Antimicrobial resistance among Acinetobacter spp. isolates has increased substantially in recent years (9). The emergence of multidrug-resistant (MDR) Acinetobacter spp. isolates has become a serious clinical concern worldwide (3). A. baumannii is generally intrinsically resistant to a number of commonly used antimicrobial agents such as aminopenicillins, cephalosporins, and chloroamphenicol. In addition, it has shown a remarkable capacity to acquire resistance to broad-spectrum-β-lactams, aminoglycosides, fluoroquinolones, and tetracyclines (10). Such extensive antimicrobial resistance in A. baumannii may be due in part to the organism's relatively impermeable outer membrane and its environmental exposure to a large reservoir of resistance genes (11).

Outbreaks by MDR A. baumannii isolates have occurred worldwide, and even isolates resistant to most commercially available agents (pandrug resistance, or PDR) are also emerging (3, 12). Of particular concern is resistance to carbapenems such as imipenem and meropenem. Carbapenems are usually recommended as a potent antimicrobial agent against A. baumannii infections (13). However, carbapenem resistance in A. baumannii is emerging in many parts of the world and the resistance rate has increased to about 30% (14). Thus, few antimicrobial agents can be reliably used for effective therapy against MDR or PDR Acinetobacter infections. Although polymyxins such as polymyxin B and colistin have not typically been included in regimens to treat Acinetobacter infections since the 1980s because of their neurotoxicity and nephrotoxicity, they are now considered as one of the last resorts against MDR or PDR Acinetobacter infections (15, 16). So far, colistin or polymyxin B resistance rates among Acinetobacter isolates are very low worldwide (17). However, some investigators have reported the emergence of heteroresistance or resistance to colistin following colistin treatment (18, 19). In addition, high resistance rates against polymyxin B and colistin among Acinetobacter isolates from South Korea have been recently reported (20). Even A. baumannii isolates showing nonsusceptibilities to all antimicrobials including polymyxins and tigecycline have been found in several countries including South Korea (21, 22).

High mortality in P. aeruginosa infections is attributable to the intrinsic resistance to many antimicrobial agents and the development of the MDR phenotype in healthcare settings. The increasing prevalence of MDR among P. aeruginosa isolates from ICU patients in the United States - from 4% in 1993 to 14% in 2002 (23) - is noteworthy. As in A. baumannii and Enterobacteriaceae such as Escherichia coli and Klebsiella pneumonia, carbapenems play a significant role in the treatment of P. aeruginosa infections. However, in contrast to Enterobacteriaceae, carbapenem resistance is not unusual in P. aeruginosa. The rate of imipenem resistance among P. aeruginosa isolates has been estimated as 7~23% (7). In South Korea, imipenem-resistant P. aeruginosa isolates have increased from 17% in 1997 to 26% in 2009 according to data from the Korean Nationwide Surveillance of Antimicrobial Resistance (KONSAR) program (24).

Carbapenems such as imipenem and meropenem enter Gram-negative bacteria through outer membrane proteins and acylate the penicillin-binding proteins (PBPs). Carbapenems inhibit the peptidase domain of PBPs and can interfere with peptide cross-linking. As a result, the peptidoglycan is weakened and the cell bursts due to osmotic pressure (25). Carbapenems exhibit an overall broad in vitro antimicrobial spectrum including Gram-positive and Gram negative bacteria (26, 27). Against Acinetobacter and Pseudomonas infections, carbapenem is the most potent and widely-used agent (10, 28). However, carbapenem resistance in non-fermenting bacteria such as Acinetobacter spp. and P. aeruginosa is increasing worldwide and poses a major public health threat. The mechanisms of carbapenem resistance include the production of β-lactamases, efflux pumps, and mutations altering the expression and/or function of porins and PBPs (25).

Genetic and biochemical basis of carbapenem resistance in Acinetobacter spp. have mostly been related to the production of β-lactamases. So far, two intrinsic β-lactamases, AmpC-type cephalosporinase and oxacillinase (OXA-51-like), have been identified in most A. baumannii isolates. However, these intrinsic enzymes are expressed at very low levels and do not enhance the full carbapenem resistance in A. baumannii (29). Instead of these intrinsic β-lactamases, several other acquired β-lactamases have been identified as inducing carbapenem resistance in A. baumannii. These acquired enzymes belong either to the class B enzymes (also known as metallo-β-lactamases, MBLs) or to the class D enzymes (also known as oxacillinases). MBLs such as VIM and IMP confer a high level of carbapenem resistance in A. baumannii isolates, as well as resistance to all β-lactams except aztreonam. However, isolates with SIM-1 can display imipenem minimum inhibitory concentrations (MICs) of 8~16 mg/l. Oxacillinases represented by OXA-23, -24/40, and -58 are able to hydrolyze imipenem, but not always meropenem, and are grouped in a particular subgroup of β-lactamases termed carbapenem-hydrolyzing oxacillinases (CHDLs) (30). Compared with MBLs, the carbapenem resistance level by oxacillinases in A. baumannii is much lower. However, bla_OXA-23, bla_OXA-58, and bla_OXA40 genes play significant roles in carbapenem resistance (31). In addition, reduced susceptibility to carbapenems has also attributed to the modification of PBPs and porins, or to the up-regulation of the AdeABC efflux system in A. baumannii (29).

So far, MBLs are the major determinants of β-lactamase-mediated resistance to carbapenems in P. aeruginosa. As in A. baumannii, the VIM and IMP enzymes are by far the most common MBLs found in carbapenem-resistant P. aeruginosa isolates (32). While IMP-type MLBs predominate in P. aeruginosa isolates from Asia, VIM-type MBLs are prevalent in Europe (28). However, this distinction is blurred, as both enzymes become disseminated worldwide. In addition to VIM and IMP, GIM-1 has been found in P. aeruginosa isolates from Germany (33), and SPM-1 is prevalent in P. aeruginosa isolates from Brazil (34). Among the class A β-lactamases (or carbapenemases) such as GES, IMI, KPC, NMC-A, and SME, GES and KPC enzymes have been identified in P. aeruginosa. KPC enzymes showing activity against most β-lactams have primarily been described in Klebsiella pneumoniae, and rarely in P. aeruginosa. For GES, GES-2, and GES-5 have been reported in P. aeruginosa isolates.

Among the five families of efflux pump systems so far described in bacteria, the Resistance Nodulation Division (RND) family is the most significant in the antimicrobial resistance of P. aeruginosa. Of the RND-type efflux pump systems, MexAB-OprM, MexCD-OprJ, and MexXY-OprM contribute to the resistance to carbapenems (7). However, the efflux pump is a minor contributor to carbapenem resistance in P. aeruginosa (28). The most common mechanism of resistance to the carbapenems in P. aeruginosa is loss or alteration of the outer membrane porin protein OprD (35). OprD is the major means for the entry of carbapenems, and inactivation of OprD is the main cause of non-MBL-mediated carbapenem resistance in P. aeruginosa. OprD inactivation frequently operates in conjunction with other mechanisms such as derepressed ampC or MexAB-OprM.

Polymyxins (polymyxin B and colistin), which are a group of cyclic decapeptides produced by Bacillus polymyxa, and which have been known since 1949, bind to the anionic bacterial outer membrane, leading to a detergent effect that disrupts membrane integrity (16). They show a high affinity for the lipid moiety of lipopolysaccharide (LPS) and can preferentially displace Mg²⁺ and Ca²⁺ from cationic binding sites. Colistin was largely replaced by aminoglycosides in the 1970s because of its neurotoxicity and nephrotoxicity (36). However, colistin and polymyxin B are now considered as a therapy of last resort against infections by MDR Gram-negative bacteria, in particular A. baumannii, P. aeruginosa, and K. pneumoniae (36). Colisitn resistance rate is now relatively low worldwide, probably due to its low use over the last 50 years. However, colistin (or polymyxin B)-resistant A. baumannii or P. aeruginosa isolates have been identified (20, 37, 38).

In several Gram-negative species, colistin resistance is related to the modification of the lipid A moiety of the LPS outer membrane component. Polymyxin resistance in Salmonella enterica and P. aeruginosa has been linked to the PmrAB and PhoPQ two-component systems, which are involved in modifying the LPS core and lipid A regions with ethanolamine and the addition of aminoarabinose to lipid A (39~41). Recently, mutations in pmrA and pmrB in colistin-resistant derivatives of A. baumannii isolate were identified (42). In addition, it was reported that the complete loss of LPS production may mediate the colistin resistance in A. baumannii (43). Very recently, it was shown that the addition of phosphoethanolamine to lipid A is critical to polymyxin resistance in A. baumannii (44).

In P. aeruginosa, substitution of the LPS lipid A with aminoarabinose contributes to polymyxin resistance (40). This modification is carried out by the products of the araBCADTEF-ugd locus, which is regulated by two-component systems, PmrAB and PhoPQ. It has been reported that mutations in phoQ and pmrB promote the polymyxin B resistance in clinical P. aeruginosa isolates (45, 46). Another two-component system, ParRS, also regulates arnBCADTEF-ugd expression, with a mutation in parR being associated with polymyxin resistance (47). However, the mechanism of polymyxin resistance in A. baumannii and P. aeruginosa is not fully understood.

According to our recent studies on the colistin resistance in A. baumannii and P. aeruginosa, complete correlation among colistin resistance, PmrAB or PhoPQ mutations, and PmrAB or PhoPQ overexpression was not identified (48). Thus, PmrAB or PhoPQ overexpression associated with their amino acid alterations is only partially responsible for colistin resistance.

Based on band pattern typing methods such as amplified fragment length polymorphism (AFLP) and ribotyping, three clones, European clones I, II, and III, have been suggested to be responsible for a majority of hospital outbreaks caused by MDR A. baumannii isolates in European hospitals (49, 50). Recently, it was reported that these European clones have disseminated worldwide, which prompted to be re-designated as Global clones (GCs) or worldwide (WW) lineages I, II, and III (51). Of these, GCs I and II have caused the most outbreaks worldwide. In the multilocus sequence typing (MLST) schemes of Bartual et al. (52), ST92 and its close relatives is the most prevalent clone worldwide including the United States, Europe, and Asia (53~55). Of note, clone ST92 may be responsible for worldwide dissemination of the bla_OXA-23 carbapenemase gene in A. baumannii (56, 57).

Also in South Korea, ST92 has been the most frequently identified clone among imipenem-resistant A. baumannii isolates (21). However, it was recently replaced by its single-locus variants, including ST75 and ST138 (5). Although ST75 and ST138 differ from ST92 only in the gpi locus, they showed high resistance rate of carbapenems. Because the gpi locus is as a hot spot of high recombination event (21, 58), a clonal switch of A. baumannii in South Korea is probably due to recombination.

Presumably, P. aeruginosa exhibits a nonclonal epidemic population structure and recombination may be frequent and play a critical role in its evolution (59~61). Thus, a great diversity of STs in MLST has been observed and overlaps between isolates from clinical and environmental sources existed (62). In spite of the nonclonal feature of P. aeruginosa isolates, the emergence, spread, and persistence of a few MDR clones have been observed. One clear example is the MDR O12 clone, a CC/BURST Group (BG) 4. It emerged during the 1980s and includes only clinical isolates (63). ST111 and ST229 belong to the O12 or BG4 clone (64). CC111, referred as 'Major European MDR clone P12', is also a member of international groups including VIM-2 and VIM-4-producing P. aeruginosa isolates (65, 66).

In addition to the O12 clone, the O11 clone is also closely related to epidemic isolates (63). While serotype O12 isolates are a heterogeneous population, serotype O11 isolates often present low diversity (63). The O11 clone is also termed as CC/BURST Group (BG) 11. In particular, CC235, which is the most prevalent clone in nosocomial P. aeruginosa isolates, corresponds to this O11 clone. CC235 has been found in many countries such as Austria, Belgium, France, Greece, Hungary, Italy, Japan, Poland, Russia, Serbia, Singapore, Sweden, Turkey, Nigeria, Brazil, and the United States (64, 65). In addition, it was also identified in South Korea (38, 67). In ST235 P. aeruginosa isolates, diverse β-lactamases such as VIM, BEL, IMP, OXA, PER, PSE, and SPM have been identified. In a study of P. aeruginosa isolates from Mediterranean countries, ST235 was the most common clone and was related to MDR and exoX^-/exoU⁺ (61). In particular, the association between ST235 and IMP-6 in South Korea (67, 68) is noteworthy. Because IMP-6 induces high-level meropenem resistance, the combination of the worldwide clone and potent MBL is troubling.

In addition to the two international clones, some CF clones of P. aeruginosa such as CC146, CC148, and CC406 have also been reported worldwide since the late 1990s (69). These CF clones often have hypermutable phenotype (i.e., a 'pan-resistant' phenotype), but rarely possess carbapenemases (65). CC277 also has a worldwide distribution and SPM-1-positive isolates have been were found in Brazil. CC175 has been identified in many European countries, and VIM-2-producing CC175 isolates have been described in Germany.

The emergence and spread of carbapenem- or polymyxin-resistant Acinetobacter spp. and P. aeruginosa isolates is a great concern worldwide, especially in South Korea. The understanding of their epidemiology and resistance mechanisms will help to combat the threat posed by antimicrobial resistance.