High-level amikacin resistance in Escherichia coli due to phosphorylation and impaired aminoglycoside uptake

Aminoglycoside-resistance gene signatures are predictive of aminoglycoside MICs for carbapenem-resistant Klebsiella pneumoniae

BACKGROUND

Aminoglycoside-containing regimens may be an effective treatment option for infections caused by carbapenem-resistant Klebsiella pneumoniae (CR-Kp), but aminoglycoside-resistance genes are common in these strains. The relationship between the aminoglycoside-resistance genes and aminoglycoside MICs remains poorly defined.

OBJECTIVES

To identify genotypic signatures capable of predicting aminoglycoside MICs for CR-Kp.

METHODS

Clinical CR-Kp isolates (n = 158) underwent WGS to detect aminoglycoside-resistance genes. MICs of amikacin, gentamicin, plazomicin and tobramycin were determined by broth microdilution (BMD). Principal component analysis was used to initially separate isolates based on genotype. Multiple linear regression was then used to generate models that predict aminoglycoside MICs based on the aminoglycoside-resistance genes. Last, the performance of the predictive models was tested against a validation cohort of 29 CR-Kp isolates.

RESULTS

Among the original 158 CR-Kp isolates, 91.77% (145/158) had at least one clinically relevant aminoglycoside-resistance gene. As a group, 99.37%, 84.81%, 82.28% and 10.76% of the CR-Kp isolates were susceptible to plazomicin, amikacin, gentamicin and tobramycin, respectively. The first two principal components explained 72.23% of the total variance in aminoglycoside MICs and separated isolates into four groups with aac(6')-Ib, aac(6')-Ib', aac(6')-Ib+aac(6')-Ib' or no clinically relevant aminoglycoside-resistance genes. Regression models predicted aminoglycoside MICs with adjusted R2 values of 56%-99%. Within the validation cohort, the categorical agreement when comparing the observed BMD MICs with the predicated MICs was 96.55%, 89.66%, 86.21% and 82.76% for plazomicin, gentamicin, amikacin and tobramycin, respectively.

CONCLUSIONS

Susceptibility to each aminoglycoside varies in CR-Kp. Detection of aminoglycoside-resistance genes may be useful to predict aminoglycoside MICs for CR-Kp.

Amikacin: Uses, Resistance, and Prospects for Inhibition

Aminoglycosides are a group of antibiotics used since the 1940s to primarily treat a broad spectrum of bacterial infections. The primary resistance mechanism against these antibiotics is enzymatic modification by aminoglycoside-modifying enzymes that are divided into acetyl-transferases, phosphotransferases, and nucleotidyltransferases. To overcome this problem, new semisynthetic aminoglycosides were developed in the 70s. The most widely used semisynthetic aminoglycoside is amikacin, which is refractory to most aminoglycoside modifying enzymes. Amikacin was synthesized by acylation with the l-(-)-γ-amino-α-hydroxybutyryl side chain at the C-1 amino group of the deoxystreptamine moiety of kanamycin A. The main amikacin resistance mechanism found in the clinics is acetylation by the aminoglycoside 6'-N-acetyltransferase type Ib [AAC(6')-Ib], an enzyme coded for by a gene found in integrons, transposons, plasmids, and chromosomes of Gram-negative bacteria. Numerous efforts are focused on finding strategies to neutralize the action of AAC(6')-Ib and extend the useful life of amikacin. Small molecules as well as complexes ionophore-Zn⁺² or Cu⁺² were found to inhibit the acetylation reaction and induced phenotypic conversion to susceptibility in bacteria harboring the aac(6')-Ib gene. A new semisynthetic aminoglycoside, plazomicin, is in advance stage of development and will contribute to renewed interest in this kind of antibiotics.

Involvement of aph(3')-IIa in the formation of mosaic aminoglycoside resistance genes in natural environments

Intragenic recombination leading to mosaic gene formation is known to alter resistance profiles for particular genes and bacterial species. Few studies have examined to what extent aminoglycoside resistance genes undergo intragenic recombination. We screened the GenBank database for mosaic gene formation in homologs of the aph(3')-IIa (nptII) gene. APH(3')-IIa inactivates important aminoglycoside antibiotics. The gene is widely used as a selectable marker in biotechnology and enters the environment via laboratory discharges and the release of transgenic organisms. Such releases may provide opportunities for recombination in competent environmental bacteria. The retrieved GenBank sequences were grouped in three datasets comprising river water samples, duck pathogens and full-length variants from various bacterial genomes and plasmids. Analysis for recombination in these datasets was performed with the Recombination Detection Program (RDP4), and the Genetic Algorithm for Recombination Detection (GARD). From a total of 89 homologous sequences, 83% showed 99-100% sequence identity with aph(3')-IIa originally described as part of transposon Tn5. Fifty one were unique sequence variants eligible for recombination analysis. Only a single recombination event was identified with high confidence and indicated the involvement of aph(3')-IIa in the formation of a mosaic gene located on a plasmid of environmental origin in the multi-resistant isolate Pseudomonas aeruginosa PA96. The available data suggest that aph(3')-IIa is not an archetypical mosaic gene as the divergence between the described sequence variants and the number of detectable recombination events is low. This is in contrast to the numerous mosaic alleles reported for certain penicillin or tetracycline resistance determinants.

Prevalence of the aminoglycoside phosphotransferase genes aph(3')-IIIa and aph(3')-IIa in Escherichia coli, Enterococcus faecalis, Enterococcus faecium, Pseudomonas aeruginosa, Salmonella enterica subsp. enterica and Staphylococcus aureus isolates in Austria

The aminoglycoside phosphotransferase aph(3')-IIa primarily inactivates kanamycin and neomycin, whilst aph(3')-IIIa also inactivates amikacin. The aim of this study was to determine the frequency of both resistance genes in major human pathogens to obtain their baseline prevalence in the gene pool of these bacterial populations in Austria. In total, 10 541 Escherichia coli, Enterococcus faecalis, Enterococcus faecium, Pseudomonas aeruginosa, Salmonella enterica subsp. enterica and Staphylococcus aureus isolates were collected representatively without selection bias between 2008 and 2011. Isolates were analysed by aph(3')-IIIa/nptIII- and aph(3')-IIa/nptII-specific TaqMan real-time PCR. For positive strains, MICs using Etests were performed and resistance gene sequences were determined. The overall prevalence of aph(3')-IIIa/nptIII was 1.62 % (95 % confidence interval: 1.38-1.88 %). In Escherichia coli, enterococci, Staphylococcus aureus, P. aeruginosa and Salmonella spp., the aph(3')-IIIa/nptIII prevalence was 0.47 % (0-1.47 %), 37.53 % (32.84-42.40 %), 2.90 % (1.51-5.02 %), 0 % (0-0.32 %) and 0 % (0-0.037 %), respectively. Eleven of a total of 169 carriers showed single-nucleotide polymorphisms in the resistance allele. The overall prevalence of aph(3')-IIa/nptII was 0.0096 % (0-0.046 %). Escherichia coli (0-0.70 %), enterococci (0-0.75 %), Staphylococcus aureus (0-0.73 %) and P. aeruginosa (0-0.32 %) did not carry aph(3')-IIa. A single Salmonella isolate was positive, resulting in an aph(3')-IIa prevalence of 0.013 % (0-0.058 %). aph(3')-IIIa/nptIII carriers were moderately prevalent in the strains tested except for in enterococci, which appeared to be an important reservoir for aph(3')-IIIa. aph(3')-IIa/nptII genes were detected at clinically irrelevant frequencies and played no significant role in the aminoglycoside resistance gene pool during the observation period.

Evolution of antibiotic resistance at non-lethal drug concentrations

Human use of antimicrobials in the clinic, community and agricultural systems has driven selection for resistance in bacteria. Resistance can be selected at antibiotic concentrations that are either lethal or non-lethal, and here we argue that selection and enrichment for antibiotic resistant bacteria is often a consequence of weak, non-lethal selective pressures - caused by low levels of antibiotics - that operates on small differences in relative bacterial fitness. Such conditions may occur during antibiotic therapy or in anthropogenically drug-polluted natural environments. Non-lethal selection increases rates of mutant appearance and promotes enrichment of highly fit mutants and stable mutators.

Antibiotic resistance markers in genetically modified plants: a risk to human health?

Cotransformation with an antibiotic-resistance marker is often necessary in the process of creating a genetically modified (GM) plant. Concern has been expressed that the release of these markers in GM plants may result in an increase in the rate of antibiotic resistance in human pathogens. For such an event to occur, DNA must not be totally degraded in field conditions, and the antibiotic-resistance marker must encounter potential recipient bacteria and be taken up by them, before being integrated into the bacterial genome, and the genes then expressed. In addition, the new recombinant must overcome the physiological disadvantage of acquisition of a piece of foreign DNA, probably in conditions where the new gene does not provide a selective advantage. We review each of these stages, summarising the investigations that have followed each of these steps. We contrast the potential increase in the antibiotic resistance reservoir created by antibiotic-resistance markers in GM plants with the current situation created by medical antibiotic prescribing. We conclude that, although fragments of DNA large enough to contain an antibiotic-resistance gene may survive in the environment, the barriers to transfer, incorporation, and transmission are so substantial that any contribution to antibiotic resistance made by GM plants must be overwhelmed by the contribution made by antibiotic prescription in clinical practice.

Contribution of the MexXY multidrug transporter to aminoglycoside resistance in Pseudomonas aeruginosa clinical isolates

MexXY is an aminoglycoside-inducible multidrug transporter shown to contribute to intrinsic and acquired aminoglycoside resistance in laboratory isolates of Pseudomonas aeruginosa. To assess its contribution to aminoglycoside resistance in 14 clinical isolates demonstrating a panaminoglycoside resistance phenotype unlikely to be explained solely by aminoglycoside modification, expression of mexXY by these isolates was examined by reverse transcription-PCR. Elevated levels of mexXY expression were evident for most strains compared with those detected for an aminoglycoside-susceptible control strain, although there was no correlation between mexXY levels and the aminoglycoside MICs for the resistant strains, indicating that if MexXY was playing a role, other factors were also contributing. Deletion of mexXY from 9 of the 14 isolates resulted in enhanced susceptibilities to multiple aminoglycosides, confirming the contribution of this efflux system to the aminoglycoside resistance of these clinical isolates. Still, the impact of MexXY loss varied, with some strains clearly more or less dependent on MexXY for aminoglycoside resistance. Expression of mexXY also varied in these strains, with some showing high-level expression of the efflux genes independent of aminoglycoside exposure (aminoglycoside-independent hyperexpression) and others showing hyperexpression of the efflux genes that was to a greater or lesser degree aminoglycoside dependent. None of these strains carried mutations in mexZ, which encodes a negative regulator of mexXY expression, or in the mexZ-mexXY intergenic region. Thus, mexXY hyperexpression in aminoglycoside-resistant clinical isolates occurs via mutation in one or more as yet unidentified genes.

The crystal structure of aminoglycoside-3'-phosphotransferase-IIa, an enzyme responsible for antibiotic resistance

A major factor in the emergence of antibiotic resistance is the existence of enzymes that chemically modify common antibiotics. The genes for these enzymes are commonly carried on mobile genetic elements, facilitating their spread. One such class of enzymes is the aminoglycoside phosphotransferase (APH) family, which uses ATP-mediated phosphate transfer to chemically modify and inactivate aminoglycoside antibiotics such as streptomycin and kanamycin. As part of a program to define the molecular basis for aminoglycoside recognition and inactivation by such enzymes, we have determined the high resolution (2.1A) crystal structure of aminoglycoside-3'-phosphotransferase-IIa (APH(3')-IIa) in complex with kanamycin. The structure was solved by molecular replacement using multiple models derived from the related aminoglycoside-3'-phosphotransferase-III enzyme (APH(3')-III), and refined to an R factor of 0.206 (R(free) 0.238). The bound kanamycin molecule is very well defined and occupies a highly negatively charged cleft formed by the C-terminal domain of the enzyme. Adjacent to this is the binding site for ATP, which can be modeled on the basis of nucleotide complexes of APH(3')-III; only one change is apparent with a loop, residues 28-34, in a position where it could fold over an incoming nucleotide. The three rings of the kanamycin occupy distinct sub-pockets in which a highly acidic loop, residues 151-166, and the C-terminal residues 260-264 play important parts in recognition. The A ring, the site of phosphoryl transfer, is adjacent to the catalytic base Asp190. These results give new information on the basis of aminoglycoside recognition, and on the relationship between this phosphotransferase family and the protein kinases.

Low-level antibacterial resistance: a gateway to clinical resistance

The huge amount of antibiotic substances released in the human environment has probably resulted in an acceleration in the rate of bacterial evolution. It is to note that most interactions between chemotherapeutic agents and microbial populations occur at very low antibiotic concentrations. Thus, natural selection is expected to act on very small increases in the bacterial ability to resist to antibiotic inhibitory effects. On the other hand, there is a wealth of mechanisms to resist to these low antibiotic concentrations. The progressive enrichment in low-level resistant populations favours secondary selections for more specific and effective mechanisms of resistance, particularly in treated patients. These adaptations may have a biological cost in the absence of antibiotics, but frequently compensatory mutations occur, minimizing such genetic burden. In this way, a phenomenon of directional selection takes place, with low possibilities of return to susceptibility. Moreover, low antibiotic concentrations are not only able to select low-level antibiotic resistant variants, but may produce a substantial stress in bacterial populations, that eventually influences the rate of genetic variation and the diversity of adaptive responses. More attention should be devoted to the mechanisms of low-level resistance in microorganisms, as they can serve as stepping stones to develop high level, clinically relevant resistance. These mechanisms should be identified early in the development of drugs in order to adapt the therapeutic strategies (for instance dosage) to minimize the selection of low-level resistant variants, as frequently they emerge by means of concentration-specific selection. At the same time, conventional susceptibility testing should probably be able to detect low-level resistance, and not only clinically-relevant resistance. We should be vigilant of the evolutionary trends of microorganisms; for that a purpose, knowledge of the biology and epidemiology of low-level resistance is becoming a real need.

In vitro characterization of aminoglycoside adaptive resistance in Pseudomonas aeruginosa

Aminoglycoside adaptive resistance was characterized in one reference strain and four clinical isolates of Pseudomonas aeruginosa. Adaptive resistance was initiated with a 2-h gentamicin or tobramycin exposure at the MIC. Each P. aeruginosa strain demonstrated an adaptive-resistance period of between 8 and 12 h when tested with both aminoglycosides. Aminoglycoside adaptive resistance was shown to correlate with a decrease in [3H] gentamicin accumulation and a small (5%) but significant (P < 0.05) reduction in proton motive force. The mean generation time of P. aeruginosa during peak levels of adaptive resistance (i.e., maximum reductions in aminoglycoside killing) was not significantly different from that of control organisms (P < 0.05). No changes in outer membrane protein or lipopolysaccharide sodium dodecyl sulfate-polyacrylamide gel electrophoresis profiles were noted when control, adaptively resistant, and postadaptively resistant cells were compared. Cytoplasmic membrane profiles of adaptively resistant cells, however, demonstrated several band changes when compared with control and postadaptively resistant cells. We conclude that the decrease in aminoglycoside accumulation associated with adaptive resistance in P. aeruginosa may be, in part, a function of reductions in proton motive force and/or cytoplasmic membrane protein changes. However, the importance of these changes requires further investigation.

Non-canonical mechanisms of antibiotic resistance

Although the current in vitro methods used for detection and analysis of the phenotypes of antibiotic resistance in the laboratory are well established, other resistance mechanisms of resistance exist which may escape detection using the standard approach. The present article reviews some of these mechanisms which are grouped under the term 'non-canonical mechanisms' of antibiotic resistance. Such mechanisms include gene dosage, heterologous induction or selection, populational resistance and synergism between mechanisms of low resistance. The role of these mechanisms in the failure of therapy is discussed.

Purification and characterization of aminoglycoside 3'-phosphotransferase type IIa and kinetic comparison with a new mutant enzyme

Aminoglycoside 3'-phosphotransferase [APH(3')s] provide an important means for high-level resistance to neomycin- and kanamycin-type aminoglycoside antibiotics. A four-step purification which affords milligram quantities of homogeneous APH(3') type IIa [APH(3')-IIa] is described. The kinetic parameters for the turnover of five substrates by the enzyme were determined, and the pH dependence and metal activation for catalysis were investigated. All five cysteines in the amino acid sequence of the enzyme exist in their reduced forms; hence, there are no disulfide bonds in the protein. Modification of the cysteine thiols by S-cyanylation showed essentially no effect on the enzymatic activity. A mutant enzyme derived from APH-3'-IIa, which possesses a conservative Glu-182-Asp point mutation and which provides diminished resistance to G418 (R. L. Yenofsky, M. Fine, and J. W. Pellow, Proc. Natl. Acad. Sci. USA 87:3435-3439, 1990), was also purified to homogeneity. Kinetic analysis of this mutant protein indicated an increase of approximately ninefold in the Km for Mg2+ ATP. Insofar as Km may approximate Ks, this finding argues for the involvement of residue 182 in the binding of Mg2+ ATP. Thus, purified APH(3')-IIa and a point mutant derivative enzyme were characterized enzymologically, and the roles of metal cofactors and the five reduced cysteine residues were probed in the wild-type enzyme.

Bleomycin increases amikacin and streptomycin resistance in Escherichia coli harboring transposon Tn5

The antitumor antibiotic bleomycin acts as a transcriptional inducer of the neo-ble-str operon of the transposon Tn5, increasing the resistance level to streptomycin and amikacin in Tn5-containing Escherichia coli. The mechanism may involve a recA-independent induction mediated by DNA damage.

Molecular genetics of aminoglycoside resistance genes and familial relationships of the aminoglycoside-modifying enzymes

The three classes of enzymes which inactivate aminoglycosides and lead to bacterial resistance are reviewed. DNA hybridization studies have shown that different genes can encode aminoglycoside-modifying enzymes with identical resistance profiles. Comparisons of the amino acid sequences of 49 aminoglycoside-modifying enzymes have revealed new insights into the evolution and relatedness of these proteins. A preliminary assessment of the amino acids which may be important in binding aminoglycosides was obtained from these data and from the results of mutational analysis of several of the genes encoding aminoglycoside-modifying enzymes. Recent studies have demonstrated that aminoglycoside resistance can emerge as a result of alterations in the regulation of normally quiescent cellular genes or as a result of acquiring genes which may have originated from aminoglycoside-producing organisms or from other resistant organisms. Dissemination of these genes is aided by a variety of genetic elements including integrons, transposons, and broad-host-range plasmids. As knowledge of the molecular structure of these enzymes increases, progress can be made in our understanding of how resistance to new aminoglycosides emerges.

Overproduction of 3'-aminoglycoside phosphotransferase type I confers resistance to tobramycin in Escherichia coli

Escherichia coli HM69, isolated from urine, was resistant to high levels of kanamycin (MIC, > 1,000 micrograms/ml) and a low level of tobramycin (MIC, 8 micrograms/ml). Phosphocellulose paper-binding assays and molecular cloning indicated that resistance to both aminoglycosides was due to synthesis of a 3'-aminoglycoside phosphotransferase type I, an enzyme that phosphorylates kanamycin but not tobramycin. The structural gene for the enzyme was borne by an 80-kb conjugative plasmid, pIP1518, and was nearly identical to aphA1 of Tn903. Incubation of extracts of resistant cells with tobramycin or kanamycin led to a decrease (> 80%) of antibiotic activity as determined by a microbiological assay. Heat treatment showed that loss of activity was reversible and dependent upon the native enzyme. In the presence of ATP, only inactivation of kanamycin was reversible. These results suggest that resistance to low levels of tobramycin was due to formation of a complex between the enzyme and the antibiotic.

Mutations in bglY, the structural gene for the DNA-binding protein H1 of Escherichia coli, increase the expression of the kanamycin resistance gene carried by plasmid pGR71

bglY mutants of Escherichia coli K12 which show higher levels of kanamycin resistance (Kmr) in the presence of plasmid pGR71 have been previously described. In this work, we show that this increased resistance to an aminoglycoside antibiotic is not due either to low drug uptake or to alteration of its target, the ribosome. The copy number of plasmid pGR71 is not modified. The fact that increased antibiotic resistance is observed with only some of the Kmr determinants used in this study suggests a specific role for the bglY gene product. Moreover, for one such determinant, a higher level of resistance was observed when it was inserted in the chromosome but not when harbored by a plasmid. This discrepancy can be explained by the twin transcriptional-loop model, which proposes that transcription can lead to local variation in topology. A kan-lacZ fusion was constructed from the Kmr gene of plasmid pGR71 and inserted into a low copy number vector. Assay of beta-galactosidase in wild-type and mutant strains showed that expression of the antibiotic resistance gene was directly affected by H1 protein, the bglY gene product.

Introduction and maintenance of prokaryotic DNA in Ustilago violacea

A strain of the basidiomycete, Ustilago violacea, was transformed with a prokaryotic plasmid, pMP4-1, which confers resistance to neomycin. U. violacea transformants were selected at a frequency of 5 per microgram pMP4-1 DNA. Such transformants were at least 8-fold more resistant to neomycin than was the untransformed recipient U. violacea. Enzyme activity associated with the neomycin resistance gene was also found in the transformants. Southern DNA-DNA hybridization detected pMP4-1-derived sequences in both nuclear and mitochondrially-associated DNAs from transformants. The patterns of hybridization suggested integration of pMP4-1 sequences into the respective genomes. DNA from the nuclear fraction of U. violacea transformants failed to produce E. coli transformants resistant to neomycin or to carbenicillin. In contrast, DNA from the mitochondrially-associated fraction in U. violacea transformants produced E. coli transformants resistant to neomycin. The E. coli transformants contained a pMP4-1-derivative, pWP8, which was subsequently shown by Southern blot analysis to harbor U. violacea mitochondrial DNA. Thus, a prokaryotic plasmid can be used to transform the eukaryote U. violacea and acquire endogenous sequences from this organism.

Bleomycin-kanamycin resistance as a marker of the presence of transposon Tn5 in clinical strains of Escherichia coli

The aminoglycoside modifying enzyme aminoglycoside 3'-phosphotransferase II (APH(3')II) is encoded for on transposon Tn5 by the aphA gene, in the same operon as the ble gene determining bleomycin resistance. To document this linkage 82 kanamycin-resistant Escherichia coli strains of clinical origin were studied; all 18 isolates presenting bleomycin-kanamycin resistance were shown by an enzymatic assay to produce APH(3')II, and the presence of Tn5 was demonstrated by gene hybridization. Similarly, bleomycin-kanamycin resistance was shown to be linked to APH(3')II production in Salmonella spp. The epidemiology of strains with Tn5-encoded APH(3')II may thus be studied, at least in Escherichia coli, by a simple diffusion test using bleomycin and kanamycin discs.

Isolation, characterization, and cloning of a plasmid-borne gene encoding a phosphotransferase that confers high-level amikacin resistance in enteric bacilli

Clinical isolates of Klebsiella pneumoniae and Serratia marcescens at a hospital that had used amikacin as its principal aminoglycoside for the preceding 42 months demonstrated high-level resistance to amikacin (greater than or equal to 256 micrograms/ml), kanamycin (greater than or equal to 256 micrograms/ml), gentamicin (greater than or equal to 64 micrograms/ml), netilmicin (64 micrograms/ml), and tobramycin (greater than or equal to 16 micrograms/ml). The resistant strains contained an identical 6.8-kilobase plasmid, pRPG101. Transformation of pRPG101 into Escherichia coli produced high-level resistance to amikacin (greater than or equal to 256 micrograms/ml) and kanamycin (greater than or equal to 256 micrograms/ml) but unchanged susceptibilities to gentamicin, netilmicin, and tobramycin. The clinical isolates and transformants produced a novel 3'-phosphotransferase, APH(3'), that modified amikacin and kanamycin in vitro. The location and orientation of the amk gene encoding this APH(3') were determined by analysis of insertions in pRPG101 of the defective gene fusion phage Mu dII1681 (mini-Mulac). Cells containing plasmids with insertions into amk that had the lac operon fused to the amk promoter were selected as Lac+ and amikacin susceptible. A collection of these mini-Mulac insertions was mapped by restriction enzyme analysis. This characterization of amk facilitated its cloning as a 1.8-kilobase EcoRI-Bg/I fragment of pRPG101 into the pUC19 vector. E. coli strains containing this recombinant plasmid had APH(3') activity and demonstrated high-level resistance to amikacin and kanamycin (greater than or equal to 256 micrograms/ml) but were as susceptible to gentamicin, tobramycin, and netilmicin (less than or equal to 1.0 microgram/ml) as the strains harboring the original pRPG101 plasmid.

Coupled spectrofluorometric assay for aminoglycoside phosphotransferases

In order to make accurate kinetic measurements for the substrates of aminoglycoside (AG) phosphotransferases (APHs), we have developed an assay which overcomes many of the limitations of currently used assays. We have adapted the coupled spectrophotometric assay (P. R. Goldman and D. B. Northrop (1976) Biochem. Biophys. Res. Commun. 68, 230-236) for use in a spectrofluorometer. At an excitation wavelength of 340 nm, NADH will emit an intensity peak at 450 nm; NAD does not emit under these conditions. Our assay can accurately measure differences of 0.25 microM. For the APH(3')-II encoded on Tn5, we have redetermined the Km's for the AGs, amikacin (AK), kanamycin (KM), and ribostamycin (Rib), and for ATP. Our values for AK (76 microM) were lower than those derived from the spectrophotometric assay; for KM and Rib we obtained Km values of 5.1 and 3.6 microM, respectively. These values were well below the limit of accuracy (10 microM) for the spectrophotometric assay. In addition, we have begun characterization of an APH from a clinical isolate with a low Km for AK. Thus far, we have determined that this enzyme has Km's of approximately 1 microM for both AK and KM. These results show that the assay is well suited for accurate determinations of kinetic constants for low Km substrates of APH enzymes.

Effect of aminoglycoside concentration on reaction rates of aminoglycoside-modifying enzymes

Reaction rates of several reference aminoglycoside-modifying enzymes were studied at various substrate concentrations. The resulting concentration-response curves showed wide variation in threshold concentration, in curve slope, in enzyme saturation, and in substrate inhibition. Together, the curves of a defined aminoglycoside panel yielded more specific information for each individual aminoglycoside-modifying enzyme tested than did conventional substrate profiles obtained at a single substrate concentration.

Transferable amikacin resistance in Acinetobacter spp. due to a new type of 3'-aminoglycoside phosphotransferase

Acinetobacter baumannii BM2580 resistant to kanamycin and structurally related antibiotics, including amikacin, was isolated from a clinical specimen. A phosphocellulose paper-binding assay and DNA annealing studies indicated that resistance to aminoglycosides in BM2580 was due to synthesis of a new type of 3'-aminoglycoside phosphotransferase. The gene conferring resistance to kanamycin-amikacin in this strain was carried by a 63-kilobase plasmid, pIP1841, self-transferable to A. baumannii, A. haemolyticus, and A. lwoffii but not to Escherichia coli. The aminoglycoside resistance gene of pIP1841 was cloned in E. coli, where it was expressed.