Synchronous Growth
Synchronous growth is the growth of bacteria such that all the bacteria are at the same stage in their growth cycle (e.g., exponential phase, stationary phase). Because the same cellular reactions occur simultaneously throughout the bacterial population, synchronous growth permits the detection of events not normally detectable in a single cell or in a population consisting of bacteria in various stages of growth.
In a normal batch culture of fluid, or on an agar plate, bacteria in the population exhibit a range of sizes, ages, and growth rates. In contrast, the bacteria in a synchronized culture are virtually identical in terms of these parameters.
Synchronized growth is imposed in the laboratory. A population of bacteria can be filtered to obtain bacteria of a certain size range. Usually, the filter that is used has very small holes. All but the smallest bacteria in a population are excluded from passing through the filter. Because the smallest bacteria are frequently the youngest bacteria, the filtering method selects for a population comprised of bacteria that usually have just completed a division event. When the bacteria are suspended in fresh growth medium the population will subsequently grow and then divide at the same rate.
Bacteria of the same size can also be recovered using special techniques of centrifugation, where the bacteria in the fluid that is spinning around in a centrifuge are separated on the basis of their different densities. The smallest bacteria will have the lowest density and so will move furthest down the centrifuge tube.
Another method of obtaining a synchronous bacterial population involves the manipulation of some environmental factor that the bacteria depend on for growth. Typically, the factor is a nutrient that the bacteria cannot manufacture, and so is required to be present in the medium. In the alternative, an agent (e.g., an antibiotic) can be added that does not kill the bacteria but rather halts their growth at a certain point. Again, once the bacteria are added to fresh medium, the growth of all the bacteria will recommence from the point of blockage in the cell cycle.
Synchronous growth can only be maintained for a few rounds of growth and division. Ultimately, the inherent randomness of bacterial population growth again dominates. In other words, not all the bacteria will continue to divide at exactly and differences in size and other attributes will once again appear in the population. For those few generations, however, much useful information can be extracted from a synchronously growing population.
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