Multidrug-resistant strains belonging to the Enterobacter cloacae complex (ECC) group, and especially those belonging to clusters C-III, C-IV, and C-VIII, have increasingly emerged as a leading cause of health care-associated infections, with colistin used as one of the last lines of treatment. However, colistin-resistant ECC strains have emerged. The aim of this study was to prove that MgrB, the negative regulator of the PhoP/PhoQ two-component regulatory system, is involved in colistin resistance in ECC of cluster C-VIII, formerly referred to as Enterobacter hormaechei subsp. steigerwaltii. An in vitro mutant (Eh22-Mut) was selected from a clinical isolate of Eh22. The sequencing analysis of its mgrB gene showed the presence of one nucleotide deletion leading to the formation of a truncated protein of six instead of 47 amino acids. The wild-type mgrB gene from Eh22 and that of a clinical strain of Klebsiella pneumoniae used as controls were cloned, and the corresponding recombinant plasmids were used for complementation assays. The results showed a fully restored susceptibility to colistin and confirmed for the first time that mgrB gene expression plays a key role in acquired resistance to colistin in ECC strains.

25CTX-M ESBLs have been increasingly reported from human enterobacteria isolates in 26 Tunisia. NDM-1 carbapenemase was recently reported in isolates from Tunisian Hospitals. 27 During a 2-month period (December 2017 to January 2018), we have collected 23 ESBL- 28 producing Enterobacteriaceae (ESBL-E) from urines of patients hospitalized in three health 29 care facilities and from community, situated in two distinct Tunisian geographical regions. 30 They were divided into 15 Escherichia coli and eight Klebsiella pneumoniae. The aim of this 31 study was to characterize ESBL-E with regard to their β-lactamase content and their 32 epidemiological relationship. The results indicated a high rate (47%) of E. coli producing both 33 CTX-M group-1 and -9. For the first time, we demonstrated the presence of E. coli having 34 concomitantly CTX-M-15 and CTX-M-27 which belong to two sequences types (ST), ie. A-35 ST617 (2 isolates) and B2-ST131 subclade C2 (2 isolates). All four E. coli isolates carried a 36 multireplicon IncF with an identical allelic combination, F31:A4:B1. This study reports also 37 the first description of K. pneumoniae belonging to the clone ST147 carrying the 38 carbapenemase NDM-1 in the Tunisian community. In conclusion, our data confirms the need 39 for monitoring the resistance to extended-spectrum cephalosporins and to carbapenems 40 among enterobacteria in Tunisia. 41 42 43Resistance to extended-spectrum cephalosporins (ESC) is widespread among 44 Enterobacteriaceae species and is mainly due to the production of extended-spectrum ß-45 lactamases (ESBLs). The dissemination of ESBLs is become a growing concern since they 46 are considered as a major cause of morbidity and mortality. Up to the end of the 1990s, TEM-47 and SHV-type ESBLs were mainly produced by the Klebsiella pneumoniae and Enterobacter 48 spp. responsible for nosocomial infections (1, 2). Since 2000s, CTX-M ESBLs have gained 49 prominence mainly in Escherichia coli and K. pneumoniae strains and are now considered as 50 pandemic enzymes. Many bla CTX-M variants exist, but they can be divided into two main 51 clusters (groups -1 and -9); each cluster of CTX-M genotypes has a corresponding progenitor 52 gene sharing homology with different environmental Kluyvera spp. from which bla CTX-M 53 genes originated. Among CTX-M enzymes, CTX-M-15 belonging to group 1, has currently 54 been the most frequent all over the world (1, 3) 55 The majority of CTX-M type ESBL-associated E. coli infections is often concentrated within 56 specific extraintestinal pathogenic E. coli (ExPEC) lineages belonging to highly virulent 57 phylogenetic group B2, and recognizable by their sequence type (ST). The bla CTX-M-15 is the 58 dominant ESBL gene in the virulent E. coli ST131 clone, in particular in the subclade ST131-59 C2 (4). However other genetically divergent CTX-M genes also occur in this ST, such as 60 bla CTX-M-14/14-like variants in Canada, China, and Spain (5, 6). A subclade of E. coli ST131 61 producing the CTX-M-27 (group 9 enzyme, CTX-M-1...

No abstract