This image is a Gram stain of a sputum specimen from a patient with pneumonia. The Gram stain shows encapsulated lancet-shaped gram-positive cocci associated with the polymorphonuclear leukocytes. Note the clear zone surrounding the organisms. This zone is consistent with a large polysaccharide capsule that is not picked up by the Gram stain. These Gram stain findings are consistent with Streptococcus pneumoniae.
S. pneumoniae is a bacterium that can be found in the lung. This disease can be divided into two forms, lobar pneumonia and bronchial pneumonia. Bronchial pneumonia is prevalent in infants, young children, and older adults. Lobar pneumonia is more likely to occur in young adults. S. pneumoniae is a Gram-positive, lanceolate shaped coccus that is usually seen in pairs of cocci (diplococci – see image). It does not form spores and is not motile. It lacks catalase and ferments glucose to lactic acid. It doesn’t display an M protein, but it hydrolyzes inulin, and has a cell wall that contains a thick peptidoglycan layer as well as teichoic acids.
S. pneumoniae causes a variety of diseases including: otitis media (inflammation of the middle ear), sinusitis (inflammation of the nasal sinus), pneumonia (inflammation of the lung), meningitis (inflammation of the meninges surrounding the brain and spinal cord), and septicemia (bacteria that have invaded the bloodstream.) Symptoms include: abrupt onset, increasingly severe cough, high fever increasing over 2-3 days, shaking chill, chest pain that increases with breathing, and breathlessness. It is classified by purulent sputum (pus filled sputum) that may or may not be blood-tinged, and has a mortality rate of 5%.
S. pneumoniae has a variety of virulent and pathogenic mechanisms that help it to spread and cause disease. Some of these include: secretory IgA protease which cleaves IgA and promotes spread of infection, neuroaminidase a hydrolytic enzyme that cleaves terminal acetylated neuraminic acids from sugar residues, hemolysins substances that causes dissolution of red blood cells, pneumolysins substances that activate the complement system and affect the immune cells, and autolysins LytA, B, and C enzymes which help to break down part of a cell or tissue by their enzymes. The bacteria also has a capsule which is made up of a well organized layer of polysaccharides or long sugar chains that aid in protection. It contains cell wall components including: PAF receptor ( platelet activating receptor), the C-reactive protein a protein that is increased during systematic inflammation, and teichoic acids in which there are no known functions but they are found in Gram-positive bacteria.
Diagnosis of S. pneumoniae can be made by the observation of gram-positive diplococci in samples of sputum, blood, or cerebral spinal fluid. Confirmation can be determined by a susceptible optochin test, susceptible bile soluble test, and a Quellung test in which you will see swelling, and a rapid agglutination test specific for the bacteria.
Treatments for S. pneumoniae infections include, penicillin, macrolides, erythromycin, beta-lactams, and chloramphenical. However, 32% of bacteria are now resistant to penicillin.
Legend written by:
Colorado State University
Fort Collins, Colorado