Introduction

ESBL and ampC-producing Enterobacterales pose significant treatment challenges, reducing the effectiveness of conventional beta-lactam/beta-lactamase inhibitor (BL/BLI) combinations like piperacillin/tazobactam. Cefepime/enmetazobactam, a novel BL/BLI combination, is designed to restore cefepime activity and serve as a carbapenem-sparing therapy.

Aim

To evaluate the in-vitro activity of cefepime/enmetazobactam along with comparators against molecularly characterized third-generation cephalosporin-resistant Enterobacterales and P. aeruginosa isolates from India.

Methods

Isolates: 

Assessments

• MIC testing by broth microdilution 
• Time-kill assay for bactericidal activity
• Genes detected by PCR: 
  1. Extended spectrum beta-lactamases (ESBLs): CTX-M, TEM, SHV
  2. ampCs (CMY, DHA)
  3. OXA-1
  4. carbapenemases

Results

A significant proportion of isolates carried multiple resistance genes

Table 1: Molecular Profile of Isolates

Species	Key Findings
E. coli	58% CTX-M producers; 42% co-harbored CTX-M + OXA-1; 62% ampC producers (CMY, DHA, dual)
K. pneumoniae	100% CTX-M; 13% OXA-1; 5 isolates with ampC (CMY/DHA)
Enterobacter spp.	All CTX-M positive
Proteus spp.	All CTX-M positive

Cefepime/enmetazobactam demonstrated potent inhibition across species:

• All E. coli CTX-M ± OXA-1 isolates inhibited at MIC ≤ 2 mg/L; ampC producers at MIC ≤ 4 mg/L
• K. pneumoniae CTX-M isolates inhibited at MIC ≤ 1 mg/L; limited activity against OXA-48-like producers (16% susceptible)
• 100% inhibition for Enterobacter spp. and Proteus spp. at low MICs
• P. aeruginosa showed complete susceptibility

 

Table 2: Susceptibility comparison of cefepime/enmetazobactam with Piperacillin/tazobactam

Pathogen	Cefepime/Enmetazobactam (MIC)	Piperacillin/tazobactam (TZP) Susceptibility
E. coli (CTX-M ± OXA-1)	≤ 2 mg/L (100% inhibited)	78% → 58% with OXA-1
E. coli (ampC)	≤ 4 mg/L (100% inhibited)	Poor activity
K. pneumoniae (CTX-M)	≤ 1 mg/L (100% inhibited)	Variable
K. pneumoniae (OXA-48-like)	Limited activity (16% ≤ 4 mg/L)	Poor activity
Enterobacter spp.	≤ 2 mg/L (100% inhibited)	71.8%
Proteus spp.	≤ 0.25 mg/L (100% inhibited)	100%
P. aeruginosa	100% susceptible	88%

 

Cefepime/enmetazobactam achieved rapid bactericidal activity:

• ≥3 log₁₀ kill against ESBL-producing E. coli and K. pneumoniae isolates
• Comparable killing effect to carbapenems; superior to TZP in resistant isolates

Table 3: Time Kill Assay Results

Isolate Type	Cefepime/Enmetazobactam Kill	Comparison
E. coli (CTX-M + OXA-1)	≥ 3 log₁₀ kill	Comparable to carbapenem
E. coli (CTX-M + CMY)	≥ 3 log₁₀ kill	Comparable to carbapenem
K. pneumoniae (CTX-M + OXA-1)	≥ 3 log₁₀ kill	Comparable to carbapenem
K. pneumoniae (CTX-M + CMY)	≥ 3 log₁₀ kill	Comparable to carbapenem

 

Cefepime/enmetazobactam showed dramatically lower MIC90 values compared to TZP:

• MIC90 for E. coli and K. pneumoniae: 1 mg/L
• TZP MIC90 for E. coli and K. pneumoniae: 128 mg/L (indicating poor activity)

Conclusion

Spectrum of coverage for cefepime/enmetazobactam was superior to that of TZP and similar to that of meropenem, emphasising the importance of cefepime/ enmetazobactam as a carbapenem-sparing option for severe infections in regions with high ESBL prevalence.

Reference

Diagnostic Microbiology & Infectious Disease. 2025;113:116993