LETTER
Previous published reports have shown that group B Streptococcus (GBS) isolates were 100% susceptible to both penicillin (1, 2) and vancomycin (1); however, reduced susceptibilities to penicillin have been documented (3–8). GBS susceptibilities to clindamycin and erythromycin have varied (2, 9–12), and data regarding inducible clindamycin resistance are not always reported (2). We examined GBS susceptibilities to clindamycin, erythromycin, penicillin, and vancomycin from isolates obtained from prenatal screening samples.
Vaginal/rectal ESwab (Copan Diagnostics Inc., Murrieta, CA) samples submitted for routine prenatal GBS screenings were evaluated. GBS was isolated and identified using Strep B Carrot Broth (Hardy Diagnostics, Santa Maria, CA) subcultured to GBS Detect (Hardy Diagnostics, Santa Maria, CA) as previously described (13). GBS Detect is a selective medium, and it allows for the isolation of GBS, including strains that are typically nonhemolytic on sheep blood agar. Immediately following isolation, GBS isolates were subcultured to sheep blood agar plates (Remel, Lenexa, KS) and incubated aerobically for 18 to 20 h at 35°C. Antimicrobial susceptibility testing (AST) was performed according to 2013 CLSI guidelines (14) on 102 GBS isolates using vancomycin and benzylpenicillin G (0.016 to 256 μg/ml) Etest strips (bioMérieux, Durham, NC) and on 387 GBS isolates using clindamycin (2 μg) and erythromycin (15 μg) BBL Sensi-Discs (Becton, Dickinson, and Company, Sparks, MD) placed 12 mm apart to detect inducible clindamycin resistance.
A total of 102/102 (100%) GBS isolates examined were susceptible to penicillin and vancomycin (Table 1). Both the MIC50 and MIC90 for penicillin were 0.064 μg/ml, and both the MIC50 and MIC90 for vancomycin were 1.0 μg/ml (data not shown). GBS isolates were susceptible to both clindamycin and erythromycin in 195/387 (50.4%) isolates and resistant to both clindamycin and erythromycin in 106/387 (27.4%) isolates (data not shown). An additional 8% (31/387) of isolates had a positive D-test, indicating inducible clindamycin resistance. The overall susceptibilities of GBS isolates to clindamycin were 62.8% (243/387) or 70.8% (274/387), accounting for or not accounting for inducible clindamycin resistance, respectively. The overall susceptibility of GBS isolates to erythromycin was 51.7% (200/387) (Table 1).
TABLE 1.
Antibiotic susceptibilities of GBS isolates
| Agent | No. (%) of isolates |
No. of isolates tested | ||
|---|---|---|---|---|
| Susceptible | Intermediate | Resistant | ||
| Clindamycina,b | 243 (62.8) | 2 (0.5) | 142 (36.7) | 387 |
| Clindamycina,c | 274 (70.8) | 2 (0.5) | 111 (28.7) | 387 |
| Erythromycina | 200 (51.7) | 12 (3.1) | 175 (45.2) | 387 |
| Penicillind | 102 (100) | 0 | 102 | |
| Vancomycind | 102 (100) | 0 | 102 | |
Testing was performed by disk diffusion.
D-test-positive isolates are reported as resistant.
D-test data are excluded.
Testing was performed with Etest.
Current recommendations for GBS in pregnant women include AST of GBS strains isolated from penicillin-allergic women at high risk for anaphylaxis, screening for inducible clindamycin resistance when AST is performed, and reporting GBS isolates with inducible clindamycin resistance as clindamycin resistant (15). However, penicillin allergy may not always be known at the onset of labor. Our laboratory routinely performs clindamycin and erythromycin susceptibility testing on all GBS strains isolated from prenatal screening samples. Here we report rates of susceptibility of GBS to clindamycin, erythromycin, penicillin, and vancomycin that are very similar to those of the 2010 Active Bacterial Core (ABC) surveillance report by the CDC (2), although that report did not account for inducible clindamycin resistance and only 396/1,427 (28%) GBS isolates were resistant to clindamycin. In this study, without accounting for inducible clindamycin resistance, only 111/387 (28.7%) GBS isolates would be reported as resistant to clindamycin. However, 31/387 (8%) isolates had a positive D-test and 142/387 (36.7%) isolates were reported as clindamycin resistant. Finally, 102/102 (100%) GBS isolates examined were susceptible to penicillin and vancomycin; both the MIC50 and MIC90 for penicillin were 0.064 μg/ml, and both the MIC50 and MIC90 for vancomycin were 1.0 μg/ml. Monitoring the antimicrobial susceptibilities of GBS isolates remains important considering the extensive use of antibiotics used for prophylaxis during the labor of pregnant carriers. These data also demonstrate the importance of including methodologies such as the D-test in AST of GBS isolates to detect inducible clindamycin resistance.
ACKNOWLEDGMENTS
We thank the staff at the University of Michigan Health System in the Clinical Microbiology Laboratory for technical expertise related to this study.
We have no conflicts of interest to declare.
Footnotes
Published ahead of print 16 July 2014
REFERENCES
- 1.Brandon M, Dowzicky MJ. 2013. Antimicrobial susceptibility among Gram-positive organisms collected from pediatric patients globally between 2004 and 2011: results from the tigecycline evaluation and surveillance trial. J. Clin. Microbiol. 51:2371–2378. 10.1128/JCM.00157-13. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 2.Centers for Disease Control and Prevention. 2012. Antimicrobial susceptibilities among group B Streptococcus isolates—Active Bacterial Core Surveillance (ABCs), 2010. Centers for Disease Control and Prevention, Atlanta, GA. [Google Scholar]
- 3.Banno H, Kimura K, Tanaka Y, Kitanaka H, Jin W, Wachino J, Yamada K, Shibayama K, Arakawa Y. 2014. Characterization of multidrug-resistant group B streptococci with reduced penicillin susceptibility forming small non-beta-hemolytic colonies on sheep blood agar plates. J. Clin. Microbiol. 52:2169–2171. 10.1128/JCM.00226-14. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 4.Gaudreau C, Lecours R, Ismail J, Gagnon S, Jetté L, Roger M. 2010. Prosthetic hip joint infection with a Streptococcus agalactiae isolate not susceptible to penicillin G and ceftriaxone. J. Antimicrob. Chemother. 65:594–595. 10.1093/jac/dkp458. [DOI] [PubMed] [Google Scholar]
- 5.Kimura K, Suzuki S, Wachino J, Kurokawa H, Yamane K, Shibata N, Nagano N, Kato H, Shibayama K, Arakawa Y. 2008. First molecular characterization of group B streptococci with reduced penicillin susceptibility. Antimicrob. Agents Chemother. 52:2890–2897. 10.1128/AAC.00185-08. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Longtin J, Vermeiren C, Shahinas D, Tamber GS, McGeer A, Low DE, Katz K, Pillai DR. 2011. Novel mutations in a patient isolate of Streptococcus agalactiae with reduced penicillin susceptibility emerging after long-term oral suppressive therapy. Antimicrob. Agents Chemother. 55:2983–2985. 10.1128/AAC.01243-10. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Nagano N, Kimura K, Nagano Y, Yakumaru H, Arakawa Y. 2009. Molecular characterization of group B streptococci with reduced penicillin susceptibility recurrently isolated from a sacral decubitus ulcer. J. Antimicrob. Chemother. 64:1326–1328. 10.1093/jac/dkp374. [DOI] [PubMed] [Google Scholar]
- 8.Park C, Nichols M, Schrag SJ. 2014. Two cases of invasive vancomycin-resistant group B Streptococcus infection. N. Engl. J. Med. 370:885–886. 10.1056/NEJMc1308504. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Back EE, O'Grady EJ, Back JD. 2012. High rates of perinatal group B Streptococcus clindamycin and erythromycin resistance in an upstate New York hospital. Antimicrob. Agents Chemother. 56:739–742. 10.1128/AAC.05794-11. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 10.Capraro GA, Rambin ED, Vanchiere JA, Bocchini JA, Jr, Matthews-Geer JM. 2013. High rates of inducible clindamycin resistance among prenatal group B streptococcal isolates in one northwest Louisiana academic medical center. J. Clin. Microbiol. 51:2469. 10.1128/JCM.00279-13. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 11.DiPersio LP, DiPersio JR. 2006. High rates of erythromycin and clindamycin resistance among OBGYN isolates of group B Streptococcus. Diagn. Microbiol. Infect. Dis. 54:79–82. 10.1016/j.diagmicrobio.2005.07.003. [DOI] [PubMed] [Google Scholar]
- 12.Heelan JS, Hasenbein ME, McAdam AJ. 2004. Resistance of group B Streptococcus to selected antibiotics, including erythromycin and clindamycin. J. Clin. Microbiol. 42:1263–1264. 10.1128/JCM.42.3.1263-1264.2004. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 13.Berg BR, Houseman JL, Garrasi MA, Young CL, Newton DW. 2013. Culture-based method with performance comparable to that of PCR-based methods for detection of group B Streptococcus in screening samples from pregnant women. J. Clin. Microbiol. 51:1253–1255. 10.1128/JCM.02780-12. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14.Clinical and Laboratory Standards Institute. 2013. Performance standards for antimicrobial susceptibility testing; 23rd informational supplement, M100-S23, vol 33 CLSI, Wayne, PA. [Google Scholar]
- 15.Centers for Disease Control and Prevention. 2010. Prevention of perinatal group B streptococcal disease. Revised guidelines from CDC MMWR Recomm. Rep. 59(RR-10):1–33. [PubMed] [Google Scholar]