Trends in Microbiology
ReviewMechanisms of biofilm resistance to antimicrobial agents
Section snippets
Failure of the antimicrobial to penetrate the biofilm
The production of an exopolysaccharide matrix, or glycocalyx, is one of the distinguishing characteristics of biofilms. It has been suggested that this matrix, among other functions, prevents the access of antibiotics to the bacterial cells embedded in the community. We will highlight a few of the more recent studies on the subject of antibiotic diffusion through a biofilm. For a more comprehensive review of this subject, the reader is directed to a review by Stewart 10.
Either reaction of the
Slow growth and the stress response
When a bacterial cell culture becomes starved for a particular nutrient, it slows its growth. Transition from exponential to slow or no growth is generally accompanied by an increase in resistance to antibiotics 18., 19.. Slow growth of the bacteria has been observed in mature biofilms 20., 21.. Because cells growing in biofilms are expected to experience some form of nutrient limitation, it has been suggested that this physiological change can account for the resistance of biofilms to
Heterogeneity
The experimental conditions resulting in the tight control of growth described in the studies summarized above allowed investigators to focus on the effect of a specific growth rate on bacterial susceptibility to antimicrobial agents. However, when thinking about biofilms, a logical assumption is that any given cell within the biofilm will experience a slightly different environment compared with other cells within the same biofilm, and thus be growing at a different rate. Gradients of
General stress response
Recently, it has been suggested that the slow growth rate of some cells within the biofilm is not owing to nutrient limitation per se, but to a general stress response initiated by growth within a biofilm 31. This idea is an attractive possibility because the stress response results in physiological changes that act to protect the cell from various environmental stresses. Thus, the cells are protected from the detrimental effects of heat shock, cold shock, changes in pH and many chemical agents
Quorum sensing
The role of quorum sensing in biocide resistance is not yet clear. Previous work by Davies and colleagues showed that a mutant in the lasR–lasI quorum-sensing system in P. aeruginosa was unable to form a biofilm with normal architecture 37. Moreover, these authors presented data showing that lasI mutant biofilms were abnormally sensitive to treatment with SDS, although the question of whether these mutant biofilms had altered antibiotic resistance was not addressed 37. However, a recent study
Induction of a biofilm phenotype
Thus far, the mechanisms discussed have been based on general strategies to slow the effect of antimicrobial agents on cells in the biofilm. An emerging idea in the field is that a biofilm-specific phenotype is induced in a subpopulation of the community that results in the expression of active mechanisms to combat the detrimental effects of antimicrobial agents 15., 39., 40., 41..
When cells attach to a surface, they will express a general biofilm phenotype and work has begun to try to identify
Conclusion
There is no one answer to the question of why and how bacteria growing in a biofilm develop increased resistance to antimicrobial agents. We have seen that there are many possible mechanisms that account for bacterial resistance to antimicrobial compounds (summarized in Fig. 3). Depending on the bacterial complement of the biofilm, and the antimicrobial agent used to treat the biofilm, different mechanisms will account for resistance to the antimicrobial compound. Furthermore, the environmental
Questions for future research
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What other factors are important for antimicrobial resistance in multi-species biofilms?
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What genes are induced in biofilm cells that allow for increased resistance to antimicrobial agents?
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What signals are involved in rpoS regulation in biofilms?
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Are multidrug efflux pumps important for biofilm resistance to antimicrobial agents?
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What is the role of quorum sensing in the biocide resistance developed by biofilms?
Acknowledgments
We wish to thank Phil Stewart for permission to use Fig. 1, Fig. 2. This work was supported by a grant from Microbia, Inc. and The Pew Charitable Trusts (to G.A.O.). G.A.O. is a Pew Scholar in the Biomedical Sciences.
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