In vitro Antimicrobial Activity of Ampicillin-Ceftriaxone and Ampicillin-Ertapenem Combinations Against Clinical Isolates of Enterococcus faecalis with High Levels of Aminoglycoside Resistance
Maria Bruna Pasticci*, Antonella Mencacci2, Amedeo Moretti1, Nicola Palladino1, Luigi Maria Lapalorcia1, Francesco Bistoni2, Franco Baldelli1
1 Infectious Disease Section, Department of Experimental Medicine and Biochemical Sciences, University of Perugia, Perugia, Italy
2 Microbiology Section, Department of Experimental Medicine and Biochemical Sciences, University of Perugia, Perugia, Italy
This paper reports on the in vitro antimicrobial activity of ampicillin-ceftriaxone and ampicillin-ertapenem combinations against five strains of E. faecalis with high-level aminoglycoside resistance recovered from blood of septicemic patients. Double disk diffusion test and time killing curves were used. A bacteriostatic synergistic effect between ampicillin and ceftriaxone was detected using the disk diffusion assay for three of the five enterococcal strains studied. With the same three isolates enhanced bactericidal activity was also observed using time killing experiments. Overall, for these three strains, after 24 hr of contact, a decrease ≥ 2 log10 from the initial bacterial inoculum was registered with most ampicillin-ceftriaxone combinations, reaching with some of them a colony reduction ≥ 3 log10. This bactericidal interaction was negatively influenced increasing the bacterial inoculum. In all five isolates neither a bacteriostatic nor a bactericidal cooperation was observed for ampicillin combined with 2 mg/l of ertapenem.
This investigation broadened the evidence of antimicrobial synergism in vitro between ampicillin and ceftriaxone in selected strains of Enterococcus faecalis with high-level aminoglycoside resistance.
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* Address correspondence to this author at the Infectious Disease Section, Department of Experimental Medicine and Biochemical Sciences, University of Perugia, Perugia, Italy; Tel +390755784359; Fax +390755784334; E-mail: firstname.lastname@example.org