Molecular methods for rapid detection of group B streptococci

Updates on Group B Streptococcus Infection in the Field of Obstetrics and Gynecology

Group B Streptococcus (GBS, Streptococcus agalactiae) is a Gram-positive bacterium that is commonly found in the gastrointestinal and urogenital tracts. However, its colonization during pregnancy is an important cause of maternal and neonatal morbidity and mortality worldwide. Herein, we specifically looked at GBS in relation to the field of Obstetrics (OB) along with the field of Gynecology (GY). In this review, based on the clinical significance of GBS in the field of OBGY, topics of how GBS is being detected, treated, and should be prevented are addressed.

Pathogenic mechanism, detection methods and clinical significance of group B Streptococcus

Group B Streptococcus (GBS) is the main pathogen of perinatal infection. It can lead to adverse pregnancy, maternal infection, premature delivery, abortion, stillbirth and a series of adverse maternal and infant outcomes such as neonatal sepsis, meningitis or pneumonia during delivery. In order to reduce the infection of perinatal pregnant and the adverse pregnancy outcome, more attention should be paid in the clinical practice, screening efforts, universal detection of GBS infection for pregnant women and preventive treatment for the possible mother infant infection. In this study, the biological characteristics, immunophenotype, major pathogenic mechanism, laboratory test methods and clinical significance of GBS are summarized.

Comparative study of Revogene GBS LB assay and GeneXpert GBS LB assay for the detection of group B Streptococcus in prenatal screening samples

Background

Group B Streptococcal (GBS) infections in the United States are a leading cause of meningitis and sepsis in newborns. The CDC therefore recommends GBS screening for all pregnant women at 35–37 weeks of gestation and administration of intrapartum prophylaxis (in those that tested positive) as an effective means of controlling disease transmission. Several FDA approved molecular diagnostic tests are available for rapid and accurate detection of GBS in antepartum women.

Method

In this study, we report a clinical comparison of the Xpert GBS LB assay and a novel FDA-cleared test, Revogene GBS LB assay. A total of 250 vaginal-rectal swabs from women undergoing prenatal screening were submitted to the University of Wisconsin’s clinical microbiology laboratory for GBS testing.

Results

We found 96.8% of samples were concordant between the two tests, while 3.2% were discordant with a positive percent agreement of 98.0% and a negative percent agreement of 96.5% between the Revogene GBS LB assay and the GeneXpert GBS LB assay.

Conclusion

Overall, we report that both assays perform well for the detection of GBS colonization in pregnant women.

Streptococcus agalactiae Strains with Chromosomal Deletions Evade Detection with Molecular Methods

Surveillance of circulating microbial populations is critical for monitoring the performance of a molecular diagnostic test. In this study, we characterized 31 isolates of Streptococcus agalactiae (group B Streptococcus [GBS]) from several geographic locations in the United States and Ireland that contain deletions in or adjacent to the region of the chromosome that encodes the hemolysin gene cfb, the region targeted by the Xpert GBS and GBS LB assays. PCR-negative, culture-positive isolates were recognized during verification studies of the Xpert GBS assay in 12 laboratories between 2012 and 2018. Whole-genome sequencing of 15 GBS isolates from 11 laboratories revealed four unique deletions of chromosomal DNA ranging from 181 bp to 49 kb. Prospective surveillance studies demonstrated that the prevalence of GBS isolates containing deletions in the convenience sample was <1% in three geographic locations but 7% in a fourth location. Among the 15 isolates with chromosomal deletions, multiple pulsed-field gel electrophoresis types were identified, one of which appears to be broadly dispersed across the United States.

Comparative study of bacteriological culture and real-time fluorescence quantitative PCR (RT-PCR) and multiplex PCR-based reverse line blot (mPCR/RLB) hybridization assay in the diagnosis of bacterial neonatal meningitis

Background

Bacterial meningitis is more common in the neonatal period than any other time in life; however, it is still a challenge for the evidence based diagnosis. Strategy for identification of neonatal bacterial meningitis pathogens is presented by evaluating three different available methods to establish evidence-based diagnosis for neonatal bacterial meningitis.

Methods

The cerebrospinal fluid samples from 56 neonates diagnosed as bacterial meningitis in 2009 in Beijing Children’s Hospital were analyzed in the study. Two PCR based molecular assays, real-time fluorescence quantitative PCR (RT-PCR) and multiplex PCR based-reverse line blot hybridization (mPCR/RLB), were used to assess 7 common neonatal meningitis bacterial pathongens, including Escherichia coli, Staphylococcus aureus, Listerisa monocytogenes, Neisseria meningitidis, Haemophilus influenzae, Streptococcus pneumoniae, and Streptococcus agalactiae. The findings in examinations of two assays were compared with the results obtained bacterial culture tests.

Results

Bacterial meningitis was identified in five cases (9%) by CSF cultures, 25 (45%) by RT-PCR and 16 (29%) by mPCR/RLB. One strain of S. epidermidis and one of E. faecalis were identified using mPCR/RLB but not by RT-PCR. In contrast, cultures identified one strain of S. pneumoniae which was missed by both PCR assays. Overall, the bacterial pathogens in 28 cases were identified with these three methods. Both RT-PCR and mPCR/RLB assays were more sensitive than bacterial culture, (p < 0.05).

Conclusion

Our study confirmed that both RT-PCR and mPCR/RLB assays have better sensitivity than bacterial culture. They are capable of detecting the pathogens in CSF samples with negative culture results.

Real-time PCR assay provides reliable assessment of intrapartum carriage of group B Streptococcus

The objective of this study was to determine the reliability of the real-time PCR assay for determining the group B Streptococcus (GBS) status of women in labor. In this prospective study we compared the results of culture and PCR testing of vaginal and rectal samples collected by nursing staff when women were in labor. Patients' charts were also reviewed to obtain relevant information about pregnancy risk factors. Our results demonstrated a sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of 90.5%, 96.1%, 86.4%, and 97.4%, respectively, for rapid PCR. Of the 196 women evaluated, 29 (14.8%) presented with unknown GBS status, 11 (37.9%) of whom received unnecessary intrapartum antibiotics. The rapid real-time PCR test was robust and was able to reliably detect the presence of GBS in women in labor within 1 h of specimen submission to the laboratory. We recommend that the rapid PCR test be targeted to women who present in labor with unknown GBS status. In cases where the laboratory does not offer 24-h availability of testing, sample collection followed by PCR testing the next morning is still valuable and provides reliable results 24 to 48 h faster than culture and will aid appropriate decision-making regarding continuing or stopping antibiotics for neonates of women with unknown GBS status.

Diagnosis of neonatal group B Streptococcus sepsis by nested-PCR of residual urine samples

Group B streptococcus (GBS) remains the most common cause of early-onset sepsis in newborns. Laboratory gold-standard, broth culture methods are highly specific, but lack sensitivity. The aim of this study was to validate a nested-PCR and to determine whether residue volumes of urine samples obtained by non invasive, non sterile methods could be used to confirm neonatal GBS sepsis. The nested-PCR was performed with primers of the major GBS surface antigen. Unavailability of biological samples to perform life supporting exams, as well as others to elucidate the etiology of infections is a frequent problem concerning newborn patients. Nevertheless, we decided to include cases according to strict criteria: newborns had to present with signs and symptoms compatible with GBS infection; at least one of the following biological samples had to be sent for culture: blood, urine, or cerebrospinal fluid; availability of residue volumes of the samples sent for cultures, or of others collected on the day of hospitalization, prior to antibiotic therapy prescription, to be analyzed by PCR; favorable outcome after GBS empiric treatment. In only one newborn GBS infection was confirmed by cultures, while infection was only presumptive in the other three patients (they fulfilled inclusion criteria but were GBS-culture negative). From a total of 12 biological samples (5 blood, 3 CSF and 4 urine specimen), eight were tested by culture methods (2/8 were positive), and 8 were tested by PCR (7/8 were positive), and only 4 samples were simultaneously tested by both methods (1 positive by culture and 3 by PCR). In conclusion, although based on a restricted number of neonates and samples, our results suggest that the proposed nested-PCR might be used to diagnose GBS sepsis as it has successfully amplified the three types of biological samples analyzed (blood, urine and cerebrospinal fluid), and was more sensitive than culture methods as PCR in urine confirmed diagnosis in all four patients. Moreover, PCR has enabled us to use residue volumes of urine samples collected by non invasive, non sterile methods, what is technically adequate as GBS is not part of the normal urine flora, thus avoiding invasive procedures such as suprapubic bladder punction or transurethral catheterization. At the same time, the use of urine instead of blood samples could help preventing newborns blood spoliation.

Development of specific and rapid detection of bacterial pathogens in dairy products by PCR

A simple and specific method for direct detection of bovine mastitis pathogens (Streptococcus agalactiae (GBS), Staphylococcus aureus and Escherichia coli) in milk products, bacterial samples from milk and isolated bacterial DNA was developed. The method is based on polymerase chain reaction (PCR) using sequence-specific primers only for GBS and species-specific primers derived from 16S and 23S rRNA for all chosen species. The presence of the gene of surface immunogenic protein (Sip) in bovine GBS isolates, described previously only in human GBS isolates was confirmed. The GBS detection was performed with the sequence coding for surface immunogenic protein from GBS human isolates designated as Sip specific sequence (SSS); this sequence was selected for specific primer design. The sequence is unique for GBS and was designed from a consensus of all known sip genes. The specific identification was shown on a collection of 75 GBS bovine isolates from different localities in Slovakia. All isolates were positive to SSS, 16S and 23S rRNA sequence. The 16S and 23S rRNA PCR detection was also performed with S. aureus and E. coli isolates and specific PCR products were also detected. The detection limit of this assay for milk products was 6 CFU/microL (i.e. 6000 CFU/mL) for GBS and E. coli, and 16 CFU/microL for S. aureus. This rapid, sensitive and specific diagnostic method can be performed within hours and represents an innovative diagnostic tool for the detection of milk pathogens in dairy products.

Simultaneous detection and serotype identification of Streptococcus agalactiae using multiplex PCR and reverse line blot hybridization

Streptococcus agalactiae (group B streptococcus, GBS) is an important cause of sepsis in neonates and their mothers, and the elderly and immunocompromised patients. Ongoing surveillance to monitor GBS serotype distribution is needed to guide the development and assess the feasibility of GBS conjugate vaccines. The authors previously developed a molecular serotype identification method based on serotype-specific PCR and partial sequencing of cps genes. In this study, a novel 10-primer pair multiplex PCR and reverse line blot (mPCR/RLB) hybridization assay was developed for simultaneous detection and serotype identification of all nine GBS serotypes. For all 316 GBS isolates tested the mPCR/RLB results corresponded with those of conventional serotyping and individual serotype-specific PCR, and the method was more convenient and practical than either alternative.

Rapid phenotypic characterization of Vibrio isolates by pyrolysis metastable atom bombardment mass spectrometry

Pyrolysis mass spectrometry was investigated for rapid characterization of food-borne bacterial pathogens. Nine isolates of Vibrio parahaemolyticus and one isolate each of Vibrio fluvialis, Vibrio hollisae, and Vibrio vulnificus were analyzed. Pyrolysis mass spectra, generated via an alternative ionization method, metastable atom bombardment, were subject to principal component-discriminant analysis. The spectral patterns were used to distinguish Vibrio isolates differing in species, serotype and expression of the thermostable direct hemolysin gene. The patterns of similarity and dissimilarity amongst spectra in the Vibrio test set generally reflected those associated with species, serotype or hemolysin-producing genes, though the combined influence of these and other variables in the multi-dimensional data did not produce a simple clustering with respect to any one of these characteristics. These results suggested that with enough examples to model the most common combinations, the method should be able to characterize Vibrio isolates according to their phenotypic characteristics. Pyrolysis-mass spectrometry with metastable atom bombardment and pattern recognition appeared suitable for rapid infraspecific comparison of Vibrio isolates. This integrated analytical, pattern-recognition system should be examined further for potential utility in clinical and public health diagnostic contexts.

Evaluation of a real-time fluorescent PCR assay for rapid detection of Group B Streptococci in neonatal blood

Streptococcus agalactiae (Group B Streptococcus: GBS) is the major causative agent of neonatal sepsis. Neonates at risk for GBS infections are empirically administered broad-spectrum antibiotics for at least 48 h pending blood culture results. A rapid assay to expedite detection of GBS would facilitate initiation of specific antibiotic therapy. Conversely, expeditious proof of absence of infection will avoid unnecessary antibiotic use. Using the LightCycler, we evaluated a hybridization probe polymerase chain reaction (PCR) assay to detect GBS-specific cfb gene target DNA sequence in blood specimens. Both sensitivity and specificity of the real-time PCR assay was 100%. The assay demonstrated 100% specificity when tested against 26 non-GBS bacteria. This method is capable of detecting as few as approximately 100 copies or 10 pg of GBS genomic DNA. This real-time PCR method is rapid, sensitive, and specific for the detection of GBS in neonatal blood samples and holds great promise in its utility in the diagnostic laboratory.

Clinical microbiology of bacterial and fungal sepsis in very-low-birth-weight infants

Twenty percent of very-low-birth-weight (<1500 g) preterm infants experience a serious systemic infection, and despite advances in neonatal intensive care and antimicrobials, mortality is as much as threefold higher for these infants who develop sepsis than their counterparts without sepsis during their hospitalization. Outcomes may be improved by preventative strategies, earlier and accurate diagnosis, and adjunct therapies to combat infection and protect the vulnerable preterm infant during an infection. Earlier diagnosis on the basis of factors such as abnormal heart rate characteristics may offer the ability to initiate treatment prior to the onset of clinical symptoms. Molecular and adjunctive diagnostics may also aid in diagnosing invasive infection when clinical symptoms indicate infection but no organisms are isolated in culture. Due to the high morbidity and mortality, preventative and adjunctive therapies are needed. Prophylaxis has been effective in preventing early-onset group B streptococcal sepsis and late-onset Candida sepsis. Future research in prophylaxis using active and passive immunization strategies offers prevention without the risk of resistance to antimicrobials. Identification of the differences in neonatal intensive care units with low and high infection rates and implementation of infection control measures remain paramount in each neonatal intensive care unit caring for preterm infants.