Seasonal variation in the prevalence of Gram-negative bacilli in sputum and urine specimens from outpatients and inpatients.

Objectives: To determine whether the prevalence of gram-negative bacilli (GNB; Pseudomonas aeruginosa, Klebsiella pneumoniae, and Escherichia coli) in sputum and urine specimens from outpatients and inpatients differed by season and according to temperature and humidity changes.

Methods: In this retrospective study, microbiologic data for adult patients from 2008 to 2019 were retrieved from the electronic database of a hospital in Japan. Data were categorized by specimen type (sputum and urine) and specimen collection (outpatient and inpatient). Associations between variables were assessed using Spearman's rank correlation coefficients. Differences between groups were assessed using Pearson's chi-square test and analysis of discrete variance.

Results: Among inpatients, the frequencies of P. aeruginosa and K. pneumoniae isolation from sputum specimens were higher in summer and autumn. The frequency of P. aeruginosa isolation from urine specimens was higher in autumn. These seasonal trends were observed in specimens from both outpatients and inpatients. No seasonal trend was observed in the frequency of E. coli isolation. Mean monthly temperature was positively correlated with the frequency of isolating P. aeruginosa (r=0.2198, p=0.0081) and K. pneumoniae (r=0.3443, p=0.00002) from sputum as well as with the frequency of isolating K. pneumoniae (r=0.1905, p=0.0222) from urine. Mean monthly humidity was positively correlated with the frequency of isolating K. pneumoniae (r=0.2602, p=0.0016) from sputum.

Conclusions: GNB were isolated more frequently in summer and autumn than in other seasons. These seasonal trends were observed for both outpatient and inpatient specimens. Seasonality should be considered for optimal infection control of GNB in hospitals.