title: In vitro activity of gentamicin as an adjunct to penicillin against biofilm group B Streptococcus.
abstract: Group B Streptococcus (GBS) increasingly causes invasive disease in non-pregnant adults, particularly in elderly persons and those with underlying diseases. Combination therapy with penicillin plus gentamicin has been suggested for periprosthetic joint infection. The postulated synergism of this combination is based on experiments with planktonic bacteria. We aimed to assess the efficacy of this combination against sessile bacteria.
  
  
  
  Four different GBS strains were used. We compared results of MICs with those of minimal biofilm eradication concentrations (MBECs), applied chequerboard assays to the MBEC device and calculated the fractional inhibitory concentration index. Synergism was evaluated with time-kill assays against bacteria adherent to cement beads, using penicillin (0.048, 0.2 and 3 mg/L), gentamicin (4 and 12.5 mg/L) and a combination thereof. Results were evaluated via colony counting after sonication of beads and scanning electron microscopy.
  
  
  
  MBEC/MIC ratios were 2000-4000 for penicillin and 1-4 for gentamicin. In chequerboard assays, synergism was observed in all four isolates. In time-kill assays, penicillin and 12.5 mg/L gentamicin showed synergism in two isolates. In the other two isolates 12.5 mg/L gentamicin alone was as efficient as the combination therapy.
  
  
  
  These in vitro investigations show activity of 12.5 mg/L gentamicin, alone or as an adjunct to penicillin, against four strains of biofilm GBS. This concentration cannot be achieved in bone with systemic administration, but can be reached if administered locally. The combination of systemic penicillin plus local gentamicin indicates a potential application in orthopaedic-device-associated GBS infections. Studies with a larger number of strains are required to confirm our results.
authors.affiliation: Institute for Infectious Diseases, University of Bern, Bern, Switzerland.|Graduate School for Cellular and Biomedical Science, University of Bern, Bern, Switzerland.
authors.forename: Corinne
authors.initials: C
authors.lastname: Ruppen
authors.affiliation: Institute of Parasitology, University of Bern, Bern, Switzerland.
authors.forename: Andrew
authors.initials: A
authors.lastname: Hemphill
authors.affiliation: Institute for Infectious Diseases, University of Bern, Bern, Switzerland parham.sendi@ifik.unibe.ch.|Department of Infectious Diseases, Bern University Hospital, University of Bern, Bern, Switzerland.
authors.forename: Parham
authors.initials: P
authors.lastname: Sendi
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D018441:Biofilms / Q000187:drug effects*; D015169:Colony Count, Microbial; D004357:Drug Synergism; D004359:Drug Therapy, Combination / Q000379:methods; D005839:Gentamicins / Q000494:pharmacology*; D006801:Humans; D008826:Microbial Sensitivity Tests; D050296:Microbial Viability / Q000187:drug effects; D008855:Microscopy, Electron, Scanning; D010003:Osteoarthritis / Q000188:drug therapy; D010406:Penicillins / Q000494:pharmacology*; D016459:Prosthesis-Related Infections / Q000188:drug therapy; D013292:Streptococcus agalactiae / Q000187:drug effects* / Q000502:physiology*
chemical_list: D000900:Anti-Bacterial Agents; D005839:Gentamicins; D010406:Penicillins