The variability of living organisms

Today I heard a talk by John McKinney where a central theme was the variability of genetically identical bacteria. This means on the one hand the differences between individuals and on the other hand the differences in the state of one individual at different times. The organism most prominent in the talk was Mycobacterium tuberculosis (cf. my previous post concerning a talk by McKinney). This bacterium can be treated using antibiotics (95% of patients who complete treatment are cured) but requires a combination of several drugs over a long period. If a cure is possible why is it so difficult? In his studies on this question McKinney has found that variability in the population of bacteria of the kinds already mentioned must be carefully taken into account.

McKinney is now based in Lausanne, having been at Rockefeller University until a few years ago. It is natural to ask why a biomedical researcher would leave such a prestigious institution. The speaker explained a feature of his present institution which was very attractive to him. This is the expertise available there in engineering and this has allowed him to develop new experimental techniques based on microfluidics. Bacteria are grown within a microfluidic channel which is observed under a microscope over long time scales. The microfluidic system allows the conditions to be controlled very precisely. Nutrients are provided and waste removed on a continuous basis. The fate of individual bacteria can be followed very closely. The processes taking inside the cells can be followed using fluorescent labelling. It can be seen exactly when the cell divides, when and where its DNA is replicated etc. Often the population is observed in a phase of exponential growth but this kind of system could also be used as a chemostat (cf. this post) to observe a steady state population.

One of the phenomena to be understood is that the population of M. tuberculosis treated with an antibiotic is biphasic. The population decreases at an exponential rate for a while and then suddenly at a different, much smaller, exponential rate. This is a phenomenon at a population level and the new techniques can help to understand what is happening on an individual level. For example, the antibiotic isoniazid (INH) is believed to work by preventing the bacterium producing mycolic acid, a substance it needs to build its cell wall. One popular theory, the “unbalanced growth model” suggests that the volume of the bacterium grows while there is no more material available for its cell wall. As a consequence the wall thins until the bacterium bursts. The new observations on the properties of individual bacteria are inconsistent with this model. Another observation which arose while trying to understand the interaction of bacteria with antibiotics is that there is a protein which is expressed in a way whose time dependence seems to be stochastic. On the films transcription of this protein is marked by a red colour and the bacteria are seen to flash on an off in a random-looking manner,

Another film showed cells caught in a microfluidic cage. There are small connections of the fluid in the cage with the outside of diameter about a micrometer which allow nutrients and waste products to be exchanged. These connections are too small for  eukaryotic cells but on the film the cells were seen trying to squeeze their way through with apparently great energy. The aim is to cultivate bacteria in eukaryotic cells in a situation where they can be observed effectively through the microscope. The necessity of tracking the cells is avoided by locking them into the cage. This would mean that the bacteria could be observed in surroundings closer to their natural habitat.

All these observations seem to have raised more questions than they have answered but what better motor can there be for scientific progress?


One Response to “The variability of living organisms”

  1. Absolute concentration robustness « Hydrobates Says:

    […] of a given substance in cells of a given type varies widely between the individual cells. (Cf. also this previous post). It is of interest to identify mechanisms which can ensure that the steady state concentration of […]

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