Abstract
Biofilm formation may play an important role in the pathogenesis of infections caused by Enterococcus faecalis, including endocarditis. Most biofilm studies use a polystyrene dish assay to quantify biofilm biomass. However, recent studies of E. faecalis strains in tissue and animal models suggest that polystyrene dish results need to be interpreted with caution. We evaluated 158 clinical E. faecalis isolates using a polystyrene dish assay and found variation in biofilm formation, with many isolates forming little biofilm even when different types of media were used. However, all tested clinical isolates were able to form biofilms on porcine heart valve explants. Dextrose-enhanced biofilm formation in the polystyrene dish assay was found in 6/12 (50%) of clinical isolates tested and may explain some, but not all of the differences between the polystyrene dish assay and the heart valve assay. These findings suggest that in studies assessing the clinical relevance of enterococcal biofilm-forming ability, ex vivo biofilm formation on a relevant tissue surface may be warranted to validate results of in vitro assays.
Highlights
The Centers for Disease Control and Prevention and the National Institutes of Health estimate that 65–80% of all microbial infections involve biofilm formation, yet only recently have we started to understand the role of biofilms in infection [1,2]
A histogram of the isolates’ biofilm indices is skewed, with 117 (74%) of isolates having an index #0.75, and 59 (34%) having an index #0.25. These findings indicate that a large proportion of clinical E. faecalis isolates formed little or no biofilm in the standard polystyrene dish assay
The scraped cells were suspended in buffer, and quantitative culture results were compared to staining to clarify whether between-strain differences in biofilm biomass as observed in the polystyrene dish assay were due to differences in extracellular matrix (ECM) biomass or colony forming units (CFU)/mL (Figure 2)
Summary
The Centers for Disease Control and Prevention and the National Institutes of Health estimate that 65–80% of all microbial infections involve biofilm formation, yet only recently have we started to understand the role of biofilms in infection [1,2]. E. faecalis is capable of causing life-threatening infections such as endocarditis, and biofilm formation may play an important role in the pathogenesis of these infections [4,5]. The majority of in vitro studies of enterococcal biofilms have used a similar assay [8,9,10,11,12] This assay offers advantages such as ease of use and highthroughput screening of large numbers of clinical isolates or mutant libraries. Some variables in this assay, including the medium used or the plate material, can be altered, but the conditions still differ significantly from those of biofilm development during natural infections. Assessment of a given strain’s potential to form biofilms during infections on tissue, such as heart valves, based entirely on the polystyrene dish assay may have limited clinical relevance
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