Formation, Regulation, and Eradication of Bacterial Biofilm in Human Infection

Abstract

Microbial biofilms are complicated structures in which planktonic cells change to a sessile form of growth. The development of an extracellular polymeric substance (EPS) matrix, which encloses the bacterial cells and offers additional protection, supports that kind of growth. Biofilms present a significant threat to public health due to their extreme resistance to higher antibiotic concentrations. In addition, biofilms are also resistant to human immune systems. Bacterial biofilms can spread their pathogenicity through a variety of approaches, such as adhering to a solid surface, evading host defenses like phagocytosis, generating a large amount of toxins, resisting anti-microbial agents, transferring genes to generate more virulent strains, and dispersing microbial aggregates that transport the microorganisms to new locations. Consequently, there is an urgent need to replace the widespread procedure of antibiotics with novel developing approaches. Furthermore, biofilm formation has been connected with high rates of disease, health-related infections, and even death, leading to the search for alternative treatment approaches. The review intends to provide information about clinically important bacterial pathogens of the gut, mouth, skin, and lungs and insights into the different perceptions of microbial biofilms, as well as their formation, regulation, and pathogenicity. In addition, for efficient eradication or inhibition of biofilms and associated infections, nanoparticle approaches for addressing persistent bacterial infections have also been discussed.

1. Introduction

The term “biofilm” refers to a connection of microorganisms when microbial cells adhere to one another on living or inactive surfaces and are enclosed in an extracellular polymeric substance (EPS) matrix [1]. The initial identification of microbial biofilm belongs to a Dutch researcher Antoni van Leeuwenhoek, who used a simple microscope to detect “animalcules” for the first time on the surfaces of teeth [2]. A number of studies have demonstrated that bacterial biofilms are resistant to antibiotics and cannot be hindered by human immune system because the microbes that cause biofilms have a greater capacity to resist or remove antimicrobial agents, which extends the period of recovery during infection [3]. During biofilm-forming stage, certain genes of bacteria are induced, resulting in the activation of stress-related genes and the transformation of bacteria into resistant phenotypes which lead to changes in cell density, pH, osmolarity, or nutrition [4]. It has been reported that majority of bacteria have the ability to develop biofilm on almost every type of surface, which poses a significant threat to the health of people because of the diseases it causes and the resistance it provides to many antibiotics [4, 5]. According to studies, the exopolymer in biofilms inhibits the ability of leucocytes to pass through the biofilm, checking their capacity of leucocytes to degranulate, and stops them from producing reactive oxygen species (ROS), which prevents bacterial phagocytosis [6, 7, 8]. Previous investigations have stated that significant amounts of clinically important bacterial pathogens such as Enterobacter cloacae, Escherichia coli, Klebsiella pneumonia, Pseudomonas aeruginosa, Staphylococcus aureus, S. epidermidis, etc. possess the ability to develop biofilms [4, 9, 10, 11]. In addition, biofilms are also known to spread diseases through colonizing surgical instruments, which include central venous catheters, urinary catheters, joint prostheses, pacemakers, etc. [11, 12]. Furthermore, biofilm has been associated with chronic wounds, lung infections in cystic fibrosis patients, and dental caries [13].

In this chapter, we will describe the clinically important bacterial pathogens of the mouth, gut, lungs, and skin. Moreover, the formation and regulation of bacterial biofilm as well as pathogenicity, its mechanisms, and the eradication of biofilm by nanoparticles will also be discussed. This information echoes advancements in microbiome diagnostics and shows how biofilm is formed and regulated. A closer examination of biofilm provides more clarity on the inherent strengths and weaknesses of biofilm. It highlights the need to realize that biofilm is not simply a more significant number of wound pathogens but a sophisticated biological process that requires specific, targeted care. This study also highlights how different types of nanoparticles help in the eradication of bacterial biofilm and shows that nanoparticles have an excellent capacity for the eradication of bacterial biofilm and that different types of nanoparticles act in different ways in order to eradicate biofilm.

2. Body-site infection of clinically-important bacterial pathogens

The clinical importance of different bacterial pathogens is widely recognized, and regular examination is required to provide an accurate diagnosis for specific kinds of infections. Some clinically important bacterial pathogens that cause health complications worldwide and occur in the mouth, skin, lungs, and gut are given below.

2.1 Mouth

The mouth, which serves as a pathway to the digestive system, offers a habitat for a diverse and abundant microbial population, and masses of these organisms and their products develop on the surfaces of the teeth and gums [14]. These growths, commonly referred to as plaque from the mouth and classified as biofilms, contribute to the development of cavities, which leads to tooth damage [15]. The microbial community that grows around teeth is extremely complicated. The microbiome of the mouth poses a threat to maintaining overall and dental health [16]. Therefore, dental caries is a dietary-microbial disease that involves a cariogenic biofilm and continuous exposure to fermentable carbohydrates from dietary sources, such as sucrose, glucose, fructose, maltose, etc. [17]. Around 700 different types of bacteria in the mouth cavity have been detected using ribosomal identification techniques, among which Streptococcus mutans is one of the most common caries-causing bacterium [18]. It canmetabolize many types of carbohydrates, generating high-level acidity and dextran that facilitates the production of dental plaque. Many different bacterial species belonging to the genera Streptococcus and Actinomyces can be found in the plaque biofilms [19, 20]. For example, the two Gram-positive and anaerobic bacteria, Streptococcus anginosus and Actinomyces naeslundii, are commonly found in biofilms, while under healthy circumstances, Gram-negative bacteria such as Aggregatibacter Actinomycetemcomitans, Campylobacter spp., Porphyromonas spp., Prevotella intermedia, and Treponema denticola can also exist [21]. The investigations have shown that these Gram-negative bacteria might infect other parts of the body when hygiene in the mouth fails to be observed. For example, staphylococci (staph) and streptococci (strep are involved in endocarditis) [22]. In addition to this, other issues related to these biofilms include actinomycosis, dental root infections, and foul breath [23].

2.2 Skin

The human skin microbiome performs a significant role in both health and disease. The initial defense line of human body from pathogens is the skin, which protects and shields the body and provides a hostile environment for majority of bacteria [24]. Microbial biofilms are an extensively investigated mode of surface-associated growth that exhibits community-like characteristics. Furthermore, biofilms play an important role in numerous skin diseases. The usual microbiota of skin comprises a considerable number of Gram-positive bacteria, such as Staphylococci and Micrococci. Gram-positive bacteria are comparatively resistant to harsh conditions such as dryness and extreme osmosis pressures noticed in high salt or sugar mixtures [25]. The common causes of bacterial infections in skin are Staphylococcus and Streptococcus [26]. The bacteria Streptococcus pyogenes is responsible for a contagious bacterial skin infection that forms pustules and yellow, crusty sores. In certain instances, both S. aureus and S. pyogenes are present. Usually, the bacteria that cause infection penetrate through a small skin opening. Additionally, the infection has the potential to spread to neighboring body parts. However, the primary skin pathogens are coryneform bacteria like hemolytic Streptococci and S. aureus [27]. Normally, such bacteria penetrate the body from a wound in the skin, including bites from insects, etc. [28]. In vitro, single-species biofilms of skin microbiota, such as S. aureus, S. epidermidis, and Propionibacterium acnes, have been investigated [29]. Furthermore, important inter-species interactions with skin prokaryotes have been found, e.g., S. epidermidis inhibiting both P. acnes growth and S. aureus biofilms [29, 30].

2.3 Lungs

Lung infection is one of the most prominent health issues. Various systemically or respiratory problems start with lung infections. The region of the upper respiratory tract is where airborne pathogens initially come into interaction with the body’s mucous membranes [31]. Pathogens from surroundings and dust particles are continuously exposed to the pulmonary system and airways. Once there are issues with any component of this system, lung disease can occur [32]. The most common causes of bacterial lung infections in normal hosts include Streptococcus pneumoniae, Haemophilus species, Staphylococcus aureus, and Mycobacterium tuberculosis. The bacterial infections of the lungs were responsible for one-fifth of all fatalities in Europe and North America between the seventeenth and nineteenth centuries. It can remain dormant for years before establishing a chronic cavitating lung infection with highly infectious sputum. Following a significant reduction in prevalence, mainly due to advancements in public health, M. tuberculosis infections are currently reducing in rate, while multidrug-resistant strains are spreading across various areas [33, 34]. Other common microorganisms responsible for pneumonia include Staphylococcus aureus, Group A Streptococcus, Klebsiella pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis [35]. Additionally, patients with smoking-related lung disease frequently have Haemophilus influenzae infection, which can result in bronchial inflammation and patchy infiltration into the surrounding lung. H. influenzae is under-detected by the usual clinical culture approach [36].

2.4 Gut

Bacteria can enter the intestinal mucosa and replicate there, as well as spread to other organs in the body [37]. The majority of intoxications, including those carried on by Staphylococcus aureus, are identified by the symptoms appearing extremely quickly (often within a few hours) [38]. The gut microbiota is a convoluted ecology with approximately 300 to 500 different bacterial species [39]. In contrast to the lower gut, the stomach and upper intestine have less abundant microbiota [40]. Bacteria can be found in the mucosa and in the lumen, although they often are unable to penetrate the gut wall. The usual intestinal microbiota contains small populations of bacteria which can lead to disease when allowed to overgrow. For instance, an over-population of Clostridium difficile can result in serious intestinal inflammation and diarrhea. Antibiotic administration starts the procedure by inhibiting the natural microbiota [41]. The main common pathogens of the gut include Vibrio cholerae, enteropathogenic strains of E. coli, Eubacterium, Bacillus cereus, Bacteroides vulgatus, Bifidobacterium, Clostridium difficile, Fusobacterium, Peptostreptococcus, Pseudomonadota, Prevotella, Salmonella enterica, Salmonella gastroenteritis, Salmonella typhimurium, and Shigella spp [42, 43, 44, 45, 46, 47, 48]. Bacillus subtilis is a gut commensal and non-pathogenic [49, 50]. Lactobacillus johnsonii and Clostridium perfringens are both commensal [51, 52]. However, Clostridium perfringens is also an opportunistic pathogen that can lead to lethal diseases as a result of overgrowth causing gas gangrene, food poisoning, non-foodborne diarrhea, and enterocolitis [53, 54]. Bacteroides fragilis is part of the normal microbiota of the human colon and is commensal, but can cause infection if displaced into the bloodstream or surrounding tissue following surgery, disease, wounds, or trauma [55, 56].

3. Biofilm formation and regulation

Wound contamination happens within minutes when planktonic (free-swimming) microorganisms travel into the wound, anchor to the wound bed, and become attached (sessile) [57]. Bacteria produce sticky sugar strands or polymers known as extracellular polymeric substances (EPS) when attached to the wound bed. These polymers form bonds with the help of metallic ions obtained from the host and wound environment, forming a three-dimensional protective structure that grows into a complex community developed to protect the encased bacteria compared to assault through the body’s immune system or external attack.

3.1 Steps of biofilm formation

The development of a biofilm actually involves a combination of physical, chemical, and biological processes that proceed over a period of time. Detailed procedures of bacterial biofilm formation are illustrated in Figure 1.

Figure 1.

Schematic illustration of the key steps in biofilm formation: (a) planktonic bacteria attaching to the surface, (b) motility factor inhibition, (c) extracellular polymeric substances (EPS) generation, and (d) biofilm maturation and dispersal. The figure is adapted and modified from a previous study with copyright permissions [58].

3.1.1 Conditioning

The formation of a conditioning layer is the initial stage in the development of a biofilm. The components of the bulk fluid settle onto the surface at this stage, developing a substratum. In common, rough surfaces and hydrophobic materials have a preference compared to hydrophilic and smooth surfaces [59]. The microorganism adheres to such surfaces, which are consequently changed to improve a surface charge that assists in the attraction and adhesion of bacteria with opposing charges [60]. The bacteria can adhere to the surface more strongly owing to the existence of pili, fimbriae, and glycocalyx on the surface [61]. Although an initial adhesion can be reversed, if attraction dominates over repulsion, it will become irreversible.

3.1.2 Attachment and growth

When the adhesion is effective, the bacteria start to grow by taking advantage of the available nutrients. Following this stage, biological events dominate bacterial adhesion to the surface. This is the outcome of the expression of a number of genes that are in the position of producing surface proteins such as porins [62]. The polysaccharides used to create the EPS layer are transported with the help of porins. Since the biofilm matures, microbial cells start to connect to each other via the release of autoinducer signals (AIs) [63]. This communication is critical due to an established biofilm can comprise up to 100 billion bacterial cells per milliliter. The cells are divided into identical different groups, each of which is accountable for a specific task [64]. Another frequently observed phenomenon in a growing biofilm is the formation of high and wrinkled structures. However, this causes lateral pressure on the cells by pulling it towards one another. The dead cells in the biofilm concentrate in the areas that promote vertical bulging, which helps in releasing this pressure [65].

3.1.3 Metabolism

The metabolic process of the biofilm modifications with changes in the environment of the biofilm throughout the primary phase of growth of biofilm, when the metabolic activity is strong and subsequently declines as growth progresses [66]. The complex diffusion channels are employed as the cell population grows to transport nutrients, oxygen, and further components required for cell growth. These channels are used to transport the metabolic wastes and debris. In fact, shear stress has a significant impact on the expression of genes involved in glycolysis [67]. The bacteria that form biofilms have a propensity to ingest foreign DNA, which could eventually lead to the expression of exogenous proteins [68]. Furthermore, it has been demonstrated that several genes involved in the biosynthesis of fatty acids were downregulated as the biofilm formed [69]. These findings show that biofilm-forming cells have a very different metabolism from planktonic cells.

3.1.4 Dispersion

The final step is dispersion, which involves the destruction of the biofilm and the sessile cells, allowing them to resume their motile forms. Finally, biofilm makes use of its disruptive forces to spread throughout, and the bacteria colonize new regions and develop [70].

3.2 Biofilm regulation

This protective biofilm structure is comprised of proteins and smaller molecules which are strung together to form larger, more robust polymer units of sugars (polysaccharides), macromolecules like DNA, and lipids [71]. This EPS helps the bacteria contained within the structure survive by supplying nutrients, removing waste products, and preventing harmful antimicrobial molecules, antibodies, and host inflammatory cells from getting or interacting with the bacteria. A developed biofilm additionally helps in the ongoing maturation and eventual spreading recolonization of the encased bacteria, prevents molecules too large to pass through the structure, and provides a diffusion barrier to small molecules like antibiotics (Figure 2) [4, 73, 74, 75, 76].

Figure 2.

The general mechanism of biofilm resistance to antimicrobials. (A) Biofilm matrix provides a diffusion barrier to small molecules like antibiotics. (B) Inactivation of antibiotics by enzymes of the biofilm matrix. (C) Persister cells in the deeper layer of biofilm inducing adaptive SOS reaction and hence developing further resistance. The figure is adapted and modified from a previous study with copyright permissions [72].

Defensive EPS structure of bacterial biofilm protects bacterial hybridization, tolerance, and gene expression by subverting the natural infection inflammatory response in order to get rid of the body of bacteria and support the survival of bacteria [75]. It is noteworthy that this protective structure can repel treatments and promote continued biofilm growth, even if the biofilm is chemically or mechanically fractured into microcolonies, rendering the bacteria within the structure virtually invincible unless the structure is solubilized and removed. The protective EPS structure of the biofilms protects bacterial hybridization, tolerance, and gene expression patterns and promotes bacterial survival by preventing the body’s own inflammatory response that is designed to get rid of bacteria [18]. This procedure enables the biofilm to rapidly mature and develop impenetrable to conventional treatments as well as unculturable using conventional culture methods. The biofilm may function passively as a reservoir for pathogenic bacteria which are typically polymicrobial in nature, or it can take a more active role by encouraging an expanding area of inflammation and pathogenic tissue damage that favors the progression into overt infection as the biofilm develops into a more mature insoluble biomass, encouraging protected bacterial growth, mutation, and proliferation through sophisticated cell-to-cell and cell-to-surface interactions between the host and the biofilm [77, 78, 79, 80, 81]. Over time, a portion of the biofilm’s bacteria disperses as fresh-roving bacteria and micro-bacterial aggregates that release and spread, acting as the foundation for new biofilm colonies [82].

4. Bacterial biofilms: Pathogenicity and properties of the bacterial biofilms

4.1 Pathogenicity

It is well-established that biofilms contribute to the virulence of pathogens. According to statistics from the Centres for Disease Control and Prevention (CDC) and the National Institutes of Health (NIH), the prevalence of disease caused by biofilms is believed to be between 65% and 80%, particularly in developed nations [83]. Several food-borne pathogens including E. coli, Salmonella, Yersinia enterocolitica, Listeria, and Campylobacter create biofilms on the surface of food or storage equipment. Furthermore, potentially pathogenic bacteria such as Staphylococcus aureus, Enterococcus faecalis, Streptococcus, E. coli, Klebsiella, and Pseudomonas thrive on catheters, prosthetic joints, mechanical heart valves, and so on. As a result of their periodic escape from the said focus these organisms may cause persistent diseases [83, 84]. The localized depletion of nutrition in a biofilm has been proposed as an inducer of cell release or detachment from the biofilm in Pseudomonas aeruginosa [85]. However, microbially produced gas bubbles, cross-linking cations, growth status, contact surface material, shear stress, quorum sensing, and lytic bacteriophage activation have all been identified as major contributors to biofilm detachment. They can be life-threatening causing endocarditis and infections in people with cystic fibrosis, in addition to infecting long-term indwelling devices like heart valves and joint prostheses [86].

Numerous bacterial toxicities in the human body such as the development of dental plaques, infections of the middle ear in children, urinary tract infections, gingivitis, and contact lens infections are caused by biofilms. The biofilm formation takes place on contact lenses and ultimately it leads to contamination [9, 87].

4.2 Properties of the bacterial biofilms

The bacterial biofilms cause pathogenicity through a variety of unique properties.

4.2.1 Variation in phase

Small colony variations (SCVs) are a colony phenotype highlighted by small size, slow development, and virulence gene downregulation and have been identified as a pathogenic mechanism for various bacterial species such as S. epidermidis and are often linked with chronic infections [88]. Biofilms have the unique potential to generate bacterial subpopulations that are shifted to a dormant state and are known as small colony variations (SCVs). They also have low catalase activity which interferes with oxidative metabolism. SCVs generate noticeable morphological changes in biofilms, increasing adhesion, auto-aggregation, hydrophobicity and pathogenicity. These variations contribute to biofilm survival in extreme environmental conditions [58]. SCVs appear to be less sensitive to antibiotics and the immune system possibly due to their ability to survive intracellularly and induce a more anti-inflammatory setting due to higher Inter Leuken-10 (IL-10) release [89].

4.2.2 Efflux pumps

Efflux pumps are present in the periplasmic space within the bacterial membranes and have a negative influence on antibiotic accumulation and their presence is critical in pathogenesis in biofilm [90]. The mutations in regulation proteins or promoters result in the production of these efflux pumps which causes pathogenicity. These efflux pumps are energy-dependent and on the basis of mechanism by which they derive energy are generally categorized into two groups. The primary efflux pumps get their energy from constant hydrolysis of ATP whereas the secondary efflux pumps get their energy from chemical gradients created by protons or ions like sodium ions [91]. The increased expression of these efflux pumps in biofilm has also been linked to pathogenesis in P. aeruginosa biofilms by causing antibiotic resistance [92].

4.2.3 Alterations in membrane protein expression

The outer membrane channel proteins existing in bacterial membranes play an important part in transferring hydrophilic particles from the outer atmosphere to the periplasmic space in biofilm of bacteria [93]. The presence of outer membrane proteins (Omps) allows for macromolecular interaction between the cell and the environment. These proteins are established in the outer membrane of bacteria in biofilm. A larger channel outer membrane protein such as OmpF can be used in place of OmpC because it has a smaller diameter. This change limits the entry of bigger compounds with high hydrophobicity such as carbenicillin. In contrast, small hydrophilic molecules such as imipenem can pass through the OmpC channels. In biofilm, differential expression of outer membrane protein-coding genes occurs which contributes to antibiotic resistance and pathogenicity [94].

5. Bacterial biofilm and its eradication

Many types of nanoparticles with therapeutic effects against bacterial biofilm infections can be categorized according to their chemical composition or their healthcare purposes. The nano-formulations have a high attraction to the bacterial cells and the capacity to penetrate biological barriers like biofilm because of their small size, large surface area, and highly sensitive nature [95]. The sizes of the NPs are sufficiently small to penetrate into biofilms and microbial cell walls, while the large surface area of the NPs enables the effective loading of drugs [96]. Although the exact strategy of NPs reducing biofilm formation is not completely elucidated yet, multiple studies have described different processes by which NPs impact bacterial cells and biofilms (Figure 3). There are various common types of nanoparticles used for biofilm eradication, which are listed and discussed below.

Figure 3.

The primary mechanisms of nanoparticles (NPs) in biofilm eradication. (A) Interaction with functional components of biofilm via the released ions. (B) Production of reactive oxygen species (ROS) that cause bacterial destruction and EPS breakdown. (C) Antimicrobial-loaded polymeric nanoparticles penetrate into the biofilm and deliver drugs to the bacterial cell. (D) the photothermal effect, which occurs in the presence of near-infrared (NIR) light irradiation, causing an increase in local heat, which acts efficiently alongside EPS and bacterial cells. (E) Liposomes encapsulate the antimicrobial and fuse with cell membranes, allowing the antibiotic to be released directly inside the bacterial cell. The figure is adapted and modified from a previous study with copyright permissions [97].

5.1 Metal-based nanocomposites

Metal nanoparticles (MNPs) are often employed in antibacterial and antibiofilm studies due to their inherent nature, structure, and large surface-to-ratio, enabling control in fabrication, approach, and modification of their physical and chemical characteristics [98]. MNPs demonstrate significantly greater antibacterial activity compared to their micro-sized counterparts, although NPs, like common antimicrobials, lack the ability to recognize sensitive and resistant microorganisms [96]. However, their non-specificity is also one of their drawbacks because they can also attack commensal bacteria [99]. Metal oxide nanoparticles (MONPs), silver nanoparticles (AgNPs), gold nanoparticles (AuNPs), and various other metal-based nanocomposites (NCs) have shown their efficiency in preventing biofilm formation through a distinct inhibitory mechanism [96, 99].

5.1.1 Metal oxide NPs

Metal oxide nanoparticles (MONPs) that demonstrated antimicrobial activity consist of iron oxide (Fe₃O₄), zinc oxide (ZnO), titanium oxide (TiO₂), silicon oxide (SiO₂), selenium oxide (SeO₂), and aluminum oxide (Al₂O₃). The majority of the NPs impacts on microbial cells involve cellular membrane breakdown resulting from NP-cell surface interaction and consequent leak of cell substance [100]. Metal oxide NPs (MONPs) also mediate mechanisms for DNA and RNA destruction, the production of ROS, and the discharge of poisonous substantial metal ions [98]. The primary antibacterial activity of these NPs is related to oxidative stress, which is caused by the formation of ROS on the outer layers of metal oxides and subsequent breakdown of cell membranes, structure of cells, and molecules [98, 101]. The effects of oxidative stress can harm the proteins that contribute to attachment and biofilm development. Furthermore, it suppresses the development of genes that are important for bacterial cell attachment on surfaces and biofilm development [102].

5.1.2 Silver nanoparticles (AgNPs)

AgNPs are commonly used as antimicrobial agents and have greater antibacterial activity than some antibiotics as well as employed in clinically developed devices, tubes, and dressings for wounds [103]. In addition to exhibiting antimicrobial properties, AgNPs possess a large surface-to-mass ratio, which makes them an attractive choice for use as single layers on the surfaces of biomolecules [104]. The antibacterial effect of AgNPs is considered to be caused by the NPs’ breakdown and the release of Ag⁺ ions, which attach to the cell membrane and depolarize the cell wall while changing the permeability and negative charge of the membrane. Additionally, when Ag⁺ ions penetrate the target bacterium, they cause the oxidation and breakdown of cellular components, the reduced activity of respiratory chain enzymes, and the formation of ROS, which hinders the recombination of DNA and the fabrication of ATP [98, 105].

5.1.3 Gold nanoparticles (AuNPs)

AuNPs are more efficient against biofilm compared to AgNPs because they have a lower hydrophobicity index, which reduces the growth of biofilm [106, 107]. The antimicrobial process of AuNPs is believed to involve affecting the membranes of bacterial cells and inhibiting ATPase production, which leads to metabolic degradation, as well as hindering the ribosome component attaching to tRNA, attacking nicotinamide, and impacting the bacterial respiratory chain [105]. In addition to having antimicrobial qualities, AuNPs also exhibit photothermal characteristics when exposed to near-infrared (NIR) light. This is because accumulated AuNPs absorb light in a red-shifted manner, which causes a dramatic increase in localized heat. Consequently, this represents yet another potent means of eradicating bacteria from the infectious biofilm without damaging the tissues that surround it because their cells require a greater amount of heat due to their larger size than bacterial cells [108, 109].

5.1.4 Metal-polymer nanocomposites (MNCs)

The fabrication of MNPs as polymer nanocomposites increases their stability and efficiency. According to Nagvenkar et al. [110], adding ZnO NPs to polyvinyl alcohol (PVA) polymer enhanced the stability and efficiency of the ZnO-PVA nanofluid against S. aureus and E. coli [110]. The toxic effects of ZnO can be decreased through its incorporation into different materials. Banerjee et al. [111] showed that doping pancreatin (PK) on ZnONPs (ZnONPs-PK) reduced the toxic effects of ZnO and increased its anti-bacterial and anti-biofilm efficiency while decreasing its virulence towards MRSA [111]. Depan and Misra [112] found that incorporating titania NPs into silicone decreased S. aureus life and adhesive abilities by 93% when compared to stand-alone silicone [112]. Silicone-TiO₂ NPs were also more effective at breaking down the biofilm; after 6 hours of incubation, the biofilm completely disintegrated. Wang et al. [113] observed that AuNPs incorporated with graphitic carbon nitride (g-C3N4) could improve H₂O₂ efficiency by causing peroxidase-like activity that effectively breaks down H₂O₂ to OH radicals, leading to demonstrated biofilm destruction and prohibited biofilm development in vitro, reducing the growth of E. coli and S. aureus, and potentially accelerating the healing process [113]. Recently, capping MNPs with polysaccharides obtained from other microbes like yeast and algae has been shown to be effective [114].

5.2 Polymer-based nanoparticles (PNPs)

Polymer nanoparticle-based antimicrobial delivery systems are whatever they are commonly referred to as in regards to their functionality. Despite the fact that the chemical composition could be organic, inorganic, or even a mixture of both, their enhanced antibacterial transport is due to their improved stability, capacity for modification, formation at the site of infection, and monitored release ability, along with boosted cytocompatibility and biodegradable properties [98, 115, 116]. PNPs have a distinct advantage over MNPs because medications can be maintained within their cavity, allowing the drug to be delivered to the target region, whether confined or entrapped [99, 117]. Based on this, PNPs are available in two shapes, nanospheres and nanocapsules, with sizes ranging from 100 to 500 nm. The nanosphere is a polymeric matrix that contains the drug that has been adsorbed in it. The drug can be entrapped in small cavities or adsorbed onto the polymer wall of nanocapsules, which have an oily core and a polymeric shell around them [117].

5.3 Natural and synthetic polymer-based nanoparticles (PNPs)

PNPs can be either synthetic or natural, like chitosan, polycaprolactone, polylactic acid, and polylactic-co-glycolic acid (PLGA). Chitosan, a cationic heteropolysaccharide, is frequently used as a nanocarrier due to its biocompatibility, immunostimulating properties, non-toxicity, biodegradability, adhesive properties, and relatively low cost of production [115, 118]. Chitosan has a high ability to inhibit biofilm growth because of its polycationic nature, which results in electrostatic interaction with the biofilm components and damages the biofilm matrix [119]. Additionally, electrostatic interaction between positively charged chitosan and negatively charged bacterial cell surfaces leads to the destruction of bacterial cell membranes and the leakage of their constituent parts, as well as the inhibition of mRNA transcription and protein synthesis through DNA binding [100].

5.4 pH-responsive polymer-based nanoparticles

The ability to be functionalized in accordance with the conditions of the microenvironment determines the polymeric NPs. As previously discussed, the rapid pH-responsive transmission of the NPs negative to positive charge increases their capacity to accumulate and penetrate biofilms inside acidic microenvironments, which decreases drug efficacy. In addition to effectively binding to the bacterial cell surfaces and enhancing photoinactivation efficiency against Gram-negative bacteria, the pH-responsive polymeric NPs carriers have a high potential to interact with the acidic biofilm microenvironment and respond to pH variation [120, 121]. The acidic pH-responsive NPs systems have been developed as bilayers with a cationic outer shell for binding with EPS components and a hydrophobic inner shell for releasing the encapsulated drug and enhancing antimicrobial and antibiofilm activity [120, 122]. For example, Horev et al. [120] conducted an in vitro and in vivo study that demonstrated farnesol-loaded pH-activated polymer NPs had a 4-fold greater ability to inhibit S. mutans biofilms than free farnesol; additionally, the drug was concentrated at the biofilm-EPS matrix interface, which greatly improved farnesol retention and bioavailability [120].

6. Conclusion

Human pathogenic biofilms are associated with chronic and recurrent diseases that can be very severe and even fatal. Biofilm formation and regulation are multi-step complex procedures that involve the transition of bacteria from free-swimming planktonic form to biofilm-making sessile form. Pathogenicity is the capacity of a pathogen to cause disease through a variety of mechanisms. Pathogenic biofilm may cause different host reactions in a human host and use a variety of mechanisms to evade the host defense systems. Furthermore, toxins such as invasins, lipopolysaccharides (LPS), and cell wall components of biofilm can damage host cells and cause septic shock. Moreover, adhesins help in adhering the pathogen to the surfaces of the host. During infection, bacteria are able to attach to a host surface and continue to penetrate host tissues. Pathogens can “burrow” more deeply into a tissue by generating and releasing proteases and glycanases that degrade host extracellular matrix proteins and polysaccharides. Another possibility for a pathogen in biofilm is to enter the host tissue cells and have access to the intracellular environment. In recent years, nanotechnology has developed into an exciting technique for eradicating bacterial biofilm-related infections. The ability of nanoparticles (NPs) to deliver drugs to the target site in the ideal dosage range, protect them from deactivation, and increase their therapeutic efficiency with fewer side effects makes them a promising therapeutic approach. Aside from that, the small size, large surface area, and highly reactive nature of nanoparticles enable them to penetrate biological barriers like biofilm and have a high eradicative selectivity for bacterial infections. Taken together, this study systematically reviews the formation, regulation, and eradication of bacterial biofilm in human infection, which not only facilitates our understanding of bacterial biofilms but also strengthens the importance of developing novel methods and technologies to inhibit and eradicate biofilm-related human infections, which will greatly reduce the mortality rate of chronic and fatal bacterial infections.

Acknowledgments

We thank the anonymous reviewers for their thoughtful comments that greatly improve the quality of the manuscript.

Conflict of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author contributions

LW and MU conceived the framework. LW and LYZ provided platform and resources. LW contributed to project administration and student supervision. MU, HY, JJW, JWT contributed to literature review. JWT contributed to the schematic illustration. All the authors wrote and revised the manuscript. All the authors approved the submitted version of the manuscript.

Funding statement

This research was funded by Guangdong Basic and Applied Basic Research Foundation (Grant No. 2022A1515220023) and Research Foundation for Advanced Talents of Guandong Provincial People’s Hospital (Grant No. KY012023293).