Computer surveillance of shifts in the gross patient flora during hospitalization.

Data accumulating routinely in a hospital microbiology laboratory were computer-plotted according to the number of days the patient had been in the hospital when each culture was obtained. The rate at which patients were cultured fell slightly during hospitalization, while the rate at which isolates were obtained from them increased gradually. For some species, such as Escherichia coli or Staphylococcus aureus, isolation rates changed little during hospitalization. They rose as much as sevenfold for other species, such as Pseudomonas aeruginosa or Serratia marcescens, or for a particular antibiotype of Klebsiella pneumoniae while it was an endemic nosocomial problem. These differences persisted after deletion of repeat isolates from the same patient. They appeared to reflect general shifts in composition of patient flora during hospitalization. These shifts paralleled shifts in infecting flora as represented by similar plots of isolates from blood cultures.