MIC for Imipenem Against Serratia fonticola
Serratia fonticola is naturally susceptible to imipenem with MIC values typically ≤1-2 mg/L, making carbapenems highly effective first-line agents for treating infections caused by this organism. 1
Natural Susceptibility Profile
S. fonticola demonstrates uniform natural sensitivity to carbapenems, including imipenem, based on comprehensive susceptibility testing of clinical isolates 1
The MIC90 (concentration inhibiting 90% of isolates) for imipenem against Serratia species is approximately 1.0 μg/mL, indicating excellent activity 2
All tested Serratia species isolates, including S. fonticola, have shown 100% susceptibility to imipenem in multiple studies 2, 3
Clinical Interpretation
MIC values ≤4 mg/L for imipenem are considered susceptible according to standard breakpoints for Enterobacteriaceae 4
S. fonticola MIC values for imipenem fall well below this threshold, typically in the 0.5-2.0 μg/mL range, ensuring reliable clinical efficacy 1, 5
Broth microdilution is the preferred method for determining accurate MIC values, though E-test and automated systems can be used with appropriate quality controls 4
Resistance Mechanisms and Considerations
S. fonticola naturally expresses both AmpC β-lactamases and a species-specific class A β-lactamase, but these enzymes do not confer resistance to carbapenems 1
Unlike some other β-lactams, imipenem maintains activity despite the presence of these naturally occurring β-lactamases in S. fonticola 1
The species shows natural resistance to penicillin G, oxacillin, cefazolin, and cefuroxime, but carbapenems remain fully active 1
Dosing Implications
For severe infections with organisms at the higher end of the susceptibility range (MIC 2-4 mg/L), prolonged or continuous infusion of carbapenems may optimize pharmacodynamic target attainment 4
Standard imipenem dosing achieves the pharmacodynamic target of 40% fT ≥ MIC for bacteria with MIC <4 mg/L when administered as continuous infusion at 2 g/24h 4
Intermittent dosing (1g every 8 hours) achieves adequate targets for MIC <2 mg/L, which encompasses typical S. fonticola susceptibility 4
Important Caveats
While S. fonticola is naturally susceptible to imipenem, susceptibility testing should still be performed for serious infections to detect any acquired resistance mechanisms 4
MIC determination is superior to simple susceptible/resistant reporting as it allows for optimization of dosing regimens, particularly in critically ill patients 4
The medium used for susceptibility testing can affect results; broth microdilution in Mueller-Hinton or IsoSensitest broth provides the most reliable results 1