Using the bca Gene Coupled with a Tetracycline and Macrolide Susceptibility Profile to Identify the Highly Virulent ST283 Streptococcus agalactiae Strains in Thailand

Abstract

Invasive infection by Streptococcus agalactiae (GBS) is a significant cause of death in newborn babies. In Thailand, data on strain distribution in GBS, specific virulence genes, and susceptibility patterns are limited. Therefore, our study aimed to establish the sequence type (ST) distribution and to use a specific virulence gene in combination with a susceptibility profile for strain identification. Non-duplicate 277 isolates of GBS were tested for ST, virulence genes, and antimicrobial susceptibility profiles. Twenty-five STs were detected. The ST283 (29.24%) and ST1 (27.07%) were the most common STs. The absence of the bca gene was an excellent marker to rule out ST283. All isolates were susceptible to nearly all tested antibiotics; however, only ST283 revealed 100% susceptibility to tetracycline, while ST1 and other non-ST283 showed 21.33 and 4.96%, respectively. Therefore, combining the alpha-C protein (bca) positive and tetracycline susceptible revealed 100% sensitivity for ST283. However, to identify the ST283, this combination revealed 78.9% specificity, which increased to 80.2% when erythromycin or azithromycin-susceptible was added. The bca positive combined with tetracycline and erythromycin susceptibility results were a simple tool for predicting ST283. The bca negative profile with tetracycline and macrolides resistance was commonly non-ST283. The information gained by this tool would benefit patient management.