Hemolytic streptococci have been more frequently found in association with broncho- than with lobar pneumonia. In 24.5 per cent of instances of lobar pneumonia, doubtless in all instances caused by pneumococci, hemolytic streptococci have invaded the lungs and in 12.6 per cent of instances have found their way into the blood. With bronchopneumonia hemolytic streptococci have been obtained from the lungs in 29.8 per cent of instances and from the blood of the heart in 34.3 per cent.
With lobar pneumonia there is little doubt that pneumococcus has been the primary cause of pneumonia, but with bronchopneumonia pneumococci have been less frequently found. It is difficult to determine how often hemolytic streptococci have invaded a bronchopneumonic lesion, caused by pneumococci because pneumococci tend to disappear. In numerous instances in which the sputum had been studied during life, it was evident that pneumonia was primarily referable to pneumococci, and hemolytic streptococci made their appearance in the sputum late in the disease or were first recognized at autopsy.
When hemolytic streptococci occur in association with bronchopneumonia, foci of pulmonary necrosis similar to those found under the same conditions with lobar pneumonia have been repeatedly found by microscopic examination. In the patches of necrosis, cocci in chains are much more abundant than in the tissue elsewhere.
In some instances of pneumonia, caused by hemolytic streptococci, opaque gray or yellowish gray patches of necrosis occur upon a background of flaccid homogeneous consolidation which has a peculiar cloudy, gray color. This mottled consolidation may implicate an entire lower lobe and has the characteristic features neither of lobar nor of bronchopneumonia. More frequently the lesion is less widespread and necrosis occurs in one or several spots which undergo softening so that finally a small abscess cavity may be formed; it is surrounded by pneumonic consolidation which is soft and has the cloudy appearance described above. These pulmonary abscesses are almost invariably situated below the pleural surface; the adjacent pleural cavity is infected by streptococci and there is purulent inflammation of the pleura.
Streptococcus infection, which has been described, doubtless has its origin in the bronchi, for in favorable sections it is not infrequently possible to demonstrate that necrosis extends through the bronchial walls into the surrounding alveolar tissue and is followed by suppuration with abscess formation. Localization of abscesses below the pleura is in part at least referable to transmission of streptococci by way of the lymphatics.
Streptococci in the lung, as in other tissues, often invade lymphatics and produce an acute inflammatory reaction within and about these vessels. The peculiar lesion which may be designated suppurative interstitial pneumonia is a suppurative lymphangitis associated with inflammation and edema of the interstitial tissue. Lymphatics invaded by streptococci are the site of acute lymphangitis; occlusion by fibrinous thrombi occurs and finally the immensely distended lymphatics, filled with purulent fluid, take a characteristic nodular or beaded form and pus flows from them when they are cut. Streptococci are present in vast numbers. Suppurative inflammation may extend to the surrounding interstitial tissue which is distended by inflammatory edema. This interstitial suppurative pneumonia extends up to the pleural surface and empyema is almost invariably associated with it. The lesion is seldom seen in the absence of influenza.
One of the most significant characters of S. hemolyticus is its ability not only to enter the bronchi and penetrate into the tissue of the lung, but to find its way into more distant structures, namely, the pleural cavity, pericardial sac and peritoneal cavity and to penetrate into the blood. Among 121 examinations, hemolytic streptococci were found in the bronchi in 47.9 per cent; among 153 examinations of the lung it was present in approximately the same proportion, namely, 50.3 per cent; among 218 examinations of the blood it was found in 39 per cent. In 4 of 5 fatal pneumonias in which the organism has penetrated into the bronchi it has ultimately found its way into the blood.
Nonhemolytic Streptococci.—In contrast with S. hemolyticus nonhemolytic types have rarely been encountered in association with the pneumonias of influenza. S. viridans has been found only 5 times among 153 autopsies in which cultures have been made from the lung and has been invariably associated with other microorganisms. In no instances have nonhemolytic streptococci been found with empyema. In one autopsy with lobular bronchopneumonia S. viridans has been isolated from the blood of the heart and in this instance it has been found in the bronchus and lung as well. This type of streptococcus is evidently little adapted to invade the bronchi and produce lesions of the lung and adjacent tissues.
Staphylococci.—Staphylococci have been very frequently isolated from the bronchi in association with the pneumonias of influenza, being found in approximately half of our autopsies. Their isolation in cultures from the lung in a fourth of the autopsies examined is in part perhaps referable to their presence in the small bronchi cut across when the lung is punctured for cultures. S. aureus shows little ability to invade the pleura, being found in association with empyema only 3 times; in these autopsies there has been opportunity for entrance from the exterior through thoracotomy wounds in 2 instances and from a bronchus in free communication with an abscess which had ruptured into the pleural cavity in 1 instance.
Abscesses of the lung caused by staphylococci have been found in a small number of autopsies and have exhibited characters which differ from those ordinarily seen in association with S. hemolyticus. Small, sharply defined abscesses are grouped about terminal bronchi, so that they occur in one or several isolated clusters. Microscopic examination demonstrates that these abscesses have arisen by destruction of the bronchial walls and extension of suppuration into the surrounding alveolar tissue; clumps of staphylococci are found in sections through the abscess, and cultures made from the pus within the abscess cavity demonstrate the presence of S. aureus or albus, but the microorganism may be missed if the culture is made from the adjacent lung tissue. It is noteworthy that there is little tendency for the staphylococcus to infect the pleura for even though these clusters of abscesses have been situated just below the pleura, there has been no associated empyema.