Abstract
In Latin America, extended-spectrum ß-lactamases (ESBLs) are commonly found in members of the Enterobacteriaceae, whereas metallo-ß-lactamases (MBLs) have been found exclusively in Pseudomonas and Acinetobacter species (Paterson & Bonomo, 2005; Sader et al., 2005; Walsh et al., 2005). In this regard, following the appearance and widespread dissemination of ESBLs, carbapenems have been the therapy of choice, as they are stable with respect to these enzymes (Paterson & Bonomo, 2005). Unfortunately, ESBL- and MBL-producing members of the Enterobacteriaceae have been observed in Brazilian hospitals since 2003, as reported in this letter. Microbiological and epidemiological aspects associated with the emergence and early dissemination of these unusual strains are disclosed.
From June 2003 to June 2005, seven imipenem-resistant enterobacterial isolates (six isolates of Klebsiella pneumoniae and one isolate of Providencia rettgeri) were identified from six different hospitals (AF) in the city of São Paulo, in southeastern Brazil (Table 1). These strains were recovered from six patients. Identification and antimicrobial susceptibility profiles were evaluated using the VITEK system (bioMérieux). The enterobacterial isolates were found to be resistant to all extended-spectrum cephalosporins, cephamycins and carbapenems and insensitive to clinically available inhibitors. Curiously, an intermediate susceptibility to aztreonam (MICs 1216 mg l1) was detected. MICs for imipenem were subsequently determined using an agar dilution method (NCCLS, 2000) and Etest according to NCCLS guidelines and the manufacturer's instructions (AB Biodisk), respectively. The high levels of imipenem resistance (>32 mg l1) observed in these enterobacterial isolates suggested the presence of a carbapenemase. Hydrolysis of imipenem was confirmed by bioassays using either Staphylococcus aureus ATCC 25923 or Micrococcus luteus ATCC 9341, as described previously (Lincopan et al., 2005). A double disc diffusion test using specific ß-lactam inhibitors was employed to screen for MBL and ESBL production (Arakawa et al., 2000; Thomson & Sanders, 1992). Hydrolysis of imipenem was inhibited by thiol compounds (2-mercaptoacetic acid and 2-mercaptopropionic acid) or EDTA, but not by clavulanic acid, suggesting that an MBL was responsible for the carbapenemase activity. The addition of EDTA to imipenem reduced the MIC from >32 mg l1 to 1.00.5 mg l1 (Table 1). On the other hand, the reduced susceptibility to aztreonam exhibited by the strains suggested an additional ESBL production, as aztreonam is not a substrate for MBL enzymes (Walsh et al., 2005). The presence of this ESBL was also confirmed by the double disc approximation test using clavulanic acid inhibition; however, a ghost zone was observed exclusively between the aztreonam- and clavulanic acid-containing discs.