The clustered group of microorganisms in a common matrix is called biofilm. The matrix is made up of extracellular polymeric substances (EPS). In 1 gram of weight of biofilm, about 108 – 1011 cells are present and may comprise many species. The same bacteria may show different properties in biofilm than in the free-living (planktonic) state.
Biofilms form especially on foreign bodies such as prosthetic joints, prosthetic heart valves, and intravenous catheters, but they also form on native structures such as heart valves. Biofilm formation on implant surfaces shelters the bacteria and encourages the persistence of infection. Biofilms play an important role in the pathogenesis of cystic fibrosis, dental plaque formation, and wound healing.
Formation of the Biofilm
The formation of the matrix depends on nutrient availability, synthesis, and secretion of the extracellular material. Biofilms are embedded in the matrix. This matrix is self-produced from an extracellular polymeric matrix (EPS). Its EPS is also regarded as slime. It is found where there is water, in kitchens, contact lenses, and in the gut linings of animals.
The first attachment of the colonies to the surface occurs by the weak and reversible Vanderwaals force. If the colonies don’t get detached immediately, the adhesion of cells in structures such as pili helps to anchor them. The indwelling of biofilm can occur in medical devices. Production of biofilms by bacteria such as Pseudomonas is controlled by the process of quorum-sensing.
Stages of Biofilm Formation
Its formation may follow the following steps:
- Reversible attachment of planktonic cells to surfaces
Firstly, the attachment of bacterial cells to the surface occurs by random collision or environmental signaling (first messenger). The force here is weak and reversible.
- Irreversible attachment of the same cells
Exopolymeric material, a more potent adhesive compound, is formed that helps in affixing the cells more securely.
- Cell growth and early development
- Further development and maturation
The biofilm further develops to form into a 3D structure containing tightly packed cells in clusters. Thus forming a nearly impenetrable mature biofilm. These cells have channels running between them.
- Dispersion into the liquid medium
After the complete development of biofilm, some of the bacteria may release into the liquid medium. It helps in the spreading.
Factors Affecting the Attachment
There are many factors influencing the initial bacterial cell attachment. They are:
- Nature of the surface
It can attach to surfaces like dead, living tissue, or inert surfaces. Hydrophobic and rougher surfaces help in the formation of biofilm.
- Properties of medium and microbial cells in the biofilm
The characteristic features of medium, like pH, nutrient levels, ionic strength, and temperature play a vital role in microbial attachment. Properties of the cell surface of microorganisms, like the production of extracellular polymeric substances (EPS), cell surface hydrophobicity, and the presence of fimbriae and flagella, influence cell attachment greatly.
- Signals involved in biofilm formation
Biofilm formation mainly depends on the interaction between the environmental signals and signals produced by the microorganisms. Many signal-producing systems can convert environment signals into signaling pathways. The responses from two-component systems (TCS), extracytoplasmic function (ECF) signaling pathway, and quorum sensing (QS) events are induced by the systems. The c-di-GMP (cyclic guanosine monophosphate), a secondary messenger, can also trigger its formation.
Advantages of Biofilm
Biofilm is a protective matrix formed by bacteria after their attachment. Biofilms protect bacteria from antibiotics and host immune defenses such as antibodies and neutrophils. The formation of biofilm is beneficial to bacteria because of the following reasons:
- Protects the bacteria from the phagocytosis
It protects the bacteria from phagocytosis. Phagocytes can easily engulf the bacterium, so it needs to release many pro-inflammatory enzymes and cytokines. It leads to inflammation and the destruction of the nearby tissues.
When the bacterial cell attaches to the surface, it triggers the expression of the biofilm-specific gene. There is an increase in slime production due to the expression of genes that produces the cell surface polysaccharide. Then it facilitates the attachment of the more cells in it.
- Protects the bacteria from the desiccation
The free-living bacteria may undergo water stress, but biofilm can tolerate the desiccation by producing the EPS molecules. It protects the biofilm from desiccation by acting as a hydrogel that retains water.
- Helps in the gene transfer
Biofilms are also responsible for gene transfer. These gene transfers between the organisms may result in the formation of highly pathogenic microorganisms.
- Helps in capturing the nutrients
The matrix present in the biofilm helps to capture the nutrients. These nutrients are present in the Biofilm’s water phase or can be associated with the substratum in which the biofilm grows.
Disadvantages of Biofilm
Biofilm formation has been challenging to the medical sciences as it interferes with ongoing medications and is associated with different diseases. Some of the challenges due to biofilm are:
- Biofilm in the industry
Biofilms have also caused several problems in the industry. It can cause fouling of the equipment, contaminate products and damage the water distribution system. It can cause contamination of the fuel and souring of the chemicals due to the excretion of H2S by the biofilm bacteria.
- Biofilms increase antibiotic resistance
The dense extracellular matrix and the cell’s outer layer protect the entire biofilm community and increase the resistance to detergents and antibiotics. It interferes with antibiotic therapy.
It is found that the bacteria in biofilm are 1000 times more resistant than the single bacteria. The antibiotic therapy may not work, so the only treatment is the removal of the contaminated implant.
- Biofilms are associated with nosocomial infections
Biofilms are present in medical devices, shower curtains, rocks in rivers, streams, and noses. The CDC has estimated that biofilms cause the 65% of nosocomial infections. The medical devices implanted in the body can be affected by biofilm formation. These implanted devices can be catheters, artificial joints, and mechanical heart valves. When colonization of microorganisms occurs, slow but persistent infection can occur.
Microbial biofilms in the hospital environment generate high healthcare costs and prolonged patient stay, resulting in further secondary microbial infections and various health complications.
Biofilm Associated with the Human Infections
Biofilm in Artificial Prosthetics and Devices
- Candida albicans
- Coagulase-negative staphylococci
- Enterococci spp.
- Klebsiella pneumoniae
- Pseudomonas aeruginosa
- Staphylococcus aureus
- Streptococci spp.
Biofilms in the cystic fibrosis
In this condition, patients have Pseudomonas infections. Cystic fibrosis is characterized by the biofilm, which contains the Pseudomonas aeruginosa and other bacteria in the lungs of cystic fibrosis patients. Antibiotics cannot penetrate easily into it, due to which the bacteria within the matrix are not affected by the drugs.
Biofilms in the endocarditis
Biofilms may be associated with medical implants such as the prosthetic valves and in the surrounding tissues of the heart. It may then cause prosthetic valve endocarditis. The responsible microorganisms are:
- Staphylococcus aureus
- Staphylococcus epidermidis
- Streptococcus spp.
- Gram-negative bacilli
- Candida spp.
Biofilm in the Urinary Tract Infections
Biofilms can form on the inner and outer surfaces of the catheters.
When catheters are placed for a long time, there is a high chance of biofilm formation, which may result in urinary tract infections. The associated organism are:
- Streptococcus epidermidis
- Enterococcus faecalis
- Escherichia coli
- Proteus mirabilis
- Pseudomonas aeruginosa
- Klebsiella pneumoiae etc
Biofilm in Food-Borne Contamination
Listeria monocytogenes are mainly responsible for causing food-borne contamination in human beings.
Biofilm as the dental plague
It is the yellowish biofilms that are formed in the teeth. If it is not removed regularly, it can cause dental caries. It begins with the initial colonization of the pellicle and ends with the mature form of the complex biofilms.
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