By Andrew Czyzewski
In neonates with probable sepsis or meningitis, a realtime polymerase chain reaction (PCR) assay specific for a virulence gene in Streptococcus agalactiae is superior to standard bacterial culture identification, results of a pathology study show.
"The method is rapid, sensitive, specific and reproducible," say Aruni de Zoysa (Health Protection Agency, London, UK) and colleagues, adding that it is "an attractive option to supplement culture-based tests."
S. agalactiae [Lancefield group B Streptococcus (GBS)] is the leading cause of sepsis, pneumonia, and meningitis in neonates. GBS is a commensal of the genitourinary and gastrointestinal tract and is carried by 20-30% of pregnant women in the UK. Vertical transmission to the infant occurs in 50% of deliveries involving colonized women, and 1-3% of colonized neonates go on to develop invasive disease.
The incidence of early-onset disease in the world ranges from 0.5 to four cases per 1000 live births. Severe early-onset disease (occurring in the first 6 days of life) usually presents as pneumonia, septicemia, or meningitis, and has a case fatality rate of around 12.1%.
Diagnosis of GBS infection in the newborn by conventional culture methods is time-consuming and unreliable. In the absence of an available vaccine against GBS, rapid, sensitive, and specific detection of the organism is crucial for effective management of disease and for reducing the risk for mortality, particularly in the newborn.
The team therefore developed a realtime PCR assay targeting the cyl virulent gene cluster, which is responsible for expression of hemolysin.
They analyzed a total of 110 blood culture-negative samples (75 cerebrospinal fluid [CSF] and 35 ethylenediaminetetraacetic acid [EDTA] blood samples) from neonates with probable GBS sepsis or meningitis.
Of these clinical samples, 18 (16.4%) were positive: 16 (21.3%) of the 75 CSF samples and two (5.7%) of the 35 EDTA blood samples.
PCR appeared to be more sensitive than culture because it does not rely on the presence of viable organisms. In addition, PCR can provide results in less than 2 hours from receipt of sample. The sensitivity of the assay required at least 10 bacterial cells to exist in 5 µl of extracted sample.
In a specificity panel comprising 58 isolates (21 species of streptococci and 37 other bacterial species - common pathogens associated with sepsis), PCR returned negative results demonstrating 100% specificity for GBS.
"Overall, this real-time PCR assay was shown to be superior to culture methods for detection of GBS from CSF and EDTA blood samples," de Zoysa et al said.
They add that it "could be an invaluable tool in the rapid diagnosis of GBS and may improve our ability to detect and manage GBS, particularly in intrapartum and neonatal settings."
The research is published in the Journal of Medical Microbiology.
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