Non-haemolytic and non-pigmented group b streptococcus, an infrequent cause of early onset neonatal sepsis

Genomic characterisation of perinatal Western Australian Streptococcus agalactiae isolates

As a leading cause of neonatal sepsis, Streptococcus agalactiae, commonly known as Group B Streptococcus, is a major neonatal pathogen. Current global screening practices employ risk- or culture-based protocols for detection of these organisms. In Western Australia (WA), universal culture-based screening is provided, with subsequent intrapartum antibiotic prophylaxis for all S. agalactiae-positive women during labour. Widespread antibiotic exposure is not ideal and this is one of the factors driving development of vaccines against S. agalactiae. Vaccine candidates have focused on the capsule, surface proteins and pilus types, however, capsule serotypes are known to vary geographically. The aim of this study was to use genome sequencing to gain an understanding of the circulating genotypes in WA, and to assess variations in the associated gene pools. We sequenced 141 antenatal carriage (vaginal/rectal) isolates and 10 neonatal invasive disease isolates from WA. Based on the global PubMLST database, the 151 strains were characterised into 30 sequence types, with clustering of these mainly into clonal complexes 1, 12, 17, 19 and 23. Of the genes encoding eleven surface proteins that were analysed, the most prevalent were fbp, lmb and scpB which were present in ≥ 98% of isolates. A cluster of non-haemolytic isolates, one of which was a neonatal invasive disease isolate, appeared to lack the entire cyl locus. Admixture analysis of population structure revealed evidence of genetic transfer among the WA isolates across structural groups. When compared against the PubMLST S. agalactiae data, WA isolates showed high levels of strain diversity with minimal apparent clustering. This is the first whole genome sequence study of WA S. agalactiae isolates and also represents the first addition of Australian isolate data to PubMLST. This report provides insight into the distribution and diversity of vaccine targets of S. agalactiae within Western Australia, indicating that the most appropriate capsular vaccine for this population would be the proposed pentavalent (Cps Ia, Ib, II, III and V) preparation, whilst vaccines targeting surface proteins should ideally utilise Fbp, Lmb and/or ScpB.

A Nonhemolytic Group B Streptococcus Strain Exhibits Hypervirulence

Group B streptococci (GBS) are Gram-positive bacteria that are a leading cause of neonatal infections. Most invasive isolates are β-hemolytic, and hemolytic activity is critical for GBS virulence. Although nonhemolytic GBS strains are occasionally isolated, they are often thought to be virulence attenuated. In this study, we show that a nonhemolytic GBS strain (GB37) isolated from a septic neonate exhibits hypervirulence. Substitution of tryptophan to leucine (W297L) in the sensor histidine kinase CovS results in constitutive kinase signaling, leading to decreased hemolysis and increased activity of the GBS hyaluronidase, HylB. These results describe how nonpigmented and nonhemolytic GBS strains can exhibit hypervirulence.

Risk-based approach versus culture-based screening for identification of group B streptococci among women in labor

OBJECTIVE

To compare a risk-based and culture-based screening approach for identification of group B streptococci (GBS) vaginal colonization using an intrapartum rectovaginal culture as the reference standard.

METHODS

Pregnant women attending the prenatal clinic at Lillebaelt Hospital, Kolding, Denmark, between April 1, 2013, and June 30, 2014, were invited to participate in a prospective observational study. For prepartum culture-based screening, vaginal and rectal culture samples were obtained and, for reference, standard, paired vaginal and rectal culture samples were collected during labor. Risk factors for risk-based screening were previous early-onset GBS, GBS bacteriuria during pregnancy, maternal temperature ≥38.0°C intrapartum, and rupture of membranes for more than 18 hours.

RESULTS

The intrapartum rectovaginal GBS colonization rate was 30% (32/108) among participants with risk factors and 15% (123/794) among participants without risk factors. Culture-based screening demonstrated a sensitivity, specificity, positive predictive value, negative predictive value, and positive likelihood ratio in predicting intrapartum GBS carriage of 78% (95% confidence interval [CI] 71-84), 95% (94-97), 78% (70-84), 95% (94-97), and 17 (12-23), respectively; for risk-based screening, these values were 21% (15-28), 90% (87-92), 30% (22-38), 85% (83-86), and 2 (1-3), respectively.

CONCLUSIONS

Culture-based screening performed considerably better than a risk-based approach in identifying intrapartum GBS colonization.

Multicenter Diagnostic Accuracy Evaluation of the Luminex Aries Real-Time PCR Assay for Group B Streptococcus Detection in Lim Broth-Enriched Samples

The vertical transmission of group B Streptococcus (GBS) strains causing neonatal sepsis is one of the leading reasons for neonatal mortality worldwide. The gold standard for GBS detection is enriched culture with or without the aid of chromogenic agars. Given the high risk for morbidity and mortality in this population, high assay sensitivity is required to prevent the personal and economic costs of GBS disease. Nucleic acid amplification tests (NAATs) allow for objective determination of GBS colonization with a sensitivity and a specificity higher than those of traditional culture methods. In this study, we determined the analytical and clinical performance of the Aries GBS assay compared to those of the enrichment culture method, biochemical identification, and the NAATs used at the study sites. Remnant Lim broth samples were used to perform the Aries assay and reference testing. Upon first testing using enriched culture as the reference standard, the Aries GBS assay identified GBS with a 96.1% sensitivity (95% confidence interval [CI], 91.2 to 98.7%) and a 91.4% specificity (95% CI, 88.8 to 93.6%). The test performed with 100% positive agreement (95% CI, 83.2 to 100%) compared to the results of the BD Max GBS assay and 98.0% positive agreement (95% CI, 89.2 to 99.9%) compared to the results of the Cepheid Xpert GBS LB test. Repeatability and reproducibility were maintained in intra- and interlaboratory testing, regardless of the instrument, module, or user who performed the test. The Aries GBS assay can be set up in less than 5 min and produces results in 2 h. The easy setup, with minimal hands-on time, and high assay sensitivity and specificity make this a useful testing option for GBS screening in prepartum women.

Intrapartum PCR assay versus antepartum culture for assessment of vaginal carriage of group B streptococci in a Danish cohort at birth

The aim of this study was to compare the performances of two strategies for predicting intrapartum vaginal carriage of group B streptococci (GBS). One strategy was based on an antepartum culture and the other on an intrapartum polymerase chain reaction (PCR). We conducted a prospective observational study enrolling 902 pregnant women offered GBS screening before delivery by two strategies. The Culture-strategy was based on vaginal and rectal cultures at 35–37 weeks’ gestation, whereas the PCR-strategy was based on PCR assay on intrapartum vaginal swab samples. An intrapartum vaginal culture for GBS was used as the reference standard from which the performances of the 2 strategies were evaluated. The reference standard showed a GBS-prevalence of 12%. The culture-strategy performed with a sensitivity of 82%, specificity of 91%, positive predictive value (PPV) of 55%, negative predictive value (NPV) of 98%, and Likelihood ratio (LH+) of 9.2. The PCR-strategy showed corresponding values as sensitivity of 83%, specificity of 97%, PPV of 78%, NPV of 98%, and LH+ of 27.5. We conclude that in a Danish population with a low rate of early-onset neonatal infection with GBS, the intrapartum PCR assay performs better than the antepartum culture for identification of GBS vaginal carriers during labor.

Reliable Detection of Group B Streptococcus in the Clinical Laboratory

Group B streptococcus (GBS) is a leading cause of invasive neonatal infections and a significant pathogen in immunocompromised adults. Screening to detect GBS colonization in pregnant women determines the need for antibiotic prophylaxis in that pregnancy. Efficient determination of the GBS colonization status of pregnant women is crucial. Methods that maximize the probability of GBS recovery are needed. The availability of technologies such as matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS), molecular techniques, and chromogenic culture media, including Granada-type media, have changed the scenario for GBS detection and identification. This review presents and evaluates novel diagnostic tools, as well as classic identification techniques, for GBS species determination.