EXHIBIT 5.1

                           OPINION OF LATHAM & WATKINS

                          [LATHAM & WATKINS LETTERHEAD]

                                November 1, 2001

Essential Therapeutics, Inc.
1365 Main Street
Waltham, Massachusetts 02451-1624

                  Re:      Registration Statement on Form S-8
                           ----------------------------------

Ladies and Gentlemen:

         In connection with the registration by Essential Therapeutics, Inc., a
Delaware corporation (the "Company"), of an aggregate of 1,550,000 shares of the
common stock, par value $0.001 per share (the "Shares"), of the Company pursuant
to the 2001 Incentive Stock Plan (the "Plan"), on a Registration Statement on
Form S-8 (the "Registration Statement"), filed with the Securities and Exchange
Commission under the Securities Act of 1933, as amended, you have requested our
opinion with respect to the matters set forth below.

         In our capacity as your counsel in connection with such registration,
we are familiar with the proceedings taken and proposed to be taken by the
Company in connection with the authorization, issuance and sale of the Shares.
In addition, we have made such legal and factual examinations and inquiries,
including an examination of originals and copies certified or otherwise
identified to our satisfaction, of such documents, corporate records and
instruments as we have deemed necessary or appropriate for purposes of this
opinion.

         In our examination, we have assumed the genuineness of all signatures,
the authenticity of all documents submitted to us as originals, and the
conformity to authentic original documents of all documents submitted to us as
copies.

         We are opining herein as to the effect on the subject transaction only
of the General Corporation Law of the State of Delaware, and we express no
opinion with respect to the applicability thereto, or the effect thereon, of any
other laws of the State of Delaware.

         Subject to the foregoing, it is our opinion that the Shares to be
issued under the Plan have been duly authorized, and upon the issuance and
delivery of the Shares in the manner contemplated by the Plan, and assuming the
Company completes all actions and proceedings required on its part to be taken
prior to the issuance and delivery of the Shares pursuant to the terms of the
Plan, including, without limitation, collection of required payment for the
Shares, the Shares will be validly issued, fully paid and nonassessable.

         We consent to your filing this opinion as an exhibit to the
Registration Statement.

                                        Very truly yours,

                                        /s/ Latham & Watkins