The present work describes the abrupt emergence of Klebsiella pneumoniae carbapenemase (KPC) and characterizes the first 79 KPC-producing enterobacteria from Argentina (isolated from 2006 to 2010). The emergence of bla(KPC-2) was characterized by two patterns of dispersion: the first was the sporadic occurrence in diverse enterobacteria from distant geographical regions, harbouring plasmids of different incompatibility groups and bla(KPC-2) in an unusual genetic environment flanked by ISKpn8-Δbla(TEM-1) and ISKpn6-like. bla(KPC-2) was associated with IncL/M transferable plasmids; the second was the abrupt clonal spread of K. pneumoniae ST258 harbouring bla(KPC-2) in Tn4401a.

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We have characterized nine mcr-1-harboring plasmids from clinical Escherichia coli isolates previously described in Argentina and Canada. Three of these plasmids carried a mcr-1-variant called here mcr-1.5. All these E. coli isolates were not clonally related and were recovered in different years and locations. However, their mcr-1-harboring plasmids showed high identity among them and to others characterized in other countries, which strongly suggests that this plasmid-type is playing an important role in spreading this mechanism of resistance to polymyxins.

cWe studied a collection of 105 clinical enterobacteria with unusual phenotypes of quinolone susceptibility to analyze the occurrence of plasmid-mediated quinolone resistance (PMQR) and oqx genes and their implications for quinolone susceptibility. The oqxA and oqxB genes were found in 31/34 (91%) Klebsiella pneumoniae and 1/3 Klebsiella oxytoca isolates. However, the oqxAand oqxB-harboring isolates lacking other known quinolone resistance determinants showed wide ranges of susceptibility to nalidixic acid and ciprofloxacin. Sixty of the 105 isolates (57%) harbored at least one PMQR gene [qnrB19, qnrB10, qnrB2, qnrB1, qnrS1, or aac(6=)-Ib-cr)], belong to 8 enterobacterial species, and were disseminated throughout the country, and most of them were categorized as susceptible by the current clinical quinolone susceptibility breakpoints. We developed a disk diffusionbased method to improve the phenotypic detection of aac(6=)-Ib-cr. The most common PMQR genes in our collection [qnrB19, qnrB10, and aac(6=)-Ib-cr] were differentially distributed among enterobacterial species, and two different epidemiological settings were evident. First, the species associated with community-acquired infections (Salmonella spp. and Escherichia coli) mainly harbored qnrB19 (a unique PMQR gene) located in small ColE1-type plasmids that might constitute its natural reservoirs. qnrB19 was not associated with an extended-spectrum ␤-lactamase phenotype. Second, the species associated with hospital-acquired infections (Enterobacter spp., Klebsiella spp., and Serratia marcescens) mainly harbored qnrB10 in ISCR1-containing class 1 integrons that may also have aac(6=)-Ib-cr as a cassette within the variable region. These two PMQR genes were strongly associated with an extended-spectrum ␤-lactamase phenotype. Therefore, this differential distribution of PMQR genes is strongly influenced by their linkage or lack of linkage to integrons.

Accurate detection of carbapenemase-producing Gram-negative bacilli is of utmost importance for the control of nosocomial spread and the initiation of appropriate antimicrobial therapy. The modified Hodge test (MHT), a carbapenem inactivation assay, has shown poor sensitivity in detecting the worldwide spread of New Delhi metallo-␤-lactamase (NDM). Recent studies demonstrated that NDM is a lipoprotein anchored to the outer membrane in Gram-negative bacteria, unlike all other known carbapenemases. Here we report that membrane anchoring of ␤-lactamases precludes detection of carbapenemase activity by the MHT. We also show that this limitation can be overcome by the addition of Triton X-100 during the test, which allows detection of NDM. We propose an improved version of the assay, called the Triton Hodge test (THT), which allows detection of membrane-bound carbapenemases with the addition of this nonionic surfactant. This test was challenged with a panel of 185 clinical isolates (145 carrying known carbapenemase-encoding genes and 40 carbapenemase nonproducers). The THT displayed test sensitivity of >90% against NDM-producing clinical isolates, while improving performance against other carbapenemases. Ertapenem provided the highest sensitivity (97 to 100%, depending on the type of carbapenemase), followed by meropenem (92.5 to 100%). Test specificity was not affected by the addition of Triton (87.5% and 92.5% with ertapenem and meropenem, respectively). This simple inexpensive test confers a large improvement to the sensitivity of the MHT for the detection of NDM and other carbapenemases. Detection of carbapenemase producers in clinical laboratories is of major importance to define appropriate empirical antimicrobial therapy and to implement infection control measures. Acquired carbapenemases belong to three of the four known classes of ␤-lactamases, namely, Ambler class A (KPC, SME, NMC-A, IMI-1, and some allelic variants of GES), Ambler class B or metallo-␤-lactamases (MBLs) (e.g., VIM, IMP, NDM, and SPM), and Ambler class D or oxacillinases (OXAs) (e.g., OXA-48 and OXA-181) (1).The modified Hodge test (MHT) is a phenotypic screening test to identify carbapenemase producers, being recommended by the Clinical and Laboratory Standards Institute (CLSI) for Enterobacteriaceae with elevated carbapenem MICs or reduced disk diffusion inhibition zones (2). This test is based on the inactivation of a carbapenem by carbapenemase-producing strains, which enables a susceptible indicator strain to extend growth toward a disk containing this antibiotic, along the streak of inoculum of the tested strain. The MHT has shown excellent sensitivity in the detection of class A and class D carbapenemase producers (3-6). Unfortunately, the MHT performs poorly in the detection of NDM-producing isolates, with sensitivity below 50% (3-7). Because NDMs are Zn(II)-dependent enzymes, it has been suggested that the deficits of this cation in commercial media could be responsible for these false-negative results (4). Indeed, Zn(II) availabili...