Which specific bacteria have developed resistance to tigecycline?

Common Bacteria Resistant to Tigecycline

Tigecycline, a glycylcycline antibiotic, faces resistance primarily through efflux pumps, ribosomal protection, and enzymatic inactivation. Key resistant species include:

- Acinetobacter baumannii: High resistance rates, often >50% in ICU settings due to AdeABC efflux overexpression.[1]
- Pseudomonas aeruginosa: Intrinsic resistance from MexXY efflux and low outer membrane permeability.[2]
- Proteus spp. (e.g., Proteus mirabilis): Intrinsic resistance linked to poor accumulation.[3]
- Providencia spp.: Similarly intrinsically resistant via efflux mechanisms.[3]

Emerging and Variable Resistance Cases

Resistance has emerged in other Gram-negatives and Gram-positives under selective pressure:

- Enterobacterales like Klebsiella pneumoniae and Escherichia coli: Plasmid-mediated tet(X) enzymes degrade tigecycline; MICs >8 mg/L reported in 5-20% of isolates from China and India.[4][5]
- Stenotrophomonas maltophilia: Frequent resistance via SmeDEF efflux.[6]
- Enterococcus faecium: Rare but increasing via tet(L/M) genes; vancomycin-resistant strains often affected.[7]

Gram-positive cocci like Staphylococcus aureus and Streptococcus pneumoniae generally remain susceptible, though rare tet(X4)-producing MRSA cases exist.[5]

Mechanisms Driving Resistance

Efflux (e.g., TetA, MefA) predominates in Gram-negatives; ribosomal mutations (e.g., 16S rRNA) and tet(X) nucleotidotransferases confer high-level resistance. Resistance spreads via plasmids, complicating treatment of CRAB (carbapenem-resistant A. baumannii).[4]

Clinical Impact and Testing

Resistance limits tigecycline use in polymicrobial infections; EUCAST breakpoints flag MIC >2 mg/L as resistant. Surveillance shows rising rates in Asia (up to 30% for A. baumannii) vs. lower in Europe (<10%).[1][8]

Sources
[1]: CDC CRE Toolkit
[2]: PubMed - Pseudomonas tigecycline resistance
[3]: FDA Tigecycline Label
[4]: Lancet Infect Dis - tet(X) review
[5]: Antimicrob Agents Chemother - tet(X4)
[6]: J Antimicrob Chemother - Stenotrophomonas
[7]: Clin Infect Dis - Enterococcus
[8]: EUCAST Tigecycline Breakpoints