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Basics of Bacterial Growth

Bacterial growth is followed by observing changes in the population of a culture.  Watching individual cells grow would be a very difficult process - they are tiny even under a microscope!  Instead, we consider population growth as the indicator of the product of cell growth.  So we look at how numbers change as an indicator of whether maximum reproduction and growth are present.  Anything that is done physically or chemically to slow the process or stop it can be used as a method to control growth.  Think refrigerator or cooking or adding salt and sugar to foods and you'll get an idea of what the upcoming "Controlling Growth" section of MOC-Lab is all about.

So how do bacteria grow?  In previous biology classes we have learned that cell division is accomplished through a process called mitosis.  Cells end their interphase period by entering prophase where the nuclear envelope disappears and the chromosomes take their visible string-like form.  Then in metaphase the chromosomes align across the equator of the cell and a spindle apparatus connects to them.  In anaphase the chromatids separate and are pulled by the spindle into the two future-daughter cells.  In telophase the daughter cells are packaged up for separation into two cells.  One cell becomes two through a process of mitosis (karyokinesis) and cell division (cytokinesis).

If you have already completed the section on Microbial Anatomy, you should have seen problems with using this description for bacteria.  Red flags should have been raised when nuclear envelope, string-like chromosomes, and spindle apparatus were all mentioned because bacteria do not have any of these structures.  Mitosis cannot happen in bacteria (prokaryotes) because they lack the cell structures needed for the process.  Mitosis is a eukaryote process.  Instead of mitosis, prokaryotes (bacteria) replicate by a process called binary fission.

In binary fission, bacteria are split into two daughter cells through a process that insures both daughter cells are complete and equipped for life.  The process is started when a key protein called "initiator protein" (shown in red) reaches critical levels, signalling all conditions are acceptable to begin the processes of DNA replication and cell division.  

The first step is to attach the circular genome to a receptor in the cell membrane so that it is anchored there.  Then, DNA replication begins moving around the circle of DNA to produce two copies of the genome (one is pink, the other blue).  In the process, the membrane anchor ("ori") is replicated and proteins are deposited between the two oris, pushing them apart.  Just as spindle fibers separate chromosomes in eukaryotes during mitosis, the separation of the ori proteins pushes the two daughter genomes apart into future daughter cell regions of the dividing cell.  

As the process of DNA replication moves along, cytokinesis (cell division) begins.  The membrane is pulled in like a tightening belt to divide the cytoplasm into two chambers. Special proteins stuff the remaining genomes into their respective compartments.  Cell wall material is deposited outside the membrane to create two complete cells.  The number of cells increases and the population grows.  One of the excellent videos at the Howard Hughes Medical Institute shows this process. 

The number of cells in a healthy culture grows logarithmically (exponentially) rather than arithmetically.  One becomes two, two become four, four become eight, and so on until the number reaches staggering heights. The term used to describe the length of time needed to double the population is "generation time", and some bacteria under ideal conditions can have a generation time of 20 minutes. Under such conditions, how long would it take for a single cell to produce a population larger than the world's human population?  The answer is staggering!  But also remember that under less favorable conditions, the growth rate reflected in the generation time is slower, and we use this fact to our advantage daily to prevent microbes from spoiling foods and causing diseases.

Microbiologists will often study bacterial growth using batch cultures of bacteria.  A flask of growth medium is inoculated with cells and samples are observed over time, either by looking at the cloudiness (turbidity) of the culture (the cloudier a culture, the more cells are present - think fresh iced tea vs. old tea in the refrigerator), or by counting the cells using special methods.  The results are often depicted in something called the "bacterial growth curve".  

This is a graphical representation of the population changes detected by these methods.  The X axis represents time (the linear constant) and the Y axis represents the number of cells (the logarthmic variable).  Generally, the results will produce a typical pattern with four phases:  the lag phase, the exponential (or logarithmic or "log") phase, the maximum stationary phase, and the death (or decline) phase.  In the lag phase, there is little or no increase in cell numbers because the cells are taking stock of the nutrients in the medium and adjusting their metabolism by synthesizing enzymes needed to use the nutrients.  So there would be an increase in RNA and protein during this process as new enzymes are being made.  Also, that initiator protein is being made to signal the cell that it is time to start dividing!  In the log phase, the cell begins DNA replication needed to drive cell division and cells begin increasing in number.  This process quickly increases the number of cells and is only limited by the depletion of nutrients and accumulation of toxic waste from metabolism.  When these limiting factors begin to take their toll on growth, cells fight to stay alive and the culture enters maximum stationary phase.  Cell numbers will not increase anymore, as survival takes precedence over binary fission.  Cells begin to look ragged and unhealthy as they recycle waste and even their own structures to supply materials to make energy for keeping life processes going.  The result is a phenomenon rare in nature - zero population growth.  Finally, the waste and lack of resources become too great to survive and cells begin to die exponentially - the more that die, the more waste and toxic materials around and the more are affected.  Just as growth accelerated logarithmically, death accelerates logarithmically until the culture is dead.

One final observation on bacterial growth...  Bacteria differ greatly in the nutrients they use and in the products they generate - remember, we said this is the basis for how we identify bacteria!  So, microbiology labs have an abundance of media with different formulations to study how bacteria grow in them.  Some media are general purpose and will grow most common bacteria.  But others have special ingredients (or are missing key ingredients) to distinguish those with a metabolic capability from those that don't.  Some tests are as clear cut as "yes it grows in this" or "no it doesn't".  Understanding the ways media are manipulated to study microbial growth is central to any microbiology lab.

To study this topic using VUMIEtm 2012, work through Micro Digital Media Exercise 5.  Follow your instructor's directions for this assignment.  

When done, take a quiz on the topic.