Pseudomonas aeruginosa is the chief cause of disease in laboratory cultures of grasshoppers. A large number of strains of the bacterium from different localities fell into five types on the basis of reactions with bacteriophages. The five types and their strains were qualitatively similar to one another in morphology, cultural characteristics, and diagnostic biochemical reactions and to a named culture of P. aeruginosa from the American Type Culture Collection, and their characteristics agreed with those listed by most authors in the literature as diagnostic of the species. The strains and types showed some quantitative differences in production of pyocyanin, development of iridescence, and degree of haemolysis of blood and of hydrolysis of casein. The symptoms of infection and the loss from mortality are described. The factors influencing the spread of the infection are discussed and methods of suppression of the disease are recommended. The disease originates from a small number of bacteria intimately associated with a very low percentage of grasshopper eggs collected in the field. Nymphs emerging from these eggs die shortly after emergence and contaminate the food supply. Grasshoppers are normally infected by ingesting food contaminated by the bacteria but the bacteria do not multiply in the gut and most of them are rapidly killed or eliminated. The bacteria are not actively invasive, but when a small number gain access to the haemocoel they increase in numbers to about 1 × 109per insect. Shortly thereafter the host dies from the metabolic activities of the parasites, most probably as a result of protein digestion by the strong proteolytic enzymes of the bacterium. There is no evidence that the bacteria produce a specific toxin, and grasshoppers are more resistant to the poisonous action of pyocyanin than are mice.

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