Cefiderocol Broth Microdilution Reading Guidance Instructional Video

Determination of cefiderocol MIC values using the broth microdilution method.

When cefiderocol susceptibility testing is performed with the broth microdilution method, different growth patterns may be observed.

These growth patterns may appear as:

• Clear endpoint
• Reduced growth
• Reduced growth with trailing
• Regrowth
• Haze
• Aberrant growth

The CLSI M-100 Appendix H reading guidelines can assist in determining cefiderocol MIC values. This animation shows examples of the application of these guidelines for different growth patterns.

Please, pause the animation at any time to review the growth pattern.

Cefiderocol inhibited the growth of this Escherichia coli strain with a clear endpoint, as illustrated by the lack of button in the wells at concentrations of 1 µg/mL and above.

The MIC is therefore 1 µg/mL.

Cefiderocol did not inhibit the growth of this Klebsiella pneumoniae strain up to the concentration of 32 µg/mL.

In the well above, at 64 µg/mL, the growth was affected; and because the button is smaller than 1 mm, the MIC is determined as 64 µg/mL.

Cefiderocol inhibited the growth of this Pseudomonas aeruginosa strain with a clear endpoint at the concentration of 8 µg/mL compared with the growth control.

Therefore, the MIC is 8 µg/mL.

Cefiderocol did not affect growth up to the concentration of 0.06 µg/mL. At 0.12 µg/mL, the growth was affected, illustrated by a button smaller than 1 mm.

Notice that growth is still visible at higher concentrations, which is called trailing. Even more enhanced growth can be observed at 4 µg/mL.

This growth is ignored, and the MIC is determined at the first well with a button size of <1 mm, which is 0.12 µg/mL.

For this Acinetobacter baumannii strain, the MIC is recorded at the first well with >80% growth reduction compared to the growth control (which is at 2 µg/mL), even though there remains growth at higher concentrations, showing trailing.

Cefiderocol did not affect growth up to the concentration of 1 µg/mL. At 2 µg/mL, growth was affected, but the growth was not reduced by >80%. This level of growth reduction occurred at 8 µg/mL; thus, the MIC is determined at 8 µg/mL.

Notice that growth is still visible at higher concentrations, which is called trailing, but this growth is ignored.

Cefiderocol did not affect growth up to the concentration of 2 µg/mL. At 4 µg/mL, growth was affected, but the growth button remains larger than 1 mm. A growth button size of <1 mm is detected only at 8 µg/mL. Notice that growth is still visible at higher concentrations, which is called trailing.

This growth is ignored, and the MIC is determined at the first well with a button size of <1 mm, which is 8 µg/mL.

Cefiderocol did not affect growth up to the concentration of 0.12 µg/mL. At 0.25 µg/mL, growth was affected but the size of the growth button was still larger than 1 mm. The button size is <1 mm only at 0.5 µg/mL. Notice that growth is still visible at higher concentrations, which is called trailing. Larger buttons can be observed at 2 and 4 µg/mL.

This growth is ignored, and the MIC is determined at the first well with a button size of <1 mm, which is 0.5 µg/mL.

Despite observed growth reduction compared with the growth control, starting at 2 µg/mL, the MIC is only recorded above the highest cefiderocol concentration tested, because the button has not yet reached a size of <1 mm.

Therefore, the MIC is > 4 µg/mL.

For this Klebsiella pneumoniae strain, the MIC is only determined when the button reaches a size of <1 mm, despite reduced growth at 0.5 µg/mL.

Thus, the MIC is 1 µg/mL.

For this Klebsiella pneumoniae strain, the MIC is recorded at the first well with a button size of <1 mm. Thus, the MIC is 8 µg/mL.

Trailing and increased growth are both seen at higher cefiderocol concentrations.

For this Acinetobacter baumannii strain, the MIC is determined at the first well with a button size of <1 mm, despite regrowth at 64 µg/mL.

The MIC is 32 µg/mL.

For this Acinetobacter baumannii strain, the MIC is determined at the first well with a button size of <1 mm, despite the observed regrowth at higher cefiderocol concentrations.

The MIC is 16 µg/mL.

For this Acinetobacter baumannii strain, the MIC is determined when the growth is reduced by >80%, considering also the “donut-shaped” growth pattern.

Thus, the MIC is 0.5 µg/mL.

For this Acinetobacter baumannii strain, an aberrant “donut-shaped” growth pattern was observed at 0.25 µg/mL, but growth was not inhibited by >80%.

This level of growth inhibition was only achieved at 1 µg/mL; thus, the MIC is 1 µg/mL. Although, there is still residual growth at higher concentrations.