07 Oct

Counting Bacteria

Counting bacteria is critical laboratory technique.  Accurate bacterial counts are needed for grading milk, determining if a patient has a bladder infection, or for determining the level of bacteria in drinking water.  In some cases it is important to know the quantity of bacteria and in other cases it is important to know whether or not bacteria are present.  There are different methods for counting bacteria and they all have their advantages and disadvantages.  Which method is applicable depends on the application.  One can use a microscope to count bacteria by using a chamber with a grid.  The grid is of known dimensions which allows the user to get an idea of how many bacteria are present in a given sample.  The most probable number method is used by relating the growth pattern of a given species to statistical analysis from previous tests.  One can inoculate lactose in test tubes with drinking water and observe acid and gas production.  This method is only useful if statistical tables have already been set up.  The most common method of counting bacteria is the standard plate count.  In this method a sample is diluted in a series of dilution blanks.  Samples of each dilution are plated and colonies are counted in 24-48 hours.