Friday, October 22, 2010

Cholera Has a Vacation Home

I'm quickly learning how challenging it is to keep up with even a weekly commitment to a blog entry, so this one is last week's entry, a few days late. I'm gonna keep plugging away at it though.

This week's paper is Kathryn L. Cottingham, Deborah A. Chiavelli, and Ronald K. Taylor. 2003. Environmental microbe and human pathogen: the ecology and microbiology of Vibrio cholerae. Frontiers in Ecology and the Environment 1: 80–86. It is a review of the state of research on Vibrio cholerae in its aqautic environment. I picked it because I got a tip that my advisor may ask me to repeat my current experiment on V. cholerae, and I thought I should start learning more about it. V. cholerae is in the same genus as my current experimental organism, Vibrio vulnificus, and they have a lot in common. They both inhabit two environments (human and aquatic) and they both can enter a Viable But Non-Culturable (VBNC) state. V. cholerae is more interesting to funds-granting organizations since it causes more infections, so it would be beneficial to include it in any papers we generate.

When in the aquatic environment, V. cholerae can be either free-swimming, or attached to a substrate. In the attached form, it may form a biofilm, an organized aggregation of many thousands of V. cholerae cells. Biofilms provide protection for the cells against predation and antibiotics, but the mechanism of their formation is not well-known. Cells may attach to phytoplankton or zooplankton, which provides a nutrient-rich environment in which V. cholerae can multiply.

The aim of the paper appears to be not to reveal any new information about V. cholerae, but to point out that very little is known about the aquatic state. The paper raises more questions than it answers, and concludes with a list of areas for new research. These include comparing gene expression between the aquatic both free-swimming and attached) and infectious forms, and determining what, if any, genes are responsible for transition into the VBNC state.

UPDATE: I started writing this blog entry last night; this morning Haitian officials confirmed that a cholera outbreak is plaguing an area of the country where many refugees headed after the earthquake to escape the conditions in Port-au-Prince. Since V. cholera is shed in the feces and vomit of infected individuals, and can be contracted from drinking contaminated water, outbreaks are common in areas with poor sanitation, such as the temporary housing in Haiti. Read more here:

Friday, October 8, 2010


So, I haven't posted in a while. I foolishly thought that I would have oodles of free time once I quit my job and started grad school, but that has not been the case. TAing takes up a lot of time. I don't know how some people manage to get their PhD while teaching the whole time; I'm counting myself lucky that I will probably only have to teach this semester. 3 hours of class time X 2 sections, plus 3 hours of "office hours", 1 1/2 hours of TA meeting . . . and grading and creating quizzes and homework. It easily adds up to the promised 20 hours per week, and is the biggest consumer of my time at the moment. Behind that is my research work and the one class I am taking.

The work I'm doing in the lab is a short-term project called a rotation; the expectation is that I will have completed it by the end of the semester. Even though I will probably stay in the same lab for my thesis, this is not my thesis work. I've decided to try to work in a bit more literature reading in order to benefit my rotation research and help me decide what I will want to work on for my thesis eventually. So I've set myself a goal of reading one non-required journal article each weekend. "Non-required" meaning I've picked it myself because it seems interesting, not because it is required for a class or my supervisor or advisor told me to read it.

This week's selection is "Changes in Membrane Fatty Acid Composition during Entry of Vibrio vulnificus into the Viable But Nonculturable State". Day AP, Oliver JD. Journal of Microbiology. 2004 Jun;42(2):69-73.

Vibrio vulnificus is the organism I am studying for my project. It is a shellfish bacterium related to Vibrio cholerae, which causes cholera. V. vulnificus causes more than 95% of all seafood-related fatalities in the U.S., and it undergoes a transformation in the winter.

Cold temperatures cause it to become dormant to the point where it cannot be grown in the lab on normal media, yet if it is introduced into animals it still causes infection. My job this semester is to elucidate the molecular mechanism for this change. Interestingly, there are all sorts of papers showing that V. vulnificus does become dormant and what physiological changes it undergoes, but none investigating how to resuscitate it from dormancy (other than warming it back up, which seems a bit obvious), or why it is not culturable in this state.

The Day and Oliver paper was quite short, and simply showed a a change in the makeup of the fatty acids that comprise the cell membrane while the cells were going into the VBNC state. Their hypothesis is that in order to maintain membrane fluidity at lower temperatures, more unsaturated fatty acids (which have a lower melting point) are needed. In addition to measuring fatty acid ratios during entry into VBNC, they also inhibited fatty acid production by treatment with an antibiotic, cerulenin. The cells that could not produce additional fatty acids due to this treatment died after incubation at 5 degrees Celsius, indicating that this change in membrane composition is required to maintain viability. However, this only addresses one side of the VBNC (Viable But Nonculturable) state--we still don't know why the cells are not culturable if they are still alive. That's what I'm working on.