Determination of polysaccharide capsular type
a — Capsular serotyping
b — Capsular genotyping (PCR)
Description of the test
To date, 10 capsular polysaccharides (CPS) have been described: Ia, Ib, II to IX. A limited percentage of strains remain untypable.
Serotyping determines the phenotype as expressed in the capsule, while genotyping determines the presence of genes encoding the CPS. A gene may be present but not necessarily expressed. Growth conditions can alter the duration and amount of detectable CPS. Typically, typing starts with serotyping; in case of a negative or inconclusive result, genotyping is performed
a — CPS serotyping: agglutination method with specific latex.
b — CPS genotyping: 1 multiplex PCR sometimes followed by another simplex PCR for serotype IX, if not typable by the multiplex.
The distribution of serotypes varies geographically, according to disease type and patient age. In the perspective of a future vaccination, surveillance of the CPS distribution intrudes.
Purpose of the test
CPS typing
- Of strains sent to the NRC for CPS typing
- Of strains isolated in the context of epidemiological and microbial surveillance of invasive GBS infections and of GBS healthy carriers.
- From strains belonging to a suspected outbreak
- From strains cultured within the framework of technical validations
Criteria for performing this test
- GBS strains isolated from normally sterile sites or from deep wounds caused by invasive infection and regularly sent to the NRC for CPS typing and surveillance purposes.
- GBS strains isolated from healthy carriers for a specific surveillance purpose.
- Similar GBS colonies isolated on solid medium as part of a validation protocol of new diagnostic assays/culture medium
- Set of GBS strains isolated from different samples during a suspected outbreak and sent after contacting the NRC.
Instructions for the sample
- GBS strains to be routinely sent to the NRC
- All strains causing invasive diseases: neonatal infections, death in utero, diseases in pregnant and non-pregnant patients.
- Strains isolated from blood culture, cerebrospinal fluid, synovial fluid or normal sterile site.
- For neonatal infections, if possible also send strain isolated from mother (from vagino/rectal swab or breast milk).
- Under special condition: if severe infections are suspected, strains isolated from urine, lower respiratory tract and wound.
- GBS strains isolated from healthy carriers and retrieved by the NRC in accordance with a specific surveillance protocol
- In case of suspicion of outbreak, set of strains from different patients
- Storage conditions : strains can be kept alive for one week at room T° or refrigerated at 2-8°C on non-selective blood agar medium, or can be frozen in storage media (skimmed milk, or nutritive non-selective broth + 10% glycerol).
Instructions for transport
A fresh subculture on blood agar, or an abundantly inoculated nutritive agar, or an enriched pure and fresh subculture collected on a cotton swab in transport medium.
As soon as possible at room T°, within 72 hours of preparation.
Label and transport conditions according to guidelines on infectious specimens — waterproof container/packaging
Unacceptable requests
The following criteria are used to reject a sample:
- selection criteria not met,
- leakage of transport medium,
- unidentified sample,
- mandatory field on application form incomplete,
- improper transport or storage of samples.
Turn around time (and frequency of analysis)
- TAT serotyping: 10 working days
- TAT genotyping: 1-4 weeks, performed in batch at least once a month
Reporting of test results
- Reporting via regular mail and/or electronic reporting.
- Available results are reported in an annual report.