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CONSOLIDATED STATEMENTS OF BRIDGE UNITS, REDEEMABLE CONVERTIBLE PREFERRED SHARES AND STOCKHOLDERS' EQUITY (DEFICIT) (Parenthetical) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Issuance of tranche rights with preferred units |  | $ 909 | $ 2,401
Payment of offering costs | $ 3,574 |  |
Initial Public Offering [Member] |  |  |
Payment of offering costs | 3,574 |  |
Bridge Units [Member] |  |  |
Contingent prepayment option derivative liability |  | 908 |
Derivative liability |  |  | 2,307
Class A Preferred Unit [Member] |  |  |
Issuance of tranche rights with preferred units |  |  | 1,100
Payment of offering costs |  |  | 170
Class A Preferred Unit [Member] | Bridge Units [Member] |  |  |
Tranche rights derivative liability upon issuance of preferred units |  |  | $ 1,301
Class B Preferred Units [Member] |  |  |
Payment of offering costs |  | 112 |
Issuance of tranche rights derivative liability with preferred units |  | $ 909 |
Series C Preferred Stock [Member] |  |  |
Payment of offering costs | $ 176 |  |
Spero Europe, Ltd. [Member] |  |  |
Ownership percentage |  | 5.00% |
Spero Potentiator, Inc. [Member] |  |  |
Ownership percentage |  |  | 49.90%
Spero Cantab [Member] |  |  |
Ownership percentage |  | 12.50% |
Spero Gyrase, Inc. [Member] |  |  |
Ownership percentage |  | 20.00% |

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