Polymyxin D1

Extended-spectrum β-lactamase (ESBL), plasmid-mediated AmpC (pAmpC) and MCR-1 phosphoethanolamine transferase enzymes have been pointed out as the main plasmid-mediated mechanisms of resistance to third generation cephalosporins (TGC) and colistin, respectively, and are currently considered a major concern both in human and veterinary medicine. Little data on these resistance determinants prevalence in companion animal infections is available. The aim of this study was to determine the resistance profile of Escherichia coli isolated from pet infections, in Argentina, and to characterize the resistance mechanisms to TGC, as well as the presence of the plasmid-borne colistin resistance gene, mcr-1. A total of 54 E. coli isolates were collected from clinical samples in dogs and cats; from them, 20/54 (37%, CI95: [24%; 51%]) displayed resistance to TGC. In this regard, thirteen pAmpC-producing isolates were positive for blaCMY-2 genes, whereas seven ESBL- producers harboured blaCTX-M-2 (n = 4), blaCTX-M-15 (n = 2) and blaCTX-M-14 (n = 1) genes. One E. coli strain (V80), isolated from a canine urinary tract infection, showed resistance to colistin (MIC = 8 μg/ml) and whole-genome sequencing analysis revealed co-occurrence of mcr-1.1, blaCTX-M-2, aadA1, ant(2'')-Ia, catA1 and sul1 genes; the former being carried by a 60,587-bp IncI2 plasmid, previously reported in human colistin-resistant E. coli. E. coli V80 belonged to ST770 and the highly virulent phylogenetic group B2. In general, most of these multidrug-resistant isolates belonged to the phylogenetic group F (11/20) and to a lesser extent B2 (5/20), B1 (2/20), D (1/20) and E (1/20). In summary, CMY- and CTX-M-type β-lactamases may constitute the main TGC resistance mechanism in E. coli isolated from pet infections in Argentina, whereas dissemination of colistin resistance mechanism MCR-1 in the human-animal interface has been mediated by IncI2 plasmids.

The clinical efficacy of polymyxins in severe infection caused by carbapenem resistant organism (CRO) has gradually been recognized, and the course of treatment is generally 2 to 4 weeks. The most common complications after intravenous injection are nephrotoxicity and neurotoxicity, however, there are few reports on the efficacy and safety of the long course use of polymyxins. A patient with carbapenem resistant Acinetobacter baumannii (CRAB) infection after neurosurgery was admitted to the department of neurosurgical intensive care unit (NICU) of Lanzhou University Second Hospital. As the family refused the excision of brain abscess and Ommaya reservoir placement, polymyxin B was given intravenous (3.0 mg×kg-1×d-1) combined with intrathecal (5 mg once daily) injection, and high-dose sulbactam (8 g/d) was intravenously injected for anti-infection therapy. Finally, the brain abscess was absorbed and the patient was successfully cured. The total course of polymyxin B was 69 days with a cumulative dosage of 7 500 mg. There were no complications such as polymyxin-related nephrotoxicity and neurotoxicity during the period, and no symptoms of respiratory inhibition or neuromuscular blockage were observed, but polymyxin-related skin pigmentation appeared about 1 month after intravenous administration of polymyxins B, which subsided after drug withdrawal. It is suggested that long course of polymyxins B is safe and effective for intracranial infection caused by CRAB.

The increasing prevalence of polymyxin-resistant bacteria has stimulated the search for improved polymyxin lipopeptides. Here we describe the sequence and product profile for polymyxin D nonribosomal peptide synthetase from Paenibacillus polymyxa ATCC 10401. The polymyxin D synthase gene cluster comprised five genes that encoded ABC transporters (pmxC and pmxD) and enzymes responsible for the biosynthesis of polymyxin D (pmxA, pmxB, and pmxE). Unlike polymyxins B and E, polymyxin D contains d-Ser at position 3 as opposed to l-α,γ-diaminobutyric acid and has an l-Thr at position 7 rather than l-Leu. Module 3 of pmxE harbored an auxiliary epimerization domain that catalyzes the conversion of l-Ser to the d-form. Structural modeling suggested that the adenylation domains of module 3 in PmxE and modules 6 and 7 in PmxA could bind amino acids with larger side chains than their preferred substrate. Feeding individual amino acids into the culture media not only affected production of polymyxins D1 and D2 but also led to the incorporation of different amino acids at positions 3, 6, and 7 of polymyxin D. Interestingly, the unnatural polymyxin analogues did not show antibiotic activity against a panel of Gram-negative clinical isolates, while the natural polymyxins D1 and D2 exhibited excellent in vitro antibacterial activity and were efficacious against Klebsiella pneumoniae and Acinetobacter baumannii in a mouse blood infection model. The results demonstrate the excellent antibacterial activity of these unusual d-Ser3 polymxyins and underscore the possibility of incorporating alternate amino acids at positions 3, 6, and 7 of polymyxin D via manipulation of the polymyxin nonribosomal biosynthetic machinery.