Antimicrobial susceptibility profiles of amniotic fluid microorganisms: limited ampicillin coverage and clinical implications for the management of intra-amniotic infection.

OBJECTIVE

To evaluate ampicillin (ABPC) susceptibility of microorganisms isolated from amniotic fluid and assess antimicrobial susceptibility patterns, including microorganisms not identified at baseline after antibiotics initiation.

STUDY

DESIGN

Retrospective cohort study at a single tertiary center.

METHODS

We reviewed singleton pregnancies that underwent transabdominal amniocentesis for suspected intra-amniotic infection between 2014 and 2023, stratified by prior antibiotic exposure within 90 days. Amniotic fluid underwent Gram staining, aerobic and anaerobic cultures, and Ureaplasma/Mycoplasma-specific culture with molecular identification of Ureaplasma parvum, U. urealyticum, and Mycoplasma hominis. Minimum inhibitory concentrations were interpreted according to CLSI M100-S25. The primary outcome was the proportion of cases with exclusively ABPC-susceptible microorganisms. Secondary outcomes included susceptibility to other antibiotics, detection of microorganisms after antibiotic initiation, and findings among early-onset neonatal sepsis (EONS).

RESULTS

Among 631 eligible women, 137 without prior antibiotic use had positive cultures; only 17% harbored exclusively ABPC-susceptible organisms. Most ABPC-non-susceptible isolates were Ureaplasma spp. or M. hominis. Excluding these organisms and fungi, 28% of isolates were ABPC-non-susceptible, with < 50% susceptibility to other β-lactams or gentamicin. Microorganisms not identified at baseline were detected after antibiotic initiation as early as 2-7 days, and most were non-susceptible to administered agents. Microbial invasion of the amniotic cavity (MIAC) was confirmed in 9 of 10 patients with EONS; 3 involved ABPC + gentamicin-non-susceptible organisms.

CONCLUSIONS

ABPC susceptibility among MIAC-associated microorganisms was limited, and non-susceptible organisms may become detectable after antibiotic exposure. Culture-directed therapy should be prioritized, and optimized regimens targeting Ureaplasma spp. and M. hominis are needed.