Staphylococcus epidermidis, member of the group of coagulase-negative staphylococci, belongs to an opportunistic pathogen. It is reported that the major pathogenicity of S. epidermidis is attributed to its biofilm formed on the surface of infected tissues, which enhances bacterial resistance to antibiotics. Thus, how to inhibit biofilm formation and screening biofilm inhibitors will have great value in reducing bacterial drug-resistance, which is beneficial to prevent and treat biofilm-associated infections. In this chapter, we present the current knowledge on formation, antibiotic resistance, and control strategies of S. epidermidis biofilm. First, biofilm formation in S. epidermidis, including factors involved in different phases in the process of biofilm, is analyzed. Second, the mechanisms of antibiotic resistance in S. epidermidis biofilms, such as poor antibiotic penetration, slow growth, and formation of persister cells, are introduced. Finally, control strategies to S. epidermidis biofilm formation are provided.
- Staphylococcus epidermidis
- antibiotic resistance
- biofilm inhibition
There is an increasing amount of biofilm research aimed at exploring how bacteria control their biofilm formation and to discover nontoxic compounds that can attenuate biofilm formation without allowing bacteria to develop drug resistance . Special plants and Actinomycetes are both rich sources of bioactive substances, notably antibiotics, enzymes, enzyme inhibitors, and pharmacologically active agents [4, 5]. Moreover, some Actinomycete species were reported to produce inhibitors against biofilm formation by
With this background, we aim to present the current knowledge on biofilm formation of
2. Biofilm formation in
2.1 Factors involved in primary attachment in
S. epidermidisbiofilm formation
Nonspecific adhesions between bacterial cells, which are mainly attributed to the composition of compounds on the surface of bacterial cells and their hydrophobicities, play an important role in biofilm formation. Additionally, autolysin (AtlE) and teichoic acids have influences on biofilm formation [11, 12]. It is reported that lots of autolysin enhanced the cell surface hydrophobicity and increased the biofilm formation. Also, teichoic acids correlated with increased cell surface hydrophobicity, so they contributed to biofilm formation [11, 12].
In vivo primary attachment occurs to host tissue or host matrix proteins.
The SD repeat family protein Sdr G in
Some of surface proteins on bacterial cell wall are adherent to host cells via noncovalent interaction, such as hydrophobic bonds and Van der Waals’ force, which of process are involved into the polymers on bacterial cell surface, e.g., teichoic acids. Teichoic acids are main components consisting of the cell wall of Gram-positive. They bind to peptidoglycan of cell wall and influence the activity of autolysin (AtlE). AtlE, encoded by the atlE gene, is a bifunctional autolysin: one is able to mediate bacterial adhesion, and the other is to promote bacterial cell autolysis, which releases DNA out of cells, named extracellular DNA (eDNA) .
2.2 Factors responsible for cellular aggregation in
S. epidermidisbiofilm formation
Following the primary attachment of cells to a surface, bacterial cells occur to accumulate with the help of a variety of associated-accumulation factors, such as polysaccharide intercellular adhesin (PIA), accumulation-associated protein (Aap), and so on.
In the process of biofilm formation by
2.3 Biofilm formation and maturation
Cellular aggregation constantly occurs and subsequently forms biofilm. Disruptive molecules create channels in the biofilm, which are essential for nutrient accessibility in deeper biofilm layers and give the biofilm its characteristic structure, often described as mushroom-like shapes . The characteristic structure of mature biofilms with mushroom-like shapes and channels is dependent on the production of phenolsoluble modulins (PSMs) in
Of primary importance for dissemination of biofilm-associated infection, cells or cell aggregates may detach from a mature biofilm to reach the next infection sites. This may occur by mechanical forces under flow, such as present in a blood vessel, in a process often called sloughing . Additionally, the bacteria can trigger detachment by PSM production. These surfactant-like molecules work by decreasing noncovalent adhesion between bacterial cells.
3. Mechanisms of antibiotic resistance in
Several in vitro studies have demonstrated that bacteria within biofilms are more resistant against antibiotic treatment as compared to planktonic cultures of the same strains .
3.1 Antibiotic penetration of biofilms
Biofilms are typically characterized by dense, highly hydrated clusters of bacterial cells enclosed in a self-produced polymeric matrix that is primarily composed of exopolysaccharides such as polysaccharide intercellular adhesin (PIA) in staphylococci and adherent to a surface. This matrix, also termed slime or extracellular polymeric substance (EPS), impairs the access of antimicrobial agents to the bacterial cells . Additionally, either a reaction of EPS with or its adsorption to the components of the biofilm matrix can delay penetration of the antibiotics through the biofilm matrix. The effective diffusion coefficients of solutes in biofilms average about 40% of the respective diffusion coefficient in pure water .
3.2 Slow cell growth in biofilms
Slow cell growth of the bacterial has been found in mature biofilms . This phenomenon is responsible for the decreased susceptibility of bacteria in biofilms to antibiotics requiring growing organisms for their bactericidal effects. For example, penicillins and cephalosporins prefer to killing the growing bacterial cells, and the rate of killing cells is proportional to the growth rate . It is well known that most antimicrobial agents act on certain types of macromolecular synthesis to exert antimicrobial activities, such as the synthesis of enzymes, proteins, and nucleic acids (DNA or RNA). Thus, these antibiotics have little effects on bacteria with stagnant macromolecular synthesis, which leads to bacterial drug resistance.
Nutrition restriction is one of reasons that are responsible for slow cell growth. The mechanism of nutrition restriction is closely related to the osmotic restriction. Due to the existence of biofilm osmotic restriction, nutrients are not easy to pass through biofilm, which leads to the lack of nutrition in biofilm and slows down the growth rate of inner layer bacteria. This slow growth state of inner layer bacteria also forms a protective mechanism, which reduces the susceptibility of bacteria to antibiotics .
When the biofilm cells are exposed to antibiotics, the bacteria on the surface of the biofilm are killed by the drug, and the cells in the middle and deep layers of the biofilm are not affected. After the antibiotic treatment stops, the remaining bacteria will use dead bacteria as nutrients to reproduce rapidly, which can only take a few hours to reproduce [25, 26].
3.3 Formation of persister cells
Delayed penetration of the antibiotics through the biofilm matrix and slow rate of bacterial reproduction in biofilm cannot explain entirely the resistance of biofilms to one important class of antibiotics, namely the fluoroquinolones. This class of antimicrobial agents equilibrates across bacterial biofilms and exerts bactericidal effect on nondividing cells . Although a dose-dependent bactericidal action was observed in
Persister cells in biofilms are considered to the key in the extraordinary survival properties of biofilms. The dynamic features of biofilm formation and shedding of cells from one biofilm to form a new biofilm may also explain the chronic nature of biofilm infections and the need for extending antimicrobial agent treatment to disturb the dynamics of biofilm formation .
4. Control strategies to
S. epidermidisbiofilm formation
Because the expression of toxins and other virulence factors is less in
4.1 Inhibition of initial attachment
The first step of biofilm formation is bacterial adherence to the host cell surface. Direct binding to host cell surface is mediated by electrostatic and hydrophobic interactions and van der Waals forces and affected by physicochemical variables .
Found in our research, after investigating the antibiofilm activities of spent media from 185 Actinomycete strains using two
Moreover, apart from physico-chemical determinants, it was demonstrated that the major autolysine AtlE is involved in attachment to polystyrene surfaces. Therefore, AtlE may be indirectly involved in cell adhesion via releasing DNA. Treatment of
4.2 Inhibition of bacterial accumulate
After adherence to the host cell surface, biofilms develop through intercellular aggregation. The major factor involved in intercellular adhesion is polysaccharide intercellular adhesin (PIA). The de-acetylation of PIA is not only essential for biofilm formation but also crucial for
PIA biosynthesis depends on the expression of the icaADBC operon, which is controlled by a complex regulatory network. Gomes et al. studied the effect of rifampicin+gentamicin and rifampicin+clindamycin combinations on the expression of icaA and rsbU genes, responsible for poly-N-acetylglucosamine/polysaccharide intercellular adhesin (PNAG/PIA) production. The results demonstrated that this combinatorial therapy can cause a lower genetic expression of the two specific genes tested and consequently can reduce biofilm formation recidivism [36, 37].
Biofilm formation is a result of bacterial interactions and group behavior. Quorum sensing (QS) is one of the regulatory mechanisms suggested to be involved in coordinating biofilm formation. The QS system is a cell-to-cell communication system used by many bacteria to assess the cell density. Quorum sensing inhibitors (QSI) could be a novel way to fight biofilm-associated infections. The study has identified furanones and thiophenones as inhibitors of quorum sensing and biofilm formation. In this study, the effect of both the furanone and the thiophenone could be abolished by the synthetic Autoinducer-2 (AI-2) molecule (S)-4,5-dihydroxy-2,3-pentanedione (DPD), indicating that furanone and thiophenone affect biofilm formation through interference with bacterial communication .
4.3 Promotion of biofilm detachment
For the biofilm that has been formed on the surface of the host, if the biofilm can be separated by antibacterial oranti-biofilm substances, the bacteria in the biofilm can be released, and the planktonic bacteria are more easily to be killed if the biofilm is exposed to antibiotics. Biofilms are composed primarily of microbial cells and extracellular polymeric substance (EPS). EPS may account for 50–90% of the total organic carbon of biofilms and can be considered the primary matrix material of the biofilm. The components of EPS include polysaccharides, nucleic acids, lipids, and proteins .
We initially determined the dependent type of biofilm formation by
Since extracellular polysaccharides are the main compounds in biofilm matrices, namely in
Our results showed that EPS in
This work was supported by the Program for New Century Excellent Talents in University (grant number NCET-11-1071) of China, a NSFC (National Science Foundation of China) Grant 31260026, and a Fund for PhD in Xinjiang Production and Construction Corps (grant number 2009JC07) to W. Chen, a NSFC-Xinjiang joint Grant U1703236 to L. L. Zhang, and a Microbial Resources Utilization Innovation Team in Key Field of Xinjiang Production and Construction Crops (grant number 2017CB014) to C. X. Wan.
Conflict of interest
No conflict of interest declared.