Pneumococci are among the most important agents of bacterial pneumonia. Other microorganisms such as staphylococci, Haemophilus influenzae
(Experiment 22.1), and Klebsiella pneumoniae
may also be associated with serious
pulmonary disease. Bacterial agents of pneumonia cause an acute inflammation of the bronchial and/or alveolar membranes.
When the alveoli are involved, their thin membranes may be disrupted by hemorrhage of alveolar capillaries and collections of
inflammatory exudate (pus) containing many white blood cells. Laboratory diagnosis is often made by isolating the causative agent
sent for culture. However, because sputum specimens pass through the oropharynx as they are expectorated, contaminating
members of the normal throat flora may interfere with culture results by overgrowing the pathogen. The causative organism
is often found in the bloodstream during early stages of infection, and therefore, patient blood should also be cultured. In
some patients, the organisms spread from the bloodstream to the central nervous system to cause meningitis. Pneumococci can
then be isolated from the patient’s cerebrospinal fluid as well.
Pneumococci are classified in the genus Streptococcus
as the species pneumoniae
. They are gram-positive, lancet-shaped
cocci that characteristically appear in pairs (diplococci) or in short chains (see colorplate 3
). Like other streptococci, they are fastidious
microorganisms and require blood-enriched media and microaerophilic conditions for primary isolation. They are alphahemolytic
and usually produce greening of blood agar around their colonies. Streptococcus pneumoniae can be distinguished from
other alpha-hemolytic streptococci because it is lysed by bile salts and other surface active substances, including one known as
optochin (see colorplate 31
Another distinctive feature of pneumococci is that they possess a capsule, composed of a viscous polysaccharide. This
slimy capsule protects them from destruction by phagocytes that gather at sites of infection throughout the body to ingest them.
In the laboratory, the pneumococcal capsules are not readily demonstrated by usual staining techniques, but they can be made
visible under the microscope by a serological technique known as the “quellung” reaction. Quellung
is the German word for
“swelling” and describes the microscopic appearance of pneumococcal or other bacterial capsules after their polysaccharide antigen
has combined with a specific antibody present in a test serum from an immunized animal. As a result of this combination,
and precipitation of the large, complex molecule formed, the capsule appears to swell, because of increased surface tension, and
its outlines become clearly demarcated (see colorplate 10
The capsular antigen can also be detected with antibody-coated latex reagents. Colonies of suspected pneumococci
growing on blood agar plates may be tested, or, depending on the disease severity, the soluble capsular antigen may be present in
the patient’s CSF, blood, and urine (the antigen, but not necessarily the organisms, is excreted from the body by the kidneys).
Regardless of the results of direct antigen detection tests, cultures of sputum, blood, and cerebrospinal fluid (in patients with signs
and symptoms of meningitis) should always be performed. In some instances, the antigen concentration in body fluids is too low
to be detected, but cultures are positive.
Pneumococci are frequently found among the normal flora of the upper respiratory tract of healthy individuals. Their
recovery in sputum cultures is not, of itself, conclusive evidence of pneumococcal disease. This finding must be correlated with
the total picture of the patient’s clinical illness.