ALCANCES, LÍMITES Y RECOMENDACIONES

Acquisition of resistance genes on mobile genetic elements is common in many bacteria, and no less so in P. aeruginosa. Genes coding for resistance enzymes are often found on plasmids and transposons, with some even carrying multiple resistance genes on a single integron (Walsh, Toleman et al. 2005).

The presence of multiple resistance gene cassettes on a single mobile element is of particular concern, given the capacity of such mobile elements to create a multi-resistant P. aeruginosa infection literally overnight. Exposure to antimicrobial agents selects for and amplifies the presence of resistant bacterial populations within a patient, providing a source for the spread of resistance throughout a hospital. In this way, hospitals and other institutions may harbour highly resistant and difficult to treat strains of P. aeruginosa unless very thorough infection control procedures are adhered to.

1.9.2.1 Narrow spectrum β-lactamases

P. aeruginosa strains commonly acquire OXA (Ambler class D) and/or PSE (syn. CARB - Ambler class A) β-lactamases (Bert, Branger et al. 2002; Rice, Sahm et al. 2003; Weldhagen, Poirel et al. 2003), which will hydrolyse carboxy-penicillins and ureidopenicillins, narrow spectrum cephalosporins and expanded spectrum cephalosporins, but not extended spectrum cephalosporins (Bert, Branger et al. 2002). These enzymes are commonly found in P. aeruginosa, but are considered very rare in the enterobacteriaceae. Conversely, narrow spectrum Ambler class A β-lactamases, such as TEM and SHV, commonly found in enterobacteriaceae are found only rarely in P. aeruginosa (Weldhagen, Poirel et al. 2003). A French study found that only 10% of ticarcillin resistant P. aeruginosa isolates (representing just 1.9% of P. aeruginosa isolates overall) had acquired a TEM type β-lactamase (Bert, Branger et al. 2002).

1.9.2.2 Extended spectrum β-lactamases

A relatively recent occurrence (1983) in the history of antimicrobial resistance was the discovery of extended spectrum β-lactamases (ESBLs). These enzymes were identified shortly after the introduction of the extended spectrum (or third generation) cephalosporins, such as ceftazidime, cefotaxime and ceftriaxone. ESBLs are narrow spectrum β-lactamase enzymes which have mutated to

allow the binding of extended spectrum cephalosporins. Thus, these enzymes are often active against all penicillins, and cephalosporins below the fourth generation (cefepime and cefpirome) as well as the monobactam, aztreonam (Jiang, Zhang et al. 2006). Some of these enzymes show a greater affinity for specific antimicrobials, such as the CTX-M ESBLs, which more efficiently hydrolyse cefotaxime and ceftriaxone than they do ceftazidime (Rice, Sahm et al. 2003). The activity of ESBLs may be inhibited by exposure to clavulanic acid, sulbactam or tazobactam. Consistent with their spectra of activity, the effect of a specific inhibitor is dependent upon what type of enzyme is involved. For instance, the CTX-M enzymes are more readily inhibited by tazobactam than by clavulanic acid, whereas some TEM ESBLs are resistant to all β-lactamases inhibitors (Rice, Sahm et al. 2003).

Many different ESBL types have been identified in P. aeruginosa, some of which are associated with specific geographic regions, leading to the suggestion that these enzymes may perform a role within specific ecological niches. Although rates of ESBL positivity in P. aeruginosa are low compared to the enterobacteriaceae, this frequency appears to be increasing (Jiang, Zhang et al. 2006). It is thought that the TEM and SHV ESBLs found in P. aeruginosa have been acquired from enterobacteriaceae, whilst the PER and OXA classes are mutations of enzymes from P. aeruginosa itself. ESBL types described in P. aeruginosa thus far are; VEB, PER, SHV, TEM, GES, IBC and BEL class ESBLs (Weldhagen, Poirel et al. 2003; Bogaerts, Bauraing et al. 2007).

1.9.2.3 Mobile metallo-β-lactamases

Metallo-β-lactamases (Ambler class B) are metalloenzymes, relying upon metal ions rather than serine for catalysis of their reactions. These enzymes have a broad spectrum of activity including penicillins, cephalosporins (including the fourth generation cephalosporins) and imipenem, but not aztreonam. They are not inhibited by β-lactamase inhibitors, but will not function in the presence of ion chelators

due to chelation of metal ion enzymatic co-factors. The extremely wide spectrum of action of these enzymes causes serious difficulties in clinical practice, as the use of virtually all β-lactam antimicrobials is ineffective in all organisms expressing such enzymes.

A recent and extremely disturbing event has been the emergence of multiple types of new mobile metallo-β-lactamases (MMβLs), and their spread between the enterobacteriaceae and non-fermentative Gram negative rods (Rice, Sahm et al. 2003). The first MMβL identified in a strain of P. aeruginosa was IMP-1, found to be encoded on a mobile conjugative plasmid. Since this time, at least four other types of MMβL have been identified (GIM-1, VIM-1, VIM-2, SPM-1), conferring resistance to all β- lactam agents, including imipenem, with a greater or lesser degrees of activity against aztreonam. The rapid spread of these novel resistance genes worldwide has resulted in its appearance in most countries (including Australia) (Walsh, Toleman et al. 2005). SPM-1 MMβL producing strains accounting for 35% of clinical carbapenem resistant isolates of P. aeruginosa from hospitals throughout Brazil in 2003 (Gales, Menezes et al. 2003). Of even greater concern has been the identification of an integron carrying a partially deleted qac and sul genes, blaGIM-1, aacA4, aadA1 and blaOXA-2. Thus, in one

mobile genetic element, resistance to sulphonamides, quarternary ammonia compounds, β-lactam agents and aminoglycosides, treatment with any of which would select for bacterial isolates that had acquired this integron (Walsh, Toleman et al. 2005). The potential for the world-wide spread of MMβL resistance and accompanying multi-drug resistance cassettes in both the enterobacteriaceae and non-fermenting Gram negative rods, in a process analogous to the rapid spread of the ESBL phenotype remains an ominous prospect.