MIC of Imipenem for Morganella morganii
Morganella morganii typically demonstrates MIC values for imipenem ranging from 0.25 to 8 μg/mL, with most wild-type strains showing susceptibility at ≤4 μg/mL, though significant resistance has emerged in recent years, particularly in healthcare settings. 1, 2, 3
Typical MIC Range for Wild-Type Strains
Most susceptible M. morganii isolates demonstrate imipenem MIC values between 0.25-4 μg/mL, with meropenem showing 8-16 times greater potency (lower MICs) than imipenem against this organism 4
In clinical surveillance studies, carbapenem-nonsusceptible M. morganii isolates showed imipenem MICs of 8 μg/mL, while meropenem MICs were 1 μg/mL and ertapenem 0.25-0.5 μg/mL 3
The MIC interpretation must account for the fact that imipenem MIC testing for M. morganii can be problematic with lack of reproducibility, similar to issues noted with other organisms like M. abscessus 5
Emerging Resistance Patterns
Imipenem resistance in M. morganii has increased dramatically over time, with resistance rates rising from <1% to over 1% of isolates within a single year in prospective surveillance 6
A concerning 19.4% of M. morganii clinical isolates demonstrated imipenem resistance in one large case series, despite 95.8% remaining susceptible to ceftazidime 2
KPC-2 carbapenemase production is now documented in M. morganii, with these strains showing imipenem MICs of 8 μg/mL and demonstrating clonal spread within healthcare facilities 3
Clinical Implications
Meropenem and imipenem remain highly active against most M. morganii isolates and are recommended as treatment options for multidrug-resistant strains 1
The bactericidal activity of meropenem at 4× MIC was superior to imipenem (76.7% vs 26.7% of strains killed at 24 hours), with significant regrowth occurring with imipenem in 80-90% of M. morganii strains 4
Inappropriate antibiotic therapy is the only independent risk factor for mortality (odds ratio 4.8) in M. morganii bacteremia, making accurate MIC interpretation critical 2
Important Caveats
Universal resistance to cephalothin and high resistance rates to cefuroxime (90.5%) and amoxicillin-clavulanate (95.9%) limit treatment options, making carbapenems particularly important for serious infections 2
Burn units and intensive care settings are high-risk environments for development and dissemination of imipenem-resistant M. morganii 6
All M. morganii isolates in one study were ESBL producers, carrying CTX-M, SHV, TEM, and OXA genes, which contributes to the multidrug-resistant phenotype but does not directly affect carbapenem activity unless KPC is present 1