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Background: Ideal dose of the antimicrobials should be decided by considering their killing dynamics since sufficient elimination of the causative microorganisms is critical for proper antimicrobial treatment. In this study, the bactericidal activities of carbapenems by resistance mechanisms were assessed for carbapenem-resistant Pseudomonas aeruginosa. Methods: Minimal inhibition concentrations (MICs) of carbapenems were determined by broth dilution method and the resistance mechanisms were identified by PCR and DNA sequencing. The expression levels of efflux pumps were determined by reverse transcriptase real-time PCR. Time-kill curves were plotted by time-course numeration of the viable cells grown under imipenem and meropenem at 1× and 4× MICs, respectively. Results: One P. aeruginosa strain was susceptible, whereas three were resistant to carbapenems by defective OprD, efflux pump overproduction, and/or IMP-6 production. The susceptible strain had imipenem and meropenem MICs of 2 and 1 mg/L, respectively. The MICs were elevated by eight-fold by defective OprD, 16- and 32-fold by the pump overproduction, and four- and

Background: Antimicrobial resistance (AMR) is an issue not only with regard to public health, but also in terms of economic impact. AMR surveillance has mainly been carried out in general hospitals, and not in nursing hospitals. This study was conducted to investigate the AMR rate for bacterial strains isolated from nursing hospital samples.Methods: Antimicrobial susceptibility testing (AST) results from a total of 23,518 bacterial isolates recovered from clinical specimens taken in 61 nursing hosals were analyzed. AST was conducted using Vitek 2 with AST cards specific for the bacterial strains.Results: A total of 19,357 Gram-negative and 4,161 Gram-positive bacterial strains were isolated. Pseudomonas aeruginosa (n=6,384) and Escherichia coli (n=5,468) were the most prevalent bacterial species and, among Gram-positive bacteria, Staphylococcus aureus (n=1,565) was common. The AMR rate was high for the following strains: cefotaxime-resistant Klebsiella pneumoniae, 77.4%; cefotaxime-resistant E. coli, 70.6%; imipenem-resistant Acinetobacter baumannii, 90.3%; imipenem-resistant P. aeruginosa, 49.3%; oxacillin- resistant S. aureus, 81.1%, penicillin-resistant Enterococcus faecalis, 44.8%, and vancomycin-resistant Enterococcus faecium, 53.5%. AMR rate change varied by bacterial species and antimicrobial drug.Conclusion: AMR rates of major pathogens from nursing hospitals were higher than those from general hospitals with the exception of imipenem-resistant A. baumannii. Continuous monitoring and infection control strategies are needed. (Ann Clin Microbiol 2019;22:96-104)

Background: Pulmonary infection with nontuberculous mycobacteria (NTM) is increasing in South Korea. Since treatment strategy differs by NTM species, accurate identification is necessary. In this study, using Mycobacterium pulmonary isolates recently recovered from a general hospital in Seoul, the prevalence of NTM isolates was investigated.Methods: A total of 483 Mycobacterium pulmonary strains isolated between May and November 2018 from an 814-bed general hospital in South Korea were analyzed. Bacterial species were identified based on nucleotide sequences of the 16S-23S rDNA internal transcribed spacer and the rpoB gene.Results: From a total of 1,209 pulmonary specimens from patients suspected to be infected with mycobacteria, 324 deduplicate strains were isolated, comprising 90 Mycobacterium tuberculosis and 229 NTM strains. Among the NTM isolates, 61.5% (n=144) were Mycobacterium avium complex (MAC), including 92 M. avium and 52 Mycobacterium intracellulare, while 8.1% (n=19) represented Mycobacterium abscessus, including 10 M. abscessus subsp. abscessus and 9 M. abscessus subsp. massiliense. In addition, 12 (5.1%) Mycobacterium lentiflavum, 12 (5.1%) Mycobacterium gordonae, 6 (2.6%) Mycobacterium kansasii, and 5 (2.1%) Mycobacterium fortuitum were identified. In addition, Mycobacterium mucogenicum (n=2), Mycobacterium septicum (n=1), Mycobacterium colombiens (n=1), Mycobacterium asiaticum (n=1), and Mycobacterium celatum (n=1) were identified.Conclusion: Among the recently recovered Mycobacterium pulmonary strains, more than half were identified as NTM, and MAC was the most prevalent NTM, followed by M. abcessuss. (Ann Clin Microbiol 2019;22:71-76)

Background: Escherichia coli and Klebsiellapneumoniae clinical isolates producing CTX-M extendedspectrum β-lactamases (ESBLs) were assessed for antimicrobial resistance phenotypes varied by group of enzymes.Methods: A total of 1,338 blood isolates, including 959 E. coli and 379 K.pneumoniae, were studied. All the strains were collected between January and July 2017 from eight general hospitals in South Korea. The species were identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry. Antimicrobial susceptibilities were determined by disk diffusion methods and ESBL phenotypes by double-disk synergy tests using disks containing cefotaxime, ceftazidime, cefepime, aztreonam, and clavulanic acid (CA). The genes for β-lactamases were identified by PCR and sequencing.Results: Of total microbes, 31.6% (303/959) E. coli and 24.0% (91/379) K.pneumoniae were resistant to cefotaxime and 28.1% (269/959) E. coli and 20.1% (76/379) K.pneumoniae were CTX-M-type ESBL producers. Among the detected CTX-M ESBLs, 58.0% (156/269) in E. coli and 86.8% (66/76) in K. pneumoniae belonged to group 1, 46.8% (126/269) in E. coli and 14.5% (11/76) in K.pneumoniae were group 9. Ten E. coli and one K.pneumoniae isolates co-produced both groups of CTX-M ESBL. The group 1 CTX-M producers had a higher level of resistance to cefotaxime, ceftazidime, cefepime, and aztreonam and exhibited stronger synergistic activities when combined with CA compared to group 9.Conclusion: ESBL phenotypes differ by CTX-M ESBL group and phenotype testing with drugs including 4th generation cephalosporins and monobactams is critical for screening CTX-M-producers with better sensitivity. (Ann Clin Microbiol 2019;22:1-8)