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Bacterial biofilms growing on implanted medical devices are difficult to eradicate, even with aggressive antibiotic therapy. However, application of ultrasound enhances the effectiveness of the antibiotic. The possible mechanisms of this phenomenon were explored in light of the observed influence of various ultrasonic parameters on the enhanced action of gentamicin against biofilms of Pseudomonas aeruginosa. It is postulated that ultrasound increases the transport of gentamicin through the cell membranes, which is the proposed rate determining step in killing by gentamicin. It is possible that the ultrasound perturbs the cell membrane and stimulates active uptake or permits passive uptake by temporarily disrupting the membrane or other structural cell components. The cell membrane disruption could be caused by high pressure, high shear stress, or cavitation. The dependence upon peak power density suggests that acoustic pressure plays a significant role. There is also a strong frequency component that causes the killing effect to decrease as frequency increases. A mathematical analysis of oscillatory shear stress on the cell shows that the magnitude of stress increases with frequency; thus, the hypothesis of oscillatory shear inducing antibiotic uptake is discounted. In addition, the shear displacement caused by shear forces is very small, so the shear disruption caused by oscillatory flow in an acoustic field has minimal impact. The experimental data also rule out the existence of transient cavitation in the bioacoustic effect. It is possible that stable cavitation and the accompanying microstreaming contribute to the bioacoustic effect., (Copyright 1999 John Wiley & Sons, Inc.)

A video microscope system and a mathematical model were developed to observe and model the early stage of bacterial growth on polymer surfaces. Glass slides were coated with polyorthoester, poly(L-lactic acid), and polysulfone, and inserted into a laminar flow cell to expose them to bacterial cultures of Staphylococcus epidermidis, Pseudomonas aeruginosa, or Escherichia coli. The free energy of adhesion (delta Fadh) was determined from contact-angle measurements. The microscopic observations along with the mathematical model allowed measurement of the rates of adhesion, release, and growth. The growth rate of P. aeruginosa on the various surfaces correlated to the delta Fadh. The growth rates of all species on all of the surfaces were slower than the growth rates of the bacteria in suspension. The mathematical model is valid for early growth before the bacteria form a complete monolayer, and is useful in predicting and modeling early growth of bacteria on implanted biomaterials.

The degradable polymers poly(orthoester) (POE), poly(L-lactic acid) (PLA), and the nondegradable polymers polysulfone (PSF), polyethylene (PE), and poly(ether ether ketone) (PEEK) were exposed to cultures of Staphylococcus epidermidis, Pseudomonas aeruginosa, or Escherichia coli. Bacteria washed and resuspended in phosphate buffered saline (PBS) adhered to polymers in amounts nearly twice those of bacteria that were left in their growth medium, tryptic soy broth (TSB). In TSB, there was variation in adhesion from species to species, but no significant variation from polymer to polymer within one species. In PBS there were significant differences in the amounts of bacteria adhering to the various polymers with the exception, of S. epidermidis, which had similar adhesion to all polymers. As a whole, P. aeruginosa was the most adherent while S. epidermidis was the least adherent. The estimated values of the free energy of adhesion (delta Fadh) correlated with the amount of adherent P. aeruginosa. When POE, PLA, and PSF were exposed to hyaluronic acid (HA) before exposure to the bacteria, there was 50% more adhesion of E. coli and P. aeruginosa on POE and PLA. With respect to bacterial adhesion, the biodegradable polymers (POE and PLA) in general were not significantly different from the nondegradable polymers.

The effects of water content and comonomer chemistry upon the adhesion of Pseudomonas aeruginosa to poly(hydroxyethyl methacrylate)-based hydrogels were studied. Hydrogels which varied in swollen water content from 33-69 wt% were polymerized onto glass microscope slides pretreated with a vinyl silane. The hydrogel water content was varied by adding methacrylic acid (1-5 wt%) or N-vinyl pyrrolidone (NVP, 10-25 wt%) or combinations of the two comonomers. The resulting hydrogel surfaces, which were 0.1 mm thick, transparent, and adherent to the glass slide, formed the test surfaces of laminar flow cells (Re = 1.3, wall shear rate = 1.6/s). The bacteria were grown for 8 h in tryptic soy broth (TSB), washed by filtration, and collected on 0.45-microns filters, resuspended in phosphate buffered saline (PBS) at pH = 7.2, and recirculated through the flow cell and across the test surface at 0.85 mL/min for 2 h. Results show that P. aeruginosa adhered less to hydrogels with higher water contents. In the presence of TSB and possible poly(NVP) contamination, the concentration of adherent bacteria was reduced to low and uniform levels independent of the hydrogel chemistry.