3,6-Di(pyridin-2-yl)-1,2,4,5-tetrazine (pytz)-capped silver nanoparticles (TzAgNPs) inhibit biofilm formation of Pseudomonas aeruginosa: a potential approach toward breaking the wall of biofilm through reactive oxygen species (ROS) generation

Cuminaldehyde and Tobramycin Forestall the Biofilm Threats of Staphylococcus aureus: A Combinatorial Strategy to Evade the Biofilm Challenges

Staphylococcus aureus, an opportunistic Gram-positive pathogen, is known for causing various infections in humans, primarily by forming biofilms. The biofilm-induced antibiotic resistance has been considered a significant medical threat. Combinatorial therapy has been considered a reliable approach to combat antibiotic resistance by using multiple antimicrobial agents simultaneously, targeting bacteria through different mechanisms of action. To this end, we examined the effects of two molecules, cuminaldehyde (a natural compound) and tobramycin (an antibiotic), individually and in combination, against staphylococcal biofilm. Our experimental observations demonstrated that cuminaldehyde (20 μg/mL) in combination with tobramycin (0.05 μg/mL) exhibited efficient reduction in biofilm formation compared to their individual treatments (p < 0.01). Additionally, the combination showed an additive interaction (fractional inhibitory concentration value 0.66) against S. aureus. Further analysis revealed that the effective combination accelerated the buildup of reactive oxygen species (ROS) and increased the membrane permeability of the bacteria. Our findings also specified that the cuminaldehyde in combination with tobramycin efficiently reduced biofilm-associated pathogenicity factors of S. aureus, including fibrinogen clumping ability, hemolysis property, and staphyloxanthin production. The selected concentrations of tobramycin and cuminaldehyde demonstrated promising activity against the biofilm development of S. aureus on catheter models without exerting antimicrobial effects. In conclusion, the combination of tobramycin and cuminaldehyde presented a successful strategy for combating staphylococcal biofilm-related healthcare threats. This combinatorial approach holds the potential for controlling biofilm-associated infections caused by S. aureus.

1, 4-naphthoquinone efficiently facilitates the disintegration of pre-existing biofilm of Staphylococcus aureus through eDNA intercalation

1, 4-naphthoquinone, a plant-based quinone derivative, has gained much attention for its effectiveness against several biofilm-linked diseases. The biofilm inhibitory effect of 1, 4-naphthoquinone against Staphylococcus aureus has already been reported in our previous study. We observed that the extracellular DNA (eDNA) could play an important role in holding the structural integrity of the biofilm. Hence, in this study, efforts have been directed to examine the possible interactions between 1, 4-naphthoquinone and DNA. An in silico analysis indicated that 1, 4-naphthoquinone could interact with DNA through intercalation. To validate the same, UV-Vis spectrophotometric analysis was performed in which a hypochromic shift was observed when the said molecule was titrated with calf-thymus DNA (CT-DNA). Thermal denaturation studies revealed a change of 8℃ in the melting temperature (Tm) of CT-DNA when complexed with 1, 4-naphthoquinone. The isothermal calorimetric titration (ITC) assay revealed a spontaneous intercalation between CT-DNA and 1, 4-naphthoquinone with a binding constant of 0.95 ± 0.12 × 108. Furthermore, DNA was run through an agarose gel electrophoresis with a fixed concentration of ethidium bromide and increasing concentrations of 1, 4-naphthoquinone. The result showed that the intensity of ethidium bromide-stained DNA got reduced concomitantly with the gradual increase of 1, 4-naphthoquinone suggesting its intercalating nature. To gain further confidence, the pre-existing biofilm was challenged with ethidium bromide wherein we observed that it could also show biofilm disintegration. Therefore, the results suggested that 1, 4-naphthoquinone could exhibit disintegration of the pre-existing biofilm of Staphylococcus aureus through eDNA intercalation.

Cuminaldehyde exhibits potential antibiofilm activity against Pseudomonas aeruginosa involving reactive oxygen species (ROS) accumulation: a way forward towards sustainable biofilm management

Pseudomonas aeruginosa often causes various acute and chronic infections in humans exploiting biofilm. Molecules interfering with microbial biofilm formation could be explored for the sustainable management of infections linked to biofilm. Towards this direction, the antimicrobial and antibiofilm activity of cuminaldehyde, an active ingredient of the essential oil of Cuminum cyminum was tested against Pseudomonas aeruginosa. In this regard, the minimum inhibitory concentration (MIC) of cuminaldehyde was found to be 150 μg/mL against the test organism. Experiments such as crystal violet assay, estimation of total biofilm protein, fluorescence microscopy and measurement of extracellular polymeric substances (EPS) indicated that the sub-MIC doses (up to 60 µg/mL) of cuminaldehyde demonstrated considerable antibiofilm activity without showing any antimicrobial activity to the test organism. Moreover, cuminaldehyde treatment resulted in substantial accumulation of cellular reactive oxygen species (ROS) that led to the inhibition of microbial biofilm formation. To this end, the exposure of ascorbic acid was found to restore the biofilm-forming ability of the cuminaldehyde-treated cells. Besides, a noticeable reduction in proteolytic activity was also observed when the organism was treated with cuminaldehyde. Taken together, the results demonstrated that cuminaldehyde could be used as a promising molecule to inhibit the biofilm formation of Pseudomonas aeruginosa.

Tryptophan interferes with the quorum sensing and cell surface hydrophobicity of Staphylococcus aureus: a promising approach to inhibit the biofilm development

Staphylococcus aureus, a Gram-positive bacterium has been implicated in a plethora of human infections by virtue of its biofilm-forming ability. Inhibition in microbial biofilm formation has been found to be a promising approach towards compromising microbial pathogenesis. In this regard, various natural and synthetic molecules have been explored to attenuate microbial biofilm. In this study, the role of an amino acid, L-tryptophan was examined against the biofilm-forming ability of S. aureus. The compound did not execute any antimicrobial characteristics, instead, showed strong antibiofilm activity with the highest biofilm inhibition at a concentration of 50 µg/mL. Towards understanding the underlying mechanism of the same, efforts were given to examine whether tryptophan could inhibit biofilm formation by interfering with the quorum-sensing property of S. aureus. A molecular docking analysis revealed an efficient binding between the quorum-sensing protein, AgrA, and tryptophan. Moreover, the expression of the quorum-sensing gene (agrA) got significantly reduced under the influence of the test compound. These results indicated that tryptophan could interfere with the quorum-sensing property of the organism thereby inhibiting its biofilm formation. Further study revealed that tryptophan could also reduce the cell surface hydrophobicity of S. aureus by downregulating the expression of dltA. Moreover, the tested concentrations of tryptophan did not show any significant cytotoxicity. Hence, tryptophan could be recommended as a potential antibiofilm agent to manage the biofilm-associated infections caused by S. aureus.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s13205-021-02924-3.

1,4-Naphthoquinone disintegrates the pre-existing biofilm of Staphylococcus aureus by accumulating reactive oxygen species

Staphylococcus aureus causes several nosocomial and community-acquired infections in human host involving biofilm. Thus, strategies need to be explored to curb biofilm threats by either inhibiting the formation of biofilm or disintegrating the pre-existing biofilm. Towards this direction, we had already revealed the biofilm inhibiting properties of 1,4-naphthoquinone against S. aureus. In this study, we have investigated whether this compound can act on pre-existing biofilm. Hence, biofilm of S. aureus was developed first and challenged further with 1,4-naphthoquinone. Experiments such as crystal violet assay, fluorescence microscopy, and estimation of total biofilm protein were performed to confirm the biofilm disintegration properties of 1,4-naphthoquinone. The disintegration of pre-existing biofilm could be attributed to the generation of reactive oxygen species (ROS). To investigate further, we observed that extracellular DNA (eDNA) was found to play an important role in holding the biofilm network as DNaseI treatment could cause an efficient disintegration of the same. To examine the effect of ROS on the eDNA, we exposed pre-existing biofilm to either 1,4-naphthoquinone or a combination of both 1,4-naphthoquinone and ascorbic acid for different length of time. Post-incubation, ROS generation and the amount of eDNA associated with the biofilm were determined wherein an inversely proportional relationship was observed between them. The result indicated that with the increase of ROS generation, the amount of eDNA associated with biofilm got decreased substantially. Thus, the results indicated that the generation of ROS could degrade the eDNA thereby compromising the integrity of biofilm which lead to the disintegration of pre-existing biofilm.

Attenuation of Pseudomonas aeruginosa biofilm by thymoquinone: an individual and combinatorial study with tetrazine-capped silver nanoparticles and tryptophan

Microbial biofilm indicates a cluster of microorganisms having the capability to display drug resistance property, thereby increasing its proficiency in spreading diseases. In the present study, the antibiofilm potential of thymoquinone, a black seed-producing natural molecule, was contemplated against the biofilm formation by Pseudomonas aeruginosa. Substantial antimicrobial activity was exhibited by thymoquinone against the test organism wherein the minimum inhibitory concentration of the compound was found to be 20 μg/mL. Thereafter, an array of experiments (crystal violet staining, protein count, and microscopic observation, etc.) were carried out by considering the sub-MIC doses of thymoquinone (5 and 10 μg/mL), each of which confirmed the biofilm attenuating capacity of thymoquinone. However, these concentrations did not show any antimicrobial activity. Further explorations on understanding the underlying mechanism of the same revealed that thymoquinone accumulated reactive oxygen species (ROS) and also inhibited the expression of the quorum sensing gene (lasI) in Pseudomonas aeruginosa. Furthermore, by taking up a combinatorial approach with two other reported antibiofilm agents (tetrazine-capped silver nanoparticles and tryptophan), the antibiofilm efficiency of thymoquinone was expanded. In this regard, the highest antibiofilm activity was observed when thymoquinone, tryptophan, and tetrazine-capped silver nanoparticles were applied together against Pseudomonas aeruginosa. These combinatorial applications of antibiofilm molecules were found to accumulate ROS in cells that resulted in the inhibition of biofilm formation. Thus, the combinatorial study of these antibiofilm molecules could be applied to control biofilm threats as the tested antibiofilm molecules alone or in combinations showed negligible or very little cytotoxicity.

1,4-Naphthoquinone accumulates reactive oxygen species in Staphylococcus aureus: a promising approach towards effective management of biofilm threat

Staphylococcus aureus, a Gram-positive opportunistic microorganism, promotes pathogenicity in the human host through biofilm formation. Microorganisms associated with biofilm often exhibit drug-resistance property that poses a major threat to public healthcare. Thus, the exploration of new therapeutic approaches is the need of the hour to manage biofilm-borne infections. In the present study, efforts are put together to test the antimicrobial as well as antibiofilm activity of 1,4-naphthoquinone against Staphylococcus aureus. The result showed that the minimum bactericidal concentration (MBC) of this compound was found to be 100 µg/mL against Staphylococcus aureus. In this regard, an array of experiments (crystal violet, biofilm protein measurement, and microscopic analysis) related to biofilm assay were conducted with the sub-MBC concentrations (1/20 and 1/10 MBC) of 1,4-naphthoquinone. All the results of biofilm assay demonstrated that these tested concentrations (1/20 and 1/10 MBC) of the compound (1,4-naphthoquinone) showed a significant reduction in biofilm development by Staphylococcus aureus. Moreover, the tested concentrations (1/20 and 1/10 MBC) of the compound (1,4-naphthoquinone) were able to reduce the microbial motility of Staphylococcus aureus that might affect the development of biofilm. Further studies revealed that the treatment of 1,4-naphthoquinone to the organism was found to increase the cellular accumulation of reactive oxygen species (ROS) that resulted in the inhibition of biofilm formation by Staphylococcus aureus. Hence, it can be concluded that 1,4-naphthoquinone might be considered as a promising compound towards biofilm inhibition caused by Staphylococcus aureus.

Inhibition of biofilm formation of Pseudomonas aeruginosa by caffeine: a potential approach for sustainable management of biofilm

Pseudomonas aeruginosa is a potent biofilm forming organism causing several diseases on host involving biofilm. Several natural and synthetic molecules have been explored towards inhibiting the biofilm formation of Pseudomonas aeruginosa. In the current report, the role of a natural molecule namely caffeine was examined against the biofilm forming ability of P. aeruginosa. We have observed that caffeine shows substantial antimicrobial activity against P. aeruginosa wherein the minimum inhibitory concentration (MIC) of caffeine was found to be 200 μg/mL. The antibiofilm activity of caffeine was determined by performing a series of experiments using its sub-MIC concentrations (40 and 80 μg/mL). The results revealed that caffeine can significantly inhibit the biofilm development of P. aeruginosa. Caffeine has been found to interfere with the quorum sensing of P. aeruginosa by targeting the swarming motility. Molecular docking analysis further indicated that caffeine can interact with the quorum sensing proteins namely LasR and LasI. Thus, the result indicated that caffeine could inhibit the formation of biofilm by interfering with the quorum sensing of the organism. Apart from biofilm inhibition, caffeine has also been found to reduce the secretion of virulence factors from Pseudomonas aeruginosa. Taken together, the results revealed that in addition to biofilm inhibition, caffeine can also decrease the spreading of virulence factors from Pseudomonas aeruginosa.

Functional diversity performs a key role in the isolation of nitrogen-fixing and phosphate-solubilizing bacteria from soil

Functional diversity covers diverse functional traits of microorganisms in an ecosystem. Thus, we hypothesized that it could play an important role in the isolation of nitrogen-fixing and phosphate-solubilizing bacteria. These bacteria have been considered as biofertilizer for sustainable agriculture development. Soils were collected from different sites of agricultural field and performed several microbiological tests in which we observed considerable differences in heterotrophic microbial abundance and microbial activities among the microcosms. Functional diversity depends on both microbial richness and evenness. Based on the results of metabolic fingerprinting of the carbon sources of BiOLOG-ECO plates, richness and evenness was measured by determining Shannon diversity index and Gini coefficient, respectively. The results showed significant differences in both microbial richness and evenness, suggesting considerable variation of functional diversity among the microcosms. Thereafter, nitrogen-fixing and phosphate-solubilizing bacteria were isolated on Burk's and Pikovskaya media, respectively. The results revealed considerable variation of both types of bacterial abundance among the microcosms. Microcosm (T2) showing the highest functional diversity houses the maximum numbers of nitrogen-fixing and phosphate-solubilizing bacteria. Similarly, the microcosm (T5) exhibiting the lowest functional diversity houses the minimum numbers of nitrogen-fixing and phosphate-solubilizing bacteria. Thus, a strong positive correlation was observed between functional diversity and both types of bacterial abundance among the soil samples. Higher richness and evenness lead to the development of increased functional diversity that facilitates to accommodate substantial numbers of nitrogen-fixing and phosphate-solubilizing bacteria in soil. Taken together, the results demonstrated that functional diversity plays an important role in the isolation of nitrogen-fixing and phosphate-solubilizing bacteria from soil.