Phenotypic Detection of Inducible AmpC Beta-Lactamase among Pseudomonas aeruginosa

Abstract

In recent years, the prevalence of infections with multidrug resistant pseudomonas aeruginosa has steadily increased. Pseudomonas aeruginosa producing AmpC β-lactamases (AmpCs) have become a major therapeutic challenge. AmpC β-lactamases have gained importance since the late 1970s as one of the mediators of antimicrobial resistance in gram negative bacilli. These enzymes are typically produced by isolates of E.coli, Klebsiella spp., Proteus, pseudomonas and Salmonella spp. and are associated with multiple antibiotic resistance that leaves few therapeutic options. Confusion exists about the importance of their resistance mechanisms, optimal test methods, and appropriate reporting conventions. The detection of AmpC-producing P. aeruginosa is of significant clinical relevance since AmpC producers may appear susceptible to expanded-spectrum cephalosporins when initially tested. This may lead to inappropriate antimicrobial regimens and therapeutic failure. Thus, a simple and reliable detection procedure for AmpC producers is needed. Our aim was to detect the antibiotic susceptibility pattern among Pseudomonas aeruginosa isolated from different clinical samples from hospitalized patients in Ain Shams University Hospitals and to phenotypically evaluate disk antagonism test, disk potentiation test, modified three dimensional test and AmpC test for detection of inducible AmpC β-lactamases. This study was conducted on a total number of 150 pseudomonas isolates collected from different clinical specimens referred to the Central Microbiology Laboratory of Ain Shams University Hospitals for routine culture and sensitivity from march to september 2013. All isolates have been subjected to the following: Antibiotic susceptibility test, Disk antagonism test as screening test for detection inducible AmpC beta lactamases, (Disk potentiation test, Modified three dimensional test and AmpC disk test) for detection derepressed AmpC beta lactamases. The highest antibiotic resistance was to Ceftazidime and Cefepime and the least resistance to Impenem.so carbapenem and Amikacin were the most effective drugs against P. aeruginosa in our study. In our study, we detected 66 /150 were inducible for AmpC production as they were positive by screening test, 33 /150 are positive by disk potentiation test with a sensitivity of 72.7% and specificity of 98.3% for detection of AmpC production, 26/150 are positive by Modified three dimensional test with the sensitivity and specificity (100%) for detection of AmpC production, 23/150 are positive by AmpC disk test with a sensitivity of 91% and specificity of 96.1% for detection of AmpC production. We considered the isolate as AmpC derepressed positive if any two of the three used tests (disk potentiation test, MTDT and AmpC disk test) were positive. 26 out of 150 isolates were AmpC derepressed and Out of them, 24 (92.3%) were partially derepressed and they were also inducible and only 2 isolates (7.7%) were fully derepressed. 82 isolates were negative for inducible and non inducible AmpC production. In our study, 25.7% and 48.5% of inducible AmpC are susceptible to ceftazidime and tazobactam respectively, which may result in treatment failure due to selection of stably derepressed mutants with high level of resistance. The options of treatment are 4=th generation cephalosporins as cefepime, carbapenems and other non beta lactam agents. But in our study 46.9% are only susceptible to cefepime which may be due to beta-lactamases production, thus best treatment of inducible Ampc beta lactamases are carbapenems and other non beta lactam antibiotic as Amikacin.