Colonization of in-dwelling catheters by microbial biofilms is a major concern in patient health eventually leading to catheter-related blood stream infections. Biofilms are less susceptible to standard antibiotic therapies that are effective against planktonic bacteria. Standard procedure for the detection of microorganisms on the catheter tip is culture. However, viable but non-culturable cells (VBNCs) may be missed. The aim of this study was to evaluate the use of fluorescence in situ hybridization (FISH) as an indicator to visualize and quantify the effect of the antibiotics daptomycin and vancomycin on biofilms in situ . We established an in vitro catheter biofilm model of Staphylococcus epidermidis biofilms on polyurethane catheters. Biofilm activity was measured by FISH and correlated to colony forming units (CFU) data. Digital image analysis was used for quantification of total biofilm mass and the area of the FISH positive biofilm cells. FISH showed a pronounced effect of both antibiotics on the biofilms, with daptomycin having a significantly stronger effect in terms of both reduction of biofilm mass and number of FISH-positive cells. This supports the anti-biofilm capacity of daptomycin. Interestingly, neither antibiotic was able to eradicate all of the FISH-positive cells. In summary, FISH succeeded in visualization, quantification, and localization of antibiotic activity on biofilms. This technique adds a new tool to the arsenal of test systems for anti-biofilm compounds. FISH is a valuable complementary technique to CFU since it can be highly standardized and provides information on biofilm architecture and quantity and localization of survivor cells.

The reduction of blood damage is still a big challenge in blood-carrying medical devices. In vitro experiments are performed to investigate the damage-causing effects, but due to the opaqueness of blood cells, only near-wall flows can be observed. Thus, several transparent blood models to visualize the rheologic behavior of blood have been proposed and examined. Nevertheless, two-phase blood models with added particles still represent the properties of blood inadequately or are very expensive and complex to produce. In this in vitro study, the viscosity, the flow behavior and the cell deformation of human red blood cells have been compared to a novel, easy-to-produce, two-phase blood model fluid with deformable alginate microspheres. The comparison has been performed in a cone-plate rheometer, a straight and a hyperbolic converging microchannel. The viscosity of the blood model fluid with a particle fraction of 30% showed a shear-thinning behavior, comparable to that of blood at room and human body temperature within shear rates from 7 to $$2000\,\hbox {s}^{-1}$$ 2000 s - 1 . The alginate microspheres were deformable in an extensional flow and formed a cell free layer comparable to that of blood in a straight microchannel. The experiments showed a good optical accessibility of the two-phase flow with traceable movements of individual microspheres in the center of the microchannel. It could be shown that our proposed blood model fluid is a promising tool for the analysis of two-phase flows in complex flow geometries. Graphical Abstract

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