Biofilm is complex and consists of bacterial colonies that reside in an exopolysaccharide matrix that attaches to foreign surfaces in a living organism. Biofilm frequently leads to nosocomial, chronic infections in clinical settings. Since the bacteria in the biofilm have developed antibiotic resistance, using antibiotics alone to treat infections brought on by biofilm is ineffective. This review provides a succinct summary of the theories behind the composition of, formation of, and drug-resistant infections attributed to biofilm and cutting-edge curative approaches to counteract and treat biofilm. The high frequency of medical device-induced infections due to biofilm warrants the application of innovative technologies to manage the complexities presented by biofilm.
A biofilm is a community of microorganisms, such as bacteria, that are capable of living and reproducing as a collective entity known as a colony. To put it another way, biofilms are living biomass that possess a sophisticated social structure that personnel involved in this field are still attempting to decipher. The structure of biofilm serves both to shield and enable the expansion of the colony.
That a symbiotic relationship exists between prokaryotes and eukaryotes, or unicellular and multicellular organisms, is common knowledge. These symbiotic relationships are mutually beneficial. The human body consists of a microbiome that is large and complex and consists of bacteria, fungi, and viruses. Most of the microbiota in the human body resides in the gastrointestinal tract, salivary mucosa, and skin, where they facilitate various physiological functions ranging from metabolism to innate immunity. However, under certain circumstances, the growth of these symbiotic microorganisms can become uncontrollable and can lead to infections that initiate the formation of biofilms. Since their evolution, bacteria have existed in two separate states: the planktonic state (free-floating) and sessile state (adhered to a surface) [1]. Bacteria exhibit different traits between these two states because bacterial attachment to a surface causes a rapid change in the expression levels of several genes associated with maturation and production of exopolysaccharide (EPS), also known as “slime” or bacterial EPS. A protective barrier is produced because of this transition, which starts immediately after bacteria colonize both biotic and abiotic surfaces [1,2,3]. This barrier shields the bacteria from the natural defense mechanisms of the host and from external threats, such as antibiotics. Anthony van Leeuwenhoek first observed surface-associated bacteria, but the word “biofilm” was not used or defined until a manuscript by Costerton et al. [4]. The significance of biofilms was acknowledged by the American Society for Microbiology in 1993 [4]. Costerton et al. characterized biofilm more fully in 1999 as an organized population of microbes encased in a polymeric matrix produced by the microbe that is adhering to a surface [5]. Biofilms impact all facets of human life, from public health to industrial concerns, and impact the economy, use of energy, equipment degradation, contaminated products, and infections. Cutting-edge technologies such as scanning and confocal microscopy have helped scientists understand the extraordinarily intricate structure of biofilms. The use of these cutting-edge technologies has discovered that biofilms are complex populations of cells wrapped in a matrix of EPS with permeable water channels and uniform deposits of cells and collected slime. Many human diseases and the colonization of medical devices are linked to microorganisms growing in biofilms and these microorganisms are very resistant to antimicrobial treatments. Biofilm formation initiates the disease process through various mechanisms, such as the detachment of individual bacterial cells or clusters of cell aggregates, the production of endotoxins, heightened evasion from host immune system surveillance, and the establishment of a protective barrier conducive to the emergence of immune-resistant organisms.
Modern understanding of this subject defines biofilms as an immobile complex structure comprising of single or multiple species of bacteria, host cells, and cellular by-products, with the cells irreversibly attached to the substratum and surrounded by an extracellular polymeric substance produced by bacteria. A surface that provides moisture and nutrients is the ideal environment for biofilm development. Biofilms can be good, bad, or neutral [6]. Biofilms that are part of a natural environment are neutral, whereas biofilms that grow on open wounds following infection are harmful. Biofilms may play a constructive role in solving ground contamination from an oil spill. Biofilms are responsible for 70% of all microorganism-induced infections and are a significant contributor to healthcare-associated infections (HAIs) in humans. The microorganism living as part of the biofilm shows characteristic features, such as collective cooperation, source capturing, and increased survival against treatment with antimicrobials. Increased survival and evasion of the host immune system make biofilms responsible for persistent chronic infections [7]. The complexity of biofilm activity and behavior requires multidisciplinary research to develop an effective solution against the devastation that can be caused by this structure.
The makeup of biofilm is 10% microbial mass and 90% water [8]. A range of 50–90% of the entirety of the organic component of biofilms is attributed to the polysaccharides that form the matrix [9]. Chains of polysaccharides are woven together in a dense, mesh-like structure [10,11]. The hydroxyl groups on the polysaccharide increase mechanical strength by interacting with each other [10,11]. The biofilm architecture can have positively charged ions, such as Ca 2+ or Mg 2+, which form supportive cross bridges between polymers and allow biofilms to grow to thicknesses of up to 300 µm. In other instances, the polysaccharides in biofilms can be neutral or polyanionic, as in the EPS of Gram-negative bacteria [12]. Biofilms also can have uronic acids, such as D-glucuronic, D-galacturonic, and mannuronic acids or ketal-linked pyruvates that bestow anionic properties [5,12]. Anionic properties allow the association of divalent cations to inter-link strands of polymer and provide a greater binding force to matured biofilm [13]. In the case of Gram-positive bacteria, such as staphylococci, the chemical composition of EPS is entirely different and is principally cationic. Hussain et al. have reported that the slime of coagulase-negative bacteria consists of a teichoic acid mixed with small quantities of proteins [14]. The different charges and ions in the biofilm provide structural integrity to the EPS, which confer biofilms with the property to withstand environments of extreme shearing forces, such as on waterfall impact points.
Bacteria growing in biofilm are sessile and are responsible for most physiological processes in the biofilm environment [15]. The sessile bacterial biofilm communities have different growth, gene expression, transcription, and translation rates. These functional characteristics are acquired by the sessile bacterial biofilm communities in the process of adaptation to microenvironments that have higher osmolarity, scarcer nutrients, and increased cell density. The resulting structure of a biofilm is extremely viscoelastic and has a rubbery behavior [16]. The most frequently found bacteria in a biofilm are Pseudomonas aeruginosa, Staphylococcus epidermidis, Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Streptococcus viridans, Staphylococcus aureus, and Enterococcus faecalis [17]. Staphylococci are a diverse group of Gram-positive bacteria and generally found in the skin and mucosa of mammalians. Bacteria belonging to this genus are responsible for nearly 80% of infections caused by implantable devices in humans [18,19,20,21,22,23,24,25,26]. A recent NIH study reports that 70% of all human microbial infections are due to biofilms and lead to various diseases including non-healing chronic wounds, endocarditis, periodontitis, cystic rhinosinusitis, fibrosis, meningitis, osteomyelitis kidney infections, prosthesis, and implantable device-related infections [2,3,18,27,28,29,30,31]. Extreme care in the manufacturing process seeks to maintain sterility of an implantable device, but a device that becomes contaminated during and after implantation that can cause serious device-associated infections that require removal and can result in death.
The formation of the three-dimensional architecture of biofilm is a several step process and involves adsorption, adhesion, microcolony formation, maturation, and dispersion. A biofilm surface’s solid–liquid intersection with an aqueous medium (such as blood or water) offers the perfect condition for microbe attachment and growth. The close association of the cells in the biofilm colony creates the conditions encouraging the development of gradient in nutrition availability, exchange of genes, and quorum sensing (QS).
Biofilms are formed in a turbulent flow environment where the Reynolds number (Re) is more than 5000. Re, a non—dimensional number used in fluid mechanics, measures the proportion of inertial to viscous forces and aids in the prediction of fluid flow patterns in various contexts. A lower Re number suggests laminar flow and a higher Re number reflects turbulent flow. Turbulent flow enhances biofilm formation. Smooth and rough surfaces have been reported as being colonized with equal ease; a biofilm surface’s physical attributes affect bacterial adhesion only to a marginal level [32]. Studies performed to comprehend the mechanism of biofilm development in the 1970s were limited by the available research tools. The advent of advanced instrumentation and technologies has revealed that biofilms are adaptive structures employed by microorganisms as a defensive shield to create an advantageous environment that helps them retain nutrients and guarantee survival in an unfavorable environment [33]. Further, biofilms produced by different microorganisms have a great degree of similarity, but they also can have minor traits that are unique to a particular species [3,18,29,34]. Studies performed in the early 21st century describe how biofilm formation is governed by natural forces and physiological events [3]. The establishment and development of biofilm is characterized by five stages: (i) initial attachment (reversible and irreversible) of single bacteria, (ii) bacterial aggregation (iii) microcolony formation, (iv) maturation, and (v) dispersion/detachment [35,36,37,38,39]. Production of bacterial adhesins, which aid in adhesion to surfaces, is a hallmark of the first phase. The initial colonization of any surface is performed by free floating planktonic bacteria. These free floaters attach to the surface, multiply, become sessile, acquire various additional characteristics based on the environment, and attach covalently to the surface.
Attachment begins when the free-floating planktonic bacteria encounter any surface. They cling to the surface via bacterial appendages, such as flagella and or pili, or by physical forces [3,28,29]. The process is more likely a chance encounter and the attachment at this stage is transient and easily reversible [29,31]. The degree of adherence of the bacteria to the encountered surface is governed by a wide range of circumstances, including material composition, surface properties of the bacteria cell, temperature, and pressure [31]. The forces that control the degree of attachment may include hydrophobic, steric, electrostatic interactions.
