title: Effective antibiotics in combination against extreme drug-resistant Pseudomonas aeruginosa with decreased susceptibility to polymyxin B.
abstract: OBJECTIVE
  Extreme drug-resistant Pseudomonas aeruginosa (XDR-PA) with decreased susceptibility to polymyxin B (PB) has emerged in Singapore, causing infections in immunocompromised hosts. Combination therapy may be the only viable therapeutic option until new antibiotics become available. The objective of this study is to assess the in vitro activity of various antibiotics against local XDR-PA isolates.
  
  
  METHODS
  PA isolates from all public hospitals in Singapore were systematically collected between 2006 and 2007. MICs were determined according to CLSI guidelines. All XDR-PA isolates identified were genotyped using a PCR-based method. Time-kill studies (TKS) were performed with approximately 10(5) CFU/ml at baseline using clinically achievable unbound concentrations of amikacin (A), levofloxacin (L), meropenem (M), rifampicin (R) and PB alone and in combination. Bactericidal activity (primary endpoint) was defined as a ≥3 log(10) CFU/ml decrease in the colony count from the initial inoculum at 24 hours.
  
  
  RESULTS
  22 clinical XDR-PA isolates with PB MIC 2-16 µg/ml were collected. From clonal typing, 5 clonal groups were identified and nine isolates exhibited clonal diversity. In TKS, meropenem plus PB, amikacin plus meropenem, amikacin plus rifampicin, amikacin plus PB exhibited bactericidal activity in 8/22, 3/22, 1/22 and 6/22 isolates at 24 hours respectively. Against the remaining ten isolates where none of the dual-drug combination achieved bactericidal activity against, only the triple-antibiotic combinations of ARP and AMP achieved bactericidal activity against 7/10 and 6/10 isolates respectively.
  
  
  CONCLUSIONS
  Bactericidal activity with sustained killing effect of ≥99.9% is critical for eradicating XDR-PA infections, especially in immunocompromised hosts. These findings underscore the difficulty of developing combination therapeutic options against XDR-PA, demonstrating that at least 3 antibiotics are required in combination and that efficacy is strain dependant.
authors.affiliation: Singapore General Hospital, Department of Pharmacy, Singapore, Singapore.
authors.forename: Tze-Peng
authors.initials: TP
authors.lastname: Lim
authors.forename: Winnie
authors.initials: W
authors.lastname: Lee
authors.forename: Thean-Yen
authors.initials: TY
authors.lastname: Tan
authors.forename: Suranthran
authors.initials: S
authors.lastname: Sasikala
authors.forename: Jocelyn
authors.initials: J
authors.lastname: Teo
authors.forename: Li-Yang
authors.initials: LY
authors.lastname: Hsu
authors.forename: Thuan-Tong
authors.initials: TT
authors.lastname: Tan
authors.forename: Nur
authors.initials: N
authors.lastname: Syahidah
authors.forename: Andrea L
authors.initials: AL
authors.lastname: Kwa
mesh_terms: D000583:Amikacin / Q000008:administration & dosage; D000900:Anti-Bacterial Agents / Q000008:administration & dosage / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial; D004357:Drug Synergism; D004359:Drug Therapy, Combination / Q000379:methods*; D006801:Humans; D064704:Levofloxacin; D000077731:Meropenem; D008826:Microbial Sensitivity Tests; D015242:Ofloxacin / Q000008:administration & dosage; D010802:Phylogeny; D011112:Polymyxin B / Q000494:pharmacology*; D011552:Pseudomonas Infections / Q000188:drug therapy; D011550:Pseudomonas aeruginosa / Q000378:metabolism*; D012293:Rifampin / Q000008:administration & dosage; D013845:Thienamycins / Q000008:administration & dosage; D013997:Time Factors; D001618:beta-Lactamases / Q000378:metabolism
chemical_list: D000900:Anti-Bacterial Agents; D013845:Thienamycins; D064704:Levofloxacin; D000583:Amikacin; D015242:Ofloxacin; D001618:beta-Lactamases; D000077731:Meropenem; D011112:Polymyxin B; D012293:Rifampin