The range of environmental factors altering biofilm formation appears to be indicative of the highly diverse habitats in which staphylococci are able to form biofilms. For example, the presence of oleic acid induces
biofilm formation. This probably results from an ionic interaction of the positively charged PIA with the negatively charged oleic acid. The effect is even more pronounced under oxygen-limited conditions
, a fact consistent with the observation that anaerobiosis is an important stimulus for
. A mature biofilm reveals an architecture that ensures the provision of nutrients and oxygen to all cells in the biofilm
. As they grow, bacteria begin to arrange in a three-dimensional structure composed of an array of pillars and mushroom-shaped structures. These structures are connected by convoluted channels that deliver nutrients and contribute to the elimination of waste. The maturation of biofilms has been studied by imaging and transcription profiling studies
. A primary discovery that emerged from microarray experiments is that persistence within a mature biofilm requires an adaptive response that limits the deleterious effects of pH reduction associated with anaerobic metabolism
. The cell envelope is a very active compartment as the expression of genes that encode binding proteins, proteins involved in the synthesis of murein and glucosaminoglycan, PIA, and other enzymes involved in the cell-envelope metabolism appears to be significantly upregulated. Thus, a biofilm is a dynamic structure that evolves with environmental conditions, such as physical shear forces, and as a result of the processes that are sensed and regulated by the bacteria. Once cell clusters reach a sufficient size, groups of cells either detach (dispersal phase) or die. Thus, it is the cycle of cell growth, detachment, and regrowth that underlies the observed patterns of organized gene expression
In 1987, Gordon Christensen published a paper on the phenotypic variation of
S. epidermidis slime production in vitro and in vivo
[65]. Today, the “slime” they described was the exopolysaccharide PIA (polysaccharide intercellular adhesin), whose chemical structure was first described in
S. epidermidis in 1996
[66]. Later, PIA was also referred to as ß-(1,6)-N-acetylglucosamine (PNAG)
[67]. The more chemical-sounding name PNAG is not really a correct description of the glucosamine polymer as it ignores the fact that N-deacetylation takes place at certain intervals, which is essential for biofilm formation. PIA represents a linear homoglycan of at least 130 beta-1,6-linked 2-deoxy-2-amino-D-glucopyranosyl residues which are from 80 to 85% N-acetylated. The rest are non-N-acetylated and positively charged. Since a correct chemical description was cumbersome, the name PIA was chosen in the initial description of the structure
[66]. PIA is a polymer of partially de-
N-acetylated ß-1,6-linked
N-acetylglucosamine (dPNAG).
4. Roles of Biofilm in the Tolerance to Multiple Drugs
In a biofilm, the bacterial cells are attached to a surface where, depending on the nutrient content of the environment, they multiply more or less actively and form a multilayered structure. The maturation to a three-dimensional biofilm is also called the accumulation phase. Such biofilms are formed in humid or marine environments in water pipes, on ship hulls, and other on stainless steel surfaces where they cause biofouling
[68], which causes enormous costs
[69][70]. Typically, such a biofilm consists of a heterogeneous spectrum of micro- and macro-organisms whose cells are embedded in a self-produced matrix and whose metabolic products lead to the corrosion of the metal
[71]. In particular, the production of extracellular polymeric substances (EPSs) by microorganisms facilitates adhesion to material surfaces such as metals. These complex biofilm structures are highly resistant to extreme stress conditions, and only aggressive bactericidal detergents or harsh physical treatments such as sonication exhibit antifouling properties
[72].
There are similarities and differences between biofouling and biofilm-associated infections. They have in common that microorganisms primarily bind to surfaces and change these surfaces by their binding so that further microorganisms can bind and thus form a robust biofilm, whereby EPSs make an important contribution to the compactness of the biofilm. While biofouling is a mixture of various microorganisms, biofilm-associated infection is usually due to a single bacterial species. The National Institutes of Health (NIH) evaluated that biofilm-producing bacteria are involved in 65% of all microbial infections and are responsible for 80% of chronic infections. The annual incidence of biofilm-related infections in the United States represents roughly 2 million cases, causing 268,000 estimated deaths, and is accompanied by USD 18 billion in direct costs for the therapy of these infections
[2][73]. The bacterial species frequently involved in such infections are
S. epidermidis,
S. aureus,
Enterococcus,
Bacillus, and
Candida spp. The origin of these microorganisms may be from the skin or from other indwelling devices such as central venous catheters or dental work
[74].
With biofilm-associated infection, the largest problem is that many therapeutic approaches fail because a high proportion of the bacterial cells in a biofilm matrix are “phenotypically” insensitive to most antibiotics. Researchers deliberately speak here not of resistance, since the latter implies certain resistance genes in the classical sense. In 1994, after penicillin was marketed, it was observed that staphylococci can enter a physiological state called persistence (or multidrug tolerance) in which lethal antibiotics failed to kill them
[75]. Multiple factors appear to contribute to the global insensitivity of biofilm bacteria
[13][76]:
-
Enhanced antimicrobial resistance is a general phenomenon of biofilms and is the result of numerous specific factors which depend on the species involved, the environment of the biofilm, and the antimicrobial agent used;
-
The implant material on which a biofilm is formed is not or is only scarcely perfused, preventing antibiotic diffusion at a sufficiently high concentration;
-
The penetration and diffusion of antibiotics into a thick biofilm is hampered;
-
The growth rate of bacterial cells in a biofilm is reduced (most antibiotics are efficient against actively growing bacteria);
-
The physiology of cells in a biofilm differs from that of planktonic cells.
The phenomenon of the general antibiotic insensitivity of bacterial cells in a biofilm is characterized by the fact that biofilm-associated cells are insensitive, whereas “the same” cells in suspension are sensitive
[77]. This suggests that insensitivity is not related to classical antibiotic resistance gene but to an altered physiological state in the biofilm mode of growth. Kim Lewis called the small fraction of essentially invulnerable cells in a biofilm “persisters” that exhibit multidrug tolerance (MDT)
[78]. In
Escherichia coli, the toxin–antitoxin (TA) modules RelE-RelB and HipB-HipA (high-persistence) seam to play a role in the persister phenotype. The overproduction of RelE or HipA causes an increase in the persister population. HipA inhibits translation by the phosphorylation of EF-Tu
[79], stimulates the RelA-dependent synthesis of (p)ppGpp
[80], and phosphorylates glutamyl-tRNA synthetase (GltX), which becomes inactivated by phosphorylation by HipA
[81]. RelE cleaves mRNA at the ribosomal A site with high codon specificity
[82]. The overexpression of RelE or HipA leads to a slowdown translation and thus the growth of
E. coli, which presumably protects the cells from lethal factors such as antibiotics. It is known from ß-lactam antibiotics that they act mainly on dividing cells and are less effective on non-growing cells.
In staphylococci, the generation of persister cells is less clear than in
E. coli. There are four different families of TA systems described, but their physiological roles are elusive
[83]. The chromosomal
mazEF system encodes the RNase toxin MazF and the antitoxin MazE
[84]. MazF specifically targets UACAU sequences of
spa (staphylococcal protein A) and
rsbW (anti-sigmaB factor) in
S. aureus mRNA in vivo, whereas translational reporter fusions indicated that the protein levels of the encoded products were unaffected. Despite a comparable growth rate to the wild-type, an
S. aureus mazEF deletion mutant was more susceptible to β-lactam antibiotics, suggesting that the genes involved in antibiotic stress response or cell wall metabolism are controlled by this TA system
[84].
Long before
E. coli, a connection between reduced growth and increased antibiotic tolerance was described in staphylococci in the form of “small colony variants” (SCVs)
[85]. From patients with persistent and relapsing infections,
S. aureus SCVs were isolated which were auxotrophs for menadione, hemin, and/or a CO
2 supplementation. All these SCVs were resistant to aminoglycosides. The phenotype of such respiratory deficient mutants was further analyzed in a stable
hemB mutant of
S. aureus [86]. Such a
hemB mutant showed the typical SCV phenotype, such as slow growth and a resistance to aminoglycosides; it also showed decreased pigmentation, low coagulase activity, reduced hemolytic activity, and a high persistence in endothelial cells. Respiratory mutants, both those that are naturally occurring or genetically constructed, demonstrate the importance of the metabolism in virulence and drug tolerance
[87]. In
S. aureus, there are many global regulators that impact virulence factor expression in SCVs
[88].
5. Staphylococcal Biofilm in the Clinical Situation
At the end of the 1990s in the United States, experts estimated that biofilms were associated with 65% of nosocomial infections and that the annual cost of treatment of these biofilm-associated infections was higher than USD 1 billion
[2][89].
S. aureus and other staphylococci are frequently found on implanted materials such as catheters, hip prosthesis, or surgical materials
[5][90][91][92]. A recent study identified methicillin-resistant coagulase negative staphylococci as a major cause of biofilm-associated infections and possibly responsible for critical clinical situations. This interesting study relied on the analysis of numerous samples originating from hospital environments and from various hospital wards. The authors identified different staphylococcal species that produce bacterial biofilms:
Staphylococcus haemolyticus,
S. epidermidis,
S. hominis, and
S. warneri. The authors isolated approximately 300 MR-CoNS among the 558 samples from community and hospital environments.
S. haemolyticus and
S. epidermidis were the predominant species, representing roughly 73% of the CoNS identified. Significant biofilm production was detected in 91% of isolates, suggesting that the absence of production is marginal in clinical and environmental CoNS
[93]. The staphylococci isolates that were derived from hospital wards were more associated with biofilm production than the community-derived isolates. Distinguished from the isolates identified in hospital wards, environmental strains were devoid of
icaAD and
bap genes and thus produced mainly proteinaceous biofilms.
Recent studies documented biofilms as community phenomena by assessing the interaction between bacteria and surface-associated-biofilm-producing organisms. Toledo-Silva reported nicely that numerous non-
aureus species of staphylococci were able to interact with biofilm-producing
S. aureus. The authors isolated
S. chromogenes,
S. epidermidis, and
S. simulans from bovine milk samples and showed that
S. chromogenes (devoid of
ica) stimulates the biofilm formation of
S. aureus and alters the dispersion of
S. aureus-formed biofilm. The study highlighted possible interactions between CoNS and
S. aureus in the biofilm communities, most likely through interactions between the respective
agr quorum systems
[94]. Further research is needed to study bacterial biofilms as community phenomena.