Biofilms are structured communities of bacteria that play a role in antibiotic resistant infections on catheters, which are tubes inserted through a narrow opening into a body cavity or for removing fluids, and other medical prosthetic devices. Bacteria can exist in the body as a single cell or as biofilms. Despite attempts at reducing infections due to biofilm formation on indwelling catheters, biofilms still cause over 80% of microbial infections and often result in hospitalization or even death. It is shown that biofilms may be up to a 1,000-fold more resistant to antibiotics than planktonic bacteria, which are one celled. A reason may be because of a slime formed by bacteria within the biofilm retards the contact of antibiotics to the bacteria. Another reason may be that bacteria in biofilms grow in a slower rate compared to those that only have a single cell, making them more resistant to antibiotics. Moreover, recent studies have shown that exposing catheter surfaces with surface acoustic waves have decreased the chances of infection. Additionally, it is hypothesized that surface acoustic waves would be effective on various different surfaces despite the type of bacteria. The aim of this study was to see the effects of surface acoustic waves on the capacity of the human neutrophils (type of white blood cell) to kill S. epidermidis.
This study was conducted by using a novel fibrin gel system that mimics a tissue-like environment. Low energy surface acoustic waves were delivered from a battery operated driver. The driver is designed and calibrated for each cell culture plate. Calibration was done by conducting pressure and intensity measurements with a Hydrophone needle probe method using the Precision Acoustics High Performance Hydrophone System, the accuracy of the measurements being +/- 20%. Additionally, in order to ensure that the energy delivered by these devices did not affect temperature, they assessed the temperature in each well of the plates. PBS was added to each well of 24, 48, and 96 well plates connected to the SAW driver. The plates were placed in a 37 degrees celsius incubator for 2 hours. Every 15 minutes the temperature of each well was recorded using an electronic thermometer and there were no significant differences in temperature across the plates. In the 24-transwell plates used to measure chemotaxis and bacterial killing, the temperature in both control and surface acoustic waves applied wells remained at 37 degrees celsius for six hours. Additionally, no experiments were done with the wells that had contact with the ceramic piezo element. Breeding pairs of CD18-deficient mice were provided by Dr. Claire Doersckuk. All mice were housed and bred in a barrier facility at Columbia University. Mice 6-20 weeks of age were used according to protocols approved by Columbia University’s Animal Care and Use Committee. Human neutrophils were isolated from human blood using a two layer Histopaque gradient. Murine neutrophils were isolated from bone marrow and neutrophils were resuspended in phosphate buffered saline containing glucose.
The results showed that human neutrophils killed about 70% of 10^5 and 55% of 10^6 of S. epidermidis in the absence of surface acoustic waves while applying surface acoustic waves resulted in almost killing 100% of the 10^5 colonies per unit and increased the percent of neutrophil-mediated bacterial killing from 55% to 82% in fibrin gels containing 10^6 bacteria.