Trends in Microbiology
Volume 9, Issue 1, 1 January 2001, Pages 34-39
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Review
Mechanisms of biofilm resistance to antimicrobial agents

https://doi.org/10.1016/S0966-842X(00)01913-2Get rights and content

Abstract

Biofilms are communities of microorganisms attached to a surface. It has become clear that biofilm-grown cells express properties distinct from planktonic cells, one of which is an increased resistance to antimicrobial agents. Recent work has indicated that slow growth and/or induction of an rpoS-mediated stress response could contribute to biocide resistance. The physical and/or chemical structure of exopolysaccharides or other aspects of biofilm architecture could also confer resistance by exclusion of biocides from the bacterial community. Finally, biofilm-grown bacteria might develop a biofilm-specific biocide-resistant phenotype. Owing to the heterogeneous nature of the biofilm, it is likely that there are multiple resistance mechanisms at work within a single community. Recent research has begun to shed light on how and why surface-attached microbial communities develop 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

  • What other factors are important for antimicrobial resistance in multi-species biofilms?

  • What genes are induced in biofilm cells that allow for increased resistance to antimicrobial agents?

  • What signals are involved in rpoS regulation in biofilms?

  • Are multidrug efflux pumps important for biofilm resistance to antimicrobial agents?

  • 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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