Antibiotic Resistance Profiles and Genotypes of Acinetobacter baumannii Isolates and In Vitro Interactions of Various Antibiotics in Combination with Tigecycline and Colistin

Prevalence of Extended-Spectrum β-Lactamase Genes and Antibiotic Resistance Pattern in Clinical Isolates of Acinetobacter baumannii from Patients Hospitalized in Mashhad, Iran

Background: Carbapenem-resistant Acinetobacter baumannii strains are one of the most severe factors in hospital infection worldwide, in which the beta-lactamase enzyme is one of the main resistance mechanisms. Objectives: This study aimed to evaluate the presence of carbapenem-resistant beta-lactamase genes and determine antibiotic resistance patterns in the clinical isolates of A. baumannii from patients hospitalized in the Shahid Kamyab Hospital, Mashhad, Iran. Methods: Out of 286 collected isolates from patients hospitalized in Shahid Kamyab Hospital (from March 2017 to June 2017), 31 isolates were confirmed to be A. baumannii using biochemical tests. Antibiotic susceptibility testing was conducted using the disc diffusion method according to the CLSI standard protocols. The presence of beta-lactamase genes, namely blaVEB, blaPER, blaAmpC, blaVIM, blaIMP, blaSHV, and blaTEM, was detected using polymerase chain reaction. Results: In this study, 31 isolates were identified as Acinetobacter baumannii, all of which revealed high resistance to ceftazidime, cefixime, ceftriaxone, meropenem, imipenem, cefotaxime and cephalexin. In this case, the lowest resistance (19.35%) was observed against polymixin B. Moreover, blaAmpC, blaTEM, blaSHV, blaPER, and blaVIM were observed in 93.54% (29), 51.61% (16), 48.38% (15), 41.93% (13), and 77% (24) of the isolates, respectively. However, blaVEB and blaIMP were observed in none of the isolates. Conclusions: The results showed high carbapenem resistance and high frequency of beta-lactamase resistance genes among the clinical isolates of A. baumannii.

An update on technical, interpretative and clinical relevance of antimicrobial synergy testing methodologies

Testing for antimicrobial interactions has gained popularity in the last decade due to the increasing prevalence of drug-resistant organisms and limited options for the treatment of these infections. In vitro combination testing provides information, on which two or more antimicrobials can be combined for a good clinical outcome. Amongst the various in vitro methods of drug interactions, time-kill assay (TKA), checkerboard (CB) assay and E-test-based methods are most commonly used. Comparative performance of these methods reveals the TKA as the most promising method to detect synergistic combinations followed by CB assay and E-test. Various combinations of antimicrobials have been tested to demonstrate synergistic activity. Promising results were obtained for the combinations of meropenem plus colistin and rifampicin plus colistin against Acinetobacter baumannii, colistin plus carbapenem and carbapenem plus fluoroquinolones against Pseudomonas aeruginosa and colistin/polymyxin B plus rifampicin/meropenem against Klebsiella pneumoniae. Antagonism was detected in only few instances. The presence of synergy or antagonism with a combination seems to correlate with minimum inhibitory concentration of the agent and molecular mechanism involved in the resistance. Further studies need to be conducted to assess the utility of in vitro testing to predict clinical outcome and direct therapy for drug-resistant organisms.