The armA ( a minoglycoside r esistance m ethylase) gene, which confers resistance to 4,6-disubstituted deoxystreptamines and fortimicin, was initially found in Klebsiella pneumoniae BM4536 on IncL/M plasmid pIP1204 of ca. 90 kb which also encodes the extended-spectrum β-lactamase CTX-M-3. Thirty-four enterobacteria from various countries that were likely to produce a CTX-M enzyme since they were more resistant to cefotaxime than to ceftazidime were studied. The armA gene was detected in 12 clinical isolates of Citrobacter freundii , Enterobacter cloacae , Escherichia coli , K. pneumoniae , Salmonella enterica , and Shigella flexneri , in which it was always associated with bla _CTX-M-3 on an IncL/M plasmid. Conjugation, analysis of DNA sequences, PCR mapping, and plasmid conduction experiments indicated that the armA gene was part of composite transposon Tn 1548 together with genes ant3"9 , sul1 , and dfrXII , which are responsible for resistance to streptomycin-spectinomycin, sulfonamides, and trimethoprim, respectively. The 16.6-kb genetic element was flanked by two copies of IS 6 and migrated by replicative transposition. This observation accounts for the presence of armA on self-transferable plasmids of various incompatibility groups and its worldwide dissemination. It thus appears that posttranscriptional modification of 16S rRNA confers high-level resistance to all the clinically available aminoglycosides except streptomycin in gram-negative human and animal pathogens.

Among five American Type Culture Collection (ATCC) Citrobacter strains, qnrB60 in Citrobacter freundii ATCC 6879, an isolate from the preantibiotic era, and qnrB61 in Citrobacter braakii ATCC 51113^T, a type strain belonging to the C. freundii complex, were identified. Meanwhile, a truncated qnrB-like pseudogene was identified in C. freundii ATCC 8090^T and ATCC 43864. No qnrB-like sequence was found in Citrobacter koseri ATCC 27028^T. These findings underscore the close relationship between this species and qnrB.

The ant(4′)-IIb gene of Pseudomonas aeruginosa BM4492, which encodes an aminoglycoside 4′- O -adenylyltransferase, was identified as a coding sequence of 756 bp corresponding to a protein with a calculated mass of 27,219 Da. Analysis of the deduced sequence indicated that the protein was related to aminoglycoside 4′- O -adenylyltransferases IIa and Ia found in P. aeruginosa and gram-positive bacteria, respectively. The enzyme conferred resistance to amikacin and tobramycin but not to dibekacin, gentamicin, or netilmicin. The ant(4′)-IIb gene had a chromosomal location in five of six clinical isolates of P. aeruginosa tested and was plasmid borne in the remaining strain. The ant(4′)-IIb gene was detected by PCR in some clinical strains of P. aeruginosa from the same hospital but not in members of other bacterial genera.

Carbapenemase-producing Enterobacter spp. Serratia marcescens, Citrobacter freundii, Providencia spp., and Morganella morganii (CP-ESCPM) are increasingly identified as causative agents of nosocomial infections but are still not under systematic genomic surveillance. In this study, using a combination of whole-genome sequencing and conjugation experiments, we sought to elucidate the genomic characteristics and transferability of resistance genes in clinical CP-ESCPM isolates from Bulgaria. Among the 36 sequenced isolates, NDM-1 (12/36), VIM-4 (11/36), VIM-86 (8/36), and OXA-48 (7/36) carbapenemases were identified; two isolates carried both NDM-1 and VIM-86. The majority of carbapenemase genes were found on self-conjugative plasmids. IncL plasmids were responsible for the spread of OXA-48 among E. hormaechei, C. freundii, and S. marcescens. IncM2 plasmids were generally associated with the spread of NDM-1 in C. freundii and S. marcescens, and also of VIM-4 in C. freundii. IncC plasmids were involved in the spread of the recently described VIM-86 in P. stuartii isolates. IncC plasmids carrying bla_NDM-1 and bla_VIM-86 were observed too. bla_NDM-1 was also detected on IncX3 in S. marcescens and on IncT plasmid in M. morganii. The significant resistance transfer rates we observed highlight the role of the ESCPM group as a reservoir of resistance determinants and stress the need for strengthening infection control measures.

Background: Acquired 16S rRNA methyltransferases (16S-RMTases) confer high-level resistance to aminoglycosides and are often associated with β-lactam and quinolone resistance determinants. Methods: Using PCR, whole-genome sequencing and conjugation experiments, we conducted a retrospective genomic surveillance study of 16S-RMTase-producing Enterobacterales, collected between 2006 and 2023, to explore transmission dynamics of methyltransferase and associated antibiotic resistance genes. Results: Among the 10,731 consecutive isolates, 150 (1.4%) from 13 species carried armA (92.7%), rmtB (4.7%), and rmtF + rmtB (2.7%) methyltransferase genes. The coexistence of extended-spectrum β-lactamase (bla_CTX-M-3/15, bla_SHV-12, bla_SFO-1), carbapenemase (bla_NDM-1/5, bla_VIM-1/4/86, bla_OXA-48), acquired AmpC (bla_CMY-2/4/99, bla_DHA-1, bla_AAC-1), and plasmid-mediated quinolone resistance (qnrB, qnrS, aac(6′)-Ib-cr) genes within these isolates was also detected. Methyltransferase genes were carried by different plasmids (IncL/M, IncA/C, IncR, IncFIB, and IncFII), suggesting diverse origins and sources of acquisition. armA was co-transferred with bla_CTX-M-3/15, bla_NDM-1, bla_VIM-4/86, bla_OXA-48, bla_CMY-4, aac(6′)-Ib-cr, qnrB, and qnrS, while rmtF1 was co-transferred with bla_SFO-1, highlighting the multidrug-resistant nature of these plasmids. Long-read sequencing of ST6260 K. pneumoniae isolates revealed a novel resistance association, with rmtB1 and bla_NDM-5 on the chromosome, bla_OXA-232 on a conjugative ColKP3 plasmid, and rmtF1 with bla_SFO-1 on self-transmissible IncFIB and IncFII plasmids. Conclusions: The genetic plasticity of plasmids carrying methyltransferase genes suggests their potential to acquire additional resistance genes, turning 16S-RMTase-producing Enterobacterales into a persistent public health threat.