Gram-negative bacterial infections peak during summer months Reference Perencevich, McGregor and Shardell1,Reference Blot, Hammami and Blot2 and a predominance of gram-negative infections is associated with proximity to the equator. Reference Fisman, Patrozou and Carmeli3 A Belgian national cohort study of hospital-associated bloodstream infections reported that infections due to Stenotrophomonas maltophilia peaked during summer months. Reference Blot, Hammami and Blot2 Additionally, hospital-associated bloodstream infections due to S. maltophilia demonstrated the greatest seasonal variability compared with other bacteria. Based on these findings, we hypothesized that we would find a summer peak in isolation of Stenotrophomonas maltophilia from clinical specimens in our hospital.
Methods
This retrospective study included S. maltophilia isolated from clinical specimens at our tertiary-care, level 1 trauma center from April 1, 2016, through March 31, 2022. We defined S. maltophilia isolates as healthcare-associated if a clinical specimen was obtained from a patient >48 hours after hospital admission or within 48 hours of hospitalization if a patient had been previously admitted to our hospital over the prior 30 days. Only the first isolate was included in our analysis if a patient had >1 clinical specimens that grew S. maltophilia. Statistical methods included Poisson regressions to model the counts of quarterly hospital-acquired infections of S. maltophilia. All tests were 2-sided; P < .05 was considered statistically significant.
Results
Respiratory specimens accounted for 53 (69%) of 172 S. maltophilia isolates (Fig. 1). We detected a significant increase in S. maltophilia isolates from July through December (calendar quarters 3 and 4) compared to January through June (calendar quarters 1 and 2; relative risk [RR], 1.85; 95% confidence interval [CI], 1.10–2.02; P < .001). The greatest peak was identified July through September: quarter 3 vs quarter 1: RR, 1.54 (95% CI, 1.01–2.36; P = .046); quarter 3 vs quarter 2: RR, 1.80 (95% CI, 1.15–2.81; P = .01); quarter 3 vs quarter 4: RR, 1.10 (95% CI, 0.75–1.62; P = .62). Otherwise, quarter 4 had a significantly higher peak than quarter 2 (RR, 1.63; 95% CI, 1.04–2.57; P = .03).
Fig. 1. Healthcare-associated Stenotrophomonas maltophilia clinical isolates, April 1, 2016–March 31, 2022.
Discussion
We have demonstrated seasonality of S. maltophilia among clinical isolates at our medical center. We can only speculate as to why this is the case. The concentration of bacteria in potable water peaks during summer months, and the greatest degradation of water quality occurs after overnight stagnation of potable water during the summer. Reference Zhang, Xu and Huang4 Additionally, a peak in the concentration of coliform bacteria in streams, rivers, and drinking wells has been documented during the summer months. Reference Atherholt, Procopio and Goodrow5
Nonfermenting gram-negative bacteria such as S. maltophilia have been isolated from drinking water, and hospital outbreaks of S. maltophilia infections have been traced to tap water dispensed from faucets with aerators. Reference Verweij, Meis and Christmann6 It is unclear whether nonfermenting gram-negative bacteria such as S. maltophilia have a selection advantage based on the temperature, pH, and oxygen content that might be found in water sources during summer months. Reference Bonaventura, Stepanovic and Picciani7 Our hospital has a continuous feed of monochloramine into our potable hot water in patient-care buildings to reduce the risk of hospital-acquired Legionella infections. Although a nonsignificant increase in the number of faucets and showerheads growing S. maltophilia Reference Duda, Kandiah and Stout8 and other nonfermenting gram-negative bacteria Reference Baron, Vikram, Duda, Stout and Bibby9 has been documented in hospitals with monochloramine potable water treatment, we were unable to identify any hospital outbreaks due to S. maltophilia associated with monochloramine water treatment. Various infection prevention strategies may be used to reduce risk of nosocomial infections due to S. maltophilia and other nonfermenting gram-negative bacteria. These include minimizing tap water use, removing aerators from faucets, use of splash guards around sinks, and placement of point-of-use filters in high-risk areas among other interventions. 10
Our study had several limitations. We included clinical isolates without medical record review, so we cannot be certain whether the isolates represented true infections, colonization, or possible contamination. Additionally, we were limited by a relatively small number of clinical isolates. The study was conducted at a single center, so these results may not be generalizable to other populations or settings. We did not do widespread testing of our hospital potable water to assess for the presence of S. maltophilia, which may have been epidemiologically linked to some of the clinical isolates.
In conclusion, we confirmed the seasonality of S. maltophilia isolated from clinical isolates with a significant peak observed from July through September. This finding may be due to changes in potable water composition during the summer months. Further research is needed to better understand the determinants driving seasonality of S. maltophilia and other nonfermenting gram-negative bacteria that may cause infections in healthcare settings. Additionally, the data presented herein, along with similar publications, should prompt discussion regarding potential seasonal adjustment for surveillance programs involving healthcare-associated infections caused by S. maltophilia and other pathogens that clearly display seasonality.
Acknowledgments
Financial support
No financial support was provided relevant to this article.
Conflicts of interest
All authors report no conflicts of interest relevant to this article.
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