Growing bacteria on petri dishes to determine bacterial growth has been accepted since the 1970s (Gerassi, 1976). The dishes are inoculated with bacteria, inverted and incubated, and the visible colonies are counted in order to determine how well the bacteria grew. While this technique is still common in today's professional and educational laboratories, it does have some limitations.
Only the visible colonies can be counted, which leaves out any too small to be seen with the naked eye. Bacterial plate counting also works off the theory that every colony will continue to go through binary fission and create more colonies. In any bacterial sample, there are always a few colonies that die off; these loses are not reflected in the plate counts.
Membrane filtration is used to clean up solutions such as water and media. The membrane traps microbes so the solution is not as contaminated when used for experiments or when drinking water.
However, membrane filtration does have it limitations. One of the more common problems with this filtration method is clogs. As water is filtered, small suspended solids such as iron, clay or dirt can be big enough not to pass through the filtration pores and block water flow. The same thing can happen in other solutions -- big precipitates can clog pores and slow down the filtration process.
Bunsen burners are the traditional method for sterilizing an inoculating loop. The loop is passed through the flame before being used in experiments. However, an open flame can be hazardous; this limitation is mostly seen in educational laboratories. Any flammable material accidentally left on the lab bench, such as a purse or notebook, can catch fire. To circumvent this, some labs have switched to using bacteria incinerators. These devises heat up on the inside electronically -- there is no open flame. The inoculating loop is placed inside the incinerator to be sterilized. If a student gets too close or accidentally passes a long-sleeved arm over it, there is no fire hazard.
The quadrant streak method is commonly used in the laboratory setting in order to get isolated colonies from a broth that contains more than one bacteria. When done correctly, isolated colonies appear in the fourth quadrant, ready to be placed in a new tube of broth for isolation. This will generate a pure sample of one of the bacterial strains.
However, this technique is limited by human error. If a person has not practiced this technique, the fourth quadrant will either not grow isolated colonies or grow too many colonies that can't be distinguished from one another. Also, that quadrant may be covered with fungal growth, which usually obscures the colonies underneath.
Human error can play a role in most microbiology techniques, whether traditional or more modern. The trick is to continue practicing the technique in question so protocol execution isn't as sloppy.