**4.2. Resistance to fluoroquinolones**

reported from Germany [41] and Australia [84] and did not spread elsewhere. Finally, the only

Ambler class A carbapenemase KPC was first reported in *P. aeruginosa* isolates in Colombia [64] but KPC-producing *P. aeruginosa* isolates have not been reported from other continents except Latin America. KPCs present high rates of carbapenem hydrolysis and inactivate all

Enzymes GES/IBC belong to the same enzymatic class but their carbapenemase activity is not as high as that of the KPCs. It may become important however if combined with diminished outer membrane permeability or efflux over-expression. For *P. aeruginosa*, GES-2 has been

Class D carbapenemases like OXA-198 have been found in *P. aeruinosa* isolates although such findings are rather rare for this species [87]. The most clinically important carbapenemases are

Among the various efflux systems of *P. aeruginosa*, MexAB-OprM, MexXY-OprM and MexCD-OprJ play an important role in developing beta-lactam resistance [88]. Between these three, MexAB-OprM accommodates the broadest range of beta-lactams [24], is by far the better exporter of meropenem [24] and is most frequently related to beta-lactam resistance in clinical *P. aeruginosa* isolates [33,89]. The efflux pumps may be over-expressed in some isolates [90] contributing thus, together with other mechanisms in the development of multi-drug resist‐

OprD is a specific porin of the outer membrane of *P. aeruginosa* through which carbapenems (mainly imipenem) enter into the periplasmic space [91]. Diminished expression [92] or mutational loss [93] of this porin is the most common mechanism of resistance to carbapenems [24,94] and is frequently associated with efflux pumps and/or AmpC over-expression [36,38].

VIM enzymes SPM-1 GIM-1 AIM-1 NDM-1

report for NDM-1 in *P. aeruginosa* was made from Serbia [76].

reported in South Africa [85] and IBC-2 in Greece [86].

**Ambler molecular class Bush-Jacoby-Madeiros group Carbapenemases**

B 3 IMP enzymes

A 2f KPC

**Table 5.** Clinically important carbapenemases found in *P. aeruginosa* isolates.

other beta-lactams including aztreonam.

summarized in Table 5.

40 Infection Control

*4.1.4. Efflux systems over-expression*

*4.1.5. Diminished permeability*

ance [24].

High-level resistance to fluoroquinolones is mediated by target site modifications. Efflux plays a contributing role as well [96,97] and the two mechanisms often coexist [32,98-100].

#### *4.2.1. DNA gyrase and topoisomerase IV mutations*

Gyrase and topoisomerase are comprised by two subunits each. DNA gyrase (GyrA and GyrB) is the main target of fluoroquinolones in *P. aeruginosa*. Consequently, mutations are most common for this enzyme rather than for topoisomerase IV (ParC and ParE) [98-102]. Highly resistant isolates have multiple mutations in *gyrA* and/or *parC* [98,101-103] while mutations regarding the other subunits are less frequently encountered [100-102,104].

#### *4.2.2. Efflux pumps contribution*

Four efflux pumps contribute to fluoroquinolone resistance: MexAB-OprM, MexCD-OprJ, MexEF-OprN and MexXY-OprM [105] as a consequence of mutational events in their repressor genes [24]. Among these, MexAB-OprM, MexCD-OprJ, and MexEF-OprN have been associ‐ ated to fluoroquinolone resistance in clinical isolates [31,105-107] whereas MexXY-OprM has only been linked rarely to such type of resistance [106].

#### **4.3. Resistance to aminoglycosides**

Acquired resistance to aminoglycosides is mediated by transferable aminoglycosidemodifying enzymes (AMEs), rRNA methylases and derepression of endogenous efflux systems [24,108,109].

#### *4.3.1. Aminoglycoside-modifying enzymes*

Modification and subsequent inactivation of aminoglycosides is achieved by three deferent mechanisms: (1) acetylation, by aminoglycoside acetyltransferases (AACs), (2) adenylation, by aminoglycoside nucleotidyltransferases (ANTs), and (3) phosphorylation, by aminoglycoside posphoryltransferases (APHs) [108].

Genes encoding AMEs are typically found on integrons together with other genes responsible for transferable resistance for other antibiotic classes. This way AMEs be‐ come important determinants for the development of multi-drug resistance in *P. aerugino‐ sa* and other species [24,108,109].

Enzymatic families that acetylate the 3 and 6' position of the antibiotic are the most common. Five subfamilies of AAC(3) and two of AAC(6') have been described for *P. aeruginosa,* each one presenting different preferences for aminoglycoside substrates (Table 6).

Among the nucleotidyltransferases, ANT(2')-I is the most frequently encountered in *P. aerugiosa.* This enzyme is present in isolates showing resistance to gentamicin and tobramycin but not to amikacin [109].

Almost all phosphoryltransferases of *P. aeruginosa* act in the 3' position of the aminoglycoside molecule [24]. However, they have less clinical importance because of the fact that they inactivate aminoglycosides that are not routinely used for the treatment of *P. aeruginosa* infections such as kanamycin and neomycin [109]. The enzymes of this family that inactivate anti-pseudomonal aminoglycosides are APH(3')-VI [110-112], APH(3')-IIb-like [113] and APH(2'') [110]. Despite being reported in some cases, these enzymes remain rare for clinical *P. aeruginosa* isolates [24].
