Hacking into bacterial biofilms: a new therapeutic challenge
© Bordi and de Bentzmann; licensee Springer. 2011
Received: 23 March 2011
Accepted: 13 June 2011
Published: 13 June 2011
Microbiologists have extensively worked during the past decade on a particular phase of the bacterial cell cycle known as biofilm, in which single-celled individuals gather together to form a sedentary but dynamic community within a complex structure, displaying spatial and functional heterogeneity. In response to the perception of environmental signals by sensing systems, appropriate responses are triggered, leading to biofilm formation. This process involves various molecular systems that enable bacteria to identify appropriate surfaces on which to anchor themselves, to stick to those surfaces and to each other, to construct multicellular communities several hundreds of micrometers thick, and to detach from the community. The biofilm microbial community is a unique, highly competitive, and crowded environment facilitating microevolutionary processes and horizontal gene transfer between distantly related microorganisms. It is governed by social rules, based on the production and use of "public" goods, with actors and recipients. Biofilms constitute a unique shield against external aggressions, including drug treatment and immune reactions. Biofilm-associated infections in humans have therefore generated major problems for the diagnosis and treatment of diseases. Improvements in our understanding of biofilms have led to innovative research designed to interfere with this process.
Biofilm notion is based on single-celled unicellular individuals (bacteria, fungi, or yeasts) forming a sedentary community within a complex structure, displaying spatial and functional heterogeneity . Bacterial biofilms account for a particular problem for human health, because they are responsible for several infectious diseases, associated with many inert surfaces, including medical devices for internal or external use. They are additionally suspected to be present in hospital water networks and as reservoirs may lead to subsequent acquired infections after patients' hospitalization.
The presence of biofilms is probably underestimated, principally because of the need for in vivo diagnosis . Early studies described biofilms as an aspect of microbial physiology , which almost all bacterial species can adopt. The multicellular structure of the biofilm makes it possible for the bacteria concerned to undergo dormancy and hibernation, enabling them to survive and to disseminate their genomes. It may therefore be considered as a step in the bacterial cell cycle.
Biofilms also display unique properties, such as multidrug tolerance and resistance to both opsonization and phagocytosis, enabling them to survive in hostile environmental conditions and to resist selective pressures . It seems that host immunity is totally ineffective at clearing these microcommunities and evidence has been obtained that shows that immune cells are paralyzed with impeded phagocytosis capacities  or decreased burst response after phagocytosis with lowered production of reactive oxygen species . This community also is unique in that it brings together different species in a structure in which they can cooperate, rather than compete. The biofilm thus constitutes a microbial society, with its own set of social rules and patterns of behavior, including altruism and cooperation, both of which favor the success of the group [7, 8] with task-sharing behavior, on the one hand, and competition , on the other. Certain subpopulations may display specialization. All of these patterns of behavior are orchestrated by communication, which may be chemical or genetic . The biofilm thus constitutes a unique way to stabilize interactions between species, inducing marked changes in the symbiotic relationships between them and affecting the function of the microbial community .
Biofilms, like other communities, form gradually over time. Whatever the bacterial species involved and the complexity of the resulting community, biofilm formation is a dynamic process highly dependent on environmental signals, passing through a four-stage universal growth cycle consisting of initiation, maturation, maintenance, and dissolution phases, regardless of the phenotype of the constituent microorganisms . Despite some common traits, generalizations cannot be made in particular when considering that it mostly associates multiple species.
Why are biofilms difficult to treat?
Biofilms in vivo are very difficult to diagnose essentially due to the lack of sampling methods and markers, but bacterial cell clusters in discrete areas in the host tissue associated with host inflammatory cells can signal such biofilm infections . Chemical, physiological, and genetic heterogeneity of the embedded bacterial population increases over both space and time  (Figure 1). This has been observed in Staphylococcus aureus biofilms, in which cells exist in at least four distinct states: aerobic growth, fermentative growth, dead, and dormant .
Multidrug resistance, more than any other property of biofilms, provides a clear demonstration that population behavior is not the sum of the contributions of single cells. Biofilms are unique multicellular constructions of bacteria from one or several species, in which horizontal genetic transfer may occur easily, thus facilitating crossbreeding of resistance genes. The bacteria within biofilms are embedded in a matrix of exopolysaccharides (EPS) that they produce themselves. This matrix limits antibiotic diffusion. The association of molecules of various types within the biofilm, including EPS and DNA, constitutes a physical barrier to the diffusion of antimicrobial agents. However, many studies have surprisingly shown that the penetration of antibiotics is not limited in bacterial biofilms. For example, fluoroquinolones diffuse rapidly within Pseudomonas aeruginosa  and Klebsiella pneumoniae  biofilms, tetracycline diffuses rapidly in Escherichia coli biofilms , and vancomycin diffuses rapidly in Staphylococcus epidermidis biofilms . Aminoglycosides are the only molecules for which poor penetration has been reported in biofilms of an alginate (the mucoid EPS)-producing strain of P. aeruginosa . As EPS differ considerably between, and even within species, the limited diffusion of antimicrobial drugs within bacterial biofilms certainly has been underestimated. Regulation of specific drug resistance-associated genes due to unique environmental stresses or starvation conditions can be observed in bacterial biofilms. These conditions may favor the emergence of dormant cells called persisters .
Persisters are in a particular physiological state with low levels of translation but a unique gene expression profile , associating the switch off of genes encoding metabolic proteins together with operons encoding toxin-antitoxin pairs switched on. The latter probably play a role in competition in addition to contribute to dormancy. However, persister cells, which are resistant to killing by antibiotics and can survive drug treatment, account for only a small proportion of the biofilm population . Indeed, when dispersed mechanically, most biofilm cells seem to be as susceptible to inhibitors as planktonic cells. A number of cells are drug-tolerant because of their particular physiological state in the biofilm, due to nutrient and oxygen limitation, for example. Some resistance mechanisms may be stronger in biofilms, given that specific efflux pumps have been shown to be more efficient in P. aeruginosa  and E. coli  biofilms. However, this mechanism is not universal, and some efflux pump inhibitors can reduce or even abolish E. coli biofilm formation .
A novel mechanism of biofilm-associated antibiotic resistance has been described recently in P. aeruginosa: released DNA, the highly anionic polymer working as a cation chelator in the extracellular matrix, creates a localized cation-limited environment. This cation-starvation is detected by P. aeruginosa, leading to the induction of LPS modification genes and resistance to antimicrobial drugs, such as cationic antimicrobial peptides and aminoglycosides . Another interesting biofilm-specific resistance mechanism also has been identified in this bacterium. The biofilm ndvB-dependent production of glucans in the periplasm leads to aminoglycoside sequestration in this cellular compartment, preventing them from reaching their ribosomal targets; this mechanism is unique to P. aeruginosa biofilms . In multimicrobial biofilms containing Candida albicans and S. aureus, such as that occurring on the surface of indwelling medical devices, resistance of S. aureus to vancomycin is higher in the polymicrobial biofilm. This increased resistance of S. aureus requires viable C. albicans and is mediated in part by the C. albicans matrix , although C. albicans growth and sensitivity to amphotericin B are not altered in the polymicrobial biofilm.
It is now widely accepted that life in a sedentary community confers a unique type of bacterial resistance, known as biofilm-associated antimicrobial resistance. This resistance is highly problematic for effective therapeutic decisions, especially when considering that many resistance phenotypes are shut down when bacterial samples are isolated from patients and examined for clinical bacteriological phenotypes.
How are biofilms built?
Our understanding of the molecular basis of bacterial biofilm development has benefited from improvements in genetics, genomics, and the development of visualization techniques unraveling the processes involved in biofilm development, physiology, and adaptation. A plethora of systems that enable bacteria to identify and anchor themselves onto appropriate surfaces, to stick to each other, to construct multicellular communities several hundreds of micrometers thick, and to detach from the community has been identified and characterized in many biofilm-forming bacteria (Figure 1). It is not possible to identify general molecular profiles for a given bacterial species, because some genes are important for biofilm formation under both static and dynamic conditions, whereas others are important only under dynamic biofilm conditions . However, throughout the bacterial kingdom, these genes can be separated into those encoding appendages consisting of oligomerized subunits responsible for motility (type IV pili or TFP, flagella) or with other functions (fimbriae, other types of pili, curli), EPS, surface adhesins, or other secreted elements. The molecular machines responsible for assembling or secreting these systems are, of course, highly dependent on the simple or double membranes of Gram-positive and Gram-negative bacteria, respectively [36, 37]. Their role in biofilm initiation and structuring also is highly dependent on environmental conditions and the surfaces encountered by the bacteria . Each bacterial species has its own adhesion toolkit, containing a number of molecules, different for each species that may be used antagonistically or synergistically, depending on the situation with which the bacterium is faced. Global expression at the patient bed is required to understand how bacteria form biofilms in patients, especially when considering that in vivo bacterial situations can widely differ with in vitro behavior .
What signals trigger biofilm structuration?
Two-component system (TCS) and extracytoplasmic function (ECF) signaling pathways are the major signaling mechanisms used by bacteria to monitor external and internal stimuli (e.g., nutrients, ions, temperature, redox states) and translate these signals into adaptive responses. The TCS pathways (Figure 2A) include two proteins: a histidine kinase (HK) protein, called "sensor," and a cognate partner, called "response regulator" (RR). Upon detection of the stimulus, the HK is activated and auto-phosphorylates on a conserved histidine residue. The phosphoryl group is then transferred onto a conserved aspartate residue on the cognate RR . Phosphorylation results in RR activation, which is most frequently a transcriptional regulator. As an example, the GacS (HK)/GacA (RR) TCS is one of the major systems involved in the control of P. aeruginosa biofilm formation. Once activated by an unknown signal, the GacS/GacA TCS switches on the transcription of the rsm genes. The rsm genes encode two small non-coding RNA (sRNA), RsmY and RsmZ, of which the expression level is a key player in controlling switch between planktonic and biofilm lifestyles . High expression of rsmY and rsmZ leads to high biofilm formation, whereas a reduced expression of them is associated with an impaired biofilm formation. The Gac regulatory pathway has been linked to two additional HK RetS and LadS. Although RetS has been demonstrated to antagonize GacS, thus repressing genes required for biofilm formation , LadS reinforces GacS-dependent activation of genes required for biofilm formation . In parallel, the Gac system activates antibiotic resistance toward aminoglycosides (gentamicin and amikacin) and chloramphenicol , thus linking once more biofilm lifestyle and antimicrobial resistance. TCS-dependent regulation of biofilm formation is widespread in many bacteria as illustrated by the positive control exerted by the GraS (HK)/GraR (RR) TCS on S. aureus biofilm induction .
The second major signaling mechanism used by bacteria and probably underestimated is the ECF signaling pathway (Figure 2B), which involves an alternative sigma factor, an anti-sigma factor located preferentially in the cytoplasmic membrane, sequestering and inhibiting its cognate sigma factor  and one or several periplasmic or outer membrane proteins required for the activation of the pathway . Upon perception of the extracellular signal by the periplasmic or outer membrane proteins, degradation of the anti-sigma factor induces releasing of the sigma factor, which can promote the transcription of a specific set of target genes. In P. aeruginosa, for example, AlgU ECF sigma factor controls production of the EPS alginate, which further impacts biofilm architecture . The AlgU sigma factor functions with the anti-sigma MucA, which C-terminal periplasmic domain is cleaved by the AlgW protease in response to an unknown signal .
Bacteria also use a multicellular response to coordinate expression of genes required for biofilm in a population density-dependent manner, called quorum sensing (QS) (Figure 2C), defined as a bacterial communication process utilizing small, diffusible molecules termed autoinducers or pheromones. Autoinducers are different between Gram-negative and Gram-positive bacteria, using preferentially N-acyl-homoserine lactones and oligopeptides, respectively. Autoinducers accumulate outside reflecting the growing population density and, upon reaching a concentration threshold, regulate virulence and pathogenicity genes. Detection of autoinducer threshold may utilize a HK, or autoinducers can enter passively or actively the cell and bind a regulator protein, both combinations trigger a specific genetic response . In S. aureus, transition between planktonic and biofilm lifestyles is predominantly controlled by QS. The S. aureus QS is encoded by the agr operon where agrD encodes the autoinducer. Once produced, exported, and present at the threshold concentration, AgrD pheromone controls through the TCS AgrCA, the expression of the small non-coding RNA, RNAIII; thus RNAIII down-regulates genes encoding adhesins required for biofilm formation [54, 55]. It thereby promotes dispersion together with extracellular protease activity , linking inversely the bacterial population size and biofilm formation together with probable resistance to glycopeptides antibiotics . Most glycopeptide-resistant S. aureus are agr dysfunctioning even though the link between agr function and glycopeptide resistance is still debated.
Finally, among signaling molecules is the intracellular second messenger c-di-GMP (Figure 2D). The amount of this messenger is tightly controlled, being increased by the activity of diguanylate cyclases (DGC) carrying GGDEF domains and decreased by the activity of phosphoesterases (PDE) carrying EAL domains. In bacteria, high c-di-GMP levels are generally associated with the stimulation of biofilm formation via the production of adhesive surface organelles or EPS synthesis and a decrease in motility.
Many proteins containing GGDEF or EAL domains are linked to various N-terminal sensory input domains, suggesting that several signals from the environment are integrated through the c-di-GMP signaling pathway. In P. aeruginosa, the FimX protein controls expression of genes encoding Type IVa pili involved in early step of biofilm formation . FimX possesses both imperfect DGC and PDE domains; however, only the PDE activity is detectable. The FimX protein is associated with a RR domain and a PAS domain; the latter is probably involved in sensing oxygen and redox potential . These signals are potentially the activating signals.
The regulatory networks controlling transition between planktonic and biofilm lifestyle are far from being elucidated and involve intricate crosstalk between regulatory pathways. These networks require extensive genetic studies to understand how bacteria integrate signals from the environment to establish into multicellular communities.
Where can we hack?
Because this biofilm lifestyle may be associated with human infectious diseases and account for 80% of bacterial chronic inflammatory and infectious diseases, several lines of research are currently focusing on the possibility of hacking into biofilm initiation, structuration or communication, and promoting dispersion  (Figure 1), even though we are far from understanding the complex genetic basis for biofilm formation in vivo.
Undoubtedly, due to antimicrobial tolerance, slow-growing cells, and EPS matrix, biofilm-associated infections do not respond consistently to therapeutically achievable concentrations of most antimicrobial agents. Practicians, therefore, must integrate these notions to direct clinical decision and further adapt antimicrobial therapy to such types of combined infectious conditions. This is particularly successful when antimicrobial lock technique (ALT) is applied in particular to combat bacterial biofilms on central veinous catheters [61, 62]. This technique corresponds to an instillation of antimicrobial drugs with bactericidal rather than bacteriostatic properties in the catheter in situ for a sufficient dwell time and at high concentrations (mg/ml). However, for most studies that evaluate ALT in patients, true elimination of bacterial biofilms has not been checked and treatment success has been based on negative culture results of blood samples or absence of clinical symptoms in patients. Because very high doses of antimicrobials are recommended, ALT can induce secondary antimicrobial resistance and potential toxicity for the patient . This technique has been tested with several other molecules, such as chelating agents, ethanol, and taurolidine-citrate and gives promising results for reduced incidence of biofilms on central-venous access devices in human studies .
Additionally, all new information concerning the functioning of biofilms may potentially lead to strategies for dismantling this microbial community  and actually requires to be validated in vivo. Much effort has focused on compounds interfering with QS autoinducers , molecules enhancing dispersion, such as NO, capsules or dispersin-like molecules and, recently, phages . Altering general regulatory pathways by protein engineering of key players also are very promising tracks (e.g., c-di-GMP proteins, global regulators) [60, 66]. For example, interfering with DGC protein activity and therefore with c-di-GMP biosynthesis would represent a promising track . Sulfathiazole is a sulfonamide that has been identified as the sole anti-biofilm molecule against E. coli strains and acts indirectly on c-di-GMP levels by targeting nucleotide synthesis rather than on DGC activity. Because anaerobic growth within biofilms could depend substantially on iron availability and is critical for biofilm-associated antimicrobial resistance, iron chelation has been proposed as an adjunct to conventional antibiotics, such as aminoglycoside administration to disrupt variable-aged P. aeruginosa biofilms [68, 69]. Increasing efforts have been dedicated to molecules interfering with adhesive structures and to the development of new surfaces for internal or external medical devices . This is illustrated by the recent demonstration of broad and high-level antimicrobial activity in vitro of antiseptic-coated as well as silver-coated endotracheal tubes to prevent adherence and biofilm formation of drug-resistant bacteria (MRSA, MDR P. aeruginosa, MDR Acinetobacter baumannii, ESBL K. pneumoniae, and MDR Enterococcus cloacae) and yeasts (C. albicans) causing ventilator-associated pneumonia (VAP) in critically ill patients. The promising development of antiseptic-coated devices requires additional animal studies and prospective, randomized clinical trials to evaluate whether they potentially induce emergence of bacterial resistance and reduce the risk of VAP in critically ill patients .
There is a very dynamic research activity in the biofilm field, especially because this bacterial lifestyle may be associated with human infectious diseases. However, the in vivo biofilms are far more complex than those studied in vitro due to the underestimation of environmental parameters or the numbers of species controlling biofilm formation. Understanding the genetic basis of biofilm formation in vitro together with the definition of biofilm signatures in vivo in infected patients is a key requirement for efficiently hacking into biofilm strategy.
The authors thank Steve Garvis for having kindly reviewed the English language of the manuscript. SdB and CB are supported by the French cystic fibrosis foundation (VLM), the foundation Bettencourt-Schueller, and CNRS institutional and ANR grants: ERA-NET ADHRES 27481, PCV-ANR 27628, ANR Jeunes Chercheurs ANR-09-JCJC-0047, Europathogenomics 2005-2010.
- Parsek MR, Tolker-Nielsen T: Pattern formation in Pseudomonas aeruginosa biofilms. Curr Opin Microbiol 2008, 11: 560–566. 10.1016/j.mib.2008.09.015PubMedView Article
- Lynch AS, Robertson GT: Bacterial and fungal biofilm infections. Annu Rev Med 2008, 59: 415–428. 10.1146/annurev.med.59.110106.132000PubMedView Article
- Henrici AT: Studies of freshwater bacteria. I. A direct microscopic technique. J Bacteriol 1933, 25: 277–287.PubMed CentralPubMed
- Weitao T: Multicellularity of a unicellular organism in response to DNA replication stress. Res Microbiol 2009, 160: 87–88. 10.1016/j.resmic.2008.09.010PubMedView Article
- Leid JG, Shirtliff ME, Costerton JW, Stoodley P: Human leukocytes adhere to, penetrate, and respond to Staphylococcus aureus biofilms. Infect Immun 2002, 70: 6339–6345. 10.1128/IAI.70.11.6339-6345.2002PubMed CentralPubMedView Article
- Jesaitis AJ, Franklin MJ, Berglund D, Sasaki M, Lord CI, Bleazard JB, Duffy JE, Beyenal H, Lewandowski Z: Compromised host defense on Pseudomonas aeruginosa biofilms: characterization of neutrophil and biofilm interactions. J Immunol 2003, 171: 4329–4339.PubMedView Article
- Shapiro JA: Thinking about bacterial populations as multicellular organisms. Annu Rev Microbiol 1998, 52: 81–104. 10.1146/annurev.micro.52.1.81PubMedView Article
- Parsek MR, Greenberg EP: Sociomicrobiology: the connections between quorum sensing and biofilms. Trends Microbiol 2005, 13: 27–33. 10.1016/j.tim.2004.11.007PubMedView Article
- Velicer GJ: Social strife in the microbial world. Trends Microbiol 2003, 11: 330–337. 10.1016/S0966-842X(03)00152-5PubMedView Article
- Weigel LM, Donlan RM, Shin DH, Jensen B, Clark NC, McDougal LK, Zhu W, Musser KA, Thompson J, Kohlerschmidt D, Dumas N, Limberger RJ, Patel JB: High-level vancomycin-resistant Staphylococcus aureus isolates associated with a polymicrobial biofilm. Antimicrob Agents Chemother 2007, 51: 231–238. 10.1128/AAC.00576-06PubMed CentralPubMedView Article
- Hansen SK, Rainey PB, Haagensen JA, Molin S: Evolution of species interactions in a biofilm community. Nature 2007, 445: 533–536. 10.1038/nature05514PubMedView Article
- Xu KD, Stewart PS, Xia F, Huang CT, McFeters GA: Spatial physiological heterogeneity in Pseudomonas aeruginosa biofilm is determined by oxygen availability. Appl Environ Microbiol 1998, 64: 4035–4039.PubMed CentralPubMed
- Stanley NR, Britton RA, Grossman AD, Lazazzera BA: Identification of catabolite repression as a physiological regulator of biofilm formation by Bacillus subtilis by use of DNA microarrays. J Bacteriol 2003, 185: 1951–1957. 10.1128/JB.185.6.1951-1957.2003PubMed CentralPubMedView Article
- Cerca N, Pier GB, Vilanova M, Oliveira R, Azeredo J: Quantitative analysis of adhesion and biofilm formation on hydrophilic and hydrophobic surfaces of clinical isolates of Staphylococcus epidermidis . Res Microbiol 2005, 156: 506–514. 10.1016/j.resmic.2005.01.007PubMed CentralPubMedView Article
- Beloin C, Valle J, Latour-Lambert P, Faure P, Kzreminski M, Balestrino D, Haagensen JA, Molin S, Prensier G, Arbeille B, Ghigo JM: Global impact of mature biofilm lifestyle on Escherichia coli K-12 gene expression. Mol Microbiol 2004, 51: 659–674.PubMedView Article
- Aparna MS, Yadav S: Biofilms: microbes and disease. Braz J Infect Dis 2008, 12: 526–530. 10.1590/S1413-86702008000600016PubMedView Article
- O'Toole G, Kaplan HB, Kolter R: Biofilm formation as microbial development. Annu Rev Microbiol 2000, 54: 49–79. 10.1146/annurev.micro.54.1.49PubMedView Article
- Hall-Stoodley L, Stoodley P: Evolving concepts in biofilm infections. Cell Microbiol 2009, 11: 1034–1043. 10.1111/j.1462-5822.2009.01323.xPubMedView Article
- Stewart PS, Franklin MJ: Physiological heterogeneity in biofilms. Nat Rev Microbiol 2008, 6: 199–210. 10.1038/nrmicro1838PubMedView Article
- Rani SA, Pitts B, Beyenal H, Veluchamy RA, Lewandowski Z, Davison WM, Buckingham-Meyer K, Stewart PS: Spatial patterns of DNA replication, protein synthesis, and oxygen concentration within bacteria biofilms reveal diverse physiological states. J Bacteriol 2007, 189: 4223–4233. 10.1128/JB.00107-07PubMed CentralPubMedView Article
- Vrany JD, Stewart PS, Suci PA: Comparison of recalcitrance to ciprofloxacin and levofloxacin exhibited by Pseudomonas aeruginosa bofilms displaying rapid-transport characteristics. Antimicrob Agents Chemother 1997, 41: 1352–1358.PubMed CentralPubMed
- Anderl JN, Franklin MJ, Stewart PS: Role of antibiotic penetration limitation in Klebsiella pneumoniae biofilm resistance to ampicillin and ciprofloxacin. Antimicrob Agents Chemother 2000, 44: 1818–1824. 10.1128/AAC.44.7.1818-1824.2000PubMed CentralPubMedView Article
- Stone G, Wood P, Dixon L, Keyhan M, Matin A: Tetracycline rapidly reaches all the constituent cells of uropathogenic Escherichia coli biofilms. Antimicrob Agents Chemother 2002, 46: 2458–2461. 10.1128/AAC.46.8.2458-2461.2002PubMed CentralPubMedView Article
- Darouiche RO, Dhir A, Miller AJ, Landon GC, Raad II, Musher DM: Vancomycin penetration into biofilm covering infected prostheses and effect on bacteria. J Infect Dis 1994, 170: 720–723. 10.1093/infdis/170.3.720PubMedView Article
- Shigeta M, Tanaka G, Komatsuzawa H, Sugai M, Suginaka H, Usui T: Permeation of antimicrobial agents through Pseudomonas aeruginosa biofilms: a simple method. Chemotherapy 1997, 43: 340–345. 10.1159/000239587PubMedView Article
- Anderson GG, O'Toole GA: Innate and induced resistance mechanisms of bacterial biofilms. Curr Top Microbiol Immunol 2008, 322: 85–105. 10.1007/978-3-540-75418-3_5PubMed
- Shah D, Zhang Z, Khodursky A, Kaldalu N, Kurg K, Lewis K: Persisters: a distinct physiological state of Escherichia coli . BMC Microbiol 2006, 6: 53. 10.1186/1471-2180-6-53PubMed CentralPubMedView Article
- Lewis K: Riddle of biofilm resistance. Antimicrob Agents Chemother 2001, 45: 999–1007. 10.1128/AAC.45.4.999-1007.2001PubMed CentralPubMedView Article
- Gillis RJ, White KG, Choi KH, Wagner VE, Schweizer HP, Iglewski BH: Molecular basis of azithromycin-resistant Pseudomonas aeruginosa biofilms. Antimicrob Agents Chemother 2005, 49: 3858–3867. 10.1128/AAC.49.9.3858-3867.2005PubMed CentralPubMedView Article
- Hancock V, Klemm P: Global gene expression profiling of asymptomatic bacteriuria Escherichia coli during biofilm growth in human urine. Infect Immun 2007, 75: 966–976. 10.1128/IAI.01748-06PubMed CentralPubMedView Article
- Kvist M, Hancock V, Klemm P: Inactivation of efflux pumps abolishes bacterial biofilm formation. Appl Environ Microbiol 2008, 74: 7376–7382. 10.1128/AEM.01310-08PubMed CentralPubMedView Article
- Mulcahy H, Charron-Mazenod L, Lewenza S: Extracellular DNA chelates cations and induces antibiotic resistance in Pseudomonas aeruginosa biofilms. PLoS Pathog 2008, 4: e1000213. 10.1371/journal.ppat.1000213PubMed CentralPubMedView Article
- Mah TF, Pitts B, Pellock B, Walker GC, Stewart PS, O'Toole GA: A genetic basis for Pseudomonas aeruginosa biofilm antibiotic resistance. Nature 2003, 426: 306–310. 10.1038/nature02122PubMedView Article
- Harriott MM, Noverr MC: Candida albicans and Staphylococcus aureus form polymicrobial biofilms: effects on antimicrobial resistance. Antimicrob Agents Chemother 2009, 53: 3914–3922. 10.1128/AAC.00657-09PubMed CentralPubMedView Article
- Ramsey MM, Whiteley M: Pseudomonas aeruginosa attachment and biofilm development in dynamic environments. Mol Microbiol 2004, 53: 1075–1087. 10.1111/j.1365-2958.2004.04181.xPubMedView Article
- Bernard CS, Giraud C, Spagnolo J, de Bentzmann S: Biofilms: the secret story of microbial communities. In Bacterial pathogenesis. Edited by: Locht C, Simonet M. Horizon Press; 2011:in press.
- Giraud C, Bernard C, Ruer S, de Bentzmann S: Biological "glue" and "Velcro": molecular tools for adhesion and biofilm formation in the hairy and gluey bug Pseudomonas aeruginosa . Env Microbiol Rep 2010, 2: 343–358.View Article
- Lauderdale KJ, Malone CL, Boles BR, Morcuende J, Horswill AR: Biofilm dispersal of community-associated methicillin-resistant Staphylococcus aureus on orthopedic implant material. J Orthop Res 2010, 28: 55–61.PubMed
- Hagan EC, Lloyd AL, Rasko DA, Faerber GJ, Mobley HL: Escherichia coli global gene expression in urine from women with urinary tract infection. PLoS Pathog 2010, 6: e1001187. 10.1371/journal.ppat.1001187PubMed CentralPubMedView Article
- de Kievit TR: Quorum sensing in Pseudomonas aeruginosa biofilms. Environ Microbiol 2009, 11: 279–288. 10.1111/j.1462-2920.2008.01792.xPubMedView Article
- Senadheera D, Cvitkovitch DG: Quorum sensing and biofilm formation by Streptococcus mutans . Adv Exp Med Biol 2008, 631: 178–188. 10.1007/978-0-387-78885-2_12PubMedView Article
- Jonas K, Melefors O, Römling U: Regulation of c-di-GMP metabolism in biofilms. Future Microbiol 2009, 4: 341–358. 10.2217/fmb.09.7PubMedView Article
- Stock AM, Robinson VL, Goudreau PN: Two-component signal transduction. Annu Rev Biochem 2000, 69: 183–215. 10.1146/annurev.biochem.69.1.183PubMedView Article
- Brencic A, McFarland KA, McManus HR, Castang S, Mogno I, Dove SL, Lory S: The GacS/GacA signal transduction system of Pseudomonas aeruginosa acts exclusively through its control over the transcription of the RsmY and RsmZ regulatory small RNAs. Mol Microbiol 2009, 73: 434–445. 10.1111/j.1365-2958.2009.06782.xPubMed CentralPubMedView Article
- Goodman AL, Merighi M, Hyodo M, Ventre I, Filloux A, Lory S: Direct interaction between sensor kinase proteins mediates acute and chronic disease phenotypes in a bacterial pathogen. Genes Dev 2009, 23: 249–259. 10.1101/gad.1739009PubMed CentralPubMedView Article
- Ventre I, Goodman AL, Vallet-Gely I, Vasseur P, Soscia C, Molin S, Bleves S, Lazdunski A, Lory S, Filloux A: Multiple sensors control reciprocal expression of Pseudomonas aeruginosa regulatory RNA and virulence genes. Proc Natl Acad Sci USA 2006, 103: 171–176. 10.1073/pnas.0507407103PubMed CentralPubMedView Article
- Brinkman FS, Macfarlane EL, Warrener P, Hancock RE: Evolutionary relationships among virulence-associated histidine kinases. Infect Immun 2001, 69: 5207–5211. 10.1128/IAI.69.8.5207-5211.2001PubMed CentralPubMedView Article
- Boles BR, Thoendel M, Roth AJ, Horswill AR: Identification of genes involved in polysaccharide-independent Staphylococcus aureus biofilm formation. PLoS One 2010, 5: e10146. 10.1371/journal.pone.0010146PubMed CentralPubMedView Article
- Schurr MJ, Yu H, Martinez-Salazar JM, Boucher JC, Deretic V: Control of AlgU, a member of the sigma E-like family of stress sigma factors, by the negative regulators MucA and MucB and Pseudomonas aeruginosa conversion to mucoidy in cystic fibrosis. J Bacteriol 1996, 178: 4997–5004.PubMed CentralPubMed
- Helmann JD: The extracytoplasmic function (ECF) sigma factors. Adv Microb Physiol 2002, 46: 47–110.PubMedView Article
- Hay ID, Gatland K, Campisano A, Jordens JZ, Rehm BH: Impact of alginate overproduction on attachment and biofilm architecture of a supermucoid Pseudomonas aeruginosa strain. Appl Environ Microbiol 2009, 75: 6022–6025. 10.1128/AEM.01078-09PubMed CentralPubMedView Article
- Cezairliyan BO, Sauer RT: Control of Pseudomonas aeruginosa AlgW protease cleavage of MucA by peptide signals and MucB. Mol Microbiol 2009, 72: 368–379. 10.1111/j.1365-2958.2009.06654.xPubMed CentralPubMedView Article
- Podbielski A, Kreikemeyer B: Cell density-dependent regulation: basic principles and effects on the virulence of Gram-positive cocci. Int J Infect Dis 2004, 8: 81–95. 10.1016/j.ijid.2003.04.003PubMedView Article
- Novick RP, Projan SJ, Kornblum J, Ross HF, Ji G, Kreiswirth B, Vandenesch F, Moghazeh S: The agr P2 operon: an autocatalytic sensory transduction system in Staphylococcus aureus . Mol Gen Genet 1995, 248: 446–458. 10.1007/BF02191645PubMedView Article
- Antunes LC, Ferreira RB, Buckner MM, Finlay BB: Quorum sensing in bacterial virulence. Microbiology 2010, 156: 2271–2282. 10.1099/mic.0.038794-0PubMedView Article
- Boles BR, Horswill AR: Agr-mediated dispersal of Staphylococcus aureus biofilms. PLoS Pathog 2008, 4: e1000052. 10.1371/journal.ppat.1000052PubMed CentralPubMedView Article
- Sakoulas G, Eliopoulos GM, Moellering RC Jr, Novick RP, Venkataraman L, Wennersten C, DeGirolami PC, Schwaber MJ, Gold HS: Staphylococcus aureus accessory gene regulator (agr) group II: is there a relationship to the development of intermediate-level glycopeptide resistance? J Infect Dis 2003, 187: 929–938. 10.1086/368128PubMedView Article
- Kazmierczak BI, Lebron MB, Murray TS: Analysis of FimX, a phosphodiesterase that governs twitching motility in Pseudomonas aeruginosa . Mol Microbiol 2006, 60: 1026–1043. 10.1111/j.1365-2958.2006.05156.xPubMed CentralPubMedView Article
- Taylor BL, Zhulin IB: PAS domains: internal sensors of oxygen, redox potential, and light. Microbiol Mol Biol Rev 1999, 63: 479–506.PubMed CentralPubMed
- Wood TK, Hong SH, Ma Q: Engineering biofilm formation and dispersal. Trends Biotechnol 2011, 29: 87–94. 10.1016/j.tibtech.2010.11.001PubMed CentralPubMedView Article
- Donlan RM: Biofilm elimination on intravascular catheters: important considerations for the infectious disease practitioner. Clin Infect Dis 2011, 52: 1038–1045. 10.1093/cid/cir077PubMedView Article
- Donlan RM: Biofilms on central venous catheters: is eradication possible? Curr Top Microbiol Immunol 2008, 322: 133–161. 10.1007/978-3-540-75418-3_7PubMed
- Hunter P: The mob response. The importance of biofilm research for combating chronic diseases and tackling contamination. EMBO Rep 2008, 9: 314–317. 10.1038/embor.2008.43PubMed CentralPubMedView Article
- Estrela AB, Heck MG, Abraham WR: Novel approaches to control biofilm infections. Curr Med Chem 2009, 16: 1512–1530. 10.2174/092986709787909640PubMedView Article
- Lu TK, Collins JJ: Dispersing biofilms with engineered enzymatic bacteriophage. Proc Natl Acad Sci USA 2007, 104: 11197–11202. 10.1073/pnas.0704624104PubMed CentralPubMedView Article
- Ma Q, Yang Z, Pu M, Peti W, Wood TK: Engineering a novel c-di-GMP-binding protein for biofilm dispersal. Environ Microbiol 2011, 13: 631–642. 10.1111/j.1462-2920.2010.02368.xPubMed CentralPubMedView Article
- Antoniani D, Bocci P, Maciag A, Raffaelli N, Landini P: Monitoring of diguanylate cyclase activity and of cyclic-di-GMP biosynthesis by whole-cell assays suitable for high-throughput screening of biofilm inhibitors. Appl Microbiol Biotechnol 2010, 85: 1095–1104. 10.1007/s00253-009-2199-xPubMedView Article
- Reid DW, O'May C, Kirov SM, Roddam L, Lamont IL, Sanderson K: Iron chelation directed against biofilms as an adjunct to conventional antibiotics. Am J Physiol Lung Cell Mol Physiol 2009, 296: L857–858. 10.1152/ajplung.00058.2009PubMedView Article
- Moreau-Marquis S, O'Toole GA, Stanton BA: Tobramycin and FDA-approved iron chelators eliminate Pseudomonas aeruginosa biofilms on cystic fibrosis cells. Am J Respir Cell Mol Biol 2009, 41: 305–313. 10.1165/rcmb.2008-0299OCPubMed CentralPubMedView Article
- Dwyer A: Surface-treated catheters-a review. Semin Dial 2008, 21: 542–546. 10.1111/j.1525-139X.2008.00499.xPubMedView Article
- Raad II, Mohamed JA, Dvorak TL, Ghannoum MA, Hachem RY, Chaftari AM: The prevention of biofilm colonization by multidrug-resistant pathogens that cause ventilator-associated pneumonia with antimicrobial-coated endotracheal tubes. Biomaterials 2011, 32: 2689–2694. 10.1016/j.biomaterials.2010.12.015PubMedView Article
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.