gms | German Medical Science

Introduction: CTX-M enzymes belong to the ESBL Ambler class A β-lactamases. They are widespread, often plasmid mediated and cause clinically relevant resistance to penicillins and cephalosporins [1]. Depending on the CTX-M variant the antibiotic resistance profile can be different and difficult to characterize [2]. CTX-M-15 is a highly prevalent β-lactamase in E. coli ST-131 clones in clinical outbreaks [3]. The aim of this study was to establish a method for rapid identification and characterization of CTX-M enzymes.

Material and methods: Clinical E. coli isolates with increased resistance to β-lactam antibiotics were investigated. Three isolates from Hamburg were tested positively for CTX-M-1, CTX-M-2 and CTX-M-15 via PCR. Four Isolates from Australia showed CTX-M-15 in screening and were previously detected to be of clonal group O25b and characterized as ST131 [4]. E. coli ATCC 25922 was used as control strain. The isolates underwent CTX-M identification by amplification of the bla_ctx-m gene via PCR and Sanger sequencing.

Susceptibility testing and ESBL-characterization were done by MIC and disk-diffusion tests.

ESBL-profiles were investigated under different concentrations of cefotaxime, ceftazidime and cefepime with and without clavulanic acid, respectively. The growth curves were generated with the oCelloScope™ system, a digital microscope combined with a software that calculates the curves from the images in real time using a background corrected absorption (BCA) algorithm [5], [6].

E. coli ST131-Isolates were evaluated on the basis of their characteristic CTX-M profile.

Results: Antibiotic susceptibility is observable by stagnation or decrease of calculated bacterial count compared to the control. The graphs show also a delay of growth depending on drug concentration. Resistance is shown by no reduced growth compared to the no drug control and can usually be detected after 4–6 hours.

An ESBL-phenotype can quickly be identified by parallel-testing of certain combinations of cephalosporins with or without a β lactamase-inhibitor. The time needed for detection is significantly reduced due to visualization of early changes in the growth curves.

Differences in antibiotic susceptibility between strains with highly related enzymes like CTX M-1 and CTX-M-15 were detected.

Conclusion: The novel application of the oCelloScope™ system allows fast and easy detection of specific drug susceptibility profiles and can be used for characterization of ESBLs. This method could be extended to monitor outbreaks of high risk clones like E. coli ST131 harboring CTX-M-15.

Figure 1 [Fig. 1]