# Multilocus sequence typing database for Streptococcus agalactiae contains a spurious allele of the transketolase gene

## TL;DR

A spurious allele in the Streptococcus agalactiae MLST database was identified and removed to improve the accuracy of sequence typing.

## Contribution

Identification and removal of a contaminating tkt_134 allele in the S. agalactiae MLST database.

## Key findings

- The tkt_134 allele was found to originate from a non-universal homologous gene (tktX).
- Strains with tkt_134 also contain a canonical tkt gene with higher sequence identity.
- The tkt_134 allele and associated sequence types were removed from the MLST database in 2021.

## Abstract

The tkt (transketolase) gene is one of the seven gene fragments used in the multilocus sequence typing (MLST) system for Streptococcus agalactiae. We discovered that the tkt_134 allele is derived from a homologous gene (which we designate tktX) that is not present in all S. agalactiae; all known strains that contain a match to the tkt_134 allele also contain a gene sequence that is much closer in sequence identity to the other non-tkt_134 alleles (i.e., the canonical tkt gene) in the database. Based on these data, the tkt_134 allele has been removed from the MLST database as of September 2021, and all sequence types containing tkt_134 have also been removed.

Multilocus sequence typing (MLST) databases are a common good and remain important for research, medical, and epidemiological purposes. This remains true even in the context of widespread whole-genome sequencing. We discovered a contaminating allele of the tkt gene in the S. agalactiae MLST database that led to unstable, ambiguous, or erroneous MLST assignment. The allele has since been removed from the public database based on the results presented in this manuscript.