Cholera should be suspected in acute massive rice-water diarrhoea, certainly if there have been several cases in a short time (epidemic). The clinical picture of severe cholera is so spectacular that differential diagnosis does not present many difficulties. Milder cholera may be similar to other forms of gastro-enteritis (but not to dysentery). A child above the age of five years who develops acute dehydration, or dies as the result of acute diarrhoea, is always suggestive for cholera.

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The vibrios are very small and can best be seen in a fresh faecal specimen with the help of dark field microscopy. There is characteristic motility ("star shooting") which stops immediately after adding anti-O1 antiserum. This does not give any information on possible toxin production. Confirmation is best made via a bacteriological culture. Culturing should preferably be on a special medium in a bacteriology lab, e.g. TCBS-agar [= Thiosulphate-Citrate-Bile salts-Sucrose], polymyxin mannose tellurite agar (PMT) or an other selective medium. In order to identify the serogroup and the serotype one subsequently finds out to which antibodies (antiserum) the colonies obtained exhibit an agglutination reaction. It is also possible to find out whether the vibrios are toxicogenic (produce toxin). Definitive identification is made in a reference laboratory.

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Specimens may be transported in a transport medium, e.g. Cary-Blair. This is a kind of mild alkaline gelatine in seawater in which the bacteria will survive for 4 weeks. If it is not available, a filter paper can be soaked with faeces and transported in an airtight bag to a well-equipped laboratory. A sample treated in this way remains usable for 1 week, but the recommendation is "the faster the analysis, the more reliable". Blotting paper, soaked with liquid faeces and if possible placed in a 1% saline solution, can be kept for several weeks at 37° (not in the freezer). This is useful if there are initial transport problems. Nevertheless it is better to have a fresh faecal specimen. For specimens from the environment or from food, in which the number of bacteria is much lower than in faeces, enrichment is necessary. The specimen can be incubated for 8 hours in alkaline peptone water, after which a TCBS agar is used.

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About 10 days after infection with V. cholerae O1 the patient produces vibrocidal antibodies. They start diminishing after only one month and disappear within the year. Antibodies against cholera toxin are produced more slowly and remain for years. However, these cross-react with enterotoxin produced by ETEC bacteria [enterotoxic Escherichia coli]. The immune response to V. cholerae O139 is not well understood. The detection of antibodies is not important for the urgent care of the individual patient, but does permit retrospective diagnosis.

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Not all movement which one can detect in a fresh stool sample is caused by flagellated bacteria. Do not confuse star shooting of Vibrio cholerae with Brownian motion.

During the formation of minerals, a drop of water can get trapped in a piece of igneous rock as the rock cools from its melt. In the early 19th century the Scottish botanist Robert Brown discovered such a drop of water in a piece of quartz. Brown reasoned that the water had been inaccessible for many millions of years to spores or pollen carried by the wind and rain. When he focused his microscope on the drop of water, he saw continuous movement. Suspended in the water were scores of tiny particles, ceaselessly oscillating with a completely irregular motion. The smaller the size, the more rapid its motion. The motion was familiar to Brown: he had previously observed such oscillations during his studies of pollen grains in water. The new experiment however, ruled out the explanation he had put forward earlier, namely that some kind of vitality is retained by the molecules of the plant long after death. Brown concluded that the agitation of the particles trapped inside the quartz had to be a physical phenomenon rather than a biological one. The explanation for this so-called Brownian motion is that a material particle (dust, pollen, bacteria) is continuously bombarded by the molecules of the fluid in which it is suspended. A single molecule hardly ever has enough momentum for this effect on the suspended particle to become visible under a microscope. However, this bombardment is not symmetrical. There is random fluctuation in the velocities of the nearby molecules. When many molecules collide with the particle from the same direction at the same time, they can noticeably deflect the particle. Atoms and molecules reveal their existence through the motions of a particle suspended in a fluid! The path of the particle is a random one. What a person can see through a microscope are the effects of relatively large fluctuations in the local molecular environment. If the resolving power of the microscope were increased by factors of, say, 10, 100, 1,000, the effects of bombardment by progressively smaller groups of molecules could be detected. The path would have a fractal structure (shape is self-similar at each magnification). This can be better appreciated when one knows that at room temperature, a dust particle will sustain about 10²¹ collisions per second. Since the turn of the 19-20^th century, the study of Brownian motion had far-reaching consequences for physics, chemistry and mathematics (e.g. kinetic theory of gases, statistical mechanics, thermodynamics, entropy, diffusion, Avogadro’s number, …).