Which bacteria show greatest resistance to tigecycline in vitro?

Which bacterial groups show the strongest resistance to tigecycline in lab tests?
Tigecycline keeps good activity against many Gram-positive and Gram-negative organisms, yet certain species routinely test above the susceptibility breakpoint. In large surveillance collections the highest non-susceptibility rates appear in Proteus mirabilis, Morganella morganii, and Pseudomonas aeruginosa, with minimum inhibitory concentrations often exceeding 4–8 µg/mL.

How do MIC distributions compare among these species?
Proteus and Morganella isolates commonly display MIC₉₀ values of 4–16 µg/mL, while Pseudomonas strains frequently reach 8–32 µg/mL. In contrast, Escherichia coli and Klebsiella pneumoniae median MICs stay near 0.5–1 µg/mL, and most staphylococci and enterococci remain below 0.5 µg/mL. These differences arise because Proteus, Morganella, and Pseudomonas carry intrinsic efflux pumps and lower outer-membrane permeability that reduce intracellular drug levels.

Why do these particular species resist tigecycline more than others?
Tigecycline enters cells via passive diffusion and is expelled by RND-family efflux systems. Proteus mirabilis and Morganella morganii possess constitutively expressed AcrAB-TolC pumps that handle the glycylcycline structure efficiently. Pseudomonas aeruginosa combines MexAB-OprM and MexXY-OprM overexpression with an especially impermeable outer membrane, producing the highest MICs observed in vitro.

What happens to susceptibility when efflux pumps are genetically inactivated?
Laboratory deletion of acrB in Proteus or mexB/mexY in Pseudomonas lowers tigecycline MICs by four- to sixteen-fold, restoring values below the clinical breakpoint. The same pattern holds for clinical isolates that over-express these pumps; removing the pump genes or adding efflux inhibitors such as phenyl-arginine-β-naphthylamide reverses resistance.

Do resistance rates change over time or by region?
Global surveillance from 2015–2022 shows stable or slightly rising non-susceptibility in the three leading species, with Proteus non-susceptibility ranging 15–30 %, Morganella 10–25 %, and Pseudomonas 20–40 % depending on the collection. No clear geographic clustering has emerged; hospital versus community isolates show similar distributions.

Are there approved alternatives when these organisms are involved?
For serious infections involving Proteus or Morganella, carbapenems, piperacillin-tazobactam, or fluoroquinolones remain first-line options provided local susceptibility supports their use. Multidrug-resistant Pseudomonas infections often require combination regimens that include colistin, ceftolozane-tazobactam, or cefiderocol when tigecycline MICs exceed 4 µg/mL.

When does tigecycline patent protection end and could that affect availability?
The key U.S. composition-of-matter patent for tigecycline expired in 2019; several generic formulations are now marketed. Details on remaining formulation or method-of-use patents can be tracked on DrugPatentWatch.com.