What Is a Bacterial Biofilm?

Bacterial biofilm (or Bacterial biofilm, BF) refers to a large number of bacterial aggregation membranes formed by bacteria that adhere to contact surfaces, secrete polysaccharide matrix, fibrin, lipoprotein, etc., and surround themselves. Thing. The polysaccharide matrix usually refers to a polysaccharide protein complex, and also includes organic and inorganic substances precipitated from the periphery. Bacterial biofilm is a life phenomenon in which bacteria adapt to the natural environment and are conducive to survival. It is formed by the accumulation of microorganisms and their secretions.

Bacterial biofilms are widely present on a variety of water-containing wet surfaces, such as food, food processing equipment, water pipes, industrial pipes, ventilation equipment, medical equipment, and even the surface of human tissues and organs under pathological conditions. A structured bacterial community composed of bacterial cells on a solid surface and a hydrated matrix that encapsulates the bacteria. Bacterial biofilm is a growth method adopted when bacteria adhere to the surface of life, and is generally composed of multiple bacteria species. According to the location of bacteria in BF, they can be divided into: free bacteria, surface bacteria and inner bacteria. Free bacteria are similar to surface bacteria. They are relatively easy to obtain nutrients and oxygen, and their metabolism is usually relatively active. The bacteria are large; while the inner bacteria are encapsulated in polysaccharides, their nutrients can only be obtained and metabolized through the surrounding space. The quality water course is carried out, the metabolic rate is low, and most of them are dormant. Generally, they do not divide frequently and the bacteria are small. John R. Lawrence and others applied laser confocal scanning microscope technology in studying the structure of biofilm for the first time and found that BF had a unique three-dimensional structure: bacteria accounted for less than one third of the biofilm, and the rest were viscous substances and cells secreted by bacteria. Exopolysaccharide. The water content in BF can be as high as 97%. In addition to water and bacteria, various biological macromolecules such as proteins, polysaccharides, DNA, RNA and phospholipids are also present.

Experts estimate that almost all bacteria can form biofilms under certain conditions. Salmonella, Escherichia coli, Listeria, Staphylococcus aureus, etc. are common pathogenic bacteria that can easily cause foodborne diseases. They are present in air, water, dust or human and animal excreta. Food processors, cooks, or salespersons carry bacteria and cause food contamination; foods are contaminated before processing, or are contaminated during processing, producing toxins, causing food poisoning; inadequate packaging of cooked food products, contamination during transportation; poultry and livestock Carrying bacteria before slaughter will also cause food contamination, and bacteria are liable to form biofilms on food, various food processing contact surfaces and non-food processing contact surfaces (such as walls, sewers, dead ends, etc.). Causes food poisoning. In addition to being able to corrode pipes and metal surfaces, the metabolic activities of bacteria in biofilms can also cause animal and plant diseases and human diseases. Therefore, controlling the contamination of various pathogenic bacteria in food, especially preventing the formation of biofilms and cross-contamination of food, can effectively control the occurrence of foodborne diseases ^[1]

It is generally believed that the process of biofilm formation is divided into 4 steps: deposition of conditioned membranes; initial arrival and adsorption of bacteria; growth and reproduction; biofilm formation. When sterile medical implants (mostly biomaterial polymers) are implanted in the body, the surface is immediately surrounded by various body fluids such as saliva, blood, urine, and mucus in the gastrointestinal tract, and various glycoproteins and mucopolysaccharides , Metal ions and other components will penetrate and adsorb to its surface within minutes, forming a condition film. The conditioned membrane covers the surface of the substrate like a net, allowing bacteria to "see" the membrane when it arrives, and further adsorb it. Positive metal ions such as sodium and magnesium are often used as bridges between negatively charged bacteria and the surface of negatively charged objects (most surfaces in nature are negatively charged). The precise mechanism by which bacteria reach and adsorb on the surface of biological materials is still under study. Several theories have been proposed, including the colloidal stability theory and thermodynamic theory of Derjag uin Landau Verwey and Overbeck (DLVO). The DVLO theory describes the mechanical changes of bacteria as they reach the surface of the object, and describes the effects of the ionic strength of the medium. Thermodynamic theory analyzes the change in free energy of bacteria in the liquid phase as they reach the surface of the solid phase matrix. Bacterial concentration, existence time, temperature, hydrodynamics, nutrient concentration, and physicochemical properties of surface materials have positive or negative effects. Additional structures such as flagella and pilus on the surface of bacteria also affect the formation of biofilms. After the bacteria reach and adsorb the surface, they begin to grow and multiply and spread further. Diffusion forms include three types of daughter cell migration, simultaneous migration of mother cells and daughter cells, and rolling reproduction, which vary with the type of bacteria and affect the structural form of the biofilm. It has been reported that as soon as the bacteria reach the surface of the object, the transcriptional activities of certain genes that synthesize EPS begin to strengthen. For example, Pseudomonas aeruginosa has enhanced transcription of algC, algD, and algU. Bacteria may have a sense of contact to sense the surface of an object, thereby initiating the expression of specific genes to synthesize EPS. When bacteria form a micro-community under the encapsulation of EPS and create a micro-environment that can resist antibacterial factors and host immune mechanisms, it indicates that a mature biofilm is formed. At this time, the biofilm can buffer the changes of the microenvironment, obtain nutrients through the gaps between the communities, and exclude waste. In addition, it allows multiple bacteria to form a community and form a synergistic environment. The product of one bacteria can be used as a matrix component of another.

At present, how bacteria accumulate on the surface of cells and coordinate their behavior to form a biofilm structure, that is, the mechanism of information transmission between bacteria is under further study. Studies have shown that there are quorum sensing systems among bacteria that can sense changes in the number of bacteria. Acylated homoserine lactones (AHLs) is one of the substances that transmit information between Gram-negative bacilli, and it plays a decisive role in the final formation of biofilms. In Staphylococcus epidermidis, polysaccharide intercellular adhesin (PIA) and accumulation-associated protein (AAP) are two possible information-transmitting substances ^[2-3]

Bacteria generally do not form biofilms in liquids, but when the liquid containing nutrients is contaminated by bacteria, the surface of the object through which the liquid flows (with or without biological activity) can form a biofilm. After the cells are deposited on a solid surface, special cell surface structures (small fibers and aggregates) firmly connect the cells to the solid surface. Therefore, the roughness of the surface of the adherent material is closely related to the formation of biofilms. The rougher the surface, the better the adhesion of bacteria, which can promote the formation of BF. On the other hand, the smoother the surface, the more difficult the adhesion is to prevent the formation of BF. In addition, the chemical composition, critical tension, surface energy, hydrophilicity / hydrophobicity, and surface charge of the surface of the attached material have a greater impact on bacterial adhesion. They determine the type of plasma protein adsorption, and these proteins are important in determining bacterial adhesion. factor. And there are many binding sites on the surface of the attachment material that can interact with bacterial cell molecules and elements. Bacteria usually adhere to locations where there is a tendency to concentrate nutrients and the surface has free energy that stimulates proliferation. Kristinsson et al. Grafted other hydrophilic materials with polyurethane, making the polyurethane surface hydrophilic and significantly reducing bacterial adhesion ^[1]

Due to the use of vaccines and antibiotics and the adoption of various social measures, most infectious diseases caused by free bacteria can be quickly controlled (except for multi-drug resistant strains), and infections caused by conditional pathogens are gradually increasing. , Especially in people with reduced resistance due to various reasons and the use of insertable medical devices are more common. These infections are often related to the formation of biofilms by bacteria. Pathogens include Gram-negative bacilli, Gram-positive cocci and Candida, Staphylococcus epidermidis, Pseudomonas aeruginosa, and Enterococcus. Once the biofilm is formed, it has a natural resistance to antibiotics and the body's immunity. It is difficult to completely remove it with antibiotics. It can only kill free bacteria on the surface of the biofilm or in the blood that cause the onset of infection. When the body's resistance decreases, the bacteria that survive in the biofilm can be released again, causing infection again. The biofilm is like a mushroom nest, which causes the recurrence of the infection, which can't be cured, and a chronic infection is formed. Device-related bloodst ream infection (DR-BSI) is extremely common in hospital infections, especially in ICU, and its harm is serious. It should be given more attention.

Biofilms in the human body can fight host immunity, and their sensitivity to antibiotics is much lower than that of free bacteria of the same species. The mechanism is multifaceted. First, the EPS that composes the biofilm can block the penetration of antibiotics or other bactericides, adsorb and inactivate some antibiotic hydrolases (such as -lactamase), and greatly reduce the effective dose of antibiotics entering the biofilm. At the same time, the biofilm Obstacles to antibiotics also provide time for bacteria to regulate themselves against antibiotics. Secondly, the bacteria in the depth of the biofilm are often in a state of starvation with insufficient nutrition, and grow slowly or even without growth. Slow growth is often one of the strategies for bacteria to resist adverse environments. This bacteria is less sensitive to antibiotics than normal growing bacteria because it can at least resist antibiotics that attack from metabolic pathways. The slow bacterial growth and high tolerance to antibiotics in the membrane have been demonstrated in vitro. The third reason may be that some of the bacteria in the biofilm are naturally protected "biofilm phenotypes". It is not a response to nutritional restriction, but a biological response to surface growth. It has also been reported that the chemical composition of biological materials is also an important factor affecting the effect of antibiotics on biofilms. The resistance of biofilms to antibiotics may also explain in part why antibiotics that are sensitive in in vitro drug experiments have no effect when treated in vivo. Therefore, some people have proposed the detection of biofilm elimination concentration (BEC), but the actual application has not yet been carried out ^[2]

As the formation of biofilms on the surface of insertable medical devices is almost inevitable, related infections have become a key factor limiting their widespread use. How to prevent the formation of biofilms has become an urgent issue. First of all, we must strictly intubate the operating procedures, strictly monitor the management room, shorten the length of hospitalization of patients as much as possible, in order to reduce the chance of patients being exposed to and infected with pathogenic bacteria and conditional pathogens, and cut off the source of bacteria formed by biofilm. Antibiotic or other chemical fungicides are commonly used to prevent the surface of medical materials such as catheters. However, coating with antibiotics has many drawbacks, it may induce the development of bacterial resistance, and the choice of broad-spectrum antibiotics is also limited. It is difficult to control the toxicity of chemical fungicides such as chlorhexidine (chlorhexidine), which may lead to serious consequences. There are many researches on the application of metallic silver. Studies have shown that silver-coated biomaterials can inhibit the formation of gram-positive cocci, gram-negative bacteria, and Candida albicans biofilms. Low concentration of silver also does not have any acute or chronic toxic and side effects on the human body, nor does it have mutagenic and carcinogenic effects. Its in vitro and clinical experiments are in progress, and have good application prospects. However, some people think that these methods cannot be successful, and corresponding biological factors should be used to block the bacterial cilia or pilus-mediated adhesion process and interfere with the information transmission system between bacteria, so as to prevent the formation of biofilms, which may be the most effective Research is still ongoing. For formed biofilms, physical methods are the most effective way to completely remove them. For infections caused by implantable medical devices, removing and replacing the device is the most thorough method. Ultrasound can remove biofilms through cavity formation and foaming. Antibiotics are usually difficult to remove biofilms, but it is also possible to find drugs with particularly strong penetrating power to remove certain types of biofilms. Biological control is another possible approach. Hughes et al. Isolated a clumping enterobacterial phage with a mucopolysaccharide degrading enzyme, which can specifically degrade EPS produced by agglomerating enterobacteria, thereby destroying the biofilm and dissolving the bacteria. Although this phage cannot be widely used because it cannot degrade EPS produced by other kinds of bacteria, it provides a method possibility. The interaction of various bacteria in complex populations also provides a way to control biofilms. If the number of Fusobacterium nucleus in dental plaque is reduced, the number of obligately anaerobic Porphyromonasginivalis and Prevotella nigrens will be significantly reduced, suggesting that ecological regulation can also be used as a delicate control method.

Biofilm formation and its related infections are the key to the success of insertable medical devices. Studies have shown that different types of bacteria use different "skills" to approach and adsorb the surface of an object, colonize it, grow and reproduce, and eventually form a biofilm. Mature biofilms are highly resistant to antibiotics and other chemical fungicides. Once formed, they are more difficult to remove. There are many factors that affect the formation of biofilms. The effects of flagella, pili, quantity sensing systems, and environmental factors have been confirmed, and corresponding control measures are also being studied. The application of molecular biology technology makes the research deeper. A thorough understanding of the structure and formation process of biofilms will definitely be beneficial to the study of control methods and finally find a method to completely conquer biofilm-related infections ^[2] .