We conducted a retrospective study of 17 transplant recipients with carbapenem-resistant Klebsiella pneumoniae bacteremia, and described epidemiology, clinical characteristics and strain genotypes. Eighty-eight percent (15/17) of patients were liver or intestinal transplant recipients. Outcomes were death due to septic shock (18%), cure (24%) and persistent (>7 days) or recurrent bacteremia (29% each). Thirty- and 90-day mortality was 18% and 47%, respectively. Patients who were cured received at least one active antimicrobial agent and underwent source control interventions. Forty-one percent (7/17) of patients had intra-abdominal infections; all except one developed persistent/recurrent bacteremia despite drainage. Two patients tolerated persistent bacteremia for >300 days. All patients except one were infected with sequence type 258 (ST258), K. pneumoniae carbapenemase (KPC)-2-producing strains harboring a mutant ompK35 porin gene; the exception was infected with an ST37, KPC-3-producing strain. Seventy-one percent (12/17) of patients were infected with ST258 ompK36 mutant strains. In two patients, persistent bacteremia was caused by two strains with different ompK36 genotypes. Three ompK36 mutations were associated with significantly higher carbapenem minimum inhibitory concentrations than wild-type ompK36. Pulse-field gel electrophoresis identified a single ST258 lineage; serial strains from individual patients were indistinguishable. In conclusion, KPC-K. pneumoniae bacteremia exhibited highly diverse clinical courses following transplantation, and was caused by clonal ST258 strains with different ompK36 genotypes.

d Doripenem-colistin exerts synergy against some, but not all, Klebsiella pneumoniae carbapenemase (KPC)-producing K. pneumoniae strains in vitro. We determined if doripenem MICs and/or ompK36 porin gene mutations impacted the responses of 23 sequence type 258 (ST258), KPC-2-producing strains to the combination of doripenem (8 g/ml) and colistin (2 g/ml) during time-kill assays. The median doripenem and colistin MICs were 32 and 4 g/ml. Doripenem MICs did not correlate with KPC-2 expression levels. Five and 18 strains had wild-type and mutant ompK36, respectively. The most common mutations were IS5 promoter insertions (n ‫؍‬ 7) and insertions encoding glycine and aspartic acid at amino acid (aa) positions 134 and 135 (ins aa134-135 GD; n ‫؍‬ 8), which were associated with higher doripenem MICs than other mutations or wild-type ompK36 (all P values < 0.04). Bactericidal activity (24 h) was achieved by doripenem-colistin against 12%, 43%, and 75% of ins aa134-135 GD, IS5, and wild-type/other mutants, respectively (P ‫؍‬ 0.04). Doripenem-colistin was more active in time-kill studies than colistin at 12 and 24 h if the doripenem MIC was <8 g/ml (P ‫؍‬ 0.0007 and 0.09, respectively), but not if the MIC was >8 g/ml (P ‫؍‬ 0.10 and 0.16). Likewise, doripenem-colistin was more active at 12 and 24 h against the wild type/other mutants than ins aa134-135 GD or IS5 mutants (P ‫؍‬ 0.007 and 0.0007). By multivariate analysis, the absence of ins aa134-135 GD or IS5 mutations was the only independent predictor of doripenem-colistin responses at 24 h (P ‫؍‬ 0.002). In conclusion, ompK36 genotypes identified ST258 KPC-K. pneumoniae strains that were most likely to respond to doripenem-colistin. Carbapenem-resistant (CR) Klebsiella pneumoniae has emerged as a major pathogen (1-9), causing infections that are associated with crude mortality rates as high as 50% (1, 10-13). Observational studies indicate that outcomes may be improved with combination antimicrobial therapy compared to monotherapy (12,14,15), particularly if the regimens include a carbapenem (16). Nevertheless, optimal combinations are not defined, and treatment failure rates remain unacceptably high. Our group and others have reported bactericidal activity and synergy for doripenem-colistin against some, but not all, CR K. pneumoniae strains in vitro (17-19). Indeed, "one-size-fits-all" approaches to identifying optimal antimicrobial regimens against CR K. pneumoniae and other carbapenem-resistant Enterobacteriaceae are not likely to be effective. Carbapenem resistance is mediated through multiple mechanisms, including production of metallo-␤-lactamases and nonmetallocarbapenemases (such as Klebsiella pneumoniae carbapenemase [KPC] and OXA-type carbapenemase). Alternatively, strains with defects in outer membrane proteins (OMPs) may express AmpC ␤-lactamase or extended-spectrum ␤-lactamases (ESBLs). Levels of carbapenem resistance in individual strains can be impacted by the presence of several mechanisms. We hypothesize that the specific resistance mechanisms mani...

We characterized carbapenem resistance mechanisms among 12 Klebsiella pneumoniae carbapenemase (KPC)-producing K. pneumoniae (referred to here as KPC K. pneumoniae) clinical isolates and evaluated their effects on the activity of 2-and 3-drug combinations of colistin, doripenem, and ertapenem. All isolates were resistant to ertapenem and doripenem; 75% (9/12) were resistant to colistin. Isolates belonged to the ST258 clonal group and harbored bla KPC-2 , bla SHV-12 , and bla TEM-1 . As determined by time-kill assays, doripenem (8 g/ml) and ertapenem (2 g/ml) were inactive against 92% (11/12) and 100% (12/12) of isolates, respectively. Colistin (2.5 g/ml) exerted some activity (range, 0.39 to 2.5 log 10 ) against 78% (7/9) of colistin-resistant isolates. Colistin-ertapenem, colistin-doripenem, and colistin-doripenem-ertapenem exhibited synergy against 42% (5/12), 50% (6/12), and 67% (8/12) of isolates, respectively. Expression of ompK35 and ompK36 porins correlated with each other (R 2 ‫؍‬ 0.80). Levels of porin expression did not correlate with colistin-doripenem or colistin-ertapenem synergy. However, synergy with colistin-doripenem-ertapenem was more likely against isolates with high porin expression than those with low expression (100% [8/8] versus 0% [0/4]; P ‫؍‬ 0.002). Moreover, bactericidal activity (area under the bacterial killing curve) against isolates with high porin expression was greater for colistin-doripenem-ertapenem than colistin-doripenem or colistin-ertapenem (P < 0.049). In conclusion, colistin-carbapenem combinations may provide optimal activity against KPC K. pneumoniae, including colistin-resistant isolates. Screening for porin expression may identify isolates that are most likely to respond to a triple combination of colistin-doripenem-ertapenem. In the future, molecular characterization of KPC K. pneumoniae isolates may be a practical tool for identifying effective combination regimens.

Systematic testing of veterans in an endemic area revealed a higher incidence of Legionella pneumonia and CAP than previously reported. Widespread urine antigen testing was not limited by false positivity.

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