A novel extended-spectrum β-lactamase, SGM-1, from an environmental isolate of Sphingobium sp

Abstract

SGM-1 is a novel class A β-lactamase from an environmental isolate of Sphingobium sp. containing all of the distinct amino acid motifs of class A β-lactamases. It shares 77 to 80% amino acid sequence identity with putative β-lactamases that are present on the chromosome of all Sphingobium species whose genomes were sequenced and annotated. Thus, SGM-type β-lactamases are native to this genus. Antibiotic susceptibility testing classifies SGM-1 as an extended-spectrum β-lactamase, conferring the highest level of resistance to penicillins. Although SGM-1 contains the conserved cysteine residues characteristic of class A carbapenemases, it does not confer resistance to the carbapenem antibiotics imipenem, meropenem, or doripenem but does increase the MIC of ertapenem 8-fold. SGM-1 hydrolyzes penicillins and the monobactam aztreonam with similar catalytic efficiencies, ranging from 10(5) to 10(6) M(-1) s(-1). The catalytic efficiencies of SGM-1 for cefoxitin and ceftazidime were the lowest (10(2) to 10(3) M(-1) s(-1)) among the cephalosporins tested, while the catalytic efficiencies against all other cephalosporins varied from about 10(5) to 10(6) M(-1) s(-1). SGM-1 exhibited measurable but not significant activity toward the carbapenems tested. SGM-1 also showed high affinity for clavulanic acid, tazobactam, and sulbactam (Ki < 1 μM); however, only clavulanic acid significantly reduced the MICs of β-lactams.

Fig 1

Amino acid sequence alignment of SGM-1 (mature enzyme) and six putative SGM β-lactamases from Sphingobium using the alignment tool CLUSTAL W (GenBank accession numbers are given in parentheses): SGM-2 from S. japonicum UT26S (YP_003545618), SGM-3 from S. indicum B90A (ZP_10221377), SGM-4 from S. chlorophenolicum L-1 (YP_004552835), SGM-5 from Sphingobium sp. strain AP49 (ZP_10541948), SGM-6 from S. yanoikuyae ATCC 51230 (ZP_18898211), and SGM-7 from S. yanoikuyae XLDN2-5 (ZP_09907955). The conserved cysteine residues (C⁶⁹ and C²³⁸ are underlined. Highlighted in gray and in boldface are conserved residues (E¹⁶⁶ and R²²⁰) and motifs (S⁷⁰XXK⁷³, S¹³⁰DN, and K²³⁴TG).