Comparative evaluation of VITEK 2 for antimicrobial susceptibility testing of group B Streptococcus.
OBJECTIVE
Intrapartum antibiotic prophylaxis is recommended to prevent neonatal group B streptococcal (GBS) disease in colonized women, and penicillin or aminopenicillin constitute the first-line antibiotics. Most isolates are resistant to tetracycline, and resistance to macrolide-lincosamide-streptogramin (MLS) antibiotics is increasing. Therefore, laboratory testing for MLS resistance in GBS is now recommended for penicillin-allergic patients. The aim of this study was to compare the antimicrobial susceptibility of GBS as determined by the VITEK 2 system (bioMérieux, Marcy l'Etoile, France), agar diffusion methods and PCR-genotypic detection of resistance genes.


METHODS
One hundred and ten unrelated selected GBS clinical isolates were studied. The antibiotics tested (VITEK 2 and agar diffusion method) were benzylpenicillin, ampicillin, erythromycin, clindamycin, co-trimoxazole, tetracycline, kanamycin, streptomycin and vancomycin. A standardized double-disc (DD) diffusion test was performed for MLS antibiotics. Genotypic characterization of tetracycline, MLS and aminoglycoside resistance genes was performed by PCR.


RESULTS
All strains were susceptible to benzylpenicillin, ampicillin and vancomycin [category agreement (CA) between VITEK 2 and the diffusion method was 100%]. Ninety-five (86%) strains were resistant to tetracycline (CA was 98.9%). Eighty-one strains (73.6%) harboured an MLS resistance phenotype; 50 (61.8%) an MLS(B)-constitutive phenotype, 25 (30.8%) an MLS(B)-inducible phenotype and 6 (7.4%) an M phenotype. The agreement between data of VITEK 2 and the DD diffusion test for the detection of MLS(B)-constitutive, MLS(B)-inducible and M phenotype isolates was 76%, 36% and 100%, respectively. Almost all discrepancies were due to failure to detect erythromycin resistance by VITEK 2.


CONCLUSIONS
VITEK 2 allows accurate determination of GBS susceptibility for the majority of antibiotics, but has to be improved for erythromycin. Thus, the DD diffusion test remains the most simple and reliable method for macrolide resistance detection among this streptococcal species.
Tazi
Asmaa
A
Groupe Hospitalier Cochin-Saint Vincent de Paul, Service de Bactériologie, Assistance Publique-Hôpitaux de Paris, 27 rue du Faubourg Saint Jacques, Paris, France.
Réglier-Poupet
Hélène
H
Raymond
Josette
J
Adam
Jean-Marie
JM
Trieu-Cuot
Patrick
P
Poyart
Claire
C
D000362:Agar; D003470:Culture Media; D004269:DNA, Bacterial / Q000235:genetics; D024881:Drug Resistance, Bacterial / Q000235:genetics / Q000502:physiology; D005838:Genotype; D018942:Macrolides / Q000494:pharmacology; D008826:Microbial Sensitivity Tests / Q000295:instrumentation*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D013292:Streptococcus agalactiae / Q000187:drug effects* / Q000235:genetics
D003470:Culture Media; D004269:DNA, Bacterial; D018942:Macrolides; D000362:Agar