To allow for you to get some more familiarity with the Kirby Bauer test, below is a sample set of data recorded from a different Kirby Bauer test that was performed on 4 different bacteria (Bacillus subtilis, Staphylococcus epidermidis, Escherichia coli and Pseudomonas aeruginosa) using the 8 different antimicrobial drugs listed. Using the Interpretation chart below, determine if each bacterium is susceptible (S), intermediate (I) or resistant (R) to a particular drug. Also, list if each antimicrobial drug is broad or narrow in its spectrum of activity. Kirby Bauer Bacillus subtilis Staphylococcus epidermidis Escherichia coli Pseudomonas aeruginoa Antimicrobial Agent Zone Dia S, R, or I Zone Dia S, R, or I Zone Dia S, R, or I Zone Dia S, R, or I Broad or Narrow Chloramphenicol 17 20 38 0 Gentamicin 13 16 32 14 Penicillin 0 45 0 0 Rifampin 8 27 21 17 Streptomycin 6 16 28 0 Tetracycline 28 29 35 23 Vancomycin 19 25 0 0 Sulfanomides 11 22 33 0 Using Antiseptics and Disinfectants To Control Microbial Growth Below is a sample set of data recorded from an Antiseptic & Disinfectant susceptibility test similar to the Kirby-Bauer test. In this experiment, 2 different bacteria (Staphylococcus epidermidis, and Pseudomonas aeruginosa) were inoculated on to nutrient agar plates. Then filter disk immersed in the following 11 antiseptics & disinfectants were placed on the plates and allowed to diffuse. The zone of inhibition for each organism in the presence of each antiseptic or disinfectant was recorded in the table below. Use this data to answer the questions 1-3 below. 1. The larger the zone size, the more ____________ the bacterium is to that chemical agent. 2. Based on the sample data of antiseptics and disinfectants, which chemical agent seems to be most effective? Describe its mode of action. 3. In general, how does the susceptibility of Staphylococcus compare with the susceptibility of Pseudomonas to various antiseptics & disinfectants? Speculate on why one bacterium is more resistant than the other. Using UV Radiation To Control Microbial Growth Below is a sample set of data recorded from an experiment using ultraviolet radiation to control microbial growth. In this experiment two bacteria (Serratia marcescens and Bacillus subtilis). In the experiment each organism was inoculated onto a nutrient agar petri dish then exposed to various times of UV exposure (see chart below). After being exposed to the UV light, the petri dishes were incubated for 48 hours and the amount of growth was recorded. Use the following key to determine the amount of growth on the plates: 0 = no growth + = 1-3 colonies ++ = 4 -10 colonies +++ > 11 colonies Exposure Times 30 sec 1 min 2 min 5 min 10 min 15 min 20 min 30 min 45 min 45 min w/paper covering Serratia marcescens survival amount +++ +++ +++ ++ ++ + 0 0 0 +++ 90 sec 3 min 5 min 10 min 20 min 30 min 45 min 60 min 90 min 90 min w/plastic lid covering Bacillus subtilis survival amount 1. Notice in the table above, only the results for Serratia marcescens has been recorded. It is known that it typically takes about 3-4 times longer exposure to kill Bacillus subtilis. Based on this information, predict and fill in the table above for Bacillus subtilis. Use the appropriate amount of plus signs, “+”, as indicated in the key above to complete the chart for Bacillus subtilis. 2. What properties of Bacillus subtilis allow for it to withstand longer exposure times and not die as readily? 3. Look at the last exposure time for Serratia marcescens (highlighted in yellow). Notice if a piece of paper is placed over the petri dish during the UV exposure, growth of the organism occurs. What does this suggest about the ability for UV to penetrate through a piece of paper? 4. Research whether or not UV-C (used to control microbial growth) has the ability to penetrate through plastic. Based on this, predict and fill in the last column (90 min w/plastic lid covering) in the data table above. 5. If a bacterial species is not susceptible to an antibacterial drug at the concentration present in a particular disk, does that necessarily mean the species is completely resistant to the drug? Explain your answer.
