Role of aminoglycoside 6'-acetyltransferase in a novel multiple aminoglycoside resistance of an actinomycete strain #8: inactivation of aminoglycosides with 6'-amino group except arbekacin and neomycin

Basic and applied research on multiple aminoglycoside antibiotic resistance of actinomycetes: an old-timer's recollection

A list of our research achievements on multiple aminoglycoside antibiotic (AG) resistance in AG-producing actinomycetes is outlined. In 1979, the author discovered a novel AG (istamycin)-producing Streptomyces tenjimariensis SS-939 by screening actinomycetes with kanamycin (KM)-resistance and plasmid profiles. This discovery directed our biochemical and genetic approaches to multiple AG resistance (AGR) of AG producers. In this article, the following discoveries will be outlined: (1) AGR profiles correlating with the productivity of AGs in AG-producers, (2) Wide distribution of multiple AG resistance in AG-nonproducing actinomycetes, (3) Involvement of ribosomal resistance and AG-acetylating enzymes as underlying AGR factors, (4) Activation by single nucleotide substitution of a silent gene coding for aminoglycoside 3-N-acetyltransferase, AAC(3), in S. griseus, (5) Discovery of a novel antibiotic indolizomycin through protoplast fusion treatment between S. tenjimariensis and S. griseus strains with different AGR phenotypes, and (6) Double stage-acting activity of arbekacin (ABK; an anti-MRSA semisynthetic AG) discovered by acetylation of ABK with cloned AACs; that is both ABK and its acetylated derivatives showed remarkable antibiotic activities.

Kanamycin and its derivative, arbekacin: significance and impact

On the occasion of the 60th anniversary of the discovery (1957) of kanamycin (KM), a series of research achievements on KM and its semisynthetic derivative Arbekacin (ABK) are outlined. KM was first used clinically in 1958 and was appreciated for its remarkable curing effect on various bacterial infections, especially tuberculosis. ABK is a KM derivative rationally semisynthesized to overcome KM resistance due to enzymatic phosphorylation and acetylation. Since its approval in 1990 as an anti-MRSA drug, ABK has been and still is effectively used in chemotherapy because MRSA rarely develops high ABK-resistance. Research that illuminated the unique features of ABK enabling it to resist the development of resistance by MRSA are also described.

Aminoglycoside resistance profile and structural architecture of the aminoglycoside acetyltransferase AAC(6')-Im

Aminoglycoside 6'-acetyltransferase-Im (AAC(6')-Im) is the closest monofunctional homolog of the AAC(6')-Ie acetyltransferase of the bifunctional enzyme AAC(6')-Ie/APH(2")-Ia. The AAC(6')-Im acetyltransferase confers 4- to 64-fold higher MICs to 4,6-disubstituted aminoglycosides and the 4,5-disubstituted aminoglycoside neomycin than AAC(6')-Ie, yet unlike AAC(6')-Ie, the AAC(6')-Im enzyme does not confer resistance to the atypical aminoglycoside fortimicin. The structure of the kanamycin A complex of AAC(6')-Im shows that the substrate binds in a shallow positively-charged pocket, with the N6' amino group positioned appropriately for an efficient nucleophilic attack on an acetyl-CoA cofactor. The AAC(6')-Ie enzyme binds kanamycin A in a sufficiently different manner to position the N6' group less efficiently, thereby reducing the activity of this enzyme towards the 4,6-disubstituted aminoglycosides. Conversely, docking studies with fortimicin in both acetyltransferases suggest that the atypical aminoglycoside might bind less productively in AAC(6')-Im, thus explaining the lack of resistance to this molecule.

N6', N6''', and O4' Modifications to Neomycin Affect Ribosomal Selectivity without Compromising Antibacterial Activity

The synthesis of a series of neomycin derivatives carrying the 2-hydroxyethyl substituent on N6' and/or N6‴ both alone and in combination with a 4'-O-ethyl group is described. By means of cell-free translation assays with wild-type bacterial ribosomes and their hybrids with eukaryotic decoding A sites, we investigate how individual substituents and their combinations affect activity and selectivity at the target level. In principle, and as shown by cell-free translation assays, modifications of the N6' and N6‴ positions allow enhancement of target selectivity without compromising antibacterial activity. As with the 6'OH aminoglycoside paromomycin, the 4'-O-ethyl modification affects the ribosomal activity, selectivity, and antibacterial profile of neomycin and its 6'-N-(2-hydroxyethyl) derivatives. The modified aminoglycosides show good antibacterial activity against model Gram-positive and Gram-negative microbes including the ESKAPE pathogens Staphylococcus aureus, Klebsiella pneumoniae, Enterobacter cloacae, and Acinetobacter baumannii.

Expanding Aminoglycoside Resistance Enzyme Regiospecificity by Mutation and Truncation

Aminoglycosides (AGs) are broad-spectrum antibiotics famous for their antibacterial activity against Gram-positive and Gram-negative bacteria, as well as mycobacteria. In the United States, the most prescribed AGs, including amikacin (AMK), gentamicin (GEN), and tobramycin (TOB), are vital components of the treatment for resistant bacterial infections. Arbekacin (ABK), a semisynthetic AG, is widely used for the treatment of resistant Pseudomonas aeruginosa and methicillin-resistant Staphylococcus aureus in Asia. However, the rapid emergence and development of bacterial resistance are limiting the clinical application of AG antibiotics. Of all bacterial resistance mechanisms against AGs, the acquisition of AG-modifying enzymes (AMEs) by bacteria is the most common. It was previously reported that a variant of a bifunctional AME, the 6'-N-AG acetyltransferase-Ie/2″-O-AG phosphotransferase-Ia [AAC(6')-Ie/APH(2″)-Ia], containing a D80G point mutation and a truncation after amino acid 240 modified ABK and AMK at a new position, the 4‴-amine, therefore displaying a change in regiospecificity. In this study, we aimed to verify the altered regiospecificity of this bifunctional enzyme by mutation and truncation for the potential of derivatizing AGs with chemoenzymatic reactions. With the three variant enzymes in this study that contained either mutation only (D80G), truncation only (1-240), or mutation and truncation (D80G-1-240), we characterized their activity by profiling their substrate promiscuity, determined their kinetics parameters, and performed mass spectrometry to determine how and where ABK and AMK were acetylated by these enzymes. We found that the three mutant enzymes possessed distinct acetylation regiospecificity compared to that of the bifunctional AAC(6')-Ie/APH(2″)-Ia enzyme and the functional AAC(6')-Ie domain [AAC(6')/APH(2″)-1-194].

New plasmid-mediated aminoglycoside 6'-N-acetyltransferase, AAC(6')-Ian, and ESBL, TLA-3, from a Serratia marcescens clinical isolate

OBJECTIVES

Enterobacteriaceae clinical isolates showing amikacin resistance (MIC 64 to >256 mg/L) in the absence of 16S rRNA methyltransferase (MTase) genes were found. The aim of this study was to clarify the molecular mechanisms underlying amikacin resistance in Enterobacteriaceae clinical isolates that do not produce 16S rRNA MTases.

METHODS

PCR was performed to detect already-known amikacin resistance determinants. Cloning experiments and sequence analyses were performed to characterize unknown amikacin resistance determinants. Transfer of amikacin resistance determinants was performed by conjugation and transformation. The complete nucleotide sequence of the plasmids was determined by next-generation sequencing technology. Amikacin resistance enzymes were purified with a column chromatography system. The enzymatic function of the purified protein was investigated by thin-layer chromatography (TLC) and HPLC.

RESULTS

Among the 14 isolates, 9 were found to carry already-known amikacin resistance determinants such as aac(6')-Ia and aac(6')-Ib. Genetic analyses revealed the presence of a new amikacin acetyltransferase gene, named aac(6')-Ian, located on a 169 829 bp transferable plasmid (p11663) of the Serratia marcescens strain NUBL-11663, one of the five strains negative for known aac(6') genes by PCR. Plasmid p11663 also carried a novel ESBL gene, named blaTLA-3. HPLC and TLC analyses demonstrated that AAC(6')-Ian catalysed the transfer of an acetyl group from acetyl coenzyme A onto an amine at the 6'-position of various aminoglycosides.

CONCLUSIONS

We identified aac(6')-Ian as a novel amikacin resistance determinant together with a new ESBL gene, blaTLA-3, on a transferable plasmid of a S. marcescens clinical isolate.

Identification and characterization of a novel aac(6')-Iag associated with the blaIMP-1-integron in a multidrug-resistant Pseudomonas aeruginosa

In a continuing study from Dec 2006 to Apr 2008, we characterized nine multi-drug resistant Pseudomonas aeruginosa strains isolated from four patients in a ward at the Hiroshima University Hospital, Japan. Pulsed-field gel electrophoresis of SpeI-digested genomic DNAs from the isolates suggested the clonal expansion of a single strain; however, only one strain, NK0009, was found to produce metallo-β-lactamase. PCR and subsequent sequencing analysis indicated NK0009 possessed a novel class 1 integron, designated as In124, that carries an array of four gene cassettes: a novel aminoglycoside (AG) resistance gene, aac(6')-Iag, blaIMP-1, a truncated form of blaIMP-1, and a truncated form of aac(6')-Iag. The aac(6')-Iag encoded a 167-amino-acid protein that shows 40% identity with AAC(6')-Iz. Recombinant AAC(6')-Iag protein showed aminoglycoside 6'-N-acetyltransferase activity using thin-layer chromatography (TLC) and MS spectrometric analysis. Escherichia coli carrying aac(6')-Iag showed resistance to amikacin, arbekacin, dibekacin, isepamicin, kanamycin, sisomicin, and tobramycin; but not to gentamicin. A conjugation experiment and subsequent Southern hybridization with the gene probes for blaIMP-1 and aac(6')-Ig strongly suggested In124 is on a conjugal plasmid. Transconjugants acquired resistance to gentamicin and were resistant to virtually all AGs, suggesting that the In124 conjugal plasmid also possesses a gene conferring resistance to gentamicin.

AAC(6')-Iaf, a novel aminoglycoside 6'-N-acetyltransferase from multidrug-resistant Pseudomonas aeruginosa clinical isolates

We report here the characterization of a novel aminoglycoside resistance gene, aac(6')-Iaf, present in two multidrug-resistant (MDR) Pseudomonas aeruginosa clinical isolates. These isolates, IMCJ798 and IMCJ799, were independently obtained from two patients, one with a urinary tract infection and the other with a decubitus ulcer, in a hospital located in the western part of Japan. Although the antibiotic resistance profiles of IMCJ798 and IMCJ799 were similar to that of MDR P. aeruginosa IMCJ2.S1, which caused outbreaks in the eastern part of Japan, the pulsed-field gel electrophoresis patterns for these isolates were different from that for IMCJ2.S1. Both IMCJ798 and IMCJ799 were found to contain a novel chromosomal class 1 integron, In123, which included aac(6')-Iaf as the first cassette gene. The encoded protein, AAC(6')-Iaf, was found to consist of 183 amino acids, with 91 and 87% identity to AAC(6')-Iq and AAC(6')-Im, respectively. IMCJ798, IMCJ799, and Escherichia coli transformants carrying a plasmid containing the aac(6')-Iaf gene and its upstream region were highly resistant to amikacin, dibekacin, and kanamycin but not to gentamicin. The production of AAC(6')-Iaf in these strains was confirmed by Western blot analysis. Thin-layer chromatography indicated that AAC(6')-Iaf is a functional acetyltransferase that specifically modifies the amino groups at the 6' positions of aminoglycosides. Collectively, these findings indicate that AAC(6')-Iaf contributes to aminoglycoside resistance.

Multidrug-resistant Pseudomonas aeruginosa strain that caused an outbreak in a neurosurgery ward and its aac(6')-Iae gene cassette encoding a novel aminoglycoside acetyltransferase

We characterized multidrug-resistant Pseudomonas aeruginosa strains isolated from patients involved in an outbreak of catheter-associated urinary tract infections that occurred in a neurosurgery ward of a hospital in Sendai, Japan. Pulsed-field gel electrophoresis of SpeI-, XbaI-, or HpaI-digested genomic DNAs from the isolates revealed that clonal expansion of a P. aeruginosa strain designated IMCJ2.S1 had occurred in the ward. This strain possessed broad-spectrum resistance to aminoglycosides, beta-lactams, fluoroquinolones, tetracyclines, sulfonamides, and chlorhexidine. Strain IMCJ2.S1 showed a level of resistance to some kinds of disinfectants similar to that of a control strain of P. aeruginosa, ATCC 27853. IMCJ2.S1 contained a novel class 1 integron, In113, in the chromosome but not on a plasmid. In113 contains an array of three gene cassettes of bla(IMP-1), a novel aminoglycoside resistance gene, and the aadA1 gene. The aminoglycoside resistance gene, designated aac(6')-Iae, encoded a 183-amino-acid protein that shared 57.1% identity with AAC(6')-Iq. Recombinant AAC(6')-Iae protein showed aminoglycoside 6'-N-acetyltransferase activity by thin-layer chromatography. Escherichia coli expressing exogenous aac(6')-Iae showed resistance to amikacin, dibekacin, isepamicin, kanamycin, netilmicin, sisomicin, and tobramycin but not to arbekacin, gentamicins, or streptomycin. Alterations of gyrA and parC at the amino acid sequence level were detected in IMCJ2.S1, suggesting that such mutations confer the resistance to fluoroquinolones observed for this strain. These results indicate that P. aeruginosa IMCJ2.S1 has developed multidrug resistance by acquiring resistance determinants, including a novel member of the aac(6')-I family and mutations in drug resistance genes.

The clavulanic acid biosynthetic cluster of Streptomyces clavuligerus: genetic organization of the region upstream of the car gene

The genetic organization of the region upstream of the car gene of the clavulanic acid biosynthetic gene cluster of Streptomyces clavuligerus has been determined. Sequence analysis of a 12.1 kb region revealed the presence of 10 ORFs whose putative functions, according to database searches, are discussed. Three co-transcriptional units are proposed: ORF10-11, ORF12-13 and ORF15-16-17-18. Potential transcriptional terminators were identified downstream of ORF11 (fd) and ORF15. Targeted disruption of ORF10 (cyp) gave rise to transformants unable to produce clavulanic acid, but with a considerably higher production of cephamycin C. Transformants inactivated at ORF14 had a remarkably lower production of clavulanic acid and similar production of cephamycin C. Significant improvements of clavulanic acid production, associated with a drop in cephamycin C biosynthesis, were obtained with transformants of S. clavuligerus harbouring multiple copies of plasmids carrying different constructions from the ORF10-14 region. This information can be used to guide strain improvement programs, blending random mutagenesis and molecular cloning, to optimize the yield of clavulanic acid.