How do microorganisms really grow in nature?
Our discussion on microbial nutrition and growth ended with a hypothetical experiment: a flask of rich culture media broth at optimal temperature was inoculated with Escherichia coli, and samples were removed at various time intervals post-inoculation for cell number determination. This experiment, once it was plotted on semi-log graph paper, allowed us to demonstrate the characteristic phases of microbial growth.
During lag phase, the E. coli cells are adjusting to the new environmental conditions (extremely abundant nutrients, no waste products to inhibit growth) by producing the enzymes that they will need to carry out metabolic pathways. In exponential phase, microbial growth is occurring at the highest possible rate possible for the given culture conditions. The maximal growth rate is determined by the richness of the culture media (a rich, complex medium will produce a faster growth rate than a simple, defined medium), and whether the temperature, pH, and osmotic pressure are all at optimal values. As the nutrients in the culture begin to be limited by the increasing cell number, cellular division is offset by cell death, and stationary phase ensues. It is important to note that cell growth is still occurring, just not at the rate observed previously. As conditions in the culture continue to diminish in death phase, cellular growth is completely offset by cellular death.
We indicated in lecture that these phases of microbial growth are to some extent a phenomenon of the laboratory, and that microorganisms in nature do not typically follow these phases unless there is an artificial situation to produce them. One such situation that we are all likely passingly familiar with was the massive oil spill in the Gulf of Mexico last summer. The BP disaster deposited a tremendous amount of organic material into the environment, which was followed by a massive increase in the number of microorganisms that fed on those introduced nutrients. As the number of microorganisms increased, the available nutrients began to be depleted, and microbial death subsequently occurred, in a situation mirroring the 4 phases of growth outlined above.
Most microorganisms in nature are typically going to be in a “nutrient limiting” state, and would normally be found at an area of the growth curve above somewhere near the intersection of Lag phase and Exponential phase. It turns out that we can also duplicate this situation in the laboratory, with the help of a clever device called the chemostat. The chemostat is actually quite simple: it is a flask of culture media that has an inflow port to allow sterile, fresh media to enter, and an outflow port that acts to drain the device at a rate equal to the inflow rate. The growth rate can be mathematically modeled, and is proportional to the rate of nutrient introduction via the inflow port. The number of microorganisms in the device remains constant, as the number of new cells formed by cellular division is offset by those that are lost via the outflow drain. But unlike a culture in stationary phase, the loss of cells is not due to cell death. Consequently, the situation in the chemostat is much more like a culture caught between lag phase and exponential phase.
Apart from a device that can satisfy our curiosity about how microorganisms grow in nature, the chemostat has an essential role in the biotechnology industry. Chemostats are used as bioreactors, and allow the growth of microbes in continuous culture. These microbes are used to produce a tremendous number of useful products for society, including pharmaceuticals and chemicals in everyday use.
My fascination with this topic is not purely academic! It goes back to my days in college at the University of Delaware, where I worked on an undergraduate research project growing bacteria in a chemostat. Click here to see my first publication in Pubmed!