Department of Biology

A: We study quorum sensing in bacterial pathogens to figure out when and why and how much they change gene expression. Some genes are turned on earlier during growth (e.g., a thousand cells), whereas others are turned on very late (e.g., a billion cells). We want to know why the timing of gene expression is important during infections. We study the timing of gene expression of important genes required for causing disease, such as toxin secretion and cell motility (movement).

A: Bacteria need to work together to accomplish massive tasks. Consider how small one bacterial cell is; how could one cell possibly cause disease in a huge human host? The answer is that one cell often does not cause disease. Small numbers of bacterial cells may enter their human hosts, but they often hunker down and hide “out of sight” in host tissues while they grow and multiply to higher cell numbers. Once they reach a threshold—or “quorum”—where their numbers are great enough to enable them to work together, they change their gene expression to perform group behaviors. This can include secreting toxins and making biofilms and antibiotic resistance genes. They launch their “attack” at high numbers when billions of cells are present to make the attack effective. Infection by bacteria is simply a nutritional strategy—it is the way that bacteria live off the nutrients provided by the host. The better the attack, the more nutrients they can steal. This coordination of gene expression that launches the attack is controlled by cell-cell communication, called quorum sensing. The cells communicate using small molecules that allow them to count their numbers and inform them when they are at high densities. When quorum is reached, it is time to change gene expression and act together as a group.

A: Quorum sensing was first discovered when researchers were studying a bioluminescent bacterium called Vibrio fischeri. They realized that the bacteria only produced bioluminescence (light) when the cells were at high densities, but were completely dark at low densities. They also found that as the cells grew and grew, the enzyme required to produce light was made in a “burst” when they started growing fast. Over ~20 years, researchers—including Woody Hastings, Mike Silverman, Peter Greenberg, and Bonnie Bassler—tracked this observation and found that this was controlled by proteins in bacteria that produce and sense small molecules that are used for cell-cell communication.

A: Vibrio cholerae is the causative agent of the diarrheal disease cholera, of which there are ~3 million cases worldwide each year and over 100,000 deaths. Vibrio vulnificus and Vibrio parahaemolyticus cause the disease vibrosis in humans, which produces vomiting and fevers in some cases or skin infections in others. There are about 80,000 cases in the US every year, with ~100 deaths. The mortality rate for V. vulnificus is quite high at 50% due to rare but serious cases of septicemia. V. vulnificus, V. parahaemolyticus, and V. harveyi are also pathogens of shellfish and fish and are huge concerns for the aquaculture industries.

We use the bioluminescent vibrios as models for all the pathogenic vibrios because the quorum sensing systems are very similar in all these bacteria. We start with studying the bioluminescent species called Vibrio harveyi that is safer to work with and for which the most is known about quorum sensing. Then we extrapolate to the other pathogens to investigate how quorum sensing impacts bacterial physiology, infection, and toxicity.

A: The LuxR/HapR proteins are at the center of the quorum sensing pathway in Vibrios. They control the expression of hundreds of genes which allow the bacteria to behave as groups and work together. These LuxR/HapR proteins are the ones directly changing the expression of the genes for toxin secretion, biofilm formation, bioluminescence, and all the phenotypes that we are following. If we can learn enough about how LuxR/HapR function, we can develop drugs that target these proteins to block their activity. If the cells cannot change their gene expression, they can’t launch their “attack” of toxins and biofilms.

A: The Mycobacteria are a fascinating and diverse group of bacteria that range from soil-dwelling organisms to disease-causing pathogens. Mycobacterium tuberculosis is the causative agent of tuberculosis, which infects one-third of the world’s population and causes ~2 million deaths each year. Other human diseases from Mycobacterial species include leprosy, skin infections, and lung diseases from nontuberculous mycobacteria.

At this point, there are no concrete data showing whether quorum sensing exists in mycobacteria—though there are hints that mycobacteria communicate. Numerous pathogens use quorum sensing to control expression of genes required for infection. It is often more a question of identifying the quorum sensing genes that can be very different from species to species. If they don’t look like the quorum sensing genes that we have studied, it is very hard to find them just by looking at the genome. We are proposing to use various experiments to test for the existence of quorum sensing genes.

A: In brief, tuberculosis is the leading cause of death from bacterial infections and is the leading cause of death of HIV-infected patients. Vibrio infections bloom in conditions where contaminated food and/or water are consumed, and this is exceptionally prevalent following natural disasters. For example, massive numbers of cholera cases and deaths associated with cholera occurred following an earthquake in Haiti in 2010.

A: The goal of our research is to understand how cell-cell communication controls pathogenesis in bacteria. We are working toward the larger goal of developing drugs that target the quorum sensing pathway. When bacteria cannot use quorum sensing to count their numbers, they do not know when to launch their “attack” and cannot cause disease. Most antibiotics that are currently used to treat disease work by preventing bacterial cell growth. They are quite effective, but they also have a high potential to generate antibiotic resistance. Conversely, if we use drugs that block quorum sensing, we would stop expression of virulence genes but not kill the bacteria. This would minimize the rate of antibiotic resistance, and the immune system can clear the infection. It’s like taking the weapons away from the army but not killing the soldiers. It’s an effective strategy to stop the war with much less damage.