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Name: Argiro K.
Status: Student
Age: 20s
Location: N/A
Country: N/A
Date: June 2003

Why does bacterial growth increase slowly in the beginning and very rapidly later?

What you describe is known as the 'lag phase'. When freshly growing cells are diluted into fresh medium, of correct temperature etc. it will take a time before they grow at their maximum logarithmic growth rate (that rate depends on the medium and growth conditions). This lag phase will be longer if the starter culture had reached the stationary phase. To begin with the explanation of the latter, I imagine every cell has to adjust to the new conditions, from stationary phase ( no growth, thus fewer ribosomes around, low metabolism, etc) to the lag phase (maximum growth, full capacity of protein production, cell division, etc). That period would be the lag phase.

But when starting with freshly growing cells that are simply diluted into new medium, why do we still see a lag phase? I guess the cells experience some sort of stress, by pipetting, or centrifuging, or simply by the handling procedure. It may also be that the cells 'see' they are suddenly with fewer mates: bacteria are able to respond to cellular density by a process called 'quorum sensing'. How and why they do this is not fully understood, and I do not know if it has been investigated if quorum sensing plays a role in the lag phase. I could not find literature on this. But I would not be surprised if it does. It has been shown that quorum sensing plays a role in entering the stationary phase (see Carbonell et al: Control of Escherichia coli growth rate through cell density. Microbiol Res. 2002;157(4):257-265. ). So why not in the lag phase?

Here is a thing for you to study!

Trudy Wassenaar
Curator of the Virtual Museum of Bacteria

Population growth in any species is usually exponential. We use bacteria to demonstrate this because their generation time is every 20 minutes instead of about 25 years as in humans. But the principle is the same. 1 becomes 2, 2 becomes 4, 4 becomes 8 and so on. It takes a while for there to be noticeable growth, this is called the lag time. Once many cells are created, each of those divides 1-2-4-8 as well, so at this time the population shows rapid growth, or logarithmic growth.

Eventually there will be so many that the resources start to be used up and waste builds up. There will be more bacteria dying than being "born", and the total population starts to level off and then will decrease. Could this be model for the human population? We will have to wait and see.....


In the simplest model of ample food and no deaths, the rate of growth is proportional to the number of bacteria so the increase in the number is: dN = k*N*(dt) where dN is the increase in the number of bacteria, 'k' is a constant, N is the number of bacteria and (dt) is the time interval. If 'k' and 'N' are small then the increase in the number of bacteria dN is also small. But as N increases (again assuming ample food and no deaths) the fractional increase in the number of bacteria: dN/N is proportional to (dt).

This means that the number of new bacteria (dN) is proportional to 'N' and to (dt).

Vince Calder

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