Antibiotic Augmentation of Thermal Eradication of Staphylococcus epidermidis Biofilm Infections.

Staphylococcus epidermidis is a major contributor to bacterial infections on medical implants, currently treated by surgical removal of the device and the surrounding infected tissue at considerable morbidity and expense. In situ hyperthermia is being investigated as a non-invasive means of mitigating these bacterial biofilm infections, but minimizing damage to the surrounding tissue requires augmenting the thermal shock with other approaches such as antibiotics and discerning the minimum shock required to eliminate the biofilm. S. epidermidis biofilms were systematically shocked at a variety of temperatures (50-80 °C) and durations (1-10 min) to characterize their thermal susceptibility and compare it to other common nosocomial pathogens such as Staphylococcus aureus and Pseudomonas aeruginosa. Biofilms were also exposed to three classes of antibiotics (ciprofloxacin, tobramycin and erythromycin) separately at concentrations ranging from 0 to 128 µg mL-1 to evaluate their impact on the efficacy of thermal shock and the subsequent potential regrowth of the biofilm. S. epidermidis biofilms were shown to be more thermally susceptible to hyperthermia than other common bacterial pathogens. All three antibiotics substantially decreased the duration and/or temperature needed to eliminate the biofilms, though this augmentation did not meet the criteria of synergism immediately following thermal shock. Subsequent reincubation, however, revealed strong synergism on a longer timescale.

Thermal susceptibility and antibiotic synergism of methicillin-resistant Staphylococcus aureus biofilms.

Methicillin-Resistant Staphylococcus aureus (MRSA) biofilms are among the most dangerous infections on medical implants, typically requiring surgical explantation and replacement. This study investigated the thermal susceptibility of MRSA biofilms to thermal shocks from 60 to 80 °C for 1-30 min as well as the effect of various antibiotics (most notably methicillin) on thermal mitigation. Pre- and post-shock exposure to three different classes of antibiotics (ciprofloxacin, tobramycin, and methicillin) at concentrations ranging from 0.25 to 128 µg mL-1 were investigated. MRSA biofilms exhibited thermal susceptibility comparable to other common nosocomial pathogens, such as Pseudomonas aeruginosa, though with greater variability. Exposure to antibiotics of any class significantly decreased the degree of thermal shock required for reliable mitigation, including at subclinical concentration. These combined treatments reduced biofilm population more than the sum of thermal and chemical treatments alone, demonstrating synergism, while also indicating a critical population drop of ∼4.5 log10 beyond which the biofilms typically became non-viable.

Thermal shock susceptibility and regrowth of Pseudomonas aeruginosa biofilms.

Biofilms on implanted medical devices cause thousands of patients each year to undergo multiple surgeries to remove and replace the implant, driving billions of dollars in increased health care costs due to the lack of viable treatment options for in situ biofilm eradication. Remotely activated localised heating is under investigation to mitigate these biofilms; however, little is known about the temperatures required to kill the biofilms. To better understand the required parameters this study investigated the thermal susceptibility of biofilms as a function of their fluidic and chemical environment during growth, as well as their propensity for regrowth following thermal shock. Pseudomonas aeruginosa biofilms were cultured in shaker plate fluidic conditions in four different growth media, then thermally shocked at various temperatures and exposure times. Biofilms were re-incubated to determine their regrowth potential following thermal shocks of various intensities. Results indicate that growth media has little impact on thermal susceptibility, while fluidic conditions strongly influence susceptibility to modest thermal shocks. This effect disappears, however, with increasingly aggressive shocks, reducing biofilm populations by up to 5 orders of magnitude. Regrowth studies indicate a critical post-shock bacterial loading (∼103 CFU/cm2) below which the biofilms were no longer viable, while biofilms above that loading slowly regrew to their previous population density.