Colonization and clinical superinfection with gram-negative bacilli in influenza.

Among 197 influenza A patients admitted to Roslagstull Hospital, abundant growth of gram-negative bacilli was found in 74 cases (38%). When assays for antibodies with the patients own strains as antigen were performed on paired sera with the indirect immunofluorescence technique, titre increases were obtained in 21 out of 55 patients (38%). In this last group signs of secondary infection, as secondary pneumonia, prolonged or recurring fever or late occurrence of leukocytosis and granulocytosis, were significantly more common than in patients without gram-negative findings. An analysis of possible interference by other bacteria did not invalidate the observations. Findings of gram-negative bacilli occurred most often in patients more than 50 years of age. The bacteria were probably hospital acquired in 32 out of 55 patients. Antibody responses took place both in the IgM and the IgG fraction.