Background
Pseudomonas aeruginosa is a Gram-negative, pathogenic bacterium that can be associated with orthopaedic implant infections, causing 20-30% of all Gram-Negative fracture related infections (FRI) (1, 2). In recent years P. aeruginosa was listed by the World Health Organisation as a priority pathogen due to its prevalence in developing antibiotic resistance. Treatment of P. aeruginosa infections is therefore not simple, as its virulence mechanisms and ability to form bacterial aggregates (biofilms) complicate drug delivery and efficacy (3). Formation of biofilms on orthopaedic implants post-operation is a common cause of implant failure and often leads to patient discomfort and even revision surgeries (4, 5). Chronic biofilm formation of P. aeruginosa is poorly understood. In order to reduce implant failure and revision surgery rates a further understanding of chronic P. aeruginosa biofilm formation is required alongside new treatment and prevention methods, such as the use of combination treatments and novel antimicrobials such as plant derived chemicals e.g. coumarins.
Methods
Minimum inhibitory concentration assays (MIC) and minimum bactericidal concentration (MBC) of a number of commonly used antibiotics in the treatment of P. aeruginosa were conducted. MICs were carried out in 96-well plates using Mueller-Hinton Broth as a growth medium, a range of 0.0625 µg/ml to 128 µg/ml antibiotic was tested against a clinical P. aeruginosa isolate PaV66. Growth was quantified by measuring the optical density (OD600nm) after 20 hours of incubation. The MBC was conducted by stamping the wells from the MIC plate on Tryptic Soy Agar (TSA), after 24 hours of incubation the MBC was determined as the lowest concentration of antibiotic at which no growth was observed.
Results
The antibiotics tested were effective against growth of P. aeruginosa PaV66 at certain concentrations. Growth of PaV66 was severely inhibited at 1 µg/ml of Ciprofloxacin and 0.5 µg/ml of Tobramycin. While the MBC for both antibiotics was twice the concentration, with Ciprofloxacin showing no growth from 2 µg/ml onwards and Tobramycin at 1 µg/ml.
Discussion
The results show that both Tobramycin and Ciprofloxacin on their own are effective against
P. aeruginosa PaV66 at certain concentrations. Unfortunately, there are many different strains of P. aeruginosa with varying degrees and mechanisms of antimicrobial resistance. In order to improve efficacy of treatment, combination treatments with other antibiotics or alternative antimicrobials should be tested.
Conclusion
This study provides an insight into the efficacy of certain antibiotics against P. aeruginosa PaV66. Further testing of different antibiotics and plant derived compounds will give insight into their efficacy against P. aeruginosa and allow for informed choices on the testing of combination treatments, in the hopes of helping to discover novel methods of treatment and prevention of orthopaedic implant related P. aeruginosa infections.
References
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