Cholera loves chitin
Last year, I was lucky enough to visit the Wellcome Collection’s Dirt exhibition, which featured several microbiology treasures. Among other objects, I saw an original van Leeuwenhoek microscope and a first edition of Robert Hooke’s Micrografia. I also had the chance to get close to an original copy of John Snow’s cholera map.
Snow, who is commonly considered to be the father of modern epidemiology, is most famous for identifying where cases of cholera were occurring during an epidemic in London in 1854. This allowed him to trace the source of the outbreak – a contaminated water pump on Broad Street.
Today, very few cases of cholera are reported in the UK; however, it is endemic in many other countries and resulted in more than 100 000 deaths in 2010. It is caused by the bacterium Vibrio cholerae, which produces toxins in the small intestine of an infected person, causing them – if untreated – to produce more than ten litres of diarrhoea a day. Death comes as a result of dehydration.
As shown by Snow in 1854, people get the disease through drinking contaminated water. This is a real problem in much of the world, where clean drinking water is not accessible for local populations.
This week, I’ve been reading a paper that describes the two seasonal disease outbreaks in Bangladesh: one in spring, the other in autumn. Between the outbreaks, the V. cholerae bacteria remain in non-culturable biofilms – a slimy mass of bacteria that cling to any available surface. In this state, the bacteria are alive but dormant, not really doing anything. Some of these biofilms colonise plankton – in particular, the group of tiny aquatic crustaceans known as copepods.
The two Bangladeshi cholera outbreaks correlate with seasonal plankton blooms, but little was known about how the explosion of plankton numbers contributes to the V. cholerae life cycle by causing the dormant bacteria to wake and infect people. This paper, published in Frontiers in Microbiology, shows that the key molecule is chitin, a polymer of β-1,4-linked N-acetylglucosamine.
Chitin is the main component of crustacean shells and insect exoskeletons, among other things. It is found in more animals (and fungi) that you can think of, and V. cholerae loves it.
In fresh water, V. cholerae is able to stay in its active growth phase for up to 15 days. The researchers showed that when the bacteria were grown in water from Mathbaria, a village in Bangladesh with endemic cholera, they were active for 49 days. By contrast, when the water was supplemented with chitin, the bacteria remained in the active growth phase for up to six months.
This shows that both the Mathbaria water (which is estuarine, rather than fresh) and the chitin allow V. cholerae to grow for an extended period of time in this part of the world. Active bacteria are rarely identified by culturing them from water samples, and even less so between epidemics, so this study is important because it identifies the part that plankton blooms play in the disease life cycle.
It’s been more than 150 years since Snow’s seminal work on cholera in London. We’ve learned much about the genetics of V. cholerae since then, but this study shows that there’s still much to understand about the interaction between these microbes and the environment.
Nahar, S., Sultana, M., Naser, M., Nair, G., Watanabe, H., Ohnishi, M., Yamamoto, S., Endtz, H., Cravioto, A., Sack, R., Hasan, N., Sadique, A., Huq, A., Colwell, R., & Alam, M. (2012). Role of Shrimp Chitin in the Ecology of Toxigenic Vibrio cholerae and Cholera Transmission Frontiers in Microbiology, 2 DOI: 10.3389/fmicb.2011.00260