251. The Development of In Vitro Resistance of Selected Pathogens to Ertapenem and other Carbapenems
Session: Poster Abstract Session: Diagnostic Microbiology; Antimicrobial Sensitivities
Thursday, October 3, 2013
Room: The Moscone Center: Poster Hall C
Background: Ertapenem exposure may decrease susceptibility of organisms to ertapenem and other carbapenems. Previous studies have reported a decrease in Pseudomonas aeruginosa susceptibility to anti-pseudomonal carbapenems after ertapenem use. It has been hypothesized that ertapenem use may be a contributing factor to the increasing resistance rates seen with A. baumannii, P.aeruginosa, and to the  development of Carbapenem Resistant Enterobacteriaceae (CRE).

Methods: Twelve non-duplicate patient isolates (K. pneumoniae, P. aeruginosa, E. coli, S. marcescens, A. baumanii, and Enterobacter spp.) were inoculated at a 0.5-McFarland onto Mueller Hinton agar for a multipass procedure with doubling dilutions of ertapenem. Ertapenem E-tests were placed daily to continually expose each organism to ertapenem. A sample from the highest ertapenem concentration that permitted growth of the organism was re-inoculated daily for fifteen days. Meropenem, imipenem, and doripenem E-tests were placed for each organism on days zero, five, ten, and fifteen to determine minimum inhibitory concentrations (MIC).

Results: The mean ertapenem MIC increased from 0.5 to 10 over the fifteen day in vitro multipass. Ertapenem MICs increased for seven isolates, decreased for two isolates, and remained the same for three isolates. All meropenem, imipenem, and doripenem MICs increased from day zero to fifteen with the exception of five organisms, which demonstrated unchanged MICs over the fifteen day multipass. There were no meropenem, imipenem, or doripenem MICs that decreased after ertapenem exposure. The mechanisms of resistance observed in isolates with elevated MICs have not been determined; however, no organisms tested demonstrated a positive modified hodge test.

 

 

ERT

Anti-PSA Carb

 

# isolates

15d Interpret

# isolates

15d Interpret

# isolates

ERT S

7

Int

0

Int

3

 

Res

4

Res

3

ERT R

5

Int

-

Int

0

 

 

Res

5

Res

5

Conclusion: In-vitro exposure of selected organisms to ertapenem may cause an increase in MICs to other carbapenems. The potential for resistance to develop to other carbapenems, after exposure to ertapenem may limit its clinical utility and may influence practitioners to carefully evaluate empiric ertapenem use in patients at high risk for infection with resistant organisms.

Caitlin Handler, PharmD, S. Christian Cheatham, PharmD and Andrea Stock, PharmD, Pharmacy, Franciscan St. Francis Health, Indianapolis, IN

Disclosures:

C. Handler, None

S. C. Cheatham, None

A. Stock, None

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