Potential Application of Bacteriophages in Enrichment Culture for Improved Prenatal Streptococcus agalactiae Screening

Abstract

Vertical transmission of Streptococcus agalactiae can cause neonatal infections. A culture test in the late stage of pregnancy is used to screen for the presence of maternal S. agalactiae for intrapartum antibiotic prophylaxis. For the test, vaginal-rectal swab sampling is immediately followed by enrichment culture and bacterial identification. In some cases, Enterococcus faecalis competes with and overgrowths S. agalactiae in the enrichment culture. Consequently, the identification test occasionally yields false-negative results. Bacterial viruses, bacteriophages (phages), infect and kill specific host bacteria. In the current study, we explored the feasibility of using phages to minimize the undesirable E. faecalis outgrowth and facilitate S. agalactiae detection in an experimental setting. Phage mixture was prepared using three phages that specifically infect E. faecalis: phiEF24C, phiEF17H, and phiM1EF22. The mixture inhibited the growth of 86.7% (26/30) of E. faecalis strains tested in the enrichment broth. When single strains of E. faecalis and S. agalactiae were inoculated in the enrichment broth containing the phage mixture, bacterial growth was inhibited or facilitated, respectively. Further, several sets of S. agalactiae and E. faecalis strains were co-cultured, and bacteria were detected on chromogenic agar after the enrichment culture. S. agalactiae was dominant after plating a phage mixture-treated co-culture, while it was barely detected after plating the untreated co-culture. Considering these observations, the phage mixture can be employed in the S. agalactiae culture test to increase test accuracy.