What does it mean if Gram-negative bacteria are found in the sample? Why would they be present in the sample?

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Explanation:

In yogurt are usually found, (several) types of gram - positive bacteria. Those bacterias are used to made yogurt, and don't bring negative effects on the human body. These bacterias are Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus bacteria.

When doing a gram-staining on a yogurt sample, this staining will show a positive result if there are gram - positive bacteria present, what is most likely to happen.

In a yogurt sample, normally, no gram-negative bacteria will occur. Although, if gram - negative bacteria will be found. This means the yogurt sample is infected. Those bacteria are (probably) diarrhea causing bacteria such as E. coli and Salmonella.

If gram-negative bacteria are found in the sample, the gram-staining will be negative and show red/pink bacteria.