CAMP-negative Streptococcus agalactiae strains exhibited complete or partial chromosomal deletions of the CAMP-factor encoding gene cfb

ABSTRACT Universal antepartum group B streptococcus (GBS) screening and intrapartum antibiotic prophylaxis (IAP) have effectively reduced early-onset GBS infections. However, GBS strains with chromosomal deletions affecting the cfb gene may produce false negatives in both the CAMP test and cfb-based molecular diagnostics, potentially increasing the risk of neonatal infections. Vaginal swabs were collected from pregnant women at 35–37 weeks of gestation in our hospital and cultured on agar. Suspected GBS strains were initially identified using the CAMP test and then confirmed with the VITEK-2 system. CAMP-negative GBS strains underwent additional testing by qPCR, 16S rDNA, serotyping, and multilocus sequence typing (MLST). PCR for the cfb gene and whole-genome sequencing were performed on CAMP-negative strains. From 5,794 samples, 526 (9.1%) GBS strains, including 19 (3.6%) CAMP-negative strains and 2 strains from the same patient, were isolated. All 19 CAMP-negative strains were serotypes III and ST862. Among these strains, only one strain was cfb positive by qPCR, whereas all tested positive with a multitarget qPCR kit for cfb and cps. PCR amplification upstream of the cfb gene produced a specific band in strain PP669713 only, suggesting N-terminal cfb gene retention in PP669713 and complete cfb loss in the other strains. Whole-genome sequencing confirmed a chromosomal deletion in PP669713. Antibiotic susceptibility testing revealed no resistance to penicillin. However, CAMP-positive strains presented a greater prevalence of resistance to ciprofloxacin, and levofloxacin than CAMP-negative strains did. Our study highlights the potential risk of missed GBS detection using CAMP tests and cfb-targeted molecular assays.IMPORTANCEOur work makes several novel contributions to the field. (i) We report the first documented case of a C-terminal deletion of the cfb gene in a CAMP-negative GBS strain, demonstrating that both N-terminal and C-terminal regions are essential for cohemolytic activity. (ii) Our findings reveal that CAMP-negative GBS strains (3.6% of isolates) are more prevalent than previously recognized, with most cases resulting from complete chromosomal deletions of the cfb gene. (iii) We provide evidence that single-target molecular assays targeting only the cfb gene may miss GBS detection, highlighting the necessity for multi-target approaches in clinical diagnostics. (iv) We demonstrate a unique antibiotic resistance pattern in CAMP-negative strains, showing significantly lower resistance to certain antibiotics compared to CAMP-positive strains.