Activity of meropenem against imipenem-resistant bacteria and selection in vitro of carbapenem-resistantEnterobacteriaceae

The activity of meropenem against 106 imipenem-resistant (MIC 8 mg/l) clinical isolates, and the frequency of resistance to meropenem and imipenem among 24Enterobacteriaceae was determined. Both agents selected colonies on agar but 20–80 % were susceptible after one subculture and 72 % of the mutants reverted to susceptibility 1 to 6 months after selection. All isolates and stable mutants were inhibited by > 1 mg/l meropenem, although the MIC of imipenem was 4–16 mg/l. Three of sixXanthomonas maltophilia isolates were susceptible to meropenem (MICs 2–4 mg/l).Pseudomonas aeruginosa lacking outer membrane protein D2 were resistant to meropenem, although isolates with substantially reduced expression of this protein were susceptible. None of the imipenem-resistant gram-positive bacteria were susceptible to meropenem. There was no clear correlation between altered outer membrane protein expression and decreased susceptibility to carbapenems, and there was no apparent involvement of plasmid or chromosomal -lactamase.     

Relationship between outer membrane protein profiles and resistance to ceftazidime,imipenem, and ciprofloxacin inPseudomonas aeruginosa isolates from bacteremic patients

Outer membrane protein (OMP) profiles of 122Pseudomonas aeruginosa isolates recovered from the blood of bacteremic patients were analyzed to relate alterations in the expression of OMPs with porin activity to resistance to imipenem, ceftazidime, and ciprofloxacin. Imipenem-resistant isolates lacked or expressed reduced amounts of porin OprD. In contrast, alterations of OMP profiles were absent in most ceftazidime-resistant isolates. Six of 12 ciprofloxacin-resistant isolates had normal OMP profiles. The remaining isolates showed alterations in the expression of either OprC, OprF, or OprD. In addition, imipenem- and ceftazidime-resistant isolates displayed a -lactamase activity compatible with that of a group 1 chromosomal cephalosporinase.     

Evaluation of imipenem/imipenem+EDTA disk method for detection of metallo-beta-lactamase-producing Klebsiella pneumoniae isolated from blood cultures

The objective of this study was to evaluate the imipenem (IMP) and IMP+EDTA (IMP/IMP+EDTA) disk method for the detection of metallo-beta-lactamases (MBLs) in clinical isolates of Klebsiella pneumoniae with various MIC levels to IMP. Forty-one blood isolates of K. pneumoniae with MIC to IMP ranging from < or =0.5 to > or =16 microg/ml were examined. The MICs were determined by VITEK-2 (bioMerieux Vitek two, France). Disks of 10 microg IMP with and without the addition of 0.5 M EDTA were used for the IMP/IMP+EDTA disk method. The E-test (AB Biodisk, Solna, Sweden) for MBL detection was also used. All isolates were examined for the bla (VIM-1) gene by PCR and for clonality of VIM-1-producing isolates by pulsed-field gel electrophoresis (PFGE). All isolates with MIC values of IMP < or =0.5 microg/ml exhibited no differences in inhibition zone diameters (IZD) produced by IMP and IMP+EDTA disks, whereas the isolates with MICs > or =1 microg/ml showed an increase in IZD, ranging from 8 to 26 mm. All isolates with MIC values of > or =1 microg/ml were found positive for the bla (VIM-1) gene by PCR and for MBL production by the E-test, whereas none of isolates with MICs <0.5 microg/ml was found positive by any of the tests. DNA restriction fragments generated by PFGE of VIM-1-producing isolates were classified in four main types. The IMP/IMP+EDTA disk method is simple to perform, sensitive, and specific for detection of MBL-producing K. pneumoniae clinical isolates. K. pneumoniae isolates with MICs of IMP > or =1 microg/ml and/or IZD produced by IMP disk <19 mm should be tested for MBL production.     

Highly synergistic activity of melittin with imipenem and colistin in biofilm inhibition against multidrug-resistant strong biofilm producer strains of <Emphasis Type="Italic">Acinetobacter baumannii</Emphasis>

The rapid increase of drug resistance and failure of available antibiotics to treat biofilm-associated infections is of great health concern. Accordingly, our study aimed to evaluate the synergistic antibacterial, biofilm inhibitory, and biofilm removal activities of melittin in combination with colistin, imipenem, and ciprofloxacin against multidrug-resistant (MDR) strong biofilm producer Acinetobacter baumannii isolates. The kinetics of biofilm formation were evaluated for the isolates for 144 h. Minimum inhibitory concentrations (MICs), minimum bactericidal concentrations (MBCs), minimum biofilm inhibitory concentrations (MBICs), and biofilm removal activities for melittin and combinations with antibiotics were determined. Inhibition of biofilm-associated protein (bap) expression by melittin was evaluated with real-time polymerase chain reaction (PCR). Field emission scanning electron microscopy (FE-SEM) was used to visualize the effect of synergism on the inhibition of biofilm production. The geometric means of the fractional inhibitory concentration index (FICi) for melittin–colistin, melittin–imipenem, and melittin–ciprofloxacin combinations were calculated as 0.31, 0.24, and 0.94, respectively. Comparing the geometric means of the removal activity for melittin, colistin, imipenem, and combinations of them in both 6 and 24 h showed a significant difference between the groups (p-value < 0.05). Exposure to melittin induced a statistically significant downregulation of bap mRNA levels in all isolates at sub-MIC doses. Analysis of the FE-SEM results demonstrated that the synergism of melittin–colistin at 0.125–0.25 μg inhibited biofilm formation completely. In conclusion, our findings indicate that melittin possesses considerable potential for use in combination with colistin and imipenem to treat infections caused by MDR strong biofilm producer A. baumannii isolates.     

Laboratory investigation of a suspected outbreak caused by Providencia stuartii with intermediate resistance to imipenem at a long-term care facility

Background

Providencia stuartii survives well in natural environment and often causes opportunistic infection in residents of long-term care facilities (LTCFs). Clinical isolates of P. stuartii are usually resistant to multiple antibiotics. The bacterium is also naturally resistant to colistin and tigecycline. Treatment of infections caused by carbapenem-resistant P. stuartii is challenging.

Meropenem versus imipenem/cilastatin in the treatment of intraabdominal infections requiring surgery

In a prospective randomized study meropenem was compared with imipenem/cilastatin in the treatment of 62 patients with intraabdominal infections requiring surgery. The patients were suffering from diffuse or local peritonitis of moderate severity complicating in most cases gangrenous appendicitis, stomach perforation or gall-bladder disease. There were 30 patients in the meropenem group and 32 patients in the imipenem/cilastatin group. Both antibiotic regimens were given intravenously at a dosage of 1 g every 8 h for a mean duration of 7.7 days in the meropenem group versus 8.6 days in the imipenem/cilastatin group. Fifty-nine aerobic strains and 15 anaerobic strains were isolated from cultures of pus taken intraoperatively, the meropenem MICs ranging from 0.25 to 2 µg/ml. At follow-up at least one month after treatment the outcome was considered successful in all of 27 evaluable patients given meropenem and in all of 29 evaluable patients given imipenem/cilastatin. Both antibiotic regimens were well tolerated.     

Heteroresistance to Meropenem in Carbapenem-Susceptible Acinetobacter baumannii

The characteristics of carbapenem heteroresistance were studied in 14 apparently carbapenem-susceptible Acinetobacter baumannii isolates. The MICs for carbapenems were determined, and the isolates were genotyped by pulsed-field gel electrophoresis (PFGE) and sequence typing (ST). Population analysis, testing of the stability of the heteroresistant subpopulations, and time-killing assays were performed. The agar dilution MICs of both imipenem and meropenem for the native isolates ranged from 0.25 to 4 mg/liter. The isolates belonged to nine PFGE types and exhibited seven ST allelic profiles. Population analysis revealed subpopulations that grew in the presence of imipenem at concentrations of up to 8 mg/liter and meropenem at concentrations of up to 32 mg/liter. The meropenem-heteroresistant subpopulations of 11 isolates exhibited stable resistance with MICs that ranged from 16 to >32 mg/liter; their PFGE profiles were identical to those of the native isolates. Time-kill assays with meropenem revealed less pronounced killing for 10 isolates. These findings indicate that meropenem pressure can produce meropenem-heteroresistant subpopulations that might subsequently select for highly resistant strains.Acinetobacter baumannii is an opportunistic pathogen associated with severe hospital infections (1, 16), which often need the use of carbapenems as the treatment of last resort. However, reduced susceptibility or resistance to carbapenems is increasingly being observed among A. baumannii clinical isolates (2, 5, 25).In a preliminary work, we reported on the growth of distinct colonies within the inhibition halo around carbapenem disks or Etest strips and assessed these colonies as having the phenotypic manifestation of carbapenem heteroresistance (17). Likewise, a recent study (12) has described colistin-heteroresistant subpopulations among apparently colistin-susceptible A. baumannii clinical isolates, implying an intrinsic potential of the species to overcome drug pressure. These observations suggest that the use of carbapenems or colistin to treat severe multidrug-resistant A. baumannii infections may lead to the development of resistance.The present study aimed to evaluate in vitro the incidence of carbapenem heteroresistance, to characterize the traits of the heteroresistant subpopulations, and to assess the efficacies of carbapenems against heteroresistant A. baumannii isolates. The study included carbapenem-susceptible A. baumannii clinical isolates that cause infections; such infections are commonly treated with carbapenems, although it is recognized that the heterogeneity of resistance might affect the outcome. The study also determined how the heterogeneity of resistance might affect the treatment outcome.     

Carbapenem Heteroresistance in VIM-1-Producing Klebsiella pneumoniae Isolates Belonging to the Same Clone: Consequences for Routine Susceptibility Testing

Susceptibility results with low reproducibility by the same or different methods have been observed for metallo-beta-lactamase (MBL)-producing Enterobacteriaceae. Eighteen VIM-1-producing Klebsiella pneumoniae isolates (one per patient) belonging to a single epidemic clone in our hospital (2005 to 2008) but with different susceptibilities to carbapenems were studied. Imipenem MICs ranged from 8 to >128 mg/liter by standard CLSI microdilution, from ≤1 to >8 mg/liter by the semiautomatic Wider system, and from 0.75 to >32 mg/liter by Etest. Meropenem MICs ranged from 0.5 to 128, ≤1 to >8, and 0.38 to >32 mg/liter, respectively. Ertapenem MICs by CLSI microdilution and Etest ranged from 1 to 64 and 0.75 to >32 mg/liter, respectively. The rates of essential agreement (±1 log₂ dilution) for imipenem and meropenem MICs between the Wider system and the reference microdilution method were 45% and 49%, respectively. Those between Etest and the reference microdilution method for imipenem, meropenem, and ertapenem MICs were 33%, 67%, and 84%. The rates of very major errors for the Wider system and Etest were 33% and 28% for imipenem and 25% and 75% for meropenem, respectively. Low MIC reproducibility was observed even when the same inoculum was used (differences up to 4-fold dilutions). Heteroresistance was suspected due to the presence of colonies in the Etest inhibition zone. It was confirmed by population analysis profiles of 4 isolates displaying different imipenem MICs, with the exception of an OmpK36-porin-deficient isolate that homogeneously expressed carbapenem resistance (MIC, >128 mg/liter). Low carbapenem MIC reproducibility could be due to the presence of resistant subpopulations and variable expression of the resistance mechanisms. Since carbapenem MICs are not good markers of MBL production, reliable and reproducible phenotypic methods are needed to detect the presence of this mechanism.Resistance to carbapenems due to metallo-beta-lactamases (MBLs) in Enterobacteriaceae is increasingly recognized, and different levels of resistance to carbapenems have been observed in these isolates (18). The detection of MBL-carrying isolates in clinical laboratories presents practical difficulties when routine susceptibility testing is performed (10, 28). Automated systems do not accurately detect enzyme-mediated carbapenem resistance when it is expressed at low levels, and MBL-carrying organisms can even appear to be fully susceptible to these compounds (9, 28). Additionally, reproducible results within the same method or among different methods, such as disk diffusion or Etest, are not always obtained, and discrepancies in the susceptibility to imipenem and meropenem of carbapenemase (KPC or VIM types)-producing Klebsiella pneumoniae isolates have already been reported (10, 28).In addition, some studies describing heteroresistance to carbapenems in Acinetobacter baumannii isolates demonstrate its presence by the growth of colonies in the inhibition zone of the Etest strip (12, 19). This phenomenon has also been reported for Klebsiella pneumoniae (28), Pseudomonas aeruginosa (20), and, more recently, Enterobacter aerogenes isolates (11).The objective of this work was to analyze the carbapenem susceptibility profiles of VIM-1-producing K. pneumoniae isolates by different susceptibility testing methods. These isolates belong to a single clone that was involved in an epidemic in our hospital (2005 to 2008) affecting 18 patients in different wards (25).     

Influence of zinc on Pseudomonas aeruginosa susceptibilities to imipenem.

Serial dilution susceptibility testing of imipenem against 59 clinical isolates of Pseudomonas aeruginosa, conducted simultaneously on single lots of Difco and BBL Mueller-Hinton agar (MHA), resulted in MICs for 90% of strains tested of 8 and 16 micrograms/ml, respectively. MICs for Escherichia coli, Klebsiella pneumoniae, and Pseudomonas spp. were also higher on BBL MHA. Quantification of the cation content of the two MHAs by atomic absorption spectroscopy demonstrated that the zinc concentration in BBL MHA was 15 times greater than that measured in Difco MHA (2.61 and 0.17 micrograms/ml, respectively). Concentrations of calcium, magnesium, iron, manganese, and copper in the two agars were similar. Addition of zinc to Difco MHA resulted in increases in MICs of imipenem for P. aeruginosa but not in the MICs of ceftazidime or cefpirome for P. aeruginosa (P < 0.01). A lesser zinc effect was seen on the activity of imipenem against E. coli, K. pneumoniae, and Pseudomonas spp. The activities of ceftazidime and cefpirome were similar on both MHAs when tested against all gram-negative organisms in this study. Thus, the effect of zinc in MHA was clearly demonstrated by a significant increase in the MICs of imipenem for P. aeruginosa, and, to a lesser extent, for other gram-negative bacilli.     

Activity of imipenem against VIM-1 metallo-β-lactamase-producing Klebsiella pneumoniae in the murine thigh infection model

The in-vivo activity of imipenem against VIM-1-producing Klebsiella pneumoniae (VPKP) was assessed in a thigh infection model in neutropenic mice. Animals were infected with three VPKP isolates (imipenem MICs 2, 4 and 32 mg/L, respectively) and a susceptible clinical isolate (MIC 0.125 mg/L) that did not produce any beta-lactamase with broad-spectrum activity. Bacterial density at the site of infection was determined after imipenem treatment (30 and 60 mg/kg every 2 h for 24 h). The log(10) reduction in CFU/thigh was greatest for the wild-type isolate, intermediate for the two imipenem-susceptible VPKP isolates, and lowest for the imipenem-resistant VPKP isolate. Whilst in-vivo imipenem activity appeared reduced against in-vitro susceptible VIM-1 producers compared with a VIM-1-negative control, an increased drug dosage could moderate this reduction.     

Synergic in-vitro activity of imipenem and sulbactam against Acinetobacter baumannii

The interaction between sulbactam and imipenem was evaluated with four clinical isolates of Acinetobacter baumannii, including two isolates resistant to imipenem, one of which produced IMP-1 metallo-beta-lactamase. Two isolates (one of which was imipenem-resistant) were sulbactam-resistant by undefined mechanisms. MICs were determined by standard broth microdilution methods. Time-kill assays with imipenem and sulbactam, alone or in combination at 0.5 x MIC and 1 x MIC, showed a synergic effect in all four isolates of A. baumannii after incubation for 0, 4, 8 and > 24 h at 35 degrees C.     

In vitro antimicrobial susceptibility of viridans streptococci isolated from blood cultures

The susceptibility of 211 viridans streptococci isolated from blood cultures to eight antimicrobial agents was determined. All the isolates were susceptible to cefotaxime, ceftriaxone, imipenem and vancomycin. Thirty eight percent of the isolates were resistant to penicillin (MICs 0.25 µg/ml). Tetracycline resistance was found in 41 % of the isolates and in 7 % of these strains tetracycline resistance was combined with erythromycin resistance. FiveStreptococcus mitis isolates exhibited increased (MIC 64 µg/ml and 128 µg/ml) or high-level (MIC 500 µg/ml) resistance to gentamicin, kanamycin and tobramycin. Four of these isolates were also resistant to penicillin (MICs 16–32 µg/m). In vitro synergy was not demonstrated for combinations of penicillin and gentamicin against threeStreptococcus mitis isolates with gentamicin MICs of 1000, 128 and 64 µg/ml. Results of this study indicate the importance of monitoring antibiotic resistance trends in viridans streptococci particularly with respect to penicillin and aminoglycoside resistance.     

Lack of additive effect between mechanisms of resistance to carbapenems and other beta-lactam agents inPseudomonas aeruginosa

Eighty-nine clinical isolates resistant (n=61) or susceptible (n=28) to imipenem and exhibiting the main patterns of susceptibility to other -lactam agents (wild type pattern, penicillinase pattern, constitutive cephalosporinase pattern) were studied in order to investigate (i) the mechanism of resistance involved and (ii) whether resistance to carbapenems affects the level of resistance to other -lactam agents and, conversely, if resistance to other -lactam agents affects the level of resistance to carbapenems. For this purpose, the presence of OprD protein in the cell wall was detected by Western blot and -lactamase activity by spectrophotometric assay and isoelectric focusing. OprD expression was not detectable in the imipenem-resistant (MIC16 g/ml) strains. It was decreased in half the strains for which MICs of imipenem were 2 to 8 g/ml and was close to a normal level in the most susceptible strains (MIC 1 g/ml), thus demonstrating a direct correlation between the level of susceptibility to imipenem and the level of OprD expression. No imipenemase activity was detected in imipenem-resistant strains. Synergy between imipenem or meropenem and BRL42715 was observed for all of the strains, demonstrating the role of cephalosporinase in carbapenem resistance. Within each pattern of susceptibility, the mean MICs of -lactam agents other than carbapenems were similar, whether the strains were susceptible or resistant to imipenem. Conversely, the mean MICs of imipenem or meropenem for either the imipenem-resistant or the imipenem-susceptible strains were similar, regardless of the susceptibility of these strains to the other -lactam agents. Thus, when several mechanisms of resistance to -lactam agents are present in the same strain ofPseudomonas aeruginosa, there is no additive effect between these mechanisms.     

Instability of in vitro resistance to imipenem in Pseudomonas aeruginosa

In 3 from 19 clinical isolates of Pseudomonas aeruginosa imipenem-resistant subpopulations could be detected in vitro. In comparison to their wild strains these imipenem-resistant cells were lacking an outer-membrane-protein of 50 kD. In the subpopulation derived from Pseudomonas aeruginosa 76 resistance to imipenem was found to be instable. It could be lost within a short period of time after subcultivation in the absence of imipenem. Cells with restored sensitivity to imipenem possess the outer membrane protein of 50 kD as the wild strain does.     

Comparative in vitro activity of biapenem,a new carbapenem antibiotic

Biapenem is a new carbapenem antibiotic with high stability to human renal dehydropeptidase I. Its in vitro activity was compared with that of imipenem, meropenem, ceftazidime, ceftriaxone, piperacillin and gentamicin against a total of 650 recent clinical isolates. MICs were determined by a standard agar dilution procedure and all isolates were tested at two inocula (10⁴ and 10⁶ cfu). Biapenem inhibited 90 % of isolates ofEscherichia coli, Klebsiella spp.,Proteus mirabilis, Proteus vulgaris, Morganella morganii, Providencia stuartii, Enterobacter spp.,Citrobacter freundii, Serratia marcescens, Salmonella typhi, Shigella sonnei andYersinia enterocolitica at 2 mg/l and was as active as or two- to four-fold more active than imipenem against all these species, with the exception ofSerratia marcescens, against which imipenem was two-fold more active. Biapenem was two-fold more active than imipenem againstPseudomonas aeruginosa (MIC90 4 mg/l) and had activity similar to that of imipenem against theBacteroides fragilis group (MIC90 0.5 mg/l) but was two-fold less active than imipenem against methicillin-susceptibleStaphylococcus aureus (MIC90 0.06 mg/l) and was, like imipenem, inactive against methicillin-resistantStaphylococcus aureus.     

In vitro activity of cefoxitin and imipenem against Mycobacterium abscessus complex

The in vitro activity of cefoxitin and imipenem was compared for 43 strains of the Mycobacterium abscessus complex, mostly isolated from cystic fibrosis patients. The MICs of imipenem were lower than those of cefoxitin, although the number of imipenem-resistant strains was higher according to the CLSI breakpoints. Strain comparisons indicated that the MICs of cefoxitin were significantly higher for Mycobacterium bolletii than for M. abscessus. The MICs of both β-lactams were higher for the rough morphotype than for the smooth morphotype. The clinical impact of the in vitro difference between the activity of imipenem and that of cefoxitin remains to be determined.     

Pseudomonas aeruginosa and imipenem: correlation between membrane protein D2 and resistance in fifteen strains isolated from patients with mucoviscidosis]

A characteristic feature of imipenem-resistant strains of Pseudomonas aeruginosa is loss or decreased expression of the outer membrane protein (OMP) D2, whose molecular weight is 45 to 49 kDa. D2 was studied in 15 strains of P. aeruginosa with intermediate susceptibility or resistance to imipenem recovered from the sputum of 15 patients with cystic fibrosis. The OMP was extracted using Sarkosyl and separated by SDS-PAGE electrophoresis. Electrophoresis patterns were compared to those of reference strains 3B and 3C which are resistant and susceptible to imipenem, respectively. Expression of D2 was normal in three strains, weak or very weak in 11 strains and absent in one strain. For 12 strains, the alteration of the D2 protein was consistent with previous reports. However, the finding of normal D2 production in three strains is unusual and suggests the possible presence of another mechanism of resistance.     

Susceptibility of 114 isolates of the Bacteroides fragilis group to imipenem and eight other antimicrobial agents

The minimal inhibitory concentrations (MICs) of the newer beta-lactam antibiotic, imipenem, were compared with those of cefoxitin, cefotetan, penicillin, amoxycillin, ticarcillin and metronidazole against 114 clinical isolates of the Bacteroides fragilis group of anaerobic organisms. The ability of clavulanic acid, a beta-lactamase inhibitor, to potentiate the in vitro activity of amoxycillin and ticarcillin was also studied. Using an agar dilution technique we found imipenem to be the most active beta-lactam antibiotic tested having an MIC50 of 0.25 microgram/ml and inhibiting all isolates at a concentration of 4 micrograms/ml. Metronidazole had comparable activity with a MIC50 of 0.5 microgram/ml and all isolates inhibited by 1 microgram/ml. Cefoxitin and cefotetan showed similar activity both with a MIC50 of 8 micrograms/ml against the B. fragilis group, while penicillin, amoxycillin and ticarcillin all had a MIC50 of 16 micrograms/ml. Clavulanic acid significantly reduced the MIC50 of amoxycillin and ticarcillin to 0.5 micrograms/ml and 0.25 micrograms/ml, respectively.     

Imipenem resistance inEnterobacter

Blood cultures obtained on two separate occasions from a 37-year-old male who received multiple antibiotics (including imipenem) for treatment of repeated episodes of intraabdominal abscesses and bacteremia yielded two isolates ofEnterobacter with reduced susceptibility to imipenem, extended-spectrum cephalosporins, penicillins and aztreonam. Both isolates were unstable, giving rise to different colony types, each of which produced a single, non-inducible Bush group 1 -lactamase (pI=9.6) that hydrolyzed imipenem. Outer membrane proteins were analyzed but no differences were detected between strains with different levels of imipenem resistance. Three-dimensional tests performed in conjunction with disk diffusion susceptibility tests provided a rapid and convenient means of detecting the production of imipenem-hydrolyzing enzymes by theEnterobacter strains. These isolates provided additional evidence that overproduction of the group 1 cephalosporinase ofEnterobacter can contribute to resistance to imipenem.     

Klebsiella pneumoniae: development of a mixed population of carbapenem and tigecycline resistance during antimicrobial therapy in a kidney transplant patient

Nine isolates of Klebsiella pneumoniae were isolated from a renal transplant patient suffering from recurrent urosepsis over a period of 4 months. Imipenem resistance was detected after imipenem-ertapenem therapy. When treatment was switched to tigecycline the K. pneumoniae developed resistance to tigecycline (MIC = 8 mg/L). The nine isolates were tested by determination of agar dilution MICs, phenotypic carbapenemase, extended-spectrum beta-lactamases and metallo-beta-lactamase (MBL) testing and pulsed-field gel electrophoresis. Polymerase chain reaction and sequencing analysis were employed for identification of bla genes and mapping of the integron carrying the MBL gene. The nine isolates were clonally related and all produced the SHV-12 enzyme. Five MBL-producing isolates showed imipenem MICs ranging from 2 to 64 mg/L and all were detected by testing with imipenem and EDTA. The five isolates harboured the bla_VIM-1 gene. Three isolates showed increased tigecycline MICs (4–8 mg/L). Serial blood cultures obtained on the same day resulted in a VIM-positive/tigecycline-susceptible and a VIM-negative/tigecycline-resistant K. pneumoniae isolate. No isolate developed concurrent imipenem and tigecycline resistance. The patient had a persistent urinary tract infection and recurrent bacteraemia caused by a mixed population of Klebesiella pneumoniae isolates adapting to the selective pressure of antimicrobial therapy at the time. The present study is a worrisome example of what could happen when an immunocompromised host is subjected to the pressures of antimicrobial therapy. In addition, we report the first treatment-emergent MIC increase of tigecycline from 0.5 to 8 mg/L in K. pneumoniae.