Table A2.

Resistance mediated by mutations affecting siderophore receptors.

Target Gene	Organism(s)	Findings
piuA , piuD, pirA	P. aeruginosa , A. baumannii	Based on isogenic P. aeruginosa mutants: ○ Deficiency of PiuA resulted in 16-fold higher cefiderocol MIC (0.125→2 mg/L) [47]. In the same strain, deficiency of pirA (either alone or in addition to piuA) did not affect cefiderocol MIC [47]. ○ Deletion of piuA resulted in a 16-fold higher MIC (0.5→8 mg/L) and a 32-fold higher MIC (0.5→16 mg/L) when combined with deletion of pirA [50]. Deletion of pirA alone in the same strain had no effect on cefiderocol MIC [50]. ○ Deletion of piuD resulted in a 32-fold higher cefiderocol MIC (0.06→2 mg/L) and 64-fold a higher MIC (0.06→4 mg/L) when combined with deletion of pirA [50]. Deletion of pirA alone in the same strain resulted in a 2-fold higher MIC (0.06→0.125 mg/L) [50]. Based on clinical isolates: ○ AmpC L147F (emerging during treatment with ceftolozane/tazobactam) in combination with mutations in piuA and pirR was detected in a ceftolozane/tazobactam-resistant P. aeruginosa isolate and was associated with a 4-fold (2→8 mg/L) higher cefiderocol MIC [44]. ○ In a collection of six cefiderocol-resistant A. baumannii clinical isolates, expression of both PirA and PiuA was absent in three. In one, only expression of PiuA was absent (in combination with measurable but reduced PirA expression), and in the remaining three, mutations in both pirA and piuA were detected [52]. ○ In another collection of 12 cefiderocol-resistant A. baumannii clinical isolates, deficiency of PiuA was detected in 12 [30].
fecI **	P. aeruginosa **	In an in vitro derived cefiderocol-resistant strain with a 4-fold higher MIC (0.5→2 mg/dL), mutations in fecI were identified. fecI regulates the synthesis of the iron transporter FecA, contributing to transport of iron citrate. fecA expression was 9-fold higher in the fecI mutant [46].
cirA, fiu *	E. coli, K. pneumoniae, E. cloacae **	Based on isogenic mutants: ○ E. coli: Deletion of fiu alone resulted in a 2-fold higher cefiderocol MIC (0.063→0.125 mg/L) and a 16-fold higher MIC when combined with deletion of cirA (0.063→1 mg/L) [47]. Deletion of cirA alone did not affect the MIC [47]. ○ K. pneumoniae: Loss of CirA resulted in 4-fold higher cefiderocol MIC (0.5→2 mg/L) [57]. Based on in vitro derived (after serial passaging in the presence of cefiderocol) cefiderocol-resistant mutants: ○ In all five resistant E. clocae mutants (MIC 4→>128 mg/L), various mutations affecting the cirA gene were detected and were not associated with fitness cost [40]. ○ A mutated cirA gene was detected in a K. pneumoniae mutant (MIC 2→>128 mg/L) [57]. Based on clinical isolates: ○ Comparing a cefiderocol-resistant K. pneumoniae isolate (emerging in vivo after cefiderocol treatment) with its parental strain, various mutations affecting cirA were detected. cirA was the only gene mutating between the two strains [24]. ○ CirA mutations were also detected in six cefiderocol-resistant E. coli clinical isolates (in combination with NDM and PMB-3 mutations) [59]. ○ CirA deficiency was detected in a cefiderocol-resistant K. pneumoniae (in combination with NDM-5) [64].
fhuA, fepA, fecA **, fbpA **, efeO **, exbD **	K. pneumoniae	Based on in vitro derived cefiderocol-resistant mutants: ○ Mutations were detected in efeo (iron uptake system component), fecA (tonB-dependent receptor), and fbpA (ferric iron ABC transporter) [28]. ○ Mutation of exbD (an accessory protein related to iron transport) was associated with a >8-fold increase in cefiderocol MIC (4→>32 mg/L) [31]. Based on clinical isolates: ○ Mutations/deletions in fhuA (ferrichrome iron receptor) or fepA (ferric enterobactin receptor) were detected by whole genome sequencing in two clinical isolates with cefiderocol MIC 2 mg/L [61].
tonB **, exbD **, smlat1148 **, cirA **	S. maltophilia **	Based on in vitro derived cefiderocol-resistant mutants: ○ Among 31 mutants, 25 had mutations in tonB, and 3 had mutations in exbD. These mutations were associated with fitness cost and were reversible [58]. ○ tonB, cirA, and smlat1148 mutations were detected separately in three mutants [60]

In bold are mechanisms of resistance of which the role has been confirmed in isogenic mutant experiments (group 5 studies, see “Eligibility criteria” in Methods) and that have been detected in cefiderocol-resistant clinical isolates (group 1–3 studies, see “Eligibility criteria” in Methods). * Based only on in vitro isogenic mutant experiments (group 5 studies, see “Eligibility criteria” in Methods). ** Based only on in vitro derived mutants (group 4 studies, see “Eligibility criteria” in Methods).