Causative Agents Of Respiratory Infections

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The respiratory system is divided into two tracts. The upper respiratory tract consists of the paranasal sinuses, the nasal cavity, the pharynx, and the epiglottis. The paranasal sinuses are lined with mucous membranes that warm, humidify, and filter the air. The epiglottis seals off the airway during swallowing. The lower respiratory tract consists of the larynx, the trachea, bronchi, bronchioles, lungs, and alveoli. Mucous membranes line the trachea, bronchi, and bronchioles and form the mucociliary escalator. The mucociliary escalator prevents inhaled debris from entering the lungs. Gas exchange occurs in the alveoli, the air sacs in the lungs.

The sinus membranes can become inflamed from allergens or pathogens. This is called sinusitis and can cause sinus congestion if mucus is unable to drain into the nose. Accumulation of mucus in the sinuses can allow bacteria to multiply, causing a sinus infection.

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Bronchitis is inflammation of the bronchi or bronchioles, tracheitis is inflammation of the trachea, and laryngitis is inflammation of the larynx. Laryngitis may also cause temporary loss of voice as the vocal cords become swollen. Epiglottitis is a serious condition in which the epiglottis is inflamed. This can block the airway. Pneumonia is usually caused by inflammation of the alveoli.

Respiratory infections commonly cause sneezing, cough, and runny nose which aid in the transmission of these infections through respiratory droplets. Since air is constantly being breathed into the body, inhaling these respiratory droplets can happen very easily. It is for this reason that most microbes gain access to the body through the respiratory route.

Bacterial infections may also occur as a secondary complication of viral infections. Colds commonly cause mucus to accumulate in the respiratory tract, allowing bacterial to proliferate.

Streptococcus pyogenes can cause streptococcal pharyngitis, commonly referred to as strep throat. Many people are carriers of S. pyogenes, as humans are a natural reservoir. Transmission usually occurs through respiratory droplets. Symptoms of strep throat include inflammation of the throat, exudate in the tonsils, swollen lymph nodes in the neck, and low-grade fever.

Streptococcus pneumoniae can cause pneumococcal pneumonia. Humans are the only know reservoir of pneumococcal bacteria. Children and the elderly are at the greatest risk of this disease. S. pneumoniae first colonizes the nasopharynx which can then migrate to the lungs. Symptoms of pneumonia cause by S. pneumoniae include high fever, chills, shortness of breath, cough with sputum, and chest pain when inhaling. Pneumonia can be cause by a wide array of infectious agents. Pneumonia resembling S. pneumoniae is used to classify the category of typical pneumonia. Typical pneumonia is caused by bacteria, while atypical pneumonia is caused by viruses. Haemophilus influenzae can also cause pneumonia.

Mycobacterium tuberculosis can cause tuberculosis, also known as consumption. Mycobacterium bovis was historically another common cause until pasteurization made milk products safer to consume. Most infections of M. tuberculosis are latent and do not cause symptoms. Latent tuberculosis cases also do not usually progress to an active case. When an active case of tuberculosis does occur, symptoms include a cough that sometimes contains blood, fever, fatigue, weight loss, and night sweats. Tuberculosis is the fourth leading cause of death form an infectious disease.

Bordetella pertussis can cause Pertussis, also known as the whooping cough. Pertussis is preventable through vaccine. As a result, unvaccinated infants are at the highest risk of contracting this infection. Infants also have the most severe symptoms. Pertussis has three stages. The first stage is the catarrhal phase and lasts for 1-2 weeks. The patient has mild symptoms with runny nose, watery eyes, and a cough. The second stage is the paroxysmal stage which brings severe coughing fits that last for 2-6 weeks. The convalescent stage is the final stage, lasting about 4 weeks. Coughing fits become less frequent.

Corynebacterium diphtheriae causes diphtheria. Diphtheria in the United States is rare, as children are routinely vaccinated for it. Young children in developing countries are most likely to be affected. Diphtheria causes sore throat and low-grade fever, as well as a swollen neck. C. diphtheriae produces a dangerous toxin that kills tissue in the airway, forming a pseudomembrane on the tonsils and throat a few days after the first symptoms appear. If left untreated, the pseudomembrane will continue to grow until airflow is constricted, suffocating the patient.

The normal microbiota of the respiratory system closely resembles the normal microbiota of the mouth. Normal flora of the upper respiratory tract include staphylococci, alpha-hemolytic streptococci, nonhemolytic streptococci, Streptococcus pneumoniae, spirochetes, enterococci, diphtheroids, and members of Moraxella, Neisseria, and Haemophilus. Veillonella, Streptococcus, Pseudomonas, and Prevotella are normal microbiota of the lungs.

Cystic Fibrosis is a hereditary disease. Asymptomatic carriers contain one copy of the mutated cystic fibrosis transmembrane regulator (CFTR) gene. Those with cystic fibrosis inherited two copies of the faulty gene. Not all mutations of the CFTR gene cause cystic fibrosis. Mutations that cause a defective chloride channel result in cystic fibrosis. Intracellular chloride ions build up, causing abnormally thick mucus to accumulate in the mucous membranes. In addition, sweat contains increased levels of sodium chloride. As the mucus is thick, it settles in the lungs and allows bacteria to proliferate and form biofilm. Because of this, lung infections are common and difficult to treat in these patients. Pseudomonas aeruginosa is an infamous microorganism that infects cystic fibrosis patients. Other bacteria that predominately affect cystic fibosis patients include Burkholderia cepacia and Stenotrophomonas. Infections cause inflammation and can lead to lung failure.

Sputum gram stain reports include quantitation of epithelial cells, white blood cells, and any bacteria or fungus seen. Quantitation is reported as rare, few, moderate, or many, though this may vary depending on the laboratory. Bacteria is listed with their gram stain result, and gram positive cocci are described as being in pairs, clusters, or chains. Normal flora is noted, and any potential pathogens are identified in the sputum culture. Identification can be done through the VITEK 2 or MALDI-TOF. Sputum cultures are plated on blood, chocolate, and MacConkey agar. Oxidase and indole are performed on gram negative organisms. An acceptable sputum specimen should contain greater than or equal to 10 white blood cells with mucus and less than 25 epithelial cells per low-power field. Specimens that do not meet this criteria may be contaminated by oropharyngeal flora.

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Causative Agents Of Respiratory Infections. (2021, September 12). Edubirdie. Retrieved June 16, 2024, from
“Causative Agents Of Respiratory Infections.” Edubirdie, 12 Sept. 2021,
Causative Agents Of Respiratory Infections. [online]. Available at: <> [Accessed 16 Jun. 2024].
Causative Agents Of Respiratory Infections [Internet]. Edubirdie. 2021 Sept 12 [cited 2024 Jun 16]. Available from:

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