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.