Part V—Eureka! Mark cautiously pushed open the door to the microbiology lab and
ID: 213177 • Letter: P
Question
Part V—Eureka!
Mark cautiously pushed open the door to the microbiology lab and glanced around the room. He could
just make out the top of Anne’s head, barely visible behind several tall stacks of multi-hued bacteriological
media as she bent over a culture. “Don’t these people ever sleep?” he wondered, simultaneously admitting
to himself that his night had not been exactly restful. Mark cleared his throat, hoping to get her attention
without appearing too eager. Anne looked up, smiled, and set aside the culture she had been examining.
“Any new information on those nursery cultures, Ms. Mosel?”
“Oh, yes, of course, Dr. Maddison. We called the report up to the fl oor earlier this morning. Let me show
you what we found.” She pulled a microscope slide from a small cardboard box and placed it under her
microscope. “Take a look for yourself.”
Mark sat down and began to scan the slide. Spread out across it was a sea of round purple spheres that
looked like those he had seen on yesterday’s slide of the lesions. “So many,” he observed.
“Yes,” Anne responded. “This smear was taken from a culture plate, so there will be a large number of
organisms. Here are the plates, if you’d like to have a look.”
Mark picked up the sheep blood plate fi rst. It was covered with round, opaque creamy colonies. He held
the plate up to the light to get a better look and noticed that there was a halo around each tiny colony
where the sheep blood appeared to be missing from the agar. The plate labeled “PEA” looked nearly
identical. Mark sniffed the plate, enjoying the scent, which reminded him of the Rose Milk lotion his
grandmother used to use. Finally, he picked up the lavender plate labeled “MAC” and held it up to the
light. Except for the tracks left in the pattern of the microbiologist’s inoculating needle, the plate looked
completely barren.
“We performed catalase testing on the isolate,” interjected Anne. “You should have seen the way it
bubbled! We have a rapid agglutination test in this laboratory that we used instead of the coagulase test.
This isolate was a strong positive. There’s no doubt about its identity now, is there?”
Mark closed his eyes and rubbed his temples in an attempt to dislodge the buried bits of information.
“Why, it looks for all the world like Staphylococcus aureus!” he said, surprised by the confi dence of his
answer.
“Exactly!” replied Anne. “We have antimicrobial susceptibility testing already set up and running. Using
our automated system, we may have the results available later today.”
“Great day in the morning!” Mark jumped up from his seat, grabbed the shoulders of a very startled
Anne Mosel, and planted a kiss right in the middle of her forehead. He then turned, tossed open the laboratory
door, and trotted down the hallway toward the medical library.
Back in the library, Dr. Maddison went straight to the small pile of reference books he had been working
with the previous evening. Choosing a likely looking volume entitled Pediatric Infectious Disease, he
looked up Staphylococcus aureus in the index. There were about a dozen pages listed. “Hmmm, impetigo
is less generalized than these babies are showing… food poisoning in breast-fed babies just doesn’t
Explanation / Answer
Mark scanned the slide and found out purple spheres. In the next slide which was of sheep blood, it was full of opaque, creamy colonies and there was a halo around each tiny colony and from there the sheep blood was missing. On the third plate, it was found that strain is catalase positive. It was found that the organism that grew on the plate is Staphylococcus Aureus which is gram positive along with being catalase positive.