Synthetic Biologics Reports First Quarter 2015 Financial Results and Operational Highlights --Initiated Phase 2 for Prevention of C. difficile, Expected to Initiate Phase 2 for IBS-C by end of June 2015, and Expanded Lea

Synthetic Biologics Reports First Quarter

2015 Financial Results and Operational Highlights

--Initiated Phase 2 for Prevention

Expected to Initiate

Phase 2 for IBS-C by end of June 2015, and

Expanded Leadership Team as Company

Moves into Next Development Stage --

-- Conference Call Today, May 11,

2015, at 4:30 p.m. EDT --

MD, May 11, 2015 - Synthetic Biologics, Inc. (NYSE MKT: SYN), a microbiome-focused,

clinical-stage company developing therapeutics to protect the microbiome while targeting pathogen-specific diseases,

reported financial results for the three months ended March 31, 2015, and provided an operational update.

"We've made great progress

during the first quarter of 2015," said Jeffrey Riley, Chief Executive Officer of Synthetic Biologics. "We've

advanced our C. difficile program into Phase 2 clinical trials, plan to initiate Phase 2 clinical trials for our irritable

bowel syndrome with constipation (IBS-C) program before the end of June, and expect to have Phase 2 data from both programs focused

on protecting the microbiome by year end. Due to the nature of these microbiome programs, the time spent in the clinic is less

than the typical drug development program, and we are positioned to create value for our shareholders at multiple points during

the remainder of 2015."

Mr. Riley added, "We've recently

expanded our leadership team with the appointment of Steve Shallcross as our Chief Financial Officer beginning June 1st,

and Maureen Early joining the team in the newly created position of Vice President, Commercial. We are very optimistic about the

potential of Synthetic Biologics to meet unmet medical needs while generating significant returns for our shareholders."

Clinical Programs Update

Prevention of C. difficile infection

Pertussis (whooping cough) - SYN-005:

A separate multi-center U.S. Phase 2 trial is underway focused exclusively on cognition utilizing Trimesta with a variety of currently

Three Months Ended March 31, 2015

General and administrative expenses increased

to $1.7 million for the three months ended March 31, 2015, compared to $1.1 million for the same period in 2014. This increase

of approximately 53% is primarily the result of increased employee costs, audit fees related to the additional procedures required

under the accelerated filer status and stock compensation expense for the three months ended March 31, 2015. Non-cash charges related

to stock-based compensation were $582,000 for the three months ended March 31, 2015, compared to $255,000 for the same period in

Research and development expenses increased

to $6.5 million for the three months ended March 31, 2015, compared to $2.7 million for the same period in 2014. This increase

of approximately 139% is primarily the result of increased program costs associated with expanded clinical development, manufacturing

and research activities within our pathogen-specific, microbiome-focused pipeline, including the Company's C. difficile,

IBS-C and Pertussis programs. Research and development expenses also include a $1.0 million expense for achieving the third milestone

as set forth in the Asset Purchase Agreement with Prev ABR LLC, dated November 28, 2012, related to the C. difficile program.

Prev ABR LLC exercised its option to receive the milestone payments in shares of Synthetic Biologics' common stock, issued

in April 2015. Non-cash charges related to stock-based compensation were $246,000 for the three months ended March 31, 2015, compared

to $107,000 for the same period in 2014.

Other expense was $4.2 million for the

three months ended March 31, 2015, compared to other income of $1,000 for the same period in 2014. Other expense for the three

months ended March 31, 2015 is primarily the result of a non-cash charge of $4.2 million related to the change in fair value of

warrants. The increase in the fair value of the warrants was due to the increase in the stock price from the previous period. There

was no non-cash income or expense relating to fair value warrants for the three months ended March 31, 2014.

Cash at March 31, 2015 was $12.0 million

compared to $17.5 million at December 31, 2014.

Synthetic Biologics will hold a conference

call today, Monday, May 11, 2015, at 4:30 p.m. EDT. The dial-in information for the call is as follows: U.S. toll free:

1-888-347-5280, Canada toll free: 1-855-669-9657 and International: +1 412-902-4280.

Participants are asked to dial in 15 minutes

before the start of the call to register. Registered callers may ask to be placed in the queue for the Question & Answer Session.

The call will also be webcast over the Internet at http://www.videonewswire.com/event.asp?id=102272.

An archive of the call will be available

for approximately 90 days at the same URL, http://www.videonewswire.com/event.asp?id=102272, beginning approximately one hour after

the call's conclusion.

About Synthetic Biologics, Inc.

Synthetic Biologics, Inc. (NYSE MKT: SYN)

is a microbiome-focused, clinical-stage company developing therapeutics to protect the microbiome while targeting pathogen-specific

diseases. The Company is developing an oral biologic to protect the gut microbiome from intravenous (IV) antibiotics for the prevention

of C. difficile infection and an oral statin treatment to reduce the impact of methane producing organisms on irritable

bowel syndrome with constipation (IBS-C). In addition, the Company is developing a monoclonal antibody combination for the treatment

of Pertussis in collaboration with Intrexon Corporation (NYSE: XON), and a Phase 2 oral estriol drug for the treatment of relapsing-remitting

multiple sclerosis (MS) and cognitive dysfunction in MS. For more information, please visit Synthetic Biologics' website at www.syntheticbiologics.com.

release includes forward-looking statements on Synthetic Biologics' current expectations and projections about future events.

In some cases forward-looking statements can be identified by terminology such as "may," "should," "potential,"

"continue," "expects," "anticipates," "intends," "plans," "believes,"

"estimates," and similar expressions. These statements are based upon current beliefs, expectations and assumptions and

are subject to a number of risks and uncertainties, many of which are difficult to predict and include statements regarding the

planned Phase 2 clinical trials, expected date of data from clinical trials and the expected date of submission of IND application

for SYN-010. The forward-looking statements are subject to risks and uncertainties that could cause actual results to differ materially

from those set forth or implied by any forward-looking statements. Important factors that could cause actual results to differ

materially from those reflected in Synthetic Biologics' forward-looking statements include, among others, a failure to receive

the necessary regulatory approvals for commercialization of Synthetic Biologics' therapeutics, a failure of Synthetic Biologics'

clinical trials, and those conducted by investigators, to be commenced or completed on time or to achieve desired results, a failure

of Synthetic Biologics' clinical trials to receive anticipated funding, a failure of Synthetic Biologics' products

for the prevention and treatment of diseases to be successfully developed or commercialized, Synthetic Biologics' inability

to maintain its licensing agreements, a failure to successfully integrate new management and other factors described

in Synthetic Biologics' report on Form 10-K for the year ended December 31, 2014 and any other filings with the SEC.

The information in this release is provided only as of the date of this release, and Synthetic Biologics undertakes no obligation

to update any forward-looking statements contained in this release on account of new information, future events, or otherwise,

except as required by law.

For further information, please contact:

Synthetic Biologics: Kris Maly, VP, Corporate

Communication, (734) 332-7800, info@syntheticbiologics.com

Media: Wendy Emanuel, Wellspring Communications,

Inc., (773) 255-9580, wendy@wellspringcom.com

Investors: Michael Polyviou, EVC Group,

Inc., (212) 850-6020, mpolyviou@evcgroup.com

- Financial Tables Follow -

Synthetic Biologics, Inc. and Subsidiaries

(in thousands, except share and per share amounts)