Imipenem has a high degree of stability in the presence of beta-lactamases, including penicillinases and cephalosporinases produced by gram-negative and gram-positive bacteria. It is a potent inhibitor of beta-lactamases from certain gram-negative bacteria which are inherently resistant to many beta-lactam antibiotics, e.g., Pseudomonasaeruginosa,Serratia spp. and Enterobacter spp.

Imipenem has in vitro activity against a wide range of gram-positive and gram-negative organisms. Imipenem has been shown to be active against most strains of the following microorganisms both in vitro and in clinical infections treated with the intramuscular formulation of imipenem-cilastatin sodium as described in the INDICATIONS AND USAGE section.

Gram-positive aerobes

  Staphylococcus aureus including penicillinase-producing strains
(NOTE: Methicillin-resistant staphylococci should be reported as resistant to imipenem.)
  Group D streptococcus including   Enterococcus faecalis (formerly S. faecalis)
(NOTE: Imipenem is inactive in vitro against Enterococcus faecium [formerly S. faecium].)
  Streptococcus pneumoniae
  Streptococcus pyogenes (Group A streptococci)
  Streptococcus viridans group

Gram-negative aerobes:

  Acinetobacter spp., including A. calcoaceticus
  Citrobacterspp.
  Enterobacter cloacae
  Escherichia coli
  Haemophilus influenzae
  Klebsiella pneumoniae
  Pseudomonas aeruginosa
(NOTE: Imipenem is inactive in vitro against Xanthomonas (Pseudomonas) maltophilia and P.cepacia.)

Gram-positive anaerobes

  Peptostreptococcus spp.

Gram-negative anaerobes

  Bacteroides spp., including
  Bacteroides distasonis
  Bacteroides intermedius
(formerly B. melaninogenicus intermedius)
Bacteroides fragilis
Bacteroides thetaiotaomicron
Fusobacterium spp.

Imipenem exhibits in vitro minimal inhibitory concentrations (MICs) of 4 μg/mL or less against most ( ≥ 90%) strains of the following microorganisms; however, the safety and effectiveness of imipenem in treating clinical infections due to these microorganisms have not been established in adequate and well-controlled clinical trials.

Gram-positive aerobes

  Bacillusspp.
  Listeria monocytogenes
  Nocardiaspp.
  Group C streptococci
  Group G streptococci

Gram-negative aerobes

  Aeromonas hydrophila
  Alcaligenes spp.
  Capnocytophaga spp.
  Enterobacter agglomerans
  Haemophilus ducreyi
  Klebsiella oxytoca
  Neisseria gonorrhoeae including penicillinase-producing strains
  Pasteurella spp.
  Proteus mirabilis
  Providencia stuartii

Gram-positive anaerobes

Clostridium perfringens

Gram-negative anaerobes

  Prevotella bivia
  Prevotella disiens
  Prevotella melaninogenica
  Veillonella spp.

In vitro tests show imipenem to act synergistically with aminoglycoside antibiotics against some isolates of Pseudomonas aeruginosa.

Susceptibility Tests

Dilution techniques

Use a standardized dilution method¹ (broth, agar, microdilution) or equivalent with imipenem powder. The MIC values obtained should be interpreted according to the following criteria:

A report of “susceptible” indicates that the pathogen is likely to be inhibited by generally achievable blood levels. A report of “moderately susceptible” suggests that the organism would be susceptible if high dosage is used or if the infection is confined to tissues and fluids in which high antibiotic levels are attained. A report of “resistant” indicates that achievable concentrations are unlikely to be inhibitory and other therapy should be selected.

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