CAMP-negative group B Streptococcus in pregnant women: molecular and clinical features with implications for diagnostics and neonatal management

The Christie-Atkins-Munch-Peterson (CAMP) test and cfb-targeted polymerase chain reaction (PCR) are widely used for identification of Streptococcus agalactiae (Group B Streptococcus, GBS). However, CAMP-negative strains may evade detection. This study aimed to characterize antimicrobial resistance, molecular profiles, and pregnancy outcomes associated with CAMP-negative GBS. 55 CAMP-negative GBS strains (45 from pregnant women and 10 from non-pregnant outpatients) were collected; the 10 non-pregnant isolates were included only for molecular characterization and antimicrobial resistance profiling, while clinical pregnancy outcomes were analyzed exclusively in the pregnant cohort. A total of 66 contemporaneous CAMP-positive controls were collected (2017–2021) at Guangzhou Medical University Affiliated Women and Children’s Medical Center. Isolates were identified by VITEK-2 (bioMérieux) or matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) followed by CAMP test. Pregnancy outcomes were retrieved. Antimicrobial susceptibility, multilocus sequence typing (MLST), capsular serotyping, and 11 virulence genes were analyzed. Among 46,746 pregnant women screened, 6,108 (13.1%) were GBS-positive, including 45 (0.74%) CAMP-negative isolates. Adverse outcomes were observed in 42.2% (19/45), including premature rupture of membranes (11/45, 24.4%), low birth weight (6/45, 13.3%), and preterm delivery (2/45, 4.4%). All isolates were susceptible to β-lactams, vancomycin, linezolid, and levofloxacin, but resistant to erythromycin (18/55, 32.7%), clindamycin (31/55, 56.4%), and tetracycline (35/55, 63.6%). CAMP-negative isolates were predominantly of serotype III and sequence type (ST) 862 (50/55, 90.9%), compared with diverse types in controls. All lacked cfb and bac genes but harbored 9 remaining virulence genes. Although CAMP-negative GBS is rare in clinical settings, it can escape detection by the CAMP test and single-target PCR assays targeting the cfb gene, leading to false negative results. Integrated diagnostic strategies combining culture-based screening with multiplex PCR are therefore essential to accurate detection of GBS in maternal GBS surveillance, which is critical for optimizing neonatal GBS disease prevention and management.