Case Report CAMP-negative group B Streptococcus went unrecognized with Cepheid GeneXpert but was detected by Liofilchem ® Chromatic StrepB

Vincenzo Savini, Roberta Marrollo, Roberta De Filippis, Emanuele D'incecco, Monica Imperi, Marco Pataracchia, Giovanna Alfarone, Paola Fusilli, Eleonora Coclite, Carmine D'incecco, Paolo Fazii, Roberta Creti
2016 Int J Clin Exp Pathol   unpublished
A Streptococcus agalactiae (group B Streptococcus, GBS) strain from antenatal screening was not detected by polymerase chain reaction (PCR)-based Cepheid GeneXpert and was phenotypically CAMP-negative; nevertheless , it formed blue colonies on Liofilchem ® Chromatic StrepB, a chromogenic medium which allowed presumptive identification of such an unusual isolate. GeneXpert is a valid, genome-based support in laboratory diagnostics which helps detecting, particularly, nonhemolytic GBS variants
more » ... tic GBS variants that may escape inspection by eye with culture potentially leading to falsely negative results. The system targets the GBS cfb gene, encoding the CAMP protein, responsible for the CAMP reaction, which is historically observed in almost all isolates of this species. Such cfb-negative strains, that are believed to be rare, appear thereby as both CAMP-negative and PCR-negative thus affecting accuracy of the antenatal screening. Based on the published literature, the one we present here seems to be the first GBS isolate escaping detection with GeneXpert due to strain-specific properties, whereas this system's failure related to low bacterial counts in the rectovaginal sample has been clearly shown elsewhere. Then we emphasize that molecular tests cannot be used alone in the context of GBS carriage screening but should be combined with phenotype-based methods, including chromogenic media, what can contribute making childbirth a glad and safe event.
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