Staphylococcus aureus (S. aureus), which is one of the most common causes of indwelling device–associated, nosocomial, and community-acquired infections, can produce biofilm as a virulence factor. Methicillin-resistant S. aureus (MRSA) that is resistant to β-lactam antibiotics causes life-threatening infections. Biofilm producer strains of S. aureus that causes indwelling device–associated infections resist to antimicrobials and immune system. The combination of methicillin resistance and the ability of biofilm formation of S. aureus makes treatment difficult. Methicillin resistance of S. aureus can affect biofilm phenotype of S. aureus; the mecA gene of MRSA increases biofilm production by inactivating accessory gene regulator (agr) quorum sensing regulator system, which is a two-component regulator system of virulence factor production. The aim of this review is to determine virulence factors of S. aureus, resistance mechanisms of methicillin, and the influence of methicillin resistance on biofilm phenotype of S. aureus.
- Staphylococcus aureus
- methicillin resistance
- influence of methicillin resistance on biofilm
The biofilm has an important role in the pathogenesis of certain bacterial infections such as staphylococcal indwelling device–associated infections, wound infections, chronic urinary tract infections (UTI), cystic fibrosis pneumonia, chronic otitis media (OM), chronic rhinosinusitis, periodontitis, and recurrent tonsillitis .
The biofilm infections such as
Staphylococcus aureus and virulence
2.1. Staphylococcus aureus
While antibiotics such as methicillin are used frequently in patients, antibiotic-resistant strains may develop. After penicillin usage had become widespread to treat infections, penicillin-resistant
MRSA has become epidemic not only in nosocomial infections but also in community-associated infections . MRSA that has been a common cause of nosocomial infections worldwide also has been arising in the community in recent years . Invasive infections of MRSA have high morbidity and mortality rates . Most of invasive staphylococcal and community-acquired MRSA (CA-MRSA) infections are related to the nasal carriage of
2.2. Biofilm and pathogenesis
Biofilm plays a role in the pathogenesis of staphylococcal infections. When microorganisms exposed to stress conditions, gene expression of biofilm is induced as a stress response. The biofilm that is a slime-like glycocalyx causes bacteria to survive in the stress conditions, causes bacterial attachment and colonization on biotic or abiotic surfaces such as prosthetic surfaces that may act as a substrate for microbial adhesion, and causes bacterial spread to whole body [12–14]. The biofilm producer
2.3. Virulence of
S. aureus Biofilm
Biofilm that is a slime-like glycocalyx embedded sessile community of microorganism inside. Polysaccharide matrix, staphylococcal surface proteins, extracellular DNA (eDNA), and teichoic acids construct biofilm of
3. Mechanisms of biofilm formation and regulation by MRSA and MSSA
Biofilm is produced by distinct mechanisms in MRSA and Methicillin-sensitive Staphylococcus aureus (MSSA). Fitzpatrick et al. revealed that biofilm formation of the
Biofilm is constructed not only by polysaccharide intracellular adhesin (PIA) but also by surface proteins. In the catheter infection, biofilm formation of clinical isolates of
Three stages of
Not only biofilm formation but also virulence factors such as phenol-soluble modulins (PSMs), toxins, and degradation enzymes production are regulated by agr quorum sensing two-component regulatory system [14, 19, 20]. Activation of
Accessory gene regulator (
Supplementations of certain chemicals to growth media affect biofilm formation of
Staphylococcus aureus genome
Each strain of
Prophages have an effective role in pathogenicity of
4.2. Pathogenicity islands (PIs)
The gene of superantigen toxins (SaPIs), which is one of the secreted virulence factors of
The most known PI of
4.3. Insertion sequence (IS) and transposons (Tn)
Insertion sequences (ISs) contain inverted repeats at their terminals and the integrases gene that causes transposition. Transposons (Tn) not only contain the transposase gene but also may contain ISs that induce movement of Tn and certain genes such as antibiotic resistance genes . These elements provide a mechanism to transfer of virulence and resistance genes such as antibiotic resistance genes from place to place within the same cell or to other cell. These movable elements are excised from paired inverted repeats by transposase enzyme. While these elements are excised and inserted to new location such as within a gene that may be located within the same cell or other cell, the gene is disrupted .
Plasmids that are extrachromosomal genetic elements carry resistance genes causing antibiotic or heavy metal resistance, and virulence genes encoding for virulence factors, rather than genes involved in metabolic processes having vital functions . There are three types of plasmids of
MGEs contain the
Methicillin-resistant strains of
Methicillin resistance is not only seen in isolates of
5. The relationship between methicillin resistance and biofilm formation
The association between methicillin resistance and biofilm phenotype is taken attention according to studies executed [39–41]. Researchers determined that biofilm formation of HA-MRSA BH1CC strain is decreased by removing
Biofilm formation of MRSA is enhanced by both phenol-soluble modulin mec (PSMmec) encoded by
Like many virulence toxins of
Biofilm formation is increased by the repression of Agr system that downregulates
Biofilm formation (adherence to surfaces and intercellular aggregations) of MSSA and MRSA strains is contributed by PIA in
Agr system is repressed by expression of PBP2a that is encoded by
6. β-Lactam, methicillin, and multidrug resistance
6.1. Peptidoglycan biosynthesis of
Peptidoglycan, surface proteins such as protein A, clumping factor A, fibronectin-binding protein (FnBP), collagen-binding protein, and teichoic acids construct the cell wall of
At the beginning of peptidoglycan synthesis, UDP-
Then, transglycosylation and transpeptidation reactions are catalyzed by penicillin-binding proteins (PBPs) of which 4 types (PBP1, PBP2, PBP3, PBP4) are present in
Teichoic acids that are polymers of glycerol phosphate or ribitol residues give negative feature to cell membrane and act as receptor of
6.2. Effect of β-lactam antibiotics against cell wall
Binding of β-lactams to PBPs that have high affinity to β-lactams is lethal for
6.3. Mechanism of β-lactam resistance of
β-lactamase enzymes cause resistance of cell to β-lactam antibiotics by inactivating β-lactam antibiotics. β-lactamase inactivates β-lactam antibiotics by disrupting amide bond of β-lactam ring .
Expression of the
A study that showed the association between the antibiotic susceptibility patterns and the antibiotic resistance genes in staphylococcal isolates obtained from various clinical samples of patients revealed that 93.5% of
6.4. Mechanism of methicillin resistance of
Resistance to methicillin, oxacillin, and nafcillin that are semisynthetic β-lactamase-insensitive β-lactams has developed by acquiring of the
Structure, function, mechanism, and molecular organization of
6.5. Multidrug resistance
There are eight types of
6.6. Homogeneous and heterogeneous resistance of MRSA
Heterogeneity is a characteristic of MRSA of which resistance level varies according to contents and ingredients of culture medium in which MRSA is grown and β-lactam antibiotic used. Most of the cells of heterogeneous methicillin resistance (HeR) strains (∼99.9% or above) are susceptible to β-lactam of which concentration is low that is about 1–5 μg/mL of methicillin, whereas just a few subpopulations (such as 1 in 106 cfu/mL) grow in 50 μg/mL or above of methicillin by expressing high-level resistance. Homogeneous strains (HoR) are resistant to low concentration of β-lactam and can grow in higher concentrations of methicillin that is about 5 μg/mL or above .
Heterogeneity of MRSA is unstable and changeable according to growth conditions. HeR strains become homogeneous strains (HoR) by growth media supplemented with NaCI or sucrose for providing hypertonicity of media, or supplemented with higher concentrations of β-lactam antibiotic, or incubated at 30°C in incubator. Supplementation of growth media with EDTA or incubation at 37–43°C leads to conversion of HoR strains to HeR . This conversion of HeR and HoR in distinct culture conditions is due to the regulation of gene expression by Agr regulator system . These conversions of MRSA can be repeated by repeated culturing in changed media that have different supplementations.
Most clinical isolates of MRSA grow as HeR in routine growth conditions, and most of them show low or moderate level of resistance, whereas a few subpopulations show high-level resistance .
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