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

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	blaVIM-1	blaKPC-2	>32	>32	>32	>32	>32	8	>32	32	1	1	[16]
E. coli CP-Ec4	blaVIM-1	blaTEM-1 * blaCTX-M-15 * blaSHV-12	>32	16	>32	>32	>32	32	>32	>32	1	1	[16]
E. coli 482483	blaNDM-5	blaTEM-1 * blaCTX-M-15	>32	16	>32	>32	>32	8	>32	32	1	1	[17]
C. amalonaticus N18	blaVIM-1	* blaSHV-12	>32	8	>32	>32	>32	0.25	4	0.5	4	0.5	[18]
K. pneumoniae KL 12 SG	blaNDM-1	blaTEM-1 * blaCTX-M-15	>32	>32	>32	>32	4	0.25	4	0.25	8	0.5	This study
K. pneumoniae LC954/14	blaNDM-1	* blaCTX-M-15 * blaSHV-182	>32	>32	>32	>32	8	0.25	4	>32	1	1	[19]
C. indologenes LC650/17	blaIND-3	* blaCIA	>32	>32	>32	>32	>32	>32	>32	>32	>32	>32	[20]
E. meningoseptica LC596/11	blaB-9 blaGOB-13	* blaCME-1	>32	>32	>32	>32	>32	>32	>32	>32	>32	>32	[20]
S. maltophilia	blaL-1	* blaL-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.