[Comparison of different phenotypic methods detecting extended spectrum beta-lactamases in Pseudomonas aeruginosa and
Pinar Zarakolu1, Gökhan Metan, Gülşen Hasçelik
1Hacettepe Universitesi Tip Fakültesi, Iç Hastaliklari Anabilim Dali, Infeksiyon Hastaliklan Unitesi, Ankara.
Abstract:
Pseudomonas aeruginosa and Acinetobacter baumannii which are of great concern as the frequent causative agents of nosocomial infections, frequently exhibit resistance to beta-lactam antibiotics. One of the most common causes of this resistance is the presence of extended spectrum beta-lactamases (ESBL). There is still no standardized method for the detection of ESBLs in P. aeruginosa and A.baumannii strains. The aim of this study was to compare the results of Epsilomer test (E-test), combined disk (CD) and double disk synergy (DDS) methods for the detection of ESBL in a total of 38 P. aeruginosa and 45 A.baumannii strains with ceftazidim or cefotaxime MIC > or = 1 microg/ml, isolated from different clinical samples. Cefepime was included in the group of indicator antibiotics and the distance between clavulanic acid containing disk and indicator antibiotic disks was reduced to increase the sensitivity of DDS test. Of the P. aeruginosa strains, 78.9% was detected as ESBL producers by ceftazidime/ ceftazidime-clavulanic acid and 21.1% detected by cefotaxime/cefotaxime-clavulanic acid ratio. Among this group of strains, 31.5% was positive by DDS test and 84.2% was positive by CD method. Of the A.baumannii strains, 93.3% of them was positive by ceftazidime/ceftazidime-clavulanic acid and 6.6% was positive by cefotaxime/cefotaxime-clavulanic acid ratio. DDS test detected 20% and CD method detected 53.3% of the strains positive by E-test method. The use of cefotaxime together with ceftazidime as an indicator antibiotic increased the sensitivity of detection of ESBL enzymes. Although inclusion of cefepime and reduction of the distance between disks increased the detection of ESBL by DDS method, it was still less effective than E-test. Although, 28.9% of the P. aeruginosa and 13.3% of the A. baumannii strains were susceptible to ceftazidime by in vitro tests, they were ESBL producers. In conclusion, E-test method was found to be the most sensitive phenotypic method for the detection of ESBL in our study collection.
