PREreview of False-positive detection of Group B Streptococcus (GBS) in chromogenic media due to presence of Enterococcus faecalis in High Vaginal Swabs

Summary

This study aimed to evaluate the effectiveness of Carrot broth for detecting Group B Streptococcus (GBS) from vaginal swabs of pregnant women. To assess the sensitivity and specificity of Carrot broth in screening pregnant women for GBS, a prospective study was conducted. GBS is a major cause of neonatal diseases, typically acquired from pregnant women before or during delivery, or through horizontal transmission. Evaluating the media used to screen pregnant women is crucial for detecting GBS and preventing the transfer of bacteria to newborns. This study included 201 pregnant women, 20 of whom tested positive for GBS using Carrot Broth. Although the study's purpose is not clearly defined, the 20 GBS-positive vaginal swabs were further subcultured onto Blood Agar and Crome Agar. Typically, GBS-negative Carrot Broth is subcultured onto Blood Agar to identify non-hemolytic GBS strains, which account for less than 5%.

Major comments

·         It looks like the title and the objectives of the study do not align. Furthermore, both this study and others have reported that the presence of Enterococcus could inhibit the growth of GBS. How, then, could Enterococcus lead to a false detection of GBS? Biochemical characteristics of GBS and Enterococcus and their appearance on blood agar are different

·         In the background, include the ingredients of Carrot Broth and its mechanism of action. Specify which bacteria can grow on it and which cannot. As previously noted, Enterococcus cannot grow on this medium ((https://hardydiagnostics.com/media/assets/product/documents/StrepBCarrotBrothOneStep.pdf).). Therefore, if Enterococcus is present in the Carrot Broth used for this study, it raises a quality concern regarding the media. If the medium supports the growth of Enterococcus, it should not be used.

·         The results lack clarity in their presentation

·         Although the study aimed to evaluate the performance of Carrot Broth for detecting GBS, an appropriate gold standard method was not utilized. There is no report on sensitivity, specificity, positive predictive value (PPV), or negative predictive value (NPV). The methods employed and the results obtained do not support the claimed conclusions.

·         The significance of Gram stain and 16S rRNA sequencing is not clear to the study's objectives.

·         Why was a high vaginal swab used instead of a low vaginal and rectal swab which is recommended by the CDC and other relevant organization?

·         Most methods lack quality control.  It would have been good if the quality of Carot broth was checked.

·         Provide detailed socio-demographic and clinical information about the study participants

·         For the PCR, why was a previously used and validated primer not employed (https://www.cdc.gov/strep-lab/media/pdfs/Real-time-PCR-targets.pdf?

·         The write-up of this paper needs improvement.

·         In the results section titled "Comparison of Carrot Broth, Blood Agar, and Chrome Agar Assays for GBS Detection," there was no actual comparison made, which is misleading.

·         What were the detailed characteristics of the 20 positive Carrot Broth samples that were subcultured onto Blood Agar and Crome Agar? Subculturing Carrot broth-negative samples should only be performed to check for non-hemolytic GBS. Enterococcus and many other bacteria are not expected in Carrot Broth, and most Enterococci are not beta-hemolytic. If there is any growth, it must be carefully selected and processed. Additionally, direct molecular detection from a swab cannot be compared with swabs cultured in Carrot Broth, as it is a selective medium.

Minor comments

·         The language needs improvement.

·         Bacterial names should be italicized.

·         There should be consistency in the naming of culture media.

·         Operational definition is needed for positive and negative Carot broth. Does positive indicate only growth of GBS? Does negative indicate the absence of bacteria?

·         In the background include about burden of neonatal GBS disease and Intrapartum antibiotic prophylaxis in India.

Recommendations

·         Check the quality of the culture media (Carrot Broth) used in this study. If it passes quality control, culture the vaginal swabs. I suggest modifying the title to "Prevalence of Group B Streptococcus among Pregnant Women" or "Performance of Carrot Broth in Detecting GBS among Pregnant Women," provided there is a gold standard for comparison.

·         Include the hemolysis profile of the 20 isolates that tested positive in Carrot Broth, and recheck the isolates.

·         Include details about the study participants, such as their gestational age, how they were selected, and the reasons for excluding those who are HIV and TB positive.

Competing interests

The author declares that they have no competing interests.