Kirby Bauer Test
- Following the previous lab on Tuesday, September 8th, we were to measure the diameter of the "zone of inhibition" for each of the antibiotics we put on our Kirby Bauer Test plates
- By measuring the diameter of the zone of inhibition for each antibiotic we could decide if the bacteria were susceptible or resistant to the antibiotic
- In general, if the zone of inhibitoin is greater than or equal to 12 mm, then the bacteria is susceptible to the antibiotic.
- Also, as a rule of thumb, the longer an antibiotic, such as penicillin, has been used, the more bacteria there are that are resistant to it, if any.
Serial Dilution Technique
- to determine how many baceria are in our suspension we made in the previous lab count the number of bacterial colonies on the plate that is closest in dilution to the original sample
- after you have counted the number of colonies take that number, for example, 19 colonies, times 10 to make it the number of colonies in a full mL of the contaminated broth.
- then take that number (190) times the dilution power of the dilution that you counted the colonies on for instance 10^6th power
- that will give you 190,000,000 bacterial colonies total in the original sample
Eosin Methylene Blue & Wannitol Salt Agar
- Selective and Differential Media
Selective: agar has ingredients that prevent growth of some bacteria, and permit only certain kinds of bacteria to grow
1. EMB: prevents growth of gram+ bacteria while gram- is able to grow
2. MSA: has very high concentrations of salt; the salty environment pulls water out of the bacteria that cannot handle the salty environment and causes them to shrivel up and die
Differential: has ingredients that allow you to tell/see metabolic differences between kinds of bacteria that grow on it
1. MSA Manitol: sugar that some bacteria can ferment; the fermentation of this sugar produces acid which causes the Methyl Red in the MSA to change from red to yellow at a low pH that is caused by the acid produced
2. EMB: prevents gram+ bacteria from gowing; eosin in the EMB changes from pink to a green/blue in responst to lowered pH; lactose in EMB is fermented by some bacteria which produces acid and the eosin goes from pink to green at a lowered pH
- experiment of serial dilution = be able to do the math
- gram stain interpretation = gram +/-; shape and arrangement of bacteria; draw conclusions; cell wall structure
- interpret kirby bauer test = which antibiotic susceptible and which are not
- know mechanisms of rifampin and penicillin of affecting bacteria; how they affect cell wall/RNA polymerase