Disk-Diffusion Lab Report

Introduction Microorganisms have been long discovered to be the primary source of many infectious diseases and have led many scientists to the development of antimicrobial drugs. It is most commonly observed in many strains of bacteria that they have evolved to become resistant/susceptible against antibiotics. In 1928, it was originally proposed to perform a broth dilution method in order to investigate the resistance/susceptibility of bacteria to antimicrobials, this method was technically demanding and novel techniques soon began to make way (Hudzicki, J. 2013). By the 1950’s various microbiology labs had developed different protocols to the well-adopted disk-diffusion method; it became evident that researchers were publishing disk-diffusion

After growing the bacteria of interest on a Muller-Hinton agar plate, we added 12 different antimicrobial filter paper disks. The lack or presence of growth around the disks is how we will assay the ability of the antibiotics to inhibit growth of the pathogenic organism. On a wider scale the Kirby-Bauer susceptibility test can assist physicians in selecting treatment on their patients that carry infectious diseases; by determining bacterial resistance to antimicrobials physicians can provide accurate care in a timely manner. The Kirby-Bauer method has been standardized and is a practicable alternative to the arduous broth dilution technique, making it the best method to test our hypothesis. The main characteristics that have made the test standardized are: the incubation temperature at 37ºC, the depth of the agar, a complex media, the concentration of the antimicrobial, and that the organism is equally spread out on the agar (Bauman, R.W.,

Positioning of filter disk that contains a universal concentration of a specific antimicrobial on a Muller-Hinton (MH) agar plate completed the first procedures of the experiment. It is commonly observed that the disk absorbs water and then spreading out of the antimicrobial into the agar occurs. It’s been published that the concentration of the antimicrobial is highest closest to the disk and logarithmic reduction in concentration occurs as the distant from the disks increases (Talaro, K.P., 2008). Antimicrobial diffusion rate across the agar is dependent on the molecular weight of the compound; larger molecules will diffuse slower compared to smaller molecules. (Talaro, K.P., 2008). Each antimicrobial has an endpoint zone that will demonstrate the resistance/susceptibility to a specific bacteria (Talaro, K.P., 2008). Once the critical mass is reached, a sharp circle of bacterial growth around the disk termed the Minimal Inhibitory Concentration (MIC) is obtained by measuring the diameter of the region surrounding each antibiotic disk, we are able to determine whether our organisms was susceptible or resistant to each antimicrobial by comparing it to the literature values (Sato,B.K. et al.,