Question

Develop a simple experiment to test the given microbes in the various salt solutions and include a method to determine growth. Predict the results of your experiment. You are provided nutrient broth cultures of the following microbes: Escherichia coli, Staphylococcus epidermidis, and Halobacterium salinarium.

The availability of water is defined by a physical parameter called the water activity, Aw. It is determined by measuring the ratio of the water vapor pressure of a solution to the water vapor pressure of pure water. The values for water activity vary between 0 and 1.0, and the closer the value is to 1.0, the more water is available to a cell for metabolic purposes.

Answer 1

Bacteria are classified according to its ability to tolerate salt concentration in the medium. The bacteria which need some amount of salt in their medium are termed as halophiles. the bacteria which require 1-6% salt are called mild halophiles, moderate halophiles require 6-15% salt and extreme halophiles require 15-30% salt for their growth. If water activity is lower solute concentration is high and if water activity is higher, solute concentration such as NaCl is low. Generally microorganisms's range of water activity (A_w) for growth is 1.0 to 0.7.

In the experiments three sets (I,II,III) of petridishes containing growth medium will be taken. Each set will contain three petridishes (IA,IB,IC,IIA,IIB,IIC,IIIA,IIIB,IIIC). Set I will contain NaCl 3%, set II will contain NaCl at 10% and Set-III will contain NaCl at 30%. all the plates A of three sets will contain E.coli, all the B plates of all three sets will contain Staphylococcus epidermidis and similarly Plates C of all the three sets will contain Halobacterium salinarium. All the plates will be incubated at optimum condition and after 48 hours growth will be evaluated.  

The bacteria which will grow at plate of set III will be extremely halophiles such as Halobacterium salinarium, moderately or little halophiles will not grow in set III plates. so Plate B ad A of set-III will not show any bacterial growth. Set II will contain moderately and extreme halophiles bacteria, so plate C and Plate B of set II will contain growth of Staphylococcus epidermidis and Halobacterium salinarium, but E.coli on plate A of set II will show no growth. Plates (A,B and C) of Set-I will contain growth of E. Coli ,Staphylococcus epidermidis and Halobacterium salinarium. So by observing growth at different plates of different sets we can determine halophilicity of different bacteria.