Why do bacteria communicate and what do they say in different environments?

EMBARGOED UNTIL: Monday 5/20, 3 PM MDT

(Symposium Session 141)

Charlotte Majerczyk
University of Washington
Seattle, WA, United States
Phone: 847-987-2727
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Presentation Summary- Like higher life forms, many bacteria participate in communication and are able to execute group behaviors. One type of communication is achieved when a bacterium secretes a chemical signal that another cell can detect. These chemical signals are only active when they reach a sufficient concentration, which is often reflective of high bacterial cell numbers. For example, bacteria present at low population density can produce a signal, but, there are few bacteria around and the signal concentration is low and not detected. However, as bacterial numbers increase (and a high population density is reached), the signal concentration also increases. Ultimately, a threshold signal concentration is achieved, and other members of the community can then sense the signal. This type of cell-to-cell signaling in bacteria is called quorum sensing (QS). This term is based on the more widely used definition of a “quorum”, which represents the minimum number of members of an assembly that must be present to conduct the business of that group.

QS allows bacteria to adapt to changing environments. Diverse bacterial species use this signaling system for many different purposes. Some bacteria that cause infections in humans, animals, and plants use QS for disease progression. For this reason, scientists believe that we might be able to design new therapies that interfere with QS to treat infections. Importantly however, not all bacteria use QS for disease. There are some bacteria that have beneficial associations with their hosts (marine and terrestrial). Some of these species use QS to maintain symbiotic relationships, which can be beneficial for both participants. Furthermore, there are other bacteria that likely use QS in entirely different ways- during life in free-living soil or water habitats. There is an appreciation that bacteria use QS for many purposes and in diverse environments. However, teasing apart the specific functions of QS in each bacterial lifestyle has been a challenge. The ultimate goal of our research is to better understand the varied uses for QS so that we might be able to design novel therapies or understand how bacteria co-habituate different environments with humans and other organisms.

In this work, we investigated QS in three closely related bacterial species, Burkholderia pseudomallei (Bp), B. thailandensis (Bt), and B. mallei (Bm). Bp causes the disease of melioidosis and is endemic to mainly Southeast Asia and Northern Australia. Bp can also live in the soil. It is believed that people develop melioidosis following contact with contaminated water or soil (usually through a cut in the skin), by ingestion of contaminated food or water, or by breathing in aerosolized Bp. Thus, Bp has a dual lifestyle; it lives in the soil/ water and in an animal host when it causes disease. Bt is evolutionarily closely related to Bp. However, Bt doesn’t cause melioidosis. In fact, Bt is not considered a pathogen. Bt lives in the soil and water- it has a similar environmental reservoir to Bp. Bm is the third member of the group. Unlike Bt and Bp, Bm has never been isolated from soil or water. It has only been isolated from infected animals or humans. For this reason, Bm is classified as a host-restricted bacterium because it only lives inside animals. The natural reservoirs of Bm are equines, where Bm mostly causes chronic disease. We are interested in this group of bacteria that includes Bp, Bt, and Bm because these species occupy distinct environments yet contain nearly identical QS systems. Thus, we want to learn how QS is used by each bacterium in each lifestyle. For example, can we learn about the role of QS during bacterial life in the soil by studying Bt?  Can we learn about the role of QS in infection by studying Bp? And, can we learn about different roles of QS during infection (i.e. chronic or acute disease) by studying Bm and Bp? To begin to answer these questions, we sought to learn which factors are under QS control in each species. To do this, we employed a technique called RNAseq on QS-deficient and QS-proficient bacteria. Following this analysis, we were able to identify groups of QS controlled genes that were shared by all three species. We were also able to identify groups of QS controlled genes that are only present in the soil dwelling bacteria or in the pathogenic bacteria.

This work provides a springboard to begin to ask questions about specific QS controlled processes. For example, what is unique about the QS-controlled factors that are in all three related species (Bp, Bm, and Bt)? Does QS control of these factors benefit groups and if so, how? Additionally, we can ask if factors that are QS-controlled in only Bm and Bp have a role in disease progression. Ultimately, understanding the precise role of QS-controlled factors in each bacterial lifestyle will allow us to evaluate if we can use chemicals or putative therapies to modify bacterial behaviors. However, central to the success of this approach is that scientists must thoroughly understand the exact roles these communication systems (QS systems) in a bacterium’s life and how QS is used in the adaptation to changing environments.

In summary, numerous scientists have examined the role of quorum sensing in different bacteria, however, there is no global understanding on the role of QS in Bp, Bt, and Bm. Furthermore, most QS studies have focused on species that alternate between different lifestyles. For this reason, it has been challenging to determine which QS-controlled factors are important where. Thus, what ultimately makes the present study on Bp, Bt, and Bm unique is that we are using independent species that live in different environments to study the same QS network. This enables us to tease apart the QS-controlled factors that are unique to each bacterium and possibly each bacterial lifestyle. Moreover, we are using this group of related organisms as a tool to better understand QS and the unique roles of QS when bacteria live in the soil or when bacteria cause disease.

This work will be presented at the 2013 General Meeting of the American Society for Microbiology and is funded by the Northwest Regional Center of Excellence (NWRCE) for Biodefense and Emerging Infectious Disease Research in the laboratory of Dr. E. Peter Greenberg.