Craven, Robin R

Dunne, W. Michael, Jr.

Jennemann, Rebecca A

Weber, Carol J

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Washington University School of MedicineDigital Commons@BeckerOpen Access Publications2010Evaluation of a chromogenic agar for detection ofgroup B streptococcus in pregnant womenRobin R. CravenWashington University School of Medicine in St. LouisCarol J. WeberWashington University School of Medicine in St. LouisRebecca A. JennemannWashington University School of Medicine in St. LouisW. Michael Dunne Jr.Washington University School of Medicine in St. LouisFollow this and additional works at: http://digitalcommons.wustl.edu/open_access_pubsThis Open Access Publication is brought to you for free and open access by Digital Commons@Becker. It has been accepted for inclusion in OpenAccess Publications by an authorized administrator of Digital Commons@Becker. For more information, please contact engeszer@wustl.edu.Recommended CitationCraven, Robin R.; Weber, Carol J.; Jennemann, Rebecca A.; and Dunne, W. Michael Jr., ,"Evaluation of a chromogenic agar fordetection of group B streptococcus in pregnant women." Journal of Clinical Microbiology.48,9. 3370-3371. (2010).http://digitalcommons.wustl.edu/open_access_pubs/2396  Published Ahead of Print 30 June 2010. 10.1128/JCM.00221-10. 2010, 48(9):3370. DOI:J. Clin. Microbiol. and W. Michael Dunne Jr.Robin R. Craven, Carol J. Weber, Rebecca A. Jennemann Pregnant WomenDetection of Group B Streptococcus in Evaluation of a Chromogenic Agar forhttp://jcm.asm.org/content/48/9/3370Updated information and services can be found at: These include:REFERENCEShttp://jcm.asm.org/content/48/9/3370#ref-list-1This article cites 11 articles, 4 of which can be accessed free at:CONTENT ALERTS more»articles cite this article), Receive: RSS Feeds, eTOCs, free email alerts (when newhttp://journals.asm.org/site/misc/reprints.xhtmlInformation about commercial reprint orders: http://journals.asm.org/site/subscriptions/To subscribe to to another ASM Journal go to:  on March 12, 2014 by Washington University in St. Louishttp://jcm.asm.org/Downloaded from  on March 12, 2014 by Washington University in St. Louishttp://jcm.asm.org/Downloaded from JOURNAL OF CLINICAL MICROBIOLOGY, Sept. 2010, p. 3370–3371 Vol. 48, No. 90095-1137/10/$12.00 doi:10.1128/JCM.00221-10Copyright © 2010, American Society for Microbiology. All Rights Reserved.Evaluation of a Chromogenic Agar for Detection of Group BStreptococcus in Pregnant WomenRobin R. Craven,* Carol J. Weber, Rebecca A. Jennemann, and W. Michael Dunne, Jr.Department of Pathology and Immunology, Division of Laboratory and Genomic Medicine,Washington University School of Medicine, Barnes-Jewish Hospital, St. Louis, MissouriReceived 2 February 2010/Returned for modification 2 April 2010/Accepted 19 June 2010We compared ChromID Strepto B agar (STRB; bioMe´rieux, Inc.), a selective and differential medium forgroup B streptococcus, with culture using neomycin-nalidixic acid agar (NNA) and LIM broth. STRB alone wasmore sensitive (87.7%) than NNA alone (79.0%), while each had a sensitivity of 100% when used in conjunctionwith LIM broth.Meningitis, pneumonia, and septicemia due to group BStreptococcus (GBS) infection are leading causes of infantmortality and morbidity (7). In 1996 the American College ofObstetricians and Gynecologists (ACOG) and the CDC issuedrecommendations for intrapartum antibiotic prophylaxis ac-cording to either a risk-based or a screening-based approach toprevent GBS transmission to neonates. The risk-based guide-lines called for intrapartum prophylaxis in the event of deliveryat 37 weeks of gestation, a temperature of 100.4°F, or therupture of membranes for 18 h, while the culture-based ap-proach called for the screening of all pregnant women between35 and 37 weeks of gestation for vaginal or rectal colonizationwith GBS and subsequent intrapartum prophylaxis of thosewho were culture positive. In 2002 these guidelines were re-vised to recommend that culture-based prenatal GBS screen-ing at 35 to 37 weeks of gestation be universally adopted (8).This time point was chosen as an appropriate predictor ofintrapartum colonization (11). U.S. national estimates of early-onset (7 days of life) neonatal invasive GBS disease usingdata collected by the CDC Active Bacterial Core Surveillancesystem (ABCs) indicate a decrease in incidence since the in-stitution of GBS screening and prophylaxis guidelines, from 0.7case/1,000 live births in 1997 to 0.34 case/1,000 live births in2007 (3).The CDC and ACOG recommend that vaginal/rectal swabsbe inoculated into a selective broth medium to aid in therecovery of GBS. The broth is then to be subcultured on sheepblood agar plates after 18 to 24 h of incubation (1, 8). Theaddition of a solid agar medium inoculated upon receipt of theswab in the laboratory can allow for the identification of GBSafter 24 h of incubation, instead of the 48 h required whenswabs are inoculated only into selective broth. Neomycin-na-lidixic acid agar (NNA) medium is a GBS-selective solid agarthat can be used for primary plating, since it is significantlymore sensitive than colistin-nalidixic acid agar (CNA) for thispurpose (4). However, NNA should never be used as a substi-tute for a selective broth medium, because the combination ofa selective broth medium and NNA improves the detection ofGBS by 15% over that with either medium used alone (5).ChromID Strepto B agar (STRB; bioMe´rieux, Inc.) is anewly developed selective and differential medium for the pre-sumptive identification of GBS. STRB contains a chromogenicsubstrate that, in the presence of actively growing GBS, gen-erates pink to red colonies that are easily identified duringroutine inspection. The growth of bacteria of other specieseither is inhibited or appears in other colors (violet, blue, orcolorless). We evaluated the performance of STRB in ourlaboratory compared to that of NNA, with or without theaddition of a selective broth medium, as a suitable screeningmethod for the detection of GBS colonization among pregnantwomen.Our established protocol calls for the inoculation of vaginal/rectal swabs onto NNA, which is incubated at 35°C under 5%CO2, and into LIM broth, which is incubated at 35°C in roomair. The NNA plate is examined at 24 h for the presence ofcharacteristic beta-hemolytic colonies, which are further eval-uated by Gram staining and by catalase, PYR (pyrrolidonyl--naphthylamide), and rapid latex agglutination tests to identifyGBS. If beta-hemolytic colonies are not present after 24 h ofincubation, then (i) the NNA plate is held for an additional24 h and is examined the following day, and (ii) the LIM brothis subcultured onto 5% sheep blood agar, which is incubatedfor 48 h and is examined daily for the presence of beta-hemo-lytic colonies. To evaluate the performance of the STRB agar,primary swabs were processed by our routine protocol andwere also inoculated onto the STRB plate, which was incu-bated at 35°C for 48 h in the dark. A single swab was collectedfrom each patient, and the order of inoculation onto STRB orNNA plates was randomized. Plates were examined daily forthe presence of pink colonies, which were then subjected tolatex agglutination testing to confirm the identification of GBS.Gram staining and catalase and latex agglutination testing canbe performed directly from STRB agar, while PYR testingcannot. However, according to the manufacturer, the appear-ance of pink to red colonies that type as group B by latexagglutination is sufficient for the identification of GBS.A total of 250 prenatal screening swabs were included in the* Corresponding author. Mailing address: Department of Pathologyand Immunology, Division of Laboratory and Genomic Medicine,Washington University School of Medicine, Barnes-Jewish Hospital,St. Louis, MO 63110. Phone: (314) 747-4972. Fax: (314) 362-1461.E-mail: robincraven@gmail.com. Published ahead of print on 30 June 2010.3370 on March 12, 2014 by Washington University in St. Louishttp://jcm.asm.org/Downloaded from study. Eighty-one of these samples (32.4%) were positive forGBS by our reference method (i.e., they were recovered fromeither NNA or LIM broth). The sensitivity, specificity, positivepredictive value (PPV), and negative predictive value (NPV) ofeither combination (NNA plus LIM broth subcultured onto5% sheep blood agar or STRB plus LIM broth subculturedonto 5% sheep blood agar) were all 100%. A direct compari-son of NNA to STRB without broth amplification as a screen-ing medium revealed sensitivities of 79.0% (95% confidenceinterval [95% CI], 68.3% to 86.9%) for NNA and 87.7% (95%CI, 78.0% to 93.6%) for STRB (Table 1). For both NNA andSTRB, the specificity was 97.6% (95% CI, 93.8% to 99.3%),with four isolates being incorrectly identified as group B Strep-tococcus. The four isolates from NNA were identified as Strep-tococcus porcinus, while three of the four S. porcinus isolatesfrom NNA were also incorrectly identified on STRB, alongwith a fourth isolate that was identified as Streptococcus tho-raltensis. There is some potential for misidentification of otherstreptococcal species as GBS by using STRB. S. porcinus iso-lated from NNA would likely be evaluated further if a widezone of beta-hemolysis, which is not characteristic of GBS,were recognized. This would not be the case, however, if iso-lates were identified from STRB alone, since there would beno potential to observe the hemolysis pattern. Conversely,GBS can exhibit a nonhemolytic phenotype that may be over-looked on NNA, but these isolates would be identified usingchromogenic agar (10).Several recent publications have assessed the use of molec-ular methods for GBS screening. Real-time PCR for GBS hasnot proven to be sufficiently sensitive to replace culture-basedscreening (2, 9). An evaluation of a nonamplified nucleic acidhybridization assay indicated that in order to achieve a sensi-tivity similar to that of culture, the sample must be incubatedin LIM broth for 24 h before molecular testing (9), therebynegating the potential advantage of a rapid testing methodol-ogy. El Helali et al. report good performance (sensitivity,98.5%; specificity, 99.6%) for a rapid real-time PCR test forGBS in comparison to culture on Columbia ANC sheep bloodagar and broth medium (6). However, the authors’ suggestionthat current guidelines for antenatal testing may not be rele-vant due to the low PPV of antenatal screening (58.3%) wasbased on a comparison of PCR, not to culture of intrapartumsamples in broth medium and Columbia ANC agar, but toculture of antenatal samples on Columbia ANC agar alone, notto culture of intrapartum samples in broth medium and Co-lumbia ANC agar (6). The use of Columbia ANC agar alonewithout the addition of a selective broth medium to screen theantenatal samples would lead to decreased sensitivity andwould likely cause a falsely low PPV.In conclusion, we report that STRB used in combinationwith LIM broth performs as well as NNA with LIM broth andthat chromogenic agar has a higher sensitivity than NNA. The95% confidence intervals of the sensitivities of STRB or NNAculture alone overlap, indicating that there is not a significantdifference between these two culture methods. However, chro-mogenic agar has advantages: the ease of detecting a colorchange on this medium, as opposed to looking for weaklybeta-hemolytic colonies on NNA, and the need to perform onlyone additional test from STRB medium. The rare chance of afalse-positive result is balanced by the increased sensitivity ofSTRB and the potential for detecting nonhemolytic colonies ofGBS. real-time PCR (RT-PCR) may provide an alternative toculture in situations where a result is needed before culture canbe completed (i.e., birth at 37 weeks of gestation, or lack ofappropriate prenatal care and screening), though this wouldlikely not be a cost-effective alternative to routine antenataltesting.REFERENCES1. American College of Obstetricians and Gynecologists. 2002. ACOG Com-mittee Opinion: number 279, December 2002. Prevention of early-onsetgroup B streptococcal disease in newborns. Obstet. Gynecol. 100:1405–1412.2. Atkins, K. L., R. M. Atkinson, A. Shanks, C. A. Parvin, W. M. Dunne, andG. Gross. 2006. Evaluation of polymerase chain reaction for group B strep-tococcus detection using an improved culture method. Obstet. Gynecol.108:488–491.3. Centers for Disease Control and Prevention. 2009. Trends in perinatal groupB streptococcal disease—United States, 2000–2006. MMWR Morb. Mortal.Wkly. Rep. 58:109–112.4. Dunne, W. M., Jr. 1999. Comparison of selective broth medium plus neo-mycin-nalidixic acid agar and selective broth medium plus Columbia colistin-nalidixic acid agar for detection of group B streptococcal colonization inwomen. J. Clin. Microbiol. 37:3705–3706.5. Dunne, W. M., Jr., and C. A. Holland-Staley. 1998. Comparison of NNA agarculture and selective broth culture for detection of group B streptococcalcolonization in women. J. Clin. Microbiol. 36:2298–2300.6. El Helali, N., J. C. Nguyen, A. Ly, Y. Giovangrandi, and L. Trinquart. 2009.Diagnostic accuracy of a rapid real-time polymerase chain reaction assay foruniversal intrapartum group B streptococcus screening. Clin. Infect. Dis.49:417–423.7. Galiza, E. P., and P. T. Heath. 2009. Improving the outcome of neonatalmeningitis. Curr. Opin. Infect. Dis. 22:229–234.8. Schrag, S., R. Gorwitz, K. Fultz-Butts, and A. Schuchat. 2002. Prevention ofperinatal group B streptococcal disease. Revised guidelines from CDC.MMWR Recommend. Rep. 51:1–22.9. Scicchitano, L. M., and P. P. Bourbeau. 2009. Comparative evaluation of theAccuProbe Group B Streptococcus Culture Test, the BD GeneOhm Strep Bassay, and culture for detection of group B streptococci in pregnant women.J. Clin. Microbiol. 47:3021–3023.10. Smith, D., J. D. Perry, L. Laine, A. Galloway, and F. K. Gould. 2008.Comparison of BD GeneOhm real-time polymerase chain reaction withchromogenic and conventional culture methods for detection of group BStreptococcus in clinical samples. Diagn. Microbiol. Infect. Dis. 61:369–372.11. Yancey, M. K., A. Schuchat, L. K. Brown, V. L. Ventura, and G. R. Marken-son. 1996. The accuracy of late antenatal screening cultures in predictinggenital group B streptococcal colonization at delivery. Obstet. Gynecol.88:811–815.TABLE 1. Performance characteristics of STRB,NNA, and LIM brothMediumNo. ofGBS-positivespecimensSensitivity(%)Specificity(%)STRB 71 87.7 97.6NNA 64 79.0 97.6LIM 74 91.4 100LIM or STRB 81 100 100LIM or NNA 81 100 100VOL. 48, 2010 NOTES 3371 on March 12, 2014 by Washington University in St. Louishttp://jcm.asm.org/Downloaded from 

Evaluation of a chromogenic agar for detection of group B streptococcus in pregnant women

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Evaluation of a chromogenic agar for detection of group B streptococcus in pregnant women

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