MICROBIAL GROWTH CURVE PART TWO :STATIONARY PHASE- FACTS AND FALLACIES

In the study of fermentation technology, we rely strongly on the understanding and interpretation of the microbial growth curve in making the right decisions for different stages of the fermentation process. We use the growth curve not only to determine the efficiency of the fermentation process but also in trouble shooting exercises
Yet, despite its importance how much do we really understand about the microbial growth curve? In fact, most of us learn about the typical sigmoid growth curve even as early as out initial years in our secondary education and right up to our university days. But really how much do we know about the microbial curve?
Instead we have been indoctrinated by superficial ‘brain washings’ of knowing the lag, log and stationary phase and the verbal description or verbal description of the phases of growth; slow growth, exponential growth and no growth…
They keep telling us the stationary phase is the onset period before the microorganisms die. They tell us that the stationary phase occurs because of lacked of food or toxic conditions. And that the stationary phase is a useless phase of no importance to physiology!
THE TRUTH IS FAR FROM THAT!!!
In terms of microbial physiology, the stationary phase is the most important phase when the microbes will undergo huge changes in its metabolism (prior to kicking the bucket!) It is the time for some microbes to produce secondary metabolites such as the antibiotics or even form spores to survive.
Before going into further detail let us try to understand what factors cause the induction of the stationary phase. Bear in mind we are dealing not at the behavior of single cells but a population of millions and millions of cells of diverse types of metabolism and physiology
The stationary phase the growth curve is often visualized as a level phase occurring after the end of the log phase and before the start of the declining or the death phase. Since the growth curve is a graphical representation of mathematical data, we could therefore say that at the stationary phase, the numbers of cells remain constant where either there is no growth or loss of the population of cells or more correctly where the number of new cells added is the same as the number of cells lost. It is hard to imagine that at the stationary phase the cells are not growing at all! It is more of a steady state where the number of cells remained constant
One of the most common explanations to explain the formation of the stationary phase is when substrate becomes the limiting factor. It is a situation where there are not enough nutrients to support the growth and multiplication of all the cells.
Another hypothesis is that stationary phase is induced by the presence of toxic products which reach a certain concentration to inhibit the growth of the microorganisms.
What is often not discussed is the impact of viscosity that might have impact directly or indirectly on the induction of the stationary phase. Then it is quite fair to say that limiting nutrients or substrate are not the only factor initiating the onset of stationary phase.
With regards to toxic products build up inducing stationary phase it might be quite valid in cases where there occurs catabolite repression or the fermentation products itselves are too toxic or too acidic. This argument however does not hold where in aerobic metabolism the end products are just water or CO2
Now let us look closely at the problem. The formation of the stationary phase is commonly associated with the sigmoid growth curve of try phase is commonly associated with the sigmoid growth curve of the microorganisms in general. It is more applied in batch fed mode. Why is this so?
It is more of the continuous washout rate occurring in the fermentor where the growth of the microorganisms is controlled by controlled wasting of cells, raw substrate and waste products. You can initiate continuous phase at various point of the log phase….
I tend too see the onset of stationary phase as a major shift in microbial metabolism rather than the onset of the equivalent of menopause….:) It is a point of intense change in the metabolism of the cell when the microbes are facing extreme stress with regard to their survival. The microorganisms have to carry out drastic changes in structure and function to survive or to insulate the integrity of their metabolism from the normal mode until better conditions will return
However it is during these trying times that the microorganisms will conjure up new compounds which do not seem to serve any functions to them. Initially it seems that these moves are stupid as the microorganisms seemed to waste energy, enzymes and carbon at time when they should be trying to conserve these precious supplies.
Could it be that evolution that took billions of years is stupid or more better we are ignorant of the strategy taken by the microbes? Sometimes we have to change our paradigm and try to see the significance of things from the point of view of the microbes rather than trying to do the thinking for them






Type rest of the post here.