Tolerance of clinical vancomycin-resistant Enterococcus faecium isolates against UV-C light from a mobile source

Abstract

Background: Admission to a room previously occupied by patients carrying environmentally robust pathogens implies an increased risk of acquiring those pathogens. Therefore, 'No-touch' automated room disinfection systems, including devices based on UV-C irradiation, are discussed to improve terminal cleaning. It is still unclear if clinical isolates of relevant pathogens behave differently under UV-C irradiation compared to laboratory strains used in the approval process of disinfection procedures. In this study we analysed the susceptibility of well characterized clonally divergent vancomycin-resistant enterococci (VRE) strains, including a linezolid-resistant isolate, against UV-C radiation.

Methods: Susceptibility against UV-C of ten clonally divergent clinical isolates of VRE was determined in comparison to the commonly used test organism Enterococcus hirae ATCC 10541. Ceramic tiles contaminated with 10⁵ to 10⁶ colony forming units/25 cm² of the different enterococci were positioned at a distance of 1.0 and 1.5 m and irradiated for 20 s, resulting in a UV-C dose of 50 and 22 mJ/cm², respectively. Reduction factors were calculated after quantitative culture of the bacteria recovered from treated and untreated surfaces.

Results: Susceptibility to UV-C varied considerably among the strains studied, with the mean value of the most robust strain being up to a power of ten lower compared to the most sensitive strain at both UV-C doses. The two most tolerant strains belonged to MLST sequence types ST80 and ST1283. The susceptibility of the laboratory strain E. hirae ATCC 10541 ranged between the most sensitive and most tolerant isolates for both irradiation doses. However, for UV-C dose of 22 mJ/cm², the reduction of the most tolerant isolate of ST1283 was statistically significantly lower compared to E. hirae ATCC 10541. The most susceptible strains belonged to the MLST sequence types ST117 and ST203.

Conclusions: These results indicate that UV-C doses reported in the literature are sufficient for the reduction of commonly used reference strains of enterococci but could be insufficient for the reduction of tolerant patient VRE-isolates in a hospital setting. Therefore, for future studies, the most tolerant clinical isolates should be used to validate automated UV-C devices or longer exposure times should be expected to ensure efficacy in the real world.

Keywords: Automatic room decontamination; Disinfection; UV-C; UV-irradiation; vancomycin resistant enterococci.

Fig. 1

Minimum spanning tree of the core genome sequences of the investigated isolates. Each circle (node) represents one isolate. The number between the nodes indicates the number of allele differences. The upper numbers in each circle indicate the Sequence type (ST). The lower numbers indicate the Cluster type (CT), if applicable. White or gray nodes represent isolates carrying VanA or VanB resistance determinants, respectively

Fig. 2

Reduction of different clinical VRE isolates with UV-C doses of 50 (A) and 22 mJ/cm² (B). The boxplots represent the reduction factors determined per tested VRE isolate (n = 10) and test organism E. hirae ATCC 10541 in 5 independent experiments, comparing microbial load on ceramic tiles before and after UV-C radiation with 50 (A) and 22 mJ/cm² (B). Boxplots are in ascending order by median. Statistically significant results (p < 0.01) of pairwise Tukey post-hoc analysis are connected by brackets. Non-significant comparisons are not shown. The p-values are displayed as follows: p < 0.01 = *, p < 0.001 = **, p < 0.0001 = ***, and p < 0.00001 = ****