antibiotics Article A New Twist: The Combination of Sulbactam/Avibactam Enhances Sulbactam Activity against Carbapenem-Resistant Acinetobacter baumannii (CRAB) Isolates Fernando Pasteran 1 , Jose Cedano 2 , Michelle Baez 2 , Ezequiel Albornoz 1 , Melina Rapoport 1 , Jose Osteria 2 , Sabrina Montaña 3 , Casin Le 2 , Grace Ra 2 , Robert A. Bonomo 4,5,6 , Marcelo E. Tolmasky 2 , Mark Adams 7 , Alejandra Corso 1 and Maria Soledad Ramirez 2, *   Citation: Pasteran, F.; Cedano, J.; Baez, M.; Albornoz, E.; Rapoport, M.; Osteria, J.; Montaña, S.; Le, C.; Ra, G.; Bonomo, R.A.; et al. A New Twist: The Combination of Sulbactam/ Avibactam Enhances Sulbactam Activity against Carbapenem- Resistant Acinetobacter baumannii (CRAB) Isolates. Antibiotics 2021, 10, 577. https://doi.org/10.3390/ antibiotics10050577 Academic Editor: Maria Lina Mezzatesta Received: 29 April 2021 Accepted: 11 May 2021 Published: 13 May 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 National/Regional Reference Laboratory for Antimicrobial Resistance (NRL), Servicio Antimicrobianos, Instituto Nacional de Enfermedades Infecciosas, ANLIS “Dr. Carlos G. Malbrán”, C1282AFF Buenos Aires, Argentina; fpasteran@gmail.com or fpasteran@anlis.gob.ar (F.P.); Ealbornoz@anlis.gob.ar (E.A.); Rapoport@anlis.gob.ar (M.R.); Acorso@anlis.gob.ar (A.C.) 2 Center for Applied Biotechnology Studies, Department of Biological Science, College of Natural Sciences and Mathematics, California State University Fullerton, Fullerton, CA 92831-3599, USA; Jcedano@csu.fullerton.edu (J.C.); michelleb777@csu.fullerton.edu (M.B.); josteria@csu.fullerton.edu (J.O.); casle@Fullerton.edu (C.L.); gracera@csu.fullerton.edu (G.R.); mtolmasky@fullerton.edu (M.E.T.) 3 Laboratorio de Bacteriología Clínica, Departamento de Bioquímica Clínica, Hospital de Clínicas José de San Martín, Facultad de Farmacia y Bioquímica, C1120 Buenos Aires, Argentina; sabri.mon@hotmail.com 4 Research Service and GRECC, Louis Stokes Cleveland Department of Veterans Affairs Medical Center, Cleveland, OH 44106, USA; Robert.Bonomo@va.gov 5 Departments of Medicine, Pharmacology, Molecular Biology and Microbiology, Biochemistry, Proteomics and Bioinformatics, Case Western Reserve University School of Medicine, Cleveland, OH 44106, USA 6 CWRU-Cleveland VAMC Center for Antimicrobial Resistance and Epidemiology (Case VA CARES), Cleveland, OH 44106, USA 7 The Jackson Laboratory, Farmington, CT 06032, USA; madams@jcvi.org * Correspondence: msramirez@Fullerton.edu; Tel.: +1-657-278-4562 Abstract: An increasing number of untreatable infections are recorded every year. Many studies have focused their efforts on developing new β-lactamase inhibitors to treat multi-drug resistant (MDR) iso- lates. In the present study, sulbactam/avibactam and sulbactam/relebactam combination were tested against 187 multi-drug resistant (MDR) Acinetobacter clinical isolates; both sulbactam/avibactam and sulbactam/relebactam restored sulbactam activity. A decrease ≥2 dilutions in sulbactam MICs was observed in 89% of the isolates when tested in combination with avibactam. Sulbactam/relebactam was able to restore sulbactam susceptibility in 40% of the isolates. In addition, the susceptibility test- ing using twenty-three A. baumannii AB5075 knockout strains revealed potential sulbactam and/or sulbactam/avibactam target genes. We observed that diazabicyclooctanes (DBOs) β-lactamase in- hibitors combined with sulbactam restore sulbactam susceptibility against carbapenem-resistant Acinetobacter clinical isolates. However, relebactam was not as effective as avibactam when combined with sulbactam. Exploring novel combinations may offer new options to treat Acinetobacter spp. infections, especially for widespread oxacillinases and metallo-β-lactamases (MBLs) producers. Keywords: Acinetobacter; carbapenem-resistance; sulbactam; avibactam; relebactam 1. Introduction Infections caused by antibiotic-resistant bacteria are increasing in frequency, resulting in significant patient morbidity and mortality [1,2]. Acinetobacter baumannii is a nosocomial pathogen frequently resistant to multiple drugs. A. baumannii causes pneumonia, bac- teremia, and wound infections with associated high mortality rates. The most problematic strains are those resistant to carbapenems (carbapenem-resistant A. baumannii, CRAB). In most cases, resistance is caused by either chromosomal or plasmid-mediated class D oxacil- linases (bla OXA )[3–9], but recently, bla NDM-1 has also been increasingly observed [8,10–15]. Antibiotics 2021, 10, 577. https://doi.org/10.3390/antibiotics10050577 https://www.mdpi.com/journal/antibiotics