Serratia Marcescens

Doctors must know which antibiotic kills each disease-causing bacterium. We, Chelsea and Justin, will find out how Erythromycin, Penicillin, and Ampicillin are affected by the bacteria Serratia marcescens. Our hypothesis is that Penicillin will have the largest ring of inhibition, the ring around the disk where the antibiotic has killed bacteria. We believe Penicillin will be the most common antibiotic that we have heard of. The materials we used to make this test possible consists of a sterile nutrient agar Petri dish, the bacteria Serratia marcescens, Erythromycin antibiotic disk, Ampicillin antibiotic disk, Penicillin antibiotic disk, sterile disk of blank filter paper, marking pen, long – handled cotton swab, forceps, metric ruler, and a 37°C incubator. For our procedure we first filled our sterile Petri dish with agar, just so that the bottom of it was covered, then waited a day for it to condense. Then after it had hardened, we then we turned it over and marked the back of the Petri dish into four quadrants with dotted lines and marked them one through four. After that we then took a long – handled cotton swab and spread the bacteria Serratia marcescens over the agar in the Petri dish. Then with forceps we took the sterile disk of blank filter paper and placed it in quadrant number one for the control. Then with forceps took an Erythromycin antibiotic disk and placed it in the quadrant number two. Then again with forceps took a Penicillin antibiotic disk and placed it in quadrant number three. Then again for the last time, with forceps took an Ampicillin antibiotic disk and placed it in quadrant number four. We then, for the last step, placed the Petri disk cover back on and put it in the 37°C incubator and waited a day for the results to see how the three antibiotics, Erythromycin, Penicillin, and Ampicillin would affect the bacteria Serratia marcescens. The next day after we took the Petri dish out of the incubator we noticed immediately the affects. We first saw that each of the antibiotic disks had a zone of inhibition around them and that the sterile disk of blank filter paper had done nothing to the bacteria. And around each zone of inhibition we realized there were red clusters of bacteria, clear evidence that we used the bacteria Serratia marcescens because of the red tint it has at room temperature. Around the control disk in quadrant number one was where the most bacteria were. The Penicillin antibiotic disk, in quadrant number three, had just a little less bacterium than quadrant one. And the Erythromycin antibiotic disk in quadrant two even less bacteria. But in quadrant four was where we found the least bacteria. In conclusion, we have reason to believe that Ampicillin work the most affectively on the bacteria Serratia marcescens because it can penetrate and prevent the growth of gram – negative bacteria, unlike penicillin. And Serratia marcescens is a gram – negative bacteria. This is why penicillin only had a .2 cm radius of a zone of inhibitions where as Ampicillin had a 1 cm radius. And we also think Erythromycin worked more affectively than Penicillin because Ampicillin and Erythromycin are both used to treat some of the same diseases such as gonorrhea. But Penicillin doesn't treat any of the same diseases and Ampicillin or Erythromycin. Also both Ampicillin and Erythromycin have some of the same active ingredients, for instance titanium dioxide. And Serratia marcescens is resistant to several antibiotics because of the presence or R – Factors on plasmids. So we, Justin and Chelsea, determine the most effective antibiotic to treat an infection that Serratia marcescens caused would be Ampicillin.