A view to a kill? - Ambient bacterial load of frames and lenses of spectacles and evaluation of different cleaning methods

Abstract

Surfaces with regular contact with the human body are typically contaminated with microorganisms and might be considered as fomites. Despite spectacles being widespread across populations, little is known about their microbial contamination. Therefore, we swab-sampled 11 worn spectacles within a university setting as well as 10 worn spectacles in a nursing home setting. The microbial load was determined by aerobic cultivation. All spectacles were found to be contaminated with bacteria, with nose pads and ear clips having the highest density, i.e. at sites with direct skin contact. Summed over all sites, the median microbial load of the university spectacles (1.4 ± 10.7 x 10(3) CFU cm-2) did not differ significantly from the spectacles tested in the nursing home (20.8 ± 39.9 x 10(3) CFU cm-2). 215 dominant bacterial morphotypes were analyzed by MALDI biotyping. 182 isolates could be assigned to 10 genera, with Staphylococcus being the most common. On genus-level, bacterial diversity was greater on nursing home spectacles (10 genera) compared to the university environment (2 genera). Four cleaning methods were investigated using lenses artificially contaminated with Escherichia coli, Micrococcus luteus, a 1:2 mixture of E. coli and M. luteus, and Staphylococcus epidermidis (the dominant isolate in our study), respectively. Best cleaning results (99% -100% median germ reduction) were obtained using impregnated wipes; dry cleaning was less effective (85% -90% median germ reduction). Finally, 10 additional worn university spectacles were cleaned with wipes impregnated with an alcohol-free cleaning solution before sampling. The average bacterial load was significantly lower (0.09 ± 0.49 x 10(3) CFU cm-2) compared to the uncleaned university spectacles previously investigated. Spectacles are significantly contaminated with bacteria of mostly human skin origin-including significant amounts of potentially pathogenic ones and may contribute to eye infections as well as fomites in clinical environments.

Fig 1. Box-whisker plot showing the determined microbial counts (CFU cm^-2) of the two investigated environments (nursing home, n = 10, and university, n = 11).

Displayed are the median, 25% and 75% quartiles and outliers (open circles). Whiskers represent the lowest and highest microbial counts within the 1.5 fold of the interquartile range (IQR) (the 25% and 75% quartile). An observation is marked as an outlier if it was more than 1.5 times of the IQR away from the 25% or 75% quartile, respectively.

Fig 2. Barplot of identified bacterial taxa isolated from spectacles of two environments.

Bars show the relative abundance of isolates from university spectacles (n = 11, 143 isolates), nursing home spectacles (n = 10, 72 isolates) and cleaned university spectacles (n = 10, 76 isolates).”Unidentified” indicates a MALDI identification score below 1.7, where a reliable identification of genus-level was not made possible. Numbers (x/n) on top of the bars indicate the number of spectacles that the respective taxon was detected on.

Fig 3. Box-whisker plot showing the relative germ reduction of 4 different cleaning procedures, calculated from the microbial load before and after the cleaning.

A (orange) = cellulose-based, alcoholic lens cleaning wipes; AN (red) = cellulose-based, alcohol-free lens wipes, C (green) = dry cellulose-tissue; M (blue) = fine-grained microfabric cloth (n = 10 for all test bacteria and cleaning products, respectively). Displayed are the 25% and 75% quartiles, median and data outliers (open circles). Whiskers represent the lowest and highest microbial counts within the 1.5 fold of the interquartile range (IQR) (the 25% and 75% quartile). An observation is marked as an outlier if it was more than 1.5 times of the IQR away from the 25% or 75% quartile, respectively. Asterisks mark a statistically significant difference between the cleaning products: * p < 0.05; ** p < 0.01.