Signal Transduction and Antibiotic-Induced Stress Responses in Bacteria
In order to survive, a cell must be able to sense and respond to changes in its environment. A central problem in biology is that this information is often sensed in one cellular compartment and must be transmitted to another cellular compartment where the response is generated. Moreover, events in different compartments need to be coordinated for an optimal response. Our research focuses on signal transduction between the two cellular compartments of Gram-negative bacteria, the cytoplasm and the cell envelope.
The cell envelope of Gram-negative bacteria plays an essential role for the bacterial cell by providing a barrier between the cell and the environment, determining its morphology, and maintaining structural integrity. The ability of the cell to respond to challenges to the cell envelope is also of considerable medical importance as it is the target of several classes of antimicrobial drugs. The emergence of multidrug-resistant bacteria is a major medical problem and it is crucial that we increase our understanding of the cellular response to antimicrobials. We are using two approaches to better understand how events in the envelope are communicated to the genetic machinery in the cytoplasm where a response can be generated. The first is to investigate the signaling pathways that control the activity of a transcription factor σE, which responds to cell envelope stress. The second is to identify new envelope-sensing pathways. The goals of this research are to uncover these signaling pathways, to determine how they are integrated into the cellular regulatory network, and to investigate in molecular detail how the interactions among members of a pathway result in signal transduction.
Escherichia coli provides an excellent model system for this work because of the wealth of information about E. coli physiology and the availability of a wide range of fully developed experimental approaches. Insights gained from the work on E. coli can then be applied to other Gram-negative bacteria that are less amenable to experimentation to determine how such cell envelope-sensing pathways may be used during different aspects of the bacterial lifecycle such as pathogenesis or biofilm formation.
Envelope Sensing By The Alternative Sigma Factor σE
In E. coli, certain stresses in the cell envelope are sensed by a crucial system that activates the transcription σE. σE then directs the transcription of a set of genes required for adaptation of the cell envelope including periplasmic proteases, folding catalysts, and enzymes involved in determining the composition of the outer membrane σE is unique among the alternative sigma factors in E. coli in that it is essential for normal growth. Although much work has focused on the role of σE during the stress response, little is known about it role in the normal growth of the cell. Recent work has shown that different pathways regulate σE activity during normal growth and during the stress response indicating that there are multiple modes of communication between the cell envelope and σE in the cytoplasm. During the life cycle of a bacterium and during the growth of a bacterial culture, the cell envelope is continually changing. It is likely that σE controls important aspects of the development of the cell envelope in addition to its role in the stress response. My laboratory works to identify and characterize the signals and pathways leading to the activation of σE during normal growth and then to determine their function in the cell.
Identification of New Envelope-Sensing Pathways During Antimicrobial Challenge
To identify additional envelope sensing pathways and understand how the cell senses and responds to alterations in the cell envelope, my laboratory takes two complementary approaches. First we use DNA microarrays to characterize the cellular response to disruption of various cell envelope processes by taking advantage of the wide array of antimicrobial drugs that target different components of the cell envelope. From the results of the microarray experiments we can identify candidate envelope-sensing pathways for further studies. In a second approach, we study mutant strains with increased resistance or sensitivity to a drug to understand what factors allow the bacterium to tolerate or be more susceptible to interruption of cell envelope function. Once key pathways have been identified, we will study their function on a cellular and molecular level during both the normal growth of the bacterium and during antibiotic challenge. This work will also provide valuable insights into how antimicrobial drugs affect the cell and lead to the identification of new targets for drug design.
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