Bacterial infections adversely impact global public health. Most bacterial infections in humans are associated with biofilm formation which makes them highly resistant to commercially available antibiotics and the human immune system. Therefore, understanding the molecular mechanism of drug resistance in biofilm and developing alternative therapies and promising strategies are of utmost need.
Bacterial infections are responsible for a wide spectrum of diseases in humans causing significant morbidity and mortality on a global scale. Bacteria can survive in two different forms: free-living planktonic form and biofilm form. Bacterial biofilms are densely packed communities of bacterial cells which are surrounded by a self-produced polymeric matrix. It is reported that over 80% of microbial infections including many chronic diseases in humans are associated with biofilm. Apart from causing havoc in the form of deadly diseases, biofilm bacteria can make medical implants malfunctional or non-functional. The infection caused by planktonic form of bacteria can be comparatively more easily cured by antibiotic treatment. However, the resistance of bacteria to antibiotics and other antimicrobial agents increase by nearly a thousand fold in the biofilm state. As a consequence, conventional antibiotics often fail to completely suppress the biofilm infection despite the long-term treatment with a high dosage. It is very alarming that even the planktonic form of bacteria can also develop drug resistance. Therefore, development of new broad-spectrum antibacterial and antibiofilm agent with novel target and new approach is very necessary to combat the drug resistant bacterial infections.
The discovery of antibiotics at the beginning of the 20th century was initially thought of as a landmark step in fighting bacterial infections. However, it was observed that many bacteria had developed the potential to negate the action of antibiotics with the passage of time. On the basis of the Global Burden of Diseases, Injuries and Risk Factors Study (GBD) database, it was reported that more than one million people succumbed to death in 2019 as a result of high antibiotic resistance to bacteria. Consumption of unprescribed medicines or not following the complete dose of prescribed medicines generally results in the generation of ‘multi-drug resistant (MDR) bacteria conferring resistance to multiple drugs. MDR bacteria produce a large number of multidrug efflux pumps which expel the major portions of the antibiotics. This results in antibiotic concentrations below a certain threshold level within the cell. The phenomenon of multi-drug resistance is more pronounced in the ESKAPE group of pathogens including Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp. These pathogens have become the major cause of life-threatening nosocomial infections worldwide. Carbapenems are the most efficient drugs to treat infections caused by MDR Pseudomonas and Acinetobacter species; however their potent applications in managing these infections are threatened by the emergence of carbapenemase-producing strains. The development of carbapenem-resistant Acinetobacter baumannii (CRAB) and carbapenem-resistant Pseudomonas aeruginosa (CRPsA) has raised additional health threats worldwide. It is very difficult to treat the infection caused by these bacteria due to high levels of antimicrobial resistance and mortality. CRAB-CRPsA has been emphasized as critical pathogens in the World Health Organization (WHO) prioritization of pathogens to direct research, discovery, and development of new antibiotics for drug-resistant bacterial infections. Accordingly, WHO has identified the prevention and control of CRAB-CRPsA as an urgent priority on the global health agenda. Therefore, developing a potent agent against such MDR bacterial strains is highly desirable. It is assumed that no effective antibiotic will be available by 2050, if no new drug is discovered or developed.
Dealing with these drug-resistant bacteria effectively has become a matter of serious concern for the human healthcare system. In recent years, different strategies have been proposed to overcome the issue of microbial drug resistance. To overcome the resistance to conventional antibiotics, scientists are trying to develop novel synthetic antibiotics and modified antibiotics against biofilm. The antibiotic treatment alone often fails to disrupt bioﬁlm. Accompanied by a deeper understanding of molecular mechanisms of drug resistance, several anti-microbial peptides (AMPs), peptide mimicking molecules, enzymes, and various naturally occurring compounds are currently being applied in combination with antibiotics to disrupt the biofilm. The AMPs are small cationic, amphipathic molecules that can either directly kill the bacteria or can act indirectly by activating the host immune system. Antimicrobial peptides interact with the negatively charged bacterial cell membrane, thereby changing its electrochemical potential. This disturbs the integrity of the cell membranes leading to the permeation of proteins and other biomolecules out of the cell.
Subsequently, it destroys the cell morphology and leads to consequent death of bacteria. It has been reported that the synergistic action of AMP along with antibiotics leads to the successful elimination of MDR bacteria. The research group under Prof. Maria Teresa Machini at the University of São Paulo, Brazil has reported that cationic AMPs such as daptomycin, LL-37, and azithromycin exhibit increased antibiotic bioavailability against highly multi-drug resistant Gram-negative and methicillin-resistant S. aureus (MRSA).However, AMPs possess several drawbacks such as long synthesis time, high cost, and poor in vivo stability which limit their application despite having potent antibacterial properties. Keeping the above drawbacks in mind, polymeric peptide mimics are currently being developed which show structural similarities with anti-bacterial peptides and possess excellent antibacterial properties. Researchers at the School of Materials Science and Engineering at East China University of Science and Technology (ECUST) under Professor Runhui Liu have developed Poly(2-oxazoline): a peptide mimic that has been reported to possess potent antibacterial and antibiofilm activity against MRSA. Various immunotherapeutic approaches have also recently been developed. Antibodies can inhibit biofilm formation either through interference in the quorum-sensing pathways or can destabilize the biofilm structure by targeting biofilm matrix components.
Various research groups are employing drug repurposing, photodynamic therapy (PDT), and ultrasonication to treat biofilms. PDT has also been proposed to directly kill the bacteria through the production of reactive oxygen species (ROS) and oxidizes the biological molecules present within the cell membrane. This results in disruption of the cell membrane, organelle destruction, and DNA damage consequently leading to cell death. Apart from PDT, another physical technique used to overcome the issue of drug resistance is ultrasonication. Several studies have shown that the antibacterial agents used in combination with low-intensity ultrasonic waves at the physiotherapy level can prevent biofilm formation on medical implants.
One of the upcoming strategies to overcome the issue of high antibiotic resistance is the application of nanomaterials. Nano sized materials show improved permeability across cell membranes and are capable of simultaneously targeting multiple sites in the pathogen. Owing to their high surface-area-to-volume ratio, nanoparticles (NPs) are preferred over other agents as drug carriers. CS/fucoidan NPs and CS-coated alginate NPs have been employed to immobilize antibiotics such as gentamicin and daptomycin on their surface, thereby increasing the solubility and targeted distribution of the antibiotics. Researchers at the Institut Galien Paris-Saclay, France under the guidance of Prof. Patrick Couvreur have developed polymeric nanoparticles for the encapsulation of amikacin antibiotics within chitosan nanospheres. The release of amikacin can be enhanced through the regulation of dissociation of these nanospheres. Apart from drug delivery, nanoparticles are known to exhibit intrinsic antibacterial activity against MDR bacteria. Various metal and metal oxide nanoparticles have demonstrated potent antibacterial and antibiofilm activity. The size of the nanomaterial plays an important role in determining its antibacterial activity. In this regard, the application of nanoclusters is gaining increased prominence due to their extremely small size (< 2 nm). The research groups under Dr. S.K. Samanta and Dr. A.K. Sahoo at IIIT Allahabad, India have developed fluorescent silver nanoclusters (Ag NCs) which exhibit high antibacterial activity against drug resistant bacteria. Carbon dots (C-dots) are another novel class of carbon-based biodegradable, eco-friendly nanomaterials that are gaining a lot of attention as potent antibacterial and antibiofilm agents through their ability to produce ROS.
It is concluded that the treatment of drug resistant bacterial infections needs multidisciplinary collaboration among various research groups. Our immediate future aims should focus on the development of multi-target approaches based on the knowledge obtained from the in-vivo infection studies. Many of the already identified and/or developed novel strategies are found effective and promising. However, extensive preclinical studies along with well-designed clinical trials are very necessary to evaluate the prospects of these strategies prior to human applications.