Laboratory Detection of Imipenem or Meropenem Resistance in Gram-negative Organisms

In general, resistance to these carbapenems is rare. Within a healthcare setting, increases in species-specific carbapenem resistance should be monitored and sudden increases investigated to rule out an outbreak of resistant organisms or spurious test results.

Published reports indicate some resistance in a variety of clinical Gram-negative organisms, including Pseudomonas aeruginosa, Burkholderia cepacia, Acinetobacter species, Proteus species, Serratia marcescens, Enterobacter species, and Klebsiella pneumoniae. Stenotrophomonas maltophilia isolates are intrinsically resistant to imipenem.

Resistance to carbapenems occurs either through bacterial production of β-lactamase enzymes that hydrolyze (break down) the antimicrobial agent or through porin changes in the bacterial cell wall that reduce the permeability of the drug into the organism. In some organisms, both mechanisms may be present. DNA coding for enzyme production can be passed from organism to organism via plasmids or can occur through mutation of an existing β-lactamase enzyme. Porin changes arise through mutation.

The level of resistance is determined by the efficiency of the enzyme for hydrolyzing the drug and by the number of resistance mechanisms present in the organism. Organisms can produce more than one hydrolyzing enzyme and may show modifications in more than one porin, producing high-level resistance to the carbapenems (minimum inhibitory concentration [MIC] >16 µg/ml). Organisms with decreased susceptibility produced by porin changes alone often have lower MICs (2-8 µg/ml).

Organisms with MICs near interpretation breakpoints have greater potential for reporting errors. For example, isolates of Pseudomonas aeruginosa often have MICs that are at or near the carbapenem intermediate (8 µg/ml) and resistant (>16 µg/ml) breakpoints (3). Some species, such as Proteus mirabilis, P. vulgaris, and Morganella morganiii, often have MICs (1-4 µg/ml) just below the carbapenem intermediate breakpoint of 8 mg/ml. Most other species of Enterobacteriaceae are very susceptible (<0.5 µg/ml).

Broth microdilution methods usually detect carbapenem resistance when the tests are performed properly. However, studies have shown false resistance to imipenem in commercially prepared test panels due to degradation of the drug or to a manufacturing problem where concentrations of imipenem were too low (1,2,4,5). When performed properly, disk diffusion and agar gradient diffusion also are acceptable methods for carbapenem testing.

Imipenem degrades easily. Studies suggest meropenem may be more stable than imipenem. However, for either antimicrobial agent, storage conditions of susceptibility panels, cards, and disks must be monitored carefully and quality control results checked frequently. If possible, store supplies containing carbapenems at the coldest temperature range stated in the manufacturer’s directions. An additional test method, such as agar gradient diffusion (i.e., Etest), can be used to verify intermediate or resistant results.

Meropenem is slightly more active than imipenem against Gram-negative organisms. However, the activity is species dependent. Meropenem is a relatively new drug and more studies need to be published before susceptibilities of one carbapenem can be used to predict the other.

1. Carmeli, Y., K. Eichelberger, D. Soja, J. Dakos, L. Venkataraman, P. DeGirolami, and M. Samore. 1998. Failure of quality control measures to prevent reporting of false resistance to imipenem, resulting in a pseudo-outbreak of imipenem-resistant Pseudomonas aeruginosa. Journal of Clinical Microbiology 36:595-597.

2. Grist, R. 1992. External factors affecting imipenem performance in dried microdilution MIC plates. Journal of Clinical Microbiology 30:535-536.

3. National Committee for Clinical Laboratory Standards. 1999. Performance standards for antimicrobial susceptibility testing. NCCLS approved standard M100-S9. National Committee for Clinical Laboratory Standards, Wayne, PA.

4. O’Rourke, E.J., K.G. Lambert, K.C. Parsonnet, A.B. Macone, and D.A. Goldmann. 1991. False resistance to imipenem with a microdilution susceptibility testing system. Journal of Clinical Microbiology 29:827-829.

5. White, R.L., M.B. Kays, L.V. Friedrich, E.W. Brown, and J.R. Koonce. 1991. Pseudoresistance of Pseudomonas aeruginosa resulting from degradation of imipenem in an automated susceptibility testing system with predried panels. Journal of Clinical Microbiology 29:398-400.