60 BACTERIOLOGY 



and throat ; and if ordinary saliva is injected into a rabbit, 

 the animal usually dies rapidly of a pneumococcal septi- 

 caemia, showing that this organism can at any time, under 

 suitable circumstances, acquire special virulence and activity 

 and become pathogenic if the patient's vitality is lowered. It 

 is a very frequent cause of colds and sore throats. It may 

 co-operate along with numerous other organisms, and may be 

 found in influenzal attacks, broncho-pneumonia, and in 

 secondary pneumonias occurring during and after the acute 

 infective fevers. When virulent it may be spread by con- 

 tagion and cause epidemics of acute lobar pneumonia, broncho- 

 pneurnonia, bronchitis, and sore throat ; and, especially in 

 enfeebled patients such as drunkards, and in young children, 

 generalised infection is common. Any serous sac (pleurae, 

 pericardium, peritoneum, meninges, joints, etc.), may be 

 attacked, and middle ear disease is very often due to this 

 organism, the infection taking place from the throat along the 

 Eustachian tube, which is a small air-passage which connects 

 the middle ear and throat. 



Microscopically in its typical form it is shaped like the old- 

 fashioned blood-letting lancet, or like the flame of a candle, 

 and is usually found in pairs (hence, in addition to its commoner 

 name of Pneumococcus, it is sometimes called Diplococcus lanceo- 

 latus or Diplococcus pneumoniae), or in short chains, from which 

 another of its synonyms, Streptococcus pneumoniae, now in 

 common use, is derived. The Pneumococcus, especially when in 

 the blood or in a serous exudate, and in the sputum, usually 

 possesses a well-marked capsule around it, which may be 

 demonstrated by appropriate staining methods and gives it a 

 characteristic appearance (see Frontispiece, fig. 2). When de- 

 generated, it loses its typical shape, and may become oval or 

 even rounded in outline, in which case it is difficult to tell it 

 from other forms of streptococci, the inoculation of a mouse 

 or rabbit being the easiest test, as it then regains its typical 

 shape and capsule. 



On culture, its growth resembles that of the Streptococcus 

 pyogenes. It is a delicate organism, requiring incubation at 

 body temperature. It grows best on blood-serum or on media 

 to which blood has been added. Cultures tend rapidly to lose 

 their virulence and die out. 



3. The Pneumobacillus is a member of the Coli-Typhoid 

 group, and will be considered along with them (p. 70). 



4. MicroCQCCUS tetragenus, so named from being frequently 

 found in groups of four, or tetrads as they are called (see 

 Frontispiece, fig. 5), is occasionally found in abscesses, and 

 may sometimes give rise to more serious general infection. 



