Patient is a 60 year old male recently returned from biking/camping expedition around Lake Michigan. Patient reports eating primarily at fast food restaurants during this week-long excursion. Presents with fever, nausea, abdominal cramps and diarrhea. Stool cultures indicate the presence of a prokaryote. A Gram stain shows pink helical-shaped colonies.

Based on this result, is the microorganism Gram positive or Gram negative?

This is because initial symptoms are shown by both of the bacterias. But this Gram stain shows pink helical-shaped colonies. This confirms that infected microorganism is gram negative bacteria.

Actually A Gram stain is colored purple. When the stain combines with bacteria in a sample, the bacteria will either stay purple or turn pink or red. If the bacteria stays purple, they are Gram-positive. And If the bacteria turns pink or red, they are Gram-negative.

Reason behind it is Gram-positive bacteria have a thick mesh-like cell wall made of peptidoglycan (50-90% of cell envelope), and as a result are stained purple by crystal violet, whereas gram-negative bacteria have a thinner layer ( only 10% of cell envelope), so do not retain the purple stain and hence are converted to pink in colour.