Inhibition enzyme-linked immunosorbent assay for serotyping of group B streptococcal isolates

Abstract

Group B Streptococcus (GBS) is one of the most common organisms causing neonatal sepsis as well as serious infections in adults. Serotyping the organism is important in studying the epidemiology of the disease as well as deciding a course of treatment. There are several methods available for serotyping. Most of them need high-titered sera and are not quantitative. We are reporting a new inhibition enzyme-linked immunosorbent assay (ELISA) for serotyping which is sensitive and specific compared to the conventional methods but does not need high-titered serotype-specific antisera, as the specificity is controlled by the polysaccharide coating on the ELISA plates. The method can also be quantitative, and we have measured polysaccharide elaborated by different serotype V strains. Thus, the inhibition ELISA method will be useful in serotyping for epidemiological studies, assessing virulence, and performing strain selection for vaccine production.

FIG. 1

Serotyping by inhibition ELISA.

FIG. 2

(A) Inhibition of binding of IGIV to GBS polysaccharide type Ia by homologous and heterologous GBS polysaccharides. IGIV (1/1,000 dilution) was incubated with GBS polysaccharide types Ia, II, and III at concentrations of 2.50 to 0.08 μg/ml. For homologous inhibition, 100 μl from the preincubated mixture of type Ia GBS and IGIV was transferred to GBS type Ia polysaccharide-coated Immulon 4 plates. For heterologous inhibition, 100 μl from the preincubated mixture of IGIV with GBS polysaccharide type II or III was transferred to GBS type Ia-coated Immulon 4 plates. (B) Homologous inhibition of binding of IGIV to GBS polysaccharide Ia, II, and III coated plates. IGIV (1/1,000 dilution) was incubated with GBS polysaccharides as described above and then transferred to GBS polysaccharide Ia-, II-, and III-coated plates, respectively. Inhibition ELISA was performed as described in the text.

FIG. 3

Relative quantitation of polysaccharides elaborated by five different strains of type V GBS. The strains were grown in 5 ml of Todd-Hewitt liquid medium for 24 h, heat inactivated, and neutralized as described in Materials and Methods. One hundred microliters of IGIV (1:1,000 dilution) was incubated with serial dilutions of suspensions from different strains and 100 μl was transferred to GBS type V polysaccharide-coated plates for ELISA.