The Eli and Edythe L. Broad Institute of MIT and Harvard (), often referred to as the Broad Institute, is a biomedical and genomic research center located in Cambridge, Massachusetts, United States. The institute is independently governed and supported as a 501(c)(3) nonprofit research organization under the name Broad Institute Inc., and is partners with Massachusetts Institute of Technology, Harvard University, and the five Harvard teaching hospitals.
The following are equivalent to "We are in partnership together.":
- I am partners with you.
- You and I are partners.
- I am your partner.
- I am a partner with you.
I have deliberately listed them as pairs. The first two emphasize that I am in a dual relationship (me and you); the second emphasizes my singular role in a mutual relationship.
The example in the question is more complicated. How many partnerships are there? Are there three -- one between Broad Institute and each of MIT, Harvard, and the hospitals?* Or is there only one, the one between Broad Institute and a unified consortium of MIT, Harvard, and the hospitals?
Grammar can take you only so far, and the grammar here doesn't tell us for sure, but MIT and Harvard University are separate and independent institutions, so there are likely three partnerships involving the Broad Institute, making the meaning
Broad Institute Inc. is a partner with each of the Massachusetts Institute of Technology, Harvard University, and the five Harvard teaching hospitals
and making the plural partners appropriate.
If the singular had been used, on the other hand:
Broad Institute Inc. is a partner with the Massachusetts Institute of Technology, Harvard University, and the five Harvard teaching hospitals
you would be justified in concluding that the Broad Institute was one partner and the other was a group of two universities and five hospitals.
*I'm assuming here that the five hospitals act together as one entity. If not, my count is low.