SUSCEPTIBILITY OF SERRATIA MARCESCENS ISOLATES CAUSING NEONATAL SEPSIS TO THE PREDATORY BACTERIA BDELLOVIBRIO BACTERIOVORUS

Background/Aims: Serratia sp. are emerging as a leading cause of sepsis in neonatal intensive care units (NICU), usually presenting resistance to multiple antibiotics. The predatory bacteria Bdellovibrio bacteriovorus can feed from a broad range of human pathogens, as it has been proven in mammalian cell lines and animal models, being non-toxic and non-pathogenic for host cells. We aimed to assess the susceptibility of S. marcescens isolates causing neonatal sepsis to B. bacteriovorus predation in order to explore ecological antimicrobial alternatives to antibiotics. Methods: Eight S. marcescens clinical isolates obtained from the NICU of a tertiary hospital were used as preys. Whole genome sequencing was applied to assess the phylogenetic relationship and genotypic determinants of antimicrobial resistance of the isolates. Also, antimicrobial susceptibility testing was perform by the MicroScan Walkaway system. B. bacteriovorus HD100 strain was used as predator. Co-culture of prey and predator were performed using a 96-well Bioscreen plate. Thirty µL of Bdellovibrio suspension cells (108 or 109 PFU/mL) were added to prey cultures in HEPES broth and incubated at 30°C for 48h with shaking. The capability to prey was evaluated by quantifying the amount of reduction in optical density at 600 nm of the cultures. Pseudomonas putida KT2440 was used as a positive predation control. Results: S. marcescens isolates were found to be genetically closely related, but not identical. Antimicrobial resistant determinants were found in their genomes, some within mobile genetic elements. Phenotypically, the strains exhibited different patterns of resistance. All S. marcescens strains showed similar significant reductions in their population density. However, residual cells of prey were detected in all experiments. Conclusion: B. bacteriovorus efficiently predates in vitro S. marcescens isolates recovered from preterm neonatal patients affected with late-onset sepsis, regardless their antibiotic resistance profile. Predatory bacteria might be explored as a novel adjuvant to antibiotherapy.