Carbapenems are potent agents against multiresistant Gram negative bacilli, including P. aeruginosa, but their efficacy is increasingly compromised by the emergence and the worldwide dissemination of carbapenem resistance strains, which are implicated in large outbreaks as described in many countries [1, 25]. In Tunisia, frequencies of imipenem resistant P. aeruginosa varies between 16% and 37.6% [26–28]. At Charles Nicolle Hospital of Tunis, their frequency was stable until 2004 (1%), but increased dramatically from 2005 (25%). They were mainly isolated in surgery and intensive care unit , but in 2008, an increasing rate of multidrug-resistant P. aeruginosa was observed in the kidney transplantation unit. All strains exhibited a multidrug-resistant phenotype; they were resistant to antipseudomonal β-lactams (including aztreonam), aminoglycosides and fluoroquinolones. They remained susceptible only to colistin which is used for the treatment of our kidney transplanted patients despite its renal toxicity . MICs results are shown in table 1. All strains showed a high level of resistance to all β-lactams, particularly to carbapenems (> 512 μg/ml). Only 16 strains (67%) were positive according to the imipenem-EDTA synergy test, suggesting the presence of MBLs. The acquisition of a MBL gene alone does not necessarily confer elevated level of resistance to carbapenems. Indeed, secondary changes in regulatory system of MBL gene expression, outer membrane permeability, active efflux systems in bacterial membrane, and/or multiplication of structure gene might well be implicated in acquisition of high-level carbapenem resistance [30, 31]. Aztreonam is the only β-lactam that may remain fully active against MBL producers , however all our strains were resistant to this β-lactam, suggesting the occurrence of other mechanisms of β-lactam resistance . The eight MBL negative strains were also resistant to all antibiotic tested (Table 1), but the mechanism involved in the resistance has not been further examined.
The 16 MBLs-producer strains were positive for bla
gene and none strain harboured the bla
IMP gene (Table 1). In Tunisia, the most common MBL identified VIM-2 in accordance with the actual situation worldwide [9, 33, 34]. Historically, the first reports of MBLs genes were VIM-2 and VIM-4 types in P. aeruginosa [18, 35] and K. pneumoniae  respectively.
Serotyping identified 3 serotypes: O4 (n = 16), O11 (n = 4) and O12 (n = 1) (table 1). Only 2 strains were nontypeable with monovalent antisera. In Tunisia  as well as in many European countries , it has been repeatedly demonstrated over the past 20 years that serotypes O11 and O12 dominate among multiresistant P. aeruginosa isolates. The 16 MBL-positive strains were divided into 3 pulsotypes designed A (n = 13), B (n = 2) and C (n = 1) (Table 1). The 13 strains of pulsotype A were of serotype O4. These results imply that the dissemination of VIM-2 in our kidney transplantation unit was mainly due to the of spread clonal strains, however, unrelated VIM-2-harboring strains occurred. Outbreaks of VIM β-lactamase-producing P. aeruginosa have been also reported in Greece , Italy  and Kenya , but there is still very limited knowledge on the epidemiology of MBLs in Africa.
The emergence of acquired MBLs among P. aeruginosa represents an epidemiological risk for at least two reasons: firstly, MBLs confer resistance not only to carbapenems but to virtually all β-lactams and are frequently associated with resistance to aminoglycosides; and secondly, genes encoding for MBL enzymes are most commonly carried on mobile genetic elements (integrons, plasmids, transposons) that can spread horizontally among unrelated strains [9, 18].
Indeed, the bla
VIM-2 gene was found in strains of different genotypes (A, B and C), reflecting its ability to transfer from one bacterium to another.