To Determine In Vitro Activities of Various Drugs Tigecycline, Meropenem, and Rifampicin Alone and in Combination of Colistin Against Carbapenem–Intermediate and–Resistant Strains of Acinetobacter baumannii

Abstract Background: Acinetobacter baumannii has been one of the most troublesome pathogens with Multi Drug Resistant and Pan Drug Resistant strains in ICU settings worldwide. Treating these strains is really challenging as none of the existing antibiotic seem to perform effectively. In order to treat these strains, novel and potent synergistic antibiotic combinations must be discovered and defined from the existing antibiotic repertoire. Methods: Non repeat isolates of Acinetobacter baumannii samples were first subjected to MIC determination by Agar Dilution Test. All Meropenem resistant isolates were further subjected to Repetitive Extragenic Palindromic PCR for strain typing. MIC of each strain was determined using E strips against antibiotics Colistin, Meropenem, Tigecycline and Rifampicin. Synergy testing by E strip was performed on each strain for following antibiotic combinations a) Colistin and Meropenem, b) Colistin and Tigecycline and c) Colistin and Rifampicin. Fractional inhibitory concentration (FIC) index was employed to classify interaction outcomes as synergistic, additive, indifferent, or antagonistic. Results: Out of the total number of specimens, 80 yielded isolates confirmed as Acinetobacter baumannii . Among the 80 isolates analyzed, 48 were identified as resistant to meropenem. 48 isolates of Meropenem resistant Acinetobacter baumannii were found to be of 12 different genotypes after Repetitive Extragenic Palindromic PCR which were further subjected to Synergy testing. Synergy was found between colistin and tigecycline by one isolate only. Other isolates came to be either additive or indifferent. Other combinations i.e. colistin + meropenem and colistin + rifampicin showed either additive or indifferent results. Conclusions: Our study recommends Colistin as effective treatment modality for Multi Drug Resistant and Pan Drug Resistant strains of Acinetobacter baumannii . It was highly effective in all the isolates from ICU as shown by our In Vitro study. Colistin and tigecycline can also be tried as synergy was demonstrated in one isolate.