The Role of Multiplication of Pasteurella Pestis in Mononuclear Phagocytes in the Pathogenesis of Flea-Borne Plague

Abstract

The result of experiments conducted in this laboratory to determine the fate of virulent strains of Pasteurella pestis inoculated into an animal host by a blocked flea led to an examination and extension of the observations of Burrows (3) and Burrows and Bacon (4). These workers found that virulent strains of the plague bacillus, in contrast to avirulent strains, can occur in three forms or types: a) phagocytosis-sensitive (S-type), b) nonencapsulated phagocytosis-resistant (R-type), and c) (M-type), which is visibly well encapsulated, rich in Fraction I and is highly resistant to phagocytosis. Further, they found that polymorphonuclear leukocytes could phagocytize and destroy a large percentage of the phagocytosis-sensitive (S-type) organisms.Our findings agreed with their observations that the R- and M-forms that occur only in virulent strains of the plague bacillus were resistant to phagocytosis by polymorphonuclear leukocytes. We confirmed their findings that polymorphonuclear leukocytes could readily phagocytize and destroy the phagocytosis-sensitive S-form of both virulent and avirulent strains of P. pestis.On the other hand we have demonstrated that the phagocytosis-sensitive S-type of virulent P. pestis is ingested by and multiplies within mononuclear leukocytes of normal mice and guinea pigs. On release from the damaged monocytes the bacilli are of the encapsulated M-type and are highly resistant to phagocytosis by either neutrophils or monocytes.Our results also show that the plague blocked flca inoculates its host with the phagocytosis-sensitive S-type of virulent P. pestis which is readily phagocytized and destroyed in great numbers by the polymorphonuclear leukocytes. By contrast, those bacilli that are ingested by the monocytes in vivo rapidly multiply, and on release from the damaged cells are of the M-type and are resistant to phagocytosis by both polymorphs and monocytes, a condition that permits the establishment of bubonic plague in a susceptible host.