Why would a wound culture and sensitivity report positive for Enterobacter cloacae not mention carbapenems (antibiotics)?

Why Carbapenems May Be Missing from Enterobacter cloacae Susceptibility Reports

A wound culture and sensitivity report positive for Enterobacter cloacae may not mention carbapenems due to laboratory testing protocols that assume Enterobacter species are susceptible to carbapenems unless specific resistance mechanisms are detected or suspected. 1

Laboratory Testing Considerations

Standard Reporting Practices

• Laboratories often follow Clinical and Laboratory Standards Institute (CLSI) guidelines which have evolved over time regarding carbapenem reporting 1
• Many labs only test and report carbapenems when:
  • The isolate shows resistance to other beta-lactams
  • The isolate has elevated MICs (minimum inhibitory concentrations) for carbapenems
  • There is clinical suspicion for carbapenem resistance

Detection of Resistance Mechanisms

• Some E. cloacae strains may have carbapenem MICs that are elevated but still within the susceptible range 1
• These strains may harbor carbapenemase genes (like blaKPC) that aren't detected by routine susceptibility testing 1
• The Modified Hodge Test (MHT) is recommended for detecting carbapenemases in isolates with elevated but susceptible carbapenem MICs 1

Clinical Implications

When to Consider Carbapenem Resistance

• Request additional testing if:
  • The patient has risk factors for carbapenem-resistant organisms
  • The infection is not responding to treatment
  • The patient has had prior carbapenem exposure
  • The patient was previously colonized with carbapenem-resistant organisms

Resistance Mechanisms in E. cloacae

• E. cloacae can develop carbapenem resistance through multiple mechanisms:
  • Production of carbapenemases (KPC, NDM, VIM, OXA-48-like) 1
  • AmpC beta-lactamase overproduction (present in 97.7% of carbapenem-resistant E. cloacae) 2
  • Alterations in outer membrane proteins (porin loss or modifications) 2

What to Do When Carbapenems Are Not Reported

1. Ask the laboratory to test for carbapenem susceptibility if clinically indicated
2. Request carbapenemase testing (Modified Hodge Test or molecular methods) if carbapenem resistance is suspected 1
3. Consider the infection site and severity when selecting therapy
4. Review local antibiogram data for E. cloacae susceptibility patterns

Treatment Considerations

• For suspected ESBL or AmpC-producing E. cloacae:

  • Carbapenems remain first-line for serious infections 3
  • Ertapenem 1g IV every 24 hours for community-acquired infections 3
  • Meropenem 1g IV every 8 hours (extended or continuous infusion preferred) for severe infections 3

• For carbapenemase-producing E. cloacae:

  • Treatment should be guided by the specific carbapenemase type 1
  • For KPC-producing strains, ceftazidime/avibactam or meropenem/vaborbactam are first-line options 1
  • Imipenem/relebactam and cefiderocol may also be considered 1

Important Caveats

• Carbapenem-resistant E. cloacae is increasingly reported, with 10% of cases being community-associated 4
• Some E. cloacae strains may show resistance to both carbapenems and colistin, severely limiting treatment options 5, 6
• The prevalence of carbapenem resistance in E. cloacae varies significantly by region, with some areas reporting up to 27% resistance to ertapenem 1

Remember that wound cultures may not always represent true infection and clinical correlation is essential when interpreting culture results and selecting appropriate antimicrobial therapy.