(5) A group of students evaluated the microbicidal effectiveness of the followin
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(5) A group of students evaluated the microbicidal effectiveness of the following 6 chemicals with the Disk-Diffusion Method. CHEMICALS WITH ZONES OF INHIBITION (MM) 10% L 30 Pinesol 25 2% Iodine 70 10% Bleach 80 45 60 They concluded that 3% hydrogen peroxide, 10% bleach, and 2% iodine would be efective antiseptics since they showed the largest zones of intboon Would you agree? Explain. 5) Do you agree with the followng statement? When evaluating the effectiveness of antbiotics in the inhibition always indicate greater antibiotic effectiv eness Defend your answer. (This question has nothing to do with the previous question)Explanation / Answer
5A) In the disk diffusion method, an antimicrobial coated disk is placed on an agar plate inoculated with test bacteria. The antimicrobial diffuses through the agar and inhibits the growth of the test bacterium. The concentration of the antimicrobial decreases as it moves away from the disk. Larger the clearing or zone of inhibition, more effective is the chemical. 10% Bleach, 2% iodine and 3% hydrogen peroxide show 80mm, 70 mm and 60 mm zone of inhibition respectively. These are much larger than for the other antiseptics. Hence, they are most effective antiseptics with bleach being the best.
5B) The Kirby-Bauer disk-diffusion method, is the most widely used antibiotic susceptibility test. It relies on the inhibition of bacterial growth measured under standard conditions. Normally, a culture medium agar is inoculated with the test organism. Filter paper discs impregnated with particular antibiotic are placed on the medium. The organism grows on the agar plate while the antibiotic will inhibit the growth. If the organism is sensitive to the antibiotic, no growth will be observed around the disc containing the antibiotic. A “zone of inhibition” is observed. The size of inhibition will determine the susceptibility to the antibiotic.
Normally, a 4 mm agar plate is poured for this test. However, if agar plate is <4 mm is prepared, large zones may be seen, indicating false positive results. If the plate is > 4mm, small zone will be seen, indicating false negative results. Thick agar slows lateral diffusion of the antibiotic. Similarly, if there is inequal number of bacteria distributed in the plate, error may result. Hence, normally zone of inhibition do indicate effectiveness of the antibiotic, it depends on the agar depth, the presence of contamination etc
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