Since bacteria vary in size, structure, shape, and susceptibility to antibiotics, antimicrobial chemicals differ in their effectiveness and spectrum of activity. Scientists and doctors needed a way to effectively identify types of pathogens in patients, so they could administer a drug that would specifically target the organisms. Kirby-Bauer test, also known as the disk-diffusion method, is the most widely used way of testing antimicrobial substances. The effectiveness of a certain chemotherapeutic agent is determined by the size of a zone of inhibition on an inoculated agar plate. A zone of inhibition is the area where a filter paper disk soaked with an antimicrobial agent placed on the agar surface inhibits growth of inoculated microorganisms. Based on the size of a zone of inhibition and the relation to a specific chemotherapeutic agent used, organisms are classified as sensitive, intermediate, or resistant. We will find out which chemotherapeutic agents are most effective against which organisms. My initial hypothesis was that gram-positive bacteria such as Staphylococcus aureus would be susceptible to many chemical agents used in the lab, since they lack an outer membrane that protects the peptidoglycan cell wall. Whereas gram-negative bacteria such as Escherichia coli or Pseudomonas aeruginosa would be more resistant than gram-positive bacteria because they have an outer cell membrane. However, I also hypothesized that polymyxin B might not be as effective to gram positive S.aureus as other chemical agents, because since this agent was typically to bind to Lipid A in the outer cell membrane of gram-negative bacteria, polymyxin B would not be able to penetrate the thick peptidoglycan layer of S. aureus. Also, we will be able to infer the gram stain of my group's unknown organism based on the relationship between its sensitivity to other known organisms' sensitivities to different antimicrobial agents. Our group obtained four small and four large agar plates. Then we labeled each of the four small and four large agar plate with the name of the appropriate organism. The reason we are using large plates is to prevent Ciprofloxacin and Gentamicin from overlapping other agents' zones of inhibition. Next, we inoculated each plate with a sterile cotton swab soaked in the nutrient broth containing S. aureus by swabbing the swab on the entire surface of the plate. We made sure the swab is squeezed against the wall of the tube to remove excess culture broth as well. We swabbed the surface of the agar the total of three times, and between each swabbing, we turned the plate 1/3 of a turn to make sure we distributed the organism on the surface evenly. The rim of the agar dish was swabbed around to finish the inoculating process. Then we repeated the above steps with the rest of the organisms. After five minutes when the surface is dry, we