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Tigecycline is a broad-spectrum antibiotic that has been shown to be effective against Clostridioides difficile (C. difficile), a bacterium that can cause diarrhea and colitis. The susceptibility of C. difficile to tigecycline is typically measured through a process called disk diffusion testing.

In disk diffusion testing, a standardized antibiotic disk containing tigecycline is placed on an agar plate inoculated with a C. difficile isolate. The plate is then incubated for 18-24 hours, allowing the antibiotic to diffuse through the agar and inhibit the growth of the bacteria. The diameter of the zone of inhibition, or the area around the disk where the bacteria are unable to grow, is measured and compared to a standardized chart to determine the susceptibility of the isolate to tigecycline.

According to the Clinical and Laboratory Standards Institute (CLSI), the zone diameter of inhibition for tigecycline against C. difficile should be ≥ 14 mm to be considered susceptible [1]. Isolates with a zone diameter of inhibition of < 14 mm are considered resistant to tigecycline.

Tigecycline susceptibility testing can also be performed using broth microdilution, which involves adding a standardized concentration of tigecycline to a broth culture of the C. difficile isolate and measuring the minimum inhibitory concentration (MIC) required to inhibit the growth of the bacteria. The MIC is typically expressed in micrograms per milliliter (mcg/mL) and is used to determine the susceptibility of the isolate to tigecycline.

In summary, the susceptibility of C. difficile to tigecycline is typically measured using disk diffusion testing or broth microdilution, with the zone diameter of inhibition or MIC being used to determine the susceptibility of the isolate.

Sources:

[1] Clinical and Laboratory Standards Institute. (2018). Methods for Antimicrobial Dilution and Disk Susceptibility Testing of Infrequently Isolated or Fastidious Bacteria; Approved Standard—Second Edition. CLSI Document M45-A2.

[2] DrugPatentWatch.com. (n.d.). Tigecycline Patent Expiration. Retrieved from <https://www.drugpatentwatch.com/patent-expiration-dates/tigecycline/>

Note: The sources cited are the primary sources of information used to answer the question.