Which Bacteria Show Tigecycline Resistance?
Tigecycline, a glycylcycline antibiotic, targets multidrug-resistant Gram-negative and Gram-positive bacteria, but resistance has emerged in clinical isolates. Key examples include:
- Acinetobacter baumannii: Frequently develops resistance via efflux pumps (AdeABC overexpression) and ribosomal mutations (e.g., 16S rRNA changes). MICs often rise from ≤2 mg/L (susceptible) to >8 mg/L.[1][2]
- Klebsiella pneumoniae: Carbapenem-resistant strains (CRKP) show increased resistance through RmtB 16S rRNA methylases and efflux (AcrAB-TolC). Hypervirulent variants are particularly concerning.[1][3]
- Enterobacter cloacae complex: Resistance linked to Tet(X) enzymes and efflux overexpression; common in ICU settings with MICs up to 32 mg/L.[2][4]
- Escherichia coli: Rising in ST167 and ST410 clones via plasmid-mediated Tet(X3/X4) that degrade tigecycline, plus efflux and permeability loss.[3][5]
How Does Tigecycline Resistance Develop?
Resistance mechanisms cluster around:
- Efflux pumps: Overexpression (e.g., MexXY in Pseudomonas, though tigecycline is less affected there).
- Ribosomal protection/modification: Rmt genes methylate 16S rRNA, blocking binding.
- Enzymatic degradation: Tet(X) family oxidases, transferable via plasmids.
- Outer membrane changes: Reduced porins limit entry in Gram-negatives.
These often combine in high-risk settings like ICUs, with MIC creep observed globally since 2005.[1][2]
Are Resistance Trends Increasing?
Yes, surveillance data show rising rates:
- A. baumannii: Up to 30-50% resistance in parts of Asia/Europe.
- K. pneumoniae: 10-20% in U.S. CRKP cases.
- E. cloacae: 15% in European reports.
Mobile genetic elements accelerate spread, especially Tet(X4).[3][5]
What About Gram-Positive Bacteria?
Less common but notable:
- Staphylococcus aureus: Rare efflux (Tet(L)) or ribosomal mutations in MRSA.
- Enterococcus faecium: Emerging VRE isolates with Tet(M) and efflux.[2][4]
Treatment Challenges and Alternatives
Tigecycline fails at higher MICs; options include eravacycline (similar but efflux-resistant), omadacycline, or combinations like colistin+meropenem. Susceptibility testing is critical due to heteroresistance.[1][6]
Sources
[1]: CDC Antibiotic Resistance Threats Report
[2]: Clinical Microbiology Reviews - Tigecycline Resistance Mechanisms
[3]: Emerging Infectious Diseases - Klebsiella tigecycline resistance
[4]: Journal of Antimicrobial Chemotherapy - Enterobacter resistance
[5]: Antimicrobial Agents and Chemotherapy - Tet(X) in E. coli
[6]: IDSA Guidelines on MDR Gram-Negatives