Respiratory System Diseases

Bacterial Respiratory Diseases

Several species of bacteria cause diseases of the respiratory system, including pneumonia and tuberculosis.

Bacteria are the most common cause of pneumonia and is commonly acquired during extended hospital stays. Haemophilus influenzae opportunistically infects the mucous membranes of the lungs, and in the upper respiratory tract damage to the mucous membranes can result in systemic infection, leading to sepsis or bacterial meningitis. H. influenzae is often resistant to beta-lactam antibiotics. Fortunately, serious infections are rare, and because of vaccination the rate of H. influenzae infections is declining. Mycoplasma pneumoniae is another bacteria that causes pneumonia, though infections are most often mild in comparison to other pneumonia-causing infections. Symptoms of M. pneumoniae infection are sore throat, chest pain, fever, and cough. Acinetobacter is a genus of gram-negative bacteria with shapes ranging from coccus to rod that is a common cause of hospital-acquired, or nosocomial, pneumonia. Acinetobacter is a soil organism in nature, and respiratory infections are associated with breathing machines in hospitals. Resulting partly from its innate biology and partly from its prevalence in medical settings, Acinetobacter is resistant to many classes of antibiotics. Acinetobacter infections are treated with carbapenems or sulbactam.

Streptococcus pneumoniae is a gram-positive coccus bacterium that is a common component of the upper respiratory microbiota. When it penetrates into the lower respiratory tract, usually resulting from immune system suppression associated with other diseases or treatment, it can cause pneumonia. An inflammatory response is stimulated by many of the protein and carbohydrate structures produced on the surface of the bacterial cell wall, such as the antiphagocytic capsule and adhesin proteins. The production of invasion proteins, such as pneumolysin, that invade host cells exacerbates the inflammation response. Beta-lactam antibiotics, including penicillin, are the primary treatment option for S. pneumoniae, though resistance is common and other antibiotics such as fluoroquinolones may be required.

Legionella pneumophila is a gram-negative rod that causes an especially dangerous form of pneumonia called Legionnaires' disease. In nature L. pneumophila infects amoeba that live in aquatic environments and find humid areas of the built environment (air conditioning systems, fountains, etc.) hospitable. When water containing the amoeba, and L. pneumophila, becomes aerosolized, the bacteria are inhaled, and macrophages phagocytize them in the lungs. L. pneumophila then opportunistically infects the macrophages. The severity of Legionnaires' disease depends on the health of the host's immune system, and it is more serious in those with heart or lung diseases. L. pneumophila is difficult to culture and, therefore, to diagnose. Isolation from mucus and subsequent identification is the only definitive diagnostic available for all strains. Fluoroquinolones and macrolide antibiotics are effective at treating Legionnaires' disease once diagnosed.

Streptococcus pyogenes is a gram-positive coccus that commonly causes pharyngitis. S. pyogenes can also develop into scarlet fever and glomerulonephritis. Streptococcal pharyngitis is commonly referred to as strep throat. Signs of strep throat include sore throat, fever, pus on the tonsils, and inflamed lymph nodes. Antibiotics, usually penicillin, are given to treat strep throat and prevent complications such as rheumatic fever and streptococcal toxic shock syndrome, which is caused by superantigens, a class of antigens that cause activation of T-cells and are produced by some pathogenic viruses and bacteria most likely as a defense mechanism against the immune system.

Another example is tuberculosis, or TB, is a long-term respiratory infection caused by Mycobacterium tuberculosis. M. tuberculosis is a gram-positive bacterium with a unique waxy coating outside its cell wall that is made up of mycolic acids, long fatty acids. This waxy coating resists the Gram stain and can be specifically detected using a staining technique called an acid-fast stain. TB is an airborne bacteria that settles in a person's lungs when they breathe. TB can live in a person's body without causing them to be ill; this is called latent TB infection. If and when the bacteria become active and multiply, it is called TB disease. Signs that someone has TB are pain in the chest, a bad cough, and coughing up blood or phlegm. TB can be tested for by either a skin test, in which a small amount of sterile TB protein taken from cultures is injected in the skin on the lower part of the arm and examined in 24 to 48 hours to look for a reaction, or a blood test.

Without treatment, tuberculosis infection will eventually cause death either through asphyxiation, in which the body is deprived of oxygen, secondary infection, an infection that happens during the treatment of another infection, or pulmonary aneurysm, a blockage of an artery in the lungs by something that has traveled there in the bloodstream from somewhere else. One-fourth of the world's population is infected with TB. In 2016 10.4 million people around the world became sick with TB disease, and there were 1.7 million TB-related deaths worldwide. TB is also the leading killer of people who are HIV infected. TB is treated with antibiotics. It is important that the complete dosage of antibiotics is taken in order to prevent the bacteria from becoming immune to the antibiotics, thus causing those antibiotics to be ineffective and the disease to return.
Notable respiratory pathogens include Mycobacterium tuberculosis (scanning electron microscope), Streptococcus pneumoniae (scanning electron microscope), Staphylococcus aureus (scanning electron microscope), and Corynebacterium diphtheriae (light microscope).
Credit: NIAID (top left), CDC/Dr. Richard Facklam (top right), NIAID (bottom left), and CDC/Graham Heid (bottom right)