Summer Peaks in the Incidences of Gram-Negative Bacterial Infection Among Hospitalized Patients. 
Recognition of seasonal trends in hospital infections may improve diagnosis, use of empirical therapy, and infection prevention interventions.  There are very few data available regarding the seasonal variability of these infections.  We quantified the seasonal variation in the incidences of hospital infection caused by common bacterial pathogens and estimated the association between temperature changes and infection rates.  A cohort of all adult patients admitted to the University of Maryland Medical Center during the period from 1998 through 2005 was analyzed.  Time-series analyses were used to estimate the association of the number of infections per month caused by Pseudomonas aeruginosa, Acinetobacter baumannii, Enterobacter cloacae, Escherichia coli, Staphylococcus aureus, and enterococci with season and temperature, while controlling for long-term trends.  There were 218,594 admissions to the index hospital, and analysis of 26,624 unique clinical cultures that grew the organisms of interest identified increases in the mean monthly rates of infection caused by P. aeruginosa (28% of isolates recovered; P !  .01), E. cloacae (46%; P !  .01), E. coli (12%; P !  .01), and A. baumannii (21%; Pp.06).  For each 10 degrees F increase, we observed a 17% increase in the monthly rates of infection caused by P. aeruginosa (Pp.01) and A. baumanii (Pp.05).  Significantly higher rates of gram-negative infection were observed during the summer months, compared with other seasons.  For some pathogens, higher temperatures were associated with higher infection rates, independent of seasonality.  These findings have important implications for infection prevention, such as enhanced surveillance during the warmer months, and for choice of empirical antimicrobial therapy among hospitalized adults.  Future, quasi-experimental investigations of gram-negative infection prevention initiatives should control for seasonal variation.