Antibiofilm properties of chemically synthesized silver nanoparticles found against Pseudomonas aeruginosa
© Palanisamy et al.; licensee BioMed Central Ltd. 2014
Received: 10 October 2013
Accepted: 3 January 2014
Published: 14 January 2014
Nanomedicine is now being introduced as a recent trend in the field of medicine. It has been documented that metal nanoparticles have antimicrobial effects for bacteria, fungi and viruses. Recent advances in technology has revived the use of silver nanoparticles in the medical field; treatment, diagnosis, monitoring and control of disease. It has been used since ancient times for treating wide range of illnesses. Bacterial cells adheres to surfaces and develop structures known as biofilms. These structures are natural survival strategy of the bacteria to invade the host. They are more tolerant to commonly used antimicrobial agents, thus being more difficult to be controlled. This leads to increase in severity of infection. In this study, we have investigated the effect of silver nanoparticles in the formation of biofilm in multidrug resistant strains of Pseudomonas aeruginosa. Observation showed that biofilm formation occurred at bacterial concentration of 106 cfu/ml for the sensitive strain of P. aeruginosa while in the resistant strain, the biofilm was evident at bacterial concentration of about 103 cfu/ml. The biofilm were then tested against various concentrations of silver nanoparticles to determine the inhibitory effect of the silver nanoparticles. In the sensitive strain, 20 μg/ml of silver nanoparticles inhibited the growth optimally at bacterial concentration of 104 cfu/ml with an inhibition rate of 67%. Similarly, silver nanoparticles inhibited the formation of biofilm in the resistant strain at an optimal bacterial concentration of 105 cfu/ml with an inhibition rate of 56%. Thus, silver nanoparticles could be used as a potential alternative therapy to reduce severity of disease due to P. aeruginosa infections.
Pseudomonas aeruginosa, a gram negative bacterium, is an important opportunistic pathogen. It has been reported to be resistant to commonly used empirical antibiotic treatment and has been documented to be responsible to high rates of morbidity and mortality. P.aeruginosa is the causative pathogen for several infections which includes urinary tract infection, septicaemia, osteomyelitis and endocarditis. Thus, it poses new challenges as the emergence of multidrug resistance strains is at alarming rates.
Nanoparticles are defined as nanoscale particles with novel and distinctive physicochemical properties. Today, nanotechnology has been used in many different applications such as in the medical field; imaging and medical apparatus, sensors, fabrics, cosmetics, health products, and water remediation technologies. Silver based topical dressing has been widely used as a treatment for infections in burns, open wounds and chronic ulcers. Recently, silver nanoparticles has been used as a molecular tool and method for targeted drug delivery. Currently, nanoantibiotics is seeking our attention as it could be used as an alternative therapy. Numerous literatures has reported the use as silver nanoparticles as an antimicrobial agent. This includes the report by Kim Kuk et al. who documented the antimicrobial effect of silver nanoparticles in yeast, Escherichia coli and Staphylococcus aureus. There are also other researchers who investigated the similar effect in Bacillus megaterium, Staphylococcus aureus, Escherichia coli, Proteus vulgaris and Shigella sonnei[8, 9]. However, very minimal work has been carried out in multidrug resistant strains of P. aeruginosa.
The bacteria adheres to the site of infection which has constituents such as electrolytes, water and organic material. The constituents may serve as a source of nutrient for the bacteria. At this stage there will also be planktonic bacteria that is adsorbed to the surface reversibly. At this stage the bacteria is susceptible to antibiotics. The adherence of the bacteria is aided by the pili or flagella and constituents of the bacteria cell i.e. lipopolysaccharide and influenced by the physicochemical properties of the surface. Once the bacteria adhered irreversibly, the bacterial cells colonize the site and divides. At this point, there would be chemical signals being released. These signals suggest formation of a microbial population and there would phenotypic changes to the bacteria. When the bacterial cells senses other surrounding cells in a limited space, there would be secondary signals released (quorum sensing), which leads to autoinduction in the synthesis of extracellular matrix. The biofilm then matures and develop into three-dimensional biofilm structures. The matrix allows the interconnection of immobilized cells which serves as a digestive system and shuts them off from external extracellular enzymes[18, 19]. The bottom layers of the biofilm cells are deprived from nutrients and oxygen; hence to have a low metabolic activity. Therefore, these cells are more tolerant to commonly used antimicrobial agents than planktonic cells and treating them becomes difficult. The final stage of the development is detachment. This stage may or may not occur, depending on the environmental condition, nutrients, oxygenation, and other limitations. Cells which detach from the surfaces disperse as liquid or aerosols.
From our previous study, we found that silver nanoparticles has bactericidal effect on multidrug resistant P.aeruginosa. Therefore, in this study we investigated the effect of silver nanoparticles in the formation of biofilm in multidrug resistant strains of Pseudomonas aeruginosa.
Results and discussion
Antibiotic susceptibility of P. aeruginosa strains
Inhibition of biofilm formation by silver nanoparticles
This study shows that silver nanoparticles inhibit the formation of biofilm and strains with different antimicrobial activities have also different extent of biofilm formation. It has an effect against multidrug resistant strains of Pseudomonas aeruginosa. In order to conclude the mechanism of inhibition of silver nanoparticles in biofilm formation, the mechanism of uptake should be investigated in both the sensitive and multidrug resistant strains.
Characterisation of silver (Ag) nanoparticles
Bacterial strains and culture media
Ten clinical isolates of P. aeruginosa strains comprising of five multidrug resistant (MDR) strains were used in the study. Four strains that were susceptible to common empirical antibiotics imipenem (IPM), ceftazidime (CAZ), cefoperazone (CFP), gentamicin (GM) and ciprofloxacin (CIP) P. aeruginosa (ATCC 27853) as a control. Brain heart infusion (BHI) and Muller Hinton (MH) agar were used as culture media.
Antimicrobial susceptibility test
The method used was the agar disk diffusion method as described in Clinical and Laboratory Institute. Approximately 108 cfu/ml of bacterial colonies were lawned onto MH agar plates. Antibiotic discs were placed onto the agar plates. The plates were incubated overnight (18–24 hr) at 37°C. The diameter of zone of inhibition around the disc was observed and recorded. Experiments were conducted in triplicates and average inhibitory zone diameter with its standard deviation was determined.
Biofilm formation assay
The biofilm assay was carried in a 96-well-flat bottom tissue culture plate (Greiner-bio One, German). Briefly, The bacterial suspension was adjusted to be equivalent to 0.5 McFarland’s standard. A serial dilution was then prepared from 108 cfu/ml till 10 cfu/ml. Each well of the microtiter plate was filled with 150 ul of bacterial suspension. The plate was incubated for 24 h at 37°C. After incubation, the bacterial suspension of each well was gently removed. The wells were washed three times with 0.2 mL of phosphate buffer saline (PBS pH7.2) to remove free-floating ‘planktonic’ bacteria. Adherence of bacteria to the culture plate were stained with crystal violet (0.1%, w/v). Excess stain was rinsed off by washing with deionized water and plates were kept for drying. After drying, 95% ethanol was added to the wells and the optical densities (OD) of stained adherent bacteria were determined with a microplate reader (Spectra-Max 190, USA) at 590 nm. The optical density value was considered as the formation of biofilm on the surface of the culture plate. The experiment was performed in triplicates.
Biofilm inhibition assay
The bacterial suspension was adjusted equivalent to 0.5 McFarland’s standard. A volume of 100 μl of bacterial suspension was pipetted to each well of a microtiter plate. The silver nanoparticles was diluted two-fold from a stock concentration of 20 μg/ml. The lowest concentration used was 0.06 μg/ml. The diluted silver nanoparticles were then added to the suspension and incubated overnight at 37°C. The method for the quantification of biofilm formation was according to the method of. Briefly, 100 μl suspension of each well was transferred to fresh microtitre plate. The plate was covered, sealed and incubated under stationary conditions at 37°C. After 24 hours incubation, the medium was discarded and thoroughly washed with phosphate buffer saline (pH 7.2). A volume of 100 μl of 0.1% crystal violet was inoculated to each well and left for 30 min at ambient temperature of 25°C. Then, the stain was discarded and the plate was thoroughly washed again. The remaining stain was solubilized with 200 μl of 95% ethanol. A volume of 125 μl of the ethanol solution was then transferred into fresh microtitre plate for analysis at 590 nm.
Navindra Kumari Palanisamy, is a senior lecturer and microbiologist affliated to the Faculty of Medicine, Universiti Teknologi MARA, Sungai Buloh, 47000 Sungai Buloh, Selangor, Malaysia.
Nas Ferina, is an undergraduate student (Dip in Microbiology, Faculty of Applied Science, UiTM Kuala Pilah), pursuing this project as a fulfilment of her internship.
Zaini Mohd-Zain is an Associate Professor and microbiologist affliated to the Institute of Medical Molecular Biotechnology, Faculty of Medicine, Universiti Teknologi MARA, Sungai Buloh, 47000 Sungai Buloh, Selangor, Malaysia.
Jamal Hussaini is a senior lecturer and microbiologist affliated to the Faculty of Medicine, Universiti Teknologi MARA, Sungai Buloh, 47000 Sungai Buloh, Selangor, Malaysia.
Athirah Nur Amirulhusni is a 3rd Year medical student pursuing her Advance Medical Science (AMS) Programme at Universiti Teknologi MARA (UiTM). She works on antimicrobial activity of silver nanoparticles to gram negative bacteria.
Rajkumar Durairaj is Associate Professor at the Head of Department of Mechanical and Material Engineering, Faculty of Engineering and Science, Universiti Tunku Abdul Rahman (UTAR), Malaysia. Prior to joining UTAR, he served as a teaching staff and research assistant at University of Greenwich, UK. He’s research interest is in the area of nanoparticles, rheology and electronic materials. Rajkumar has authored and co-authored a total of 50 technical papers published in peer reviewed journal and leading international conferences. Rajkumar received BEng (1st Class Honours) in Manufacturing Engineering and a PhD from the University of Salford, UK and University of Greenwich, UK, respectively. He is a registered Chartered Engineer with Engineering Council (UK) and Senior Member of the IEEE (US).
Liew Jian Ping is a postgraduate student pursuing his Master’s in the area of synthesis and rheological characterisation of silver nanoparticles based dense suspension.
This work was supported by the excellent fund provided by Universiti Teknologi MARA (UiTM) [600RMI/ST/DANA 5/3/Dst (358/2011)] and the Fundamental Research Grant Scheme provided by the Ministry of Education Malaysia (MOE) [600-RMI/FRGS 5/3 (117/2013)].
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