The healthy lung hosts a diverse array of microbes that collectively form a microbiome. These resident microbes exist in a natural balance with the body’s immune system, and can communicate with and alert the immune system to invading pathogens. In addition to the lung microbiome, other host defenses which protect the lung from infection include: mucociliary clearance (when the cilia of the respiratory epithelium sweeps mucus and its trapped contents toward the upper respiratory tract), anti-microbial components secreted into the airways, the tight coupling of airway epithelial cells via tight junctional proteins, opsonins present in surfactant secreted by type II pneumocytes in the alveoli, and resident immune cells such as alveolar macrophages in the lung. When these barriers to infection are weakened and overcome, pathogens may descend into and infect the lungs. It is for this reason that those with an already weak immune system or other barriers, such as the old, children, or patients with preexisting conditions, are at a greater risk of developing pneumonia.

Pneumonia most commonly involves an acute respiratory infection caused by bacteria or viruses which penetrate into the lower respiratory tract. Although both bacterial and viral pneumonias are prevalent and can contribute to serious illness, viral infection is the more common cause of pneumonia. Streptococcus pneumoniae (Pneumococcus) is the most common cause of community-acquired bacterial pneumonia. Viruses that can infect the nasal cavity (causing the “common cold”) can also descend into the lungs and cause pneumonia.

Common infective causes of pneumonia

When the body responds to an ongoing infection, chemical messengers (cytokines and chemokines) are released which cause and inflammatory response and recruit immune cells to the area to help deal with the infection. It is the inflammatory response to infection which causes pneumonia through infiltration of immune cells and fluid into the lung, and by the tissue damage that occurs as the body fights the infection. Depending on the pathogen and where it colonizes along the lower respiratory tract, different inflammatory patterns (lobar pneumonias, bronchopneumonias, interstitial pneumonias) can be seen microscopically. The clinical symptoms of short breath and increased rate of breathing are caused by consolidation of the alveoli as they fill with fluid, inflammatory cells and other substances (lobar/bronchopneumonias) or by thickening of the alveolar walls which (interstitial pneumonia), both of which limit the gas exchange that can occur between the blood in the capillaries and the air in the alveoli. The abnormal buildup of fluid in the lungs, in this case caused by inflammatory mechanisms, is called pulmonary edema.

Clinical professionals can get a hint of infection by performing a physical examination on patients who show symptoms of pneumonia. This involves performing auscultation (with a stethoscope) to listen for signs of infection through abnormal sounds caused by the buildup of fluids in the lungs, using their hands/fingers to feel evidence of infection (palpitation) as the patient breathes, and watching the patient’s breathing rate and what muscles are being used during breathing as an indication of infection. Chest X-rays can be used to visualize consolidation in the lungs based on the density of the tissue in the lungs, causing areas of fluid buildup to appear white on an X-ray. Analysis of patient sputum that is coughed up from the lower airways can also give indication to infection by visual analysis (if pus or blood is present). The sputum can also be sent to a microbiology laboratory to grow and analyze the microbes in it and to test antibiotic susceptibility, which helps choose the most effective therapy for a patient. Blood tests may also be used for diagnosis, as some molecules in blood indicate inflammation at high levels (due to infection) and abnormal blood-gas levels can indicate an issue in the function of the lungs. Blood tests can also be used to monitor for infections in the blood (septicemia) which can be extremely dangerous.

The treatment of pneumonia depends largely on the identity of the microbe involved in the infection and the presentation of the patient. A healthcare team can implement changes to aid breathing and medications can be given to widen the airways and decrease swelling due to inflammation. Extra oxygen may also be given to the patient if blood oxygen saturation becomes too low. Culture and sputum results can determine the specific antimicrobial that can be used to help fight the infection.

Credits

Authors:  Noah Stewart (UBC undergraduate student) and Dr. Simon Duffy (BCIT)

Author of questions and exercises: Noah Stewart and Eva Su (UBC undergraduate students)

Medical illustrator: Sarah Pinault (UBC-O undergraduate student)

Gross anatomy video:   Dr. Jennifer Kong (BCIT & UBC)

Histopathology video:  Noah Stewart (UBC undergraduate student) with supervision of Dr. Gang Wang (UBC, Director of DHPLC)

Nurse:  Anna Kornienko (Nursing, BCIT)

Medical Radiographer:  Michealle Beauchamp (Medical Radiography, BCIT)

Microbiologist:  Marion Reagan (Medical Laboratory Sciences, BCIT)

Videoproducer:  Ian Whittlesey (BCIT)