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Journal of Clinical Microbiology, November 2006, p. 4227-4228, Vol. 44, No. 11
0095-1137/06/$08.00+0     doi:10.1128/JCM.00722-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Susceptibilities of Propionibacterium acnes Ophthalmic Isolates to Ertapenem, Meropenem, and Cefepime

Robert Shames,¹ Farhana Satti,¹ Ernestine M. Vellozzi,² and Miriam A. Smith^1*

Division of Infectious Disease,¹ Division of Microbiology, Long Island Jewish Medical Center, Albert Einstein College of Medicine, New Hyde Park, New York 11042²

Received 5 April 2006/ Returned for modification 26 June 2006/ Accepted 1 September 2006

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We investigated the in vitro susceptibilities of 23 Propionibacterium acnes ophthalmic isolates to ertapenem, meropenem, and cefepime by utilizing the Etest. The MICs ranged from 0.094 µg/ml to 0.75 µg/ml, 0.094 µg/ml to 1.5 µg/ml, and 1 µg/ml to 12 µg/ml for ertapenem, meropenem, and cefepime, respectively. Based on our excellent in vitro carbapenem susceptibility results, in vivo studies using ertapenem and meropenem in a rabbit model of P. acnes endophthalmitis are warranted.

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Propionibacterium acnes, an anaerobic, gram-positive bacillus, is known to be one of the most frequent causes of postsurgical endophthalmitis. Direct intravitreal antibiotic instillation has been shown to be safe and effective for the treatment of many aerobic organisms (8, 9, 11). Optimal treatment of P. acnes, which has been shown to cause a chronic, indolent infection after cataract surgery and lens implantation despite intravitreal vancomycin instillation, vitrectomy, and removal and exchange of the intraocular lens, remains unknown (2, 4). Despite recent research indicating the susceptibility of P. acnes to older beta-lactams, fluoroquinolones, and macrolides, in vivo success with antibiotics that are currently available for intravitreal instillation has been inconsistent, and additional antibiotic alternatives are needed (6, 10).

In the present study, the in vitro susceptibilities of P. acnes ophthalmic isolates to two of the newer carbapenems, ertapenem and meropenem, and the "fourth-generation" cephalosporin cefepime were investigated. Cefepime was of interest because of previous reports demonstrating very good in vitro activity against P. acnes using the broad-spectrum cephalosporin ceftriaxone (6, 10). In addition, cefepime provides a broad spectrum of activity and may be useful in the treatment of P. acnes infections. Further studies using a rabbit model of P. acnes endophthalmitis would be justified if this study illustrates in vitro susceptibility to any of the three study drugs.

The P. acnes isolates were identified using the Vitek Rapid ANI system (bioMérieux). The MIC (in µg/ml) of each of the test antibiotics was determined using the Etest (AB Biodisk of North America, Piscataway, NJ) (3, 10). The Division of Microbiology at the Long Island Jewish Medical Center, New Hyde Park, N.Y., supplied 23 clinical ophthalmic isolates of P. acnes obtained from eyes of infected patients in addition to control strains (P. acnes ATCC 11827, P. acnes ATCC 11828, and Bacteroides fragilis ATCC 25285) used in this study. Twelve of the 23 P. acnes strains were the same isolates used in our previous study (6). Ceftriaxone, clindamycin, and metronidazole were also added to the antibiotic test battery. These antibiotics had been evaluated against some of the P. acnes strains that were used in our previous study and served as additional experimental controls. Susceptibility testing was performed using the Etest. In our previous study, equivalent MIC results were obtained using the Etest and the Clinical and Laboratory Standards Institute (CLSI) (formerly NCCLS) method of agar dilution for testing of P. acnes isolates (5, 10). All 23 P. acnes isolates were tested in triplicate for susceptibility to the following six antibiotics: ertapenem, meropenem, cefepime, ceftriaxone, clindamycin, and metronidazole.

Test isolates and control organisms were first grown on commercially prepared 5% sheep blood agar (Becton Dickinson Microbiology Systems, BBL, Cockeysville, Md.) for 48 h at 35°C under anaerobic conditions using an anaerobic gas-generating system (BBL GasPak system) (7). Inocula for the susceptibility studies were prepared by suspending a culture grown for 48 h in reduced brucella broth (BBL) to achieve a final concentration equivalent to a 1.0 McFarland nephalometer standard (1.5 x 10⁸ CFU/ml). Etest susceptibility testing was performed on sheep blood-enriched Mueller-Hinton agar. Based on our previous study, a comparison of P. acnes growth and susceptibility studies performed using this medium with supplemented brucella agar gave equivalent results and supported the choice of sheep blood-supplemented Mueller-Hinton agar for this study (10). Etest strips of each of the test antibiotics were then placed onto the agar surface, and the plates were incubated at 35°C under anaerobic conditions. MICs were determined and recorded after 48 h of incubation (7). The MIC was defined as the point on the epsilometer scale at which the ellipse of growth inhibition met the test strip. Any intercept value that fell between two points on the scale was rounded up to the next higher value (6).

The MICs obtained for all six of the test antibiotics are listed in Table 1. The P. acnes isolates showed minor variability in their susceptibilities to both carbapenems tested. The MICs for ertapenem ranged from 0.094 µg/ml to 0.75 µg/ml, and the MICs for meropenem ranged from 0.094 µg/ml to 1.5 µg/ml. These MICs are all well within the susceptible range (4.0 µg/ml) for anaerobic organisms. The MICs for ertapenem and meropenem with respect to the control B. fragilis strain were also within the susceptible range (0.064 to 0.25 µg/ml) (5). The P. acnes strains demonstrated MICs ranging from 1.0 µg to 12.0 µg/ml when tested with cefepime. However, because of the lack of standard reference range values established for cefepime and for the testing of anaerobic organisms, the MICs obtained cannot predict the clinical utility of this antibiotic in P. acnes infections. As expected, all P. acnes strains were resistant to metronidazole (>256 µg/ml). The MIC results obtained for clindamycin and ceftriaxone were consistent with MICs reported previously for this organism (3, 6, 10).

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TABLE 1. Antimicrobial susceptibilities of 23 clinical isolates of P. acnes

Definitive treatment of P. acnes endophthalmitis is controversial. Direct intravitreal instillation of antibiotics to treat endophthalmitis caused by many aerobic organisms has been shown to be safe and effective. However, optimal management of P. acnes following cataract extraction and lens implantation remains unknown, as P. acnes can cause a chronic, indolent infection. It is possible that the organism remains secluded on the posterior portion of the capsular bag in a protected environment that prevents certain antibiotics from achieving a concentration that is high enough for eradication (1, 4).

The 23 P. acnes ophthalmic isolates demonstrated excellent in vitro susceptibilities to the above-described carbapenems. To our knowledge, there are no pharmacokinetic data for intravitreal installation of meropenem and ertapenem. Based on the MIC data for meropenem and ertapenem presented in this investigation, further studies of the pharmacokinetics, clinical utility, and efficacy of these antibiotics in an experimental rabbit model of P. acnes endophthalmitis are warranted.

 
* Corresponding author. Mailing address: Long Island Jewish Medical Center, Division of Infectious Disease, Staff House Rm. 226, Lakeville Rd., New Hyde Park, NY 11042. Phone: (718) 470-7290. Fax: (718) 470-0637. E-mail: msmith{at}lij.edu.

Published ahead of print on 13 September 2006.

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Journal of Clinical Microbiology, November 2006, p. 4227-4228, Vol. 44, No. 11
0095-1137/06/$08.00+0     doi:10.1128/JCM.00722-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Reprints and Permissions Copyright Information Books from ASM Press MicrobeWorld Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Shames, R. Articles by Smith, M. A. Search for Related Content PubMed PubMed Citation Articles by Shames, R. Articles by Smith, M. A.