The broth dilution test you did not perform this test in cla

The broth dilution test (you did not perform this test in class) can be used to determine the effectiveness of disinfectants. In this test, serial dilutions of the disinfectant were set up in the wells of a microtiter plate. Equal amounts of broth culture of Staphylococcus aureus were added to each well. After incubation, the wells were examined for bacterial growth. Wells with no growth were sub-cultured in nutrient broth without the disinfectant. Results were recorded as for growth and for no growth. With data below answer the following questions. What is the minimum bacteriocidal concentration of each disinfectant? What is the minimum bacteriostatic concentration? Which disinfectant is more effective against Staphylococcus aureus? Which disinfectant is more effective against Salmonella enteric? Which disinfectant is more effective against Bacillus anthracis?

Solution

Broth-dilution test is an efficient method to determine the concentration at which any chemical, such as the disinfectant, is able to stop the growth of bacteria or kill it. Disinfectants at certain concentration can either be bacteriocidal, that kills all the bacteria in solution, or it can be bacteriostatic, that is stop the growth of bacteria. At concentration when the disinfectants act as bacteriostatic, the bacteria will again thrive once they are inoculated in fresh media without disinfectant.

a) The minimum bacteriocidal concentration of disinfectant A is 1:80 and that of disinfectant B is 1:160. The minimum bacteriocidal concentration is the dilution at which the disinfectant efficiently kills all the bacteria in the well. Hence no growth of bacteria appears in the sub-culture.

b) The minimum bacteriostatic concentration of disinfectant A is 1:100 and that of disinfectant B is 1:170. The minimum bacteriostatic concentration is the dilution at which the disinfectant efficiently stop or halts the growth of the bacteria in the well. Hence in the sub-culture, where the disinfectant was lacking, the bacteria resumed their growth.

c) The disinfectant which can kill or inhibit the pathogen even at greater dilutions is an efficient disinfectant. Hence disinfectant B is more effective against Staphylococcus aureus, the test organism used in this broth dilution experiment.

d) Salmonella enteric is a gram negative bacteria which is a causes food intoxication. This bacteria is difficult to eradicate with most disinfectants. Hence, among the given two, the effective disinfectant B could be used against Salmonella enteric.

e) Bacillus anthracis is an endospore containing bacteria which is highly resistant to disinfectants. Hence, the more effective disinfectant B could be used to temporarily stop the growth of this bacteria.