Emergence of antibiotic resistant pathogenic bacteria poses a serious public health challenge worldwide. However, antibiotic resistance genes are not confined to the clinic; instead they are widely prevalent in different bacterial populations in the environment. Therefore, to understand development of antibiotic resistance in pathogens, we need to consider important reservoirs of resistance genes, which may include determinants that confer self-resistance in antibiotic producing soil bacteria and genes encoding intrinsic resistance mechanisms present in all or most non-producer environmental bacteria. While the presence of resistance determinants in soil and environmental bacteria does not pose a threat to human health, their mobilization to new hosts and their expression under different contexts, for example their transfer to plasmids and integrons in pathogenic bacteria, can translate into a problem of huge proportions, as discussed in this review. Selective pressure brought about by human activities further results in enrichment of such determinants in bacterial populations. Thus, there is an urgent need to understand distribution of resistance determinants in bacterial populations, elucidate resistance mechanisms, and determine environmental factors that promote their dissemination. This comprehensive review describes the major known self-resistance mechanisms found in producer soil bacteria of the genus Streptomyces and explores the relationships between resistance determinants found in producer soil bacteria, non-producer environmental bacteria, and clinical isolates. Specific examples highlighting potential pathways by which pathogenic clinical isolates might acquire these resistance determinants from soil and environmental bacteria are also discussed. Overall, this article provides a conceptual framework for understanding the complexity of the problem of emergence of antibiotic resistance in the clinic. Availability of such knowledge will allow researchers to build models for dissemination of resistance genes and for developing interventions to prevent recruitment of additional or novel genes into pathogens.
The drrAB operon of Streptomyces peucetius encodes for resistance to the antibiotics doxorubicin and daunorubicin. Subcloning of the drrAB genes in Escherichia coli has previously been shown to result in expression of DrrA and DrrB proteins and resistance to doxorubicin in a sensitive strain of E. coli. DrrA, a peripheral membrane protein, binds ATP in a UV-catalyzed reaction in a doxorubicin-dependent manner; DrrB, a hydrophobic protein, is localized to the inner membrane of E. coli (Kaur, P. (1997) J. Bacteriol. 179, 569 -575). The present study provides evidence that DrrB, the membrane component of the complex, is stably maintained in the cell only if DrrA is present. Furthermore, it was found that the catalytic component DrrA is in an active conformation only when it is in a complex with DrrB. In a subclone containing the drrB gene by itself, no DrrB protein could be detected, although a translational fusion of the first 15 amino acids of DrrB to -galactosidase indicated that DrrB is translated in the absence of DrrA. Upon co-transformation with a plasmid containing the drrA gene in trans, DrrB could again be detected in these cells. UV cross-linking studies with [␣-32 P]ATP showed that only the membrane-bound form of DrrA in cells containing both DrrA and DrrB was in a conformation competent to bind ATP. Chemical cross-linking studies also provided direct evidence for interaction between the two proteins. Based on these analyses, a model for interaction between DrrA and DrrB proteins is presented.
Background: DrrAB is dedicated to export of doxorubicin in Streptomyces peucetius, an organism that produces this anticancer drug. Whether this prototype system can export other drugs has not been investigated. Results: DrrAB exports multiple drugs efficiently. Conclusion: Substrate specificity of DrrAB overlaps with known bacterial and human multidrug resistance proteins. Significance: This study suggests common mechanisms and origin for DrrAB and other MDR proteins.
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