrointestinal tracts is one of the major connections between the interiors of the body and the outside environment.

The respiratory tract is the pathway is that pathway of the body through which fresh oxygen enters the body and removes the excess carbon dioxide which is not needed by the body. 

  • Anatomy of the respiratory system:
  • Broadly, the respiratory system of humans can broadly be divided into two distinct areas; the upper and the lower respiratory tracts.
  • The parts that consist the lower respiratory tract are:
  1. Trachea
  2. Bronchi, and
  3. Bronchioles
  • The respiratory pathway begins with the nasal and the oral passages. These passages serve to humidify the air that is inspired. These pathways extend past the nasopharynx and the oropharynx to the trachea and then to the lungs.
  • The trachea is the organ that divides into the bronchi, which then further subdivides into the bronchioles. The bronchioles are the smallest branches of the trachea which finally terminate into the alveoli.
  • Approximately 300 million alveoli are said to present in the lungs. These mainly serve as the primary, microscopic, gas exchange structures of the respiratory tract.

See the source image

  • The lungs (along with the respiratory system) and the heart lie in the thoracic cavity. 
  • The thoracic cavity has three partitions that are separated from one other by the pleura (the pleura majorly cushions the lungs and reduce the friction which may develop between the lungs, rib cage and the chest cavity. It is a two layered membrane covering the lungs.)
  • The lungs occupy the right and the left pleural cavity while the mediastinum (the space between the right and the left lungs) is occupied by the esophagus, trachea, large blood vessels along with the heart.
  • Pathogenesis of the respiratory tract:
  • The success of an organism to cause disease is mainly dependent on the organism’s ability to cause disease (pathogenesis), and
  • The human hosts ability to prevent the infection (strength of the host’s immune system)
  • The host factors that help in non-specifically protect the respiratory tract from infection are:
  1. Nasal hair
  2. Convoluted passages and the mucous lining of the nasal turbinate
  3. Secretory IgA and non-specific antibacterial substances (like lysozyme) in respiratory secretions
  4. The cilia and the mucous lining of the trachea and reflexes such as coughing and sneezing. 
  • In addition to the non-specific hosts defenses, normal flora of the nasopharynx and the oropharynx help in the prevention of colonization of the upper respiratory tract. 

Microorganism factors:

Organisms possess certain traits that promote colonization leading to infection in the host. The factors that influence the respiratory tract infections are –

  1. Adherence: 
  • The potential of a microorganism depends, in one way or the other, on its ability to establish a stable contact/foothold on the surface of the host by the process of adherence. 
  • The ability of microorganisms to adhere to the host surface is dependent on two factors:
  1. Presence of normal flora, and
  2. Overall state of the host.
  •  Surviving or growing on host tissue without causing harmful effects is called colonization. 
  • Most etiologic agents must first adhere to the mucosa of the respiratory tract to some extent before they can cause harm.
  • Example: Streptococcus pyogenes possess specific adherence factors and its gram-positive cell wall contains lipoteichoic acids and M proteins. Many gram-negative bacteria like Enterobacteriaceae, Pseudomonas spp., Bordetella pertussis, adhere by the means of proteinaceous fingerlike projections called fimbriae. 
  • Viruses possess either a hemagglutinin or other proteins that mediate that epithelial attachment.
  1. Toxins
  • Certain microorganisms are considered to be etiologic agents of disease because they possess virulence factors that are expressed in every host. 
  • Example: Corynebacterium diphtheriae. 
  • Some strains of Pseudomonas aeruginosa also produce toxins which are similar to the toxins of Diphtheria.
  •  Bordetella pertussis which is the causative agent of whooping cough produces toxins that play a role in inhibiting the activity of phagocytic cells and damaging the cells of the respiratory tract.
  1. Microorganism growth
  • Pathogens cause disease by merely growing in the host tissue, interfering with normal tissue function and attracting host immune effectors, such as neutrophils and macrophages.
  • Example: S. pyogenes, M. tuberculosis, Mycoplasma pneumoniae, etc.
  1. Avoiding the Host Response
  • Certain respiratory tract pathogens possess the ability to evade host defense mechanisms.
  •  S. pneumoniae, H. influenza, K. pneumoniae and others possess polysaccharide capsules that serve both to prevent engulfment by phagocytic host cells and to protect somatic antigens from being exposed to host immunoglobulins.
  • Organisms of the respiratory tract and agents that cause diseases: 

Pathogens may or may not cause the respiratory infection but can be present as a part of normal flora.

  •  Some of the pathogens that exist and results in the respiratory infection are referred to as true pathogens. 
  •  Some of the pathogens that are present in the body but never cause an infection until and unless they are met with the favorable conditions are called

as opportunistic pathogens.  

  • Possible pathogen: they are the pathogens that are likely to cause respiratory


  • Example: Actinomyces spp., Haemophilus influenzae, Enterobacteriaceae, etc.See the source image
  •  Rare pathogen: pathogens that may cause a respiratory infection are rare

pathogens. Example: Coxiella burnetti, Brucella spp., Salmonella spp, etc.  

  •  Definite respiratory pathogen: pathogens that always cause respiratory infections are called as definite respiratory pathogens.
  • Example: Bordetella pertussis, Blastomyces dermatitidis, Legionella spp., etc.
  • Different types of agents that cause respiratory diseases are bacteria, fungi or


  •  Bacterial agents: the bacterial agents that cause respiratory infections are

Mycoplasma spp., Streptococcus pneumoniae and Neisseria meningitides. 

  • Fungal agents: the fungal agents that cause respiratory infections are Candida

albicans, Cryptococcus neoformans and Histoplasma capsulatum.  

  •  Viral agents: the viral agents that cause respiratory infections are human

metapneumovirus, adenovirus, enteroviruses, and herpes simplex virus. 

  • Major respiratory diseases are caused by M. tuberculosis, S. pyogenes and




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