Correction: Morroni et al. Antimicrobial Activity of Aztreonam in Combination with Old and New β-Lactamase Inhibitors against MBL and ESBL Co-Producing Gram-Negative Clinical Isolates: Possible Options for the Treatment of Complicated Infections. Antibiotics 2021, 10, 1341.

Results have been implemented with additional data, which have been commented in the following paragraphs [...].

1. Text Correction

Results have been implemented with additional data, which have been commented in the following paragraphs.

In Section 2.2.1, The sentence: “In contrast, MIC values for the six Enterobacterales ranged from 1 to 8 µg/mL, so zidebactam 0.5 µg/mL, which was a sub-inhibitory dose for all strains tested, was used for testing in combination with aztreonam.” [1] was changed to Results—Checkerboard assays—Section 2.2.1:

“In contrast, MIC values for the six Enterobacterales ranged from 1 to 8 µg/mL, so for testing in combination with aztreonam, zidebactam was used both at 0.5 µg/mL, which was a sub-inhibitory dose for all strains tested, and at a 1:1 ratio.”

In Section 2.2.1, At the end of the last paragraph, the following sentence was added: “Checkerboard assays testing the combination aztreonam/zidebactam at a 1:1 ratio confirmed previous results, showing synergistic effect against C. amalonaticus N18, K. pneumoniae KL 12 SG and S. maltophilia but not against the three E. coli, K. pneumoniae LC954/14, C. indologenes and E. meningoseptica (Table 1).”

In Section 2.2.4, A correction has been made to Results—Time-kill assays with aztreonam/zidebactam 1:1 ratio—Section 2.2.4:

“Time-kill assays were also performed with aztreonam/zidebactam at 1:1 ratio against C. amalonaticus and S. maltophilia, while at 1:1 ratio was already synergistic in K. pneumoniae KL 12 SG with 0.5 mg/L concentration (Figure 3). Results were summarized in Figure S1. In C. amalonaticus synergies were also confirmed with aztreonam/zidebactam at 1:1 ratio of 1 mg/L. In S. maltophilia, although we did not detect synergies with 0.5 mg/L of zidebactam (regardless of aztreonam concentration), the two antibiotics showed synergistic effect with a 4 mg/L concentration, although the regrowth observed at 24 h suggests that this combination needs to be further analyzed in a higher number of S. maltophilia isolates.”

In Section 4.5, The sentence: “ZID, for which a recommended fixed concentration was not available, was tested in the range 0.125–32 µg/mL, alone and in association with ATM, by checkerboard assay in 96-well microtiter plates on an initial inoculum of 5 × 105 CFU/mL. The combination was considered synergistic when the fractional inhibitory concentration index (FICI) was ≤0.5” was changed to Materials and Methods—MIC evaluation and checkerboard assays—Section 4.5:

“Zidebactam (ZID), for which a recommended fixed concentration was not available, was tested in the range 0.125–32 µg/mL, alone and in association with ATM, by checkerboard assay in 96-well microtiter plates on an initial inoculum of 5 × 105 CFU/mL. Besides, as the Clinical Laboratory and Standards Institute (CLSI) in the United States of America recommends that testing of zidebactam in combination with cefepime is carried out at a 1:1 ratio, additional testing of the combination aztreonam/zidebactam at a 1:1 ratio was carried out. The combination was considered synergistic when the fractional inhibitory concentration index (FICI) was ≤0.5.”

In the section Supplementary Materials, Figure S1 was added. Figure S1 shows time-kill assays performed with aztreonam/zidebactam at 1:1 ratio against C. amalonaticus and S. maltophilia.

2. Error in Table 1

The MICs obtained using the combination aztreonam/zidebactam at a 1:1 ratio have been added in Table 1 (column #13).

Table 1

MIC (µg/mL) of aztreonam, alone and in association with BLIs, of tested strains.

Strain	Metallo-β-Lactamase	Serine-β- Lactamase	MIC ATM	MIC ATM after Addition of							MIC ZID	MIC ATM/ZID 1:1 Ratio	REF
				CLA a	TAZ b	SUL b	VAB c	AVI b	REL b	ZID d
E. coli CP-Ec3	bla VIM-1	bla KPC-2	>32	>32	>32	>32	>32	8	>32	32	1	1	[16]
E. coli CP-Ec4	bla VIM-1	blaTEM-1 * bla CTX-M-15 * bla SHV-12	>32	16	>32	>32	>32	32	>32	>32	1	1	[16]
E. coli 482483	bla NDM-5	blaTEM-1 * bla CTX-M-15	>32	16	>32	>32	>32	8	>32	32	1	1	[17]
C. amalonaticus N18	bla VIM-1	* bla SHV-12	>32	8	>32	>32	>32	0.25	4	0.5	4	0.5	[18]
K. pneumoniae KL 12 SG	bla NDM-1	blaTEM-1 * bla CTX-M-15	>32	>32	>32	>32	4	0.25	4	0.25	8	0.5	This study
K. pneumoniae LC954/14	bla NDM-1	* bla CTX-M-15 * bla SHV-182	>32	>32	>32	>32	8	0.25	4	>32	1	1	[19]
C. indologenes LC650/17	bla IND-3	* bla CIA	>32	>32	>32	>32	>32	>32	>32	>32	>32	>32	[20]
E. meningoseptica LC596/11	bla B-9 bla GOB-13	* bla CME-1	>32	>32	>32	>32	>32	>32	>32	>32	>32	>32	[20]
S. maltophilia	bla L-1	* bla L-2	>32	>32	4	8	2	1	0.5	0.5	>32	0.5	[20]

MIC, minimum inhibitory concentration; BLIs, β-lactamase inhibitors; ATM, aztreonam; CLA, clavulanate; TAZ, tazobactam; SUL, sulbactam; VAB, vaborbactam; AVI, avibactam; REL, relebactam; ZID, zidebactam. a: 2 µg/mL; b: 4 µg/mL; c: 8 µg/mL; d: 0.5 µg/mL. * ESBL.

The authors apologize for any inconvenience caused and state that the scientific conclusions are unaffected. The original publication has also been updated.