Correlation between aminoglycoside resistance profiles and DNA hybridization of clinical isolates

Molecular determination of antimicrobial resistance in Escherichia coli isolated from raw meat in Addis Ababa and Bishoftu, Ethiopia

Background

Consumption of meat contaminated by E. coli causes a serious illness and even death to affected individuals. Recently the emerging of antibiotic resistant foodborne E. coli poses serious public health risks worldwide. However, little is known about the antibiotic resistance profile of E. coli in Ethiopia. This study aimed to determine the prevalence and Antimicrobial resistance (AMR) status of E. coli isolated from different type of meat.

Methods

Overall 292 samples were collected from December 2015 to April 2016 from slaughterhouses to determine the prevalence and AMR of E. coli isolated from raw beef, mutton, chevon and chicken meat from Addis Ababa and Bishoftu, Ethiopia. The isolates were screened for AMR against commonly used antibiotics circulating in the Ethiopian market. Both phenotypic and genotypic approach were employed for AMR detection using disc diffusion test and PCR respectively.

Results

The prevalence of E. coli was 63 (21.6%), indicating one sample in every five samples harbors E. coli. Among these, the highest E. coli isolates was observed in chicken meat samples (37.0%; 27), followed by mutton (23.3%; 17), chevon (20.6%; 15) and beef (5.5%; 4). Results of disk diffusion test on the 63 isolates showed that only 4.8% of them were not resistance to all antimicrobials tested. Multiple drug resistance (resistance to ≥3 drugs) was 46.0%. Significantly high resistance to ampicillin (71.4%) and tetracycline (47.6%) was observed. Identification of genes associated with AMR was also done using PCR. The prevalence of E. coli isolates harboring resistance gene responsible for tetracycline (tet(A)), beta lactams (blaCMY) and sulphanamide (sulI) antibiotics were found 65.1, 65.1 and 54.0%, respectively. Twenty-five out of the 63 (39.7% %) E. coli isolates have got antimicrobial resistance gene to three or more classes of drugs. The associations of antimicrobial resistance phenotypes and resistance genes was also determined. The detection of resistance trait against tetracycline, sulphametazole and chloramphenicol measured either phenotypically or genotypically were high.

Conclusions

The rising levels of resistance E. coli to multiple antimicrobial dictate the urgent need to regulate and monitor antimicrobial use in both animals and humans.

Frequency of 16S rRNA Methylase and Aminoglycoside-Modifying Enzyme Genes among Clinical Isolates of Acinetobacter baumannii in Iran

BACKGROUND & OBJECTIVE

Multidrug-resistant Acinetobacter baumannii (MDR-AB) is an important nosocomial pathogen which is associated with significant morbidity and mortality, particularly in high-risk populations. Aminoglycoside-modifying enzymes (AMEs) and 16S ribosomal RNA (16S rRNA) methylation are two important mechanisms of resistance to aminoglycosides. The aim of this study was to determine the prevalence of 16S rRNA methylase (armA, rmtA, rmtB, rmtC, and rmtD), and the AME genes [aac(6')-Ib, aac(3)-I, ant(3'')-I, aph(3')-I and aac(6')-Id], among clinical isolates of A. baumannii in Tehran, Iran.

METHODS

Between November 2015 to July 2016, a total of 110 clinical strains of A. baumannii were isolated from patients in two teaching hospitals in Tehran, Iran. Antimicrobial susceptibility testing was performed according to Clinical and Laboratory Standards Institute guidelines. The presence of genes encoding the AMEs and 16S rRNA methylases responsible for resistance was investigated by multiplex polymerase chain reaction.

RESULTS

The results showed that colistin was an effective antibiotic and could be used as a last-resort treatment of infections caused by MDR-AB. The resistance rate to aminoglycosides were 100%, 96.36% and 90.9% for tobramycin, gentamicin and amikacin, respectively. In this study, AME genes of aac(6')-Ib, aac(3)-I and ant(3'')-I were most prevalent among the isolated strains.

CONCLUSION

Markedly high resistance to tobramycin, gentamicin and amikacin was noted in current study. Our results suggested that modifying enzyme genes in conjunction with methylation of 16S rRNA might contribute to aminoglycoside resistance induced in vivo in A. baumannii. Further studies are required to determine the prevalence of the aminoglycoside resistance genes in other hospitals of Iran.

Frequency distribution of genes encoding aminoglycoside modifying enzymes in uropathogenic E. coli isolated from Iranian hospital

Background

Escherichia coli is considered as the most common cause of urinary tract infection (UTI) and acquired multiple resistances to a wide range of antibiotics such as aminoglycosides. Enzymatic alteration of aminoglycosides (AMEs) by aminoglycoside- modifying enzymes is the main mechanism of resistance to these antibiotics in E. coli. The aim of this study was detection and investigation of frequency of genes encoding aminoglycoside modifying enzymes (aac(3)-IIa and ant(2′′)-Ia) in UPEC isolated from hospitalized patients in teaching hospital of Tehran, Iran.

Findings

A total of 276 UPEC were obtained from Urine samples in a hospital from Tehran. Antibiotic susceptibility to aminoglycosides was determined by disk diffusion method according CLSI guidelines in UPEC isolates. MICs of target antibiotics were determined by agar dilution method. All isolates were screened for the presence of the AMEs genes using the PCR. The results of disk diffusion showed 21%, 24.6%, 23.18%, 3.62% and 6.15% of isolates were resistant to Gentamicin, Tobramycin, Kanamicin, Amikacin and Netilmicin respectively. The agar dilution’s results (MICs) were high, 66.19% for Gentamicin. The aac (3)-IIa and ant(2″)-Ia genes were detected in (78.87%) and 47.88% of isolates respectively.

Conclusions

This study shows the high frequency of genes encoding (AMEs) aac(3)-IIa and ant(2”)-Ia genes and their relationship between different aminoglycoside resistance phenotypes.

Characterization of aminoglycoside-modifying enzymes in enterobacteriaceae clinical strains and characterization of the plasmids implicated in their diffusion

A total of 788 clinical Enterobacteriaceae were collected to describe the aminoglycoside-modifying genes (AME genes) and to characterize the plasmids that carry these genes. Among the 788 strains collected, 330 (41.8%) were aminoglycoside-resistant: 264 Escherichia coli (80%), 33 Proteus mirabilis (10%), 10 Klebsiella pneumoniae (3%), six K. oxytoca (1.8%), five Enterobacter cloacae (1.5%), three Morganella morganii (0.9%), three Providencia stuartii (0.9%), two Salmonella enterica (0.6%), and one each Citrobacter freundii, C. koseri, Proteus vulgaris, and Shigella sonnei. The most affected aminoglycoside was streptomycin (92.7%), followed by kanamycin (26.3%), gentamicin (18%), tobramycin (16.9%), netilmicin (3.6%), and amikacin (1.5%). The AME genes found were aph(3″)-Ib (65.4%), ant(3″)-Ia (37.5%), aph(3')-Ia (13.9%), aac(3)-IIa (12.4%), aac(6')-Ib (4.2%), ant(2″)-Ia (3.6%), and aph(3')-IIa (1.2%). Thirty-four percent of the strains showed more than one enzyme. The most frequent association was ant(3″)-Ia plus aph(3″)-Ib (35 strains). From 66 selected AME genes, 24 were plasmid located: 12 aac(3)-IIa, six aph(3')-Ia, three ant(3″)-Ia, two ant(2″)-Ia, and one aac(6')-Ib. These genes were located in plasmids belonging to incompatibility groups F, FIA, FIB, or HI2. In conclusion, the AME genes involved in aminoglycoside-clinical resistance were aac(3)-IIa, aac(6')-Ib, and ant(2″)-Ia, genes that confer resistance to tobramycin, gentamicin, and amikacin.

The emergence of resistance to amikacin in Serratia marcescens isolates from patients with nosocomial infection

Administration of either amikacin (1985) or gentamicin (1984, 1986-1991) as first-choice aminoglycoside did not decrease the high incidence of amikacin-resistant Serratia marcescens (ARSm) isolates responsible for nosocomial infections at the J.A. Fernández Hospital of Buenos Aires (42% in 1984, 31% in 1985 and 41% in 1987, differences not significant). In addition, a significant peak (P = 0.003) was detected in 1986, with an ARSm incidence of 70%. The incidence of ARSm decreased by 1988-1991 for reasons not related to aminoglycoside use. In the period 1984-1987 all S. marcescens isolates carried the 6'-aminoglycoside-acetyltransferase-Ic [aac(6')-Ic] gene, while in addition 20% of the isolates contained the plasmid-encoded 3'-aminoglycoside-phosphotransferase-VIa[aph(3')-VIa] and 2% the 6'-aminoglycoside-acetyltransferase-Ib [aac(6')-Ib] genes. From 1988 to 1992 resistance to amikacin was associated with only 4 ARSm isolates and correlated with the appearance of Tn1331-related sequences in these isolates. This transposon or related sequences, however, was not widely spread in the S. marcescens population under investigation. Combined use of restriction fragment length polymorphism (RFLP), ribotyping and plasmid profile analysis revealed that S. marcescens strains of the same genotype, including isolates either expressing or not the aac(6')-Ic gene, were involved in outbreaks occurring in May 1984, May 1985 and May 1986. Furthermore, these epidemiological tools permitted discrimination of different S. marcescens clones, each bearing a particular amikacin-resistance marker.

EUCAST expert rules in antimicrobial susceptibility testing

EUCAST expert rules have been developed to assist clinical microbiologists and describe actions to be taken in response to specific antimicrobial susceptibility test results. They include recommendations on reporting, such as inferring susceptibility to other agents from results with one, suppression of results that may be inappropriate, and editing of results from susceptible to intermediate or resistant or from intermediate to resistant on the basis of an inferred resistance mechanism. They are based on current clinical and/or microbiological evidence. EUCAST expert rules also include intrinsic resistance phenotypes and exceptional resistance phenotypes, which have not yet been reported or are very rare. The applicability of EUCAST expert rules depends on the MIC breakpoints used to define the rules. Setting appropriate clinical breakpoints, based on treating patients and not on the detection of resistance mechanisms, may lead to modification of some expert rules in the future.

Expert systems in clinical microbiology

This review aims to discuss expert systems in general and how they may be used in medicine as a whole and clinical microbiology in particular (with the aid of interpretive reading). It considers rule-based systems, pattern-based systems, and data mining and introduces neural nets. A variety of noncommercial systems is described, and the central role played by the EUCAST is stressed. The need for expert rules in the environment of reset EUCAST breakpoints is also questioned. Commercial automated systems with on-board expert systems are considered, with emphasis being placed on the "big three": Vitek 2, BD Phoenix, and MicroScan. By necessity and in places, the review becomes a general review of automated system performances for the detection of specific resistance mechanisms rather than focusing solely on expert systems. Published performance evaluations of each system are drawn together and commented on critically.

Sequences related to Tn1331 associated with multiple antimicrobial resistance in different Salmonella serovars

Serovars of Salmonella resistant to ampicillin, third-generation cephalosporins and aminoglycosides but sensitive to chloramphenicol, cefoxitin and ceftibuten emerged in one pediatric hospital of Buenos Aires. All isolates expressed AAC(6')-I and AAC(3)-V enzyme activities, making them resistant to all aminoglycosides marketed in Argentina by the time this investigation was performed. The cefotaxime resistance marker, the AAC(3)-V enzyme activity and Tn1331-related sequences were associated with plasmid DNAs from different Salmonella serovars.

Temporal appearance of plasmid-mediated quinolone resistance genes

One hundred fifty AAC(6')-Ib-positive gram-negative isolates collected between 1981 and 1991 were examined by PCR for the presence of the aac(6')-Ib-cr variant and other plasmid-mediated quinolone resistance (PMQR) genes. None had the aac(6')-Ib-cr variant, qnrA, qnrS, qnrC, or qepA, but two strains collected in 1988 had qnrB alleles, making these the earliest known PMQR genes.

Distribution of tetracycline and streptomycin resistance genes and class 1 integrons in Enterobacteriaceae isolated from dairy and nondairy farm soils

The prevalence of selected tetracycline and streptomycin resistance genes and class 1 integrons in Enterobacteriaceae (n = 80) isolated from dairy farm soil and nondairy soils was evaluated. Among 56 bacteria isolated from dairy farm soils, 36 (64.3%) were resistant to tetracycline, and 17 (30.4%) were resistant to streptomycin. Lower frequencies of tetracycline (9 of 24 or 37.5%) and streptomycin (1 of 24 or 4.2%) resistance were observed in bacteria isolated from nondairy soils. Bacteria (n = 56) isolated from dairy farm soil had a higher frequency of tetracycline resistance genes including tetM (28.6%), tetA (21.4%), tetW (8.9%), tetB (5.4%), tetS (5.4%), tetG (3.6%), and tetO (1.8%). Among 24 bacteria isolated from nondairy soils, four isolates carried tetM, tetO, tetS, and tetW in different combinations; whereas tetA, tetB, and tetG were not detected. Similarly, a higher prevalence of streptomycin resistance genes including strA (12.5%), strB (12.5%), ant(3'') (12.5), aph(6)-1c (12.5%), aph(3'') (10.8%), and addA (5.4%) was detected in bacteria isolated from dairy farm soils than in nondairy soils. None of the nondairy soil isolates carried aadA gene. Other tetracycline (tetC, tetD, tetE, tetK, tetL, tetQ, and tetT) and streptomycin (aph(6)-1c and ant(6)) resistance genes were not detected in both dairy and nondairy soil isolates. A higher distribution of multiple resistance genes was observed in bacteria isolated from dairy farm soil than in nondairy soil. Among 36 tetracycline- and 17 streptomycin-resistant isolates from dairy farm soils, 11 (30.6%) and 9 (52.9%) isolates carried multiple resistance genes encoding resistance to tetracycline and streptomycin, respectively, which was higher than in bacteria isolated from nondairy soils. One strain each of Citrobacter freundii and C. youngae isolated from dairy farm soils carried class 1 integrons with different inserted gene cassettes. Results of this small study suggest that the presence of multiple resistance genes and class 1 integrons in Enterobacteriaceae in dairy farm soil may act as a reservoir of antimicrobial resistance genes and could play a role in the dissemination of these antimicrobial resistance genes to other commensal and indigenous microbial communities in soil. However, additional longer-term studies conducted in more locations are needed to validate this hypothesis.

Antibiotic resistance in gram-negative bacteria: the role of gene cassettes and integrons

Resistance of gram-negative organisms to antibiotics such as beta-lactams, aminoglycosides, trimethoprim and chloramphenicol is caused by many different acquired genes, and a substantial proportion of these are part of small mobile elements known as gene cassettes. A gene cassette consists of the gene and a downstream sequence, known as a 59-base element (59-be), that acts as a specific recombination site. Gene cassettes can move into or out of a specific receptor site (attl site) in a companion element called an integron, and integration or excision of the cassettes is catalysed by a site-specific recombinase (Intl) that is encoded by the integron. At present count there are 40 different cassette-associated resistance genes and three distinct classes of integron, each encoding a distinct Intl integrase. The same cassettes are found in all three classes of integron, indicating that cassettes can move freely between different integrons. Integrons belonging to class I often contain a further antibiotic resistance gene, sull, conferring resistance to sulphonamides. The sull gene is found in a conserved region (3'-CS) that is not present in all members of this class. Class I integrons of the sull type are most prevalent in clinical isolates and have been found in many different organisms. Even though most of them are defective transposon derivatives, having lost at least one of the transposition genes, they are none the less translocatable and consequently found in many different locations. The transposon Tn7 is the best known representative of class 2 integrons, and Tn7 and relatives are also found in many different species.

Antimicrobial resistance of Acinetobacter spp. in Europe

Bacteria of the genus Acinetobacter are ubiquitous in nature. These organisms were invariably susceptible to many antibiotics in the 1970s. Since that time, acinetobacters have emerged as multiresistant opportunistic nosocomial pathogens. The taxonomy of the genus Acinetobacter underwent extensive revision in the mid-1980s, and at least 32 named and unnamed species have now been described. Of these, Acinetobacter baumannii and the closely related unnamed genomic species 3 and 13 sensu Tjernberg and Ursing (13TU) are the most relevant clinically. Multiresistant strains of these species causing bacteraemia, pneumonia, meningitis, urinary tract infections and surgical wound infections have been isolated from hospitalised patients worldwide. This review provides an overview of the antimicrobial susceptibilities of Acinetobacter spp. in Europe, as well as the main mechanisms of antimicrobial resistance, and summarises the remaining treatment options for multiresistant Acinetobacter infections.

Versatility of aminoglycosides and prospects for their future

Aminoglycoside antibiotics have had a major impact on our ability to treat bacterial infections for the past half century. Whereas the interest in these versatile antibiotics continues to be high, their clinical utility has been compromised by widespread instances of resistance. The multitude of mechanisms of resistance is disconcerting but also illuminates how nature can manifest resistance when bacteria are confronted by antibiotics. This article reviews the most recent knowledge about the mechanisms of aminoglycoside action and the mechanisms of resistance to these antibiotics.

Epidemiology of rifampin ADP-ribosyltransferase (arr-2) and metallo-beta-lactamase (blaIMP-4) gene cassettes in class 1 integrons in Acinetobacter strains isolated from blood cultures in 1997 to 2000

We characterized two new gene cassettes in an Acinetobacter isolate: one harbored the metallo-beta-lactamase (IMP-4) gene bla(IMP-4), the other harbored the rifampin ADP-ribosyltransferase (ARR-2) gene arr-2, and both arrayed with the aminoglycoside acetyltransferase [AAC(6')-Ib(7)] gene cassette aacA4 in two separate class 1 integrons. The epidemiology of these gene cassettes in isolates from blood cultures obtained from 1997 to 2000 was studied. Isolates bearing either the bla(IMP-4) or the arr-2 gene cassette or both represented 17.5% (10 of 57) of isolates in 1997, 16.1% (10 of 62) in 1998, 2.5% (1 of 40) in 1999, and 0% (0 of 58) in 2000. These two gene cassettes, probably borne on two separate integrons, were found in at least three genomic DNA groups, with evidence of clonal dissemination in the intensive care unit during 1997 to 1998. Seventeen of the 52 Acinetobacter baumannii (genomic DNA group 2) isolates from 1997 to 2000 harbored intI1, but only one was positive for these gene cassettes, whereas 20 of the 21 intI1-positive isolates of all other genomic DNA groups were positive for either or both of them. Reduced susceptibility to imipenem and rifampin was seen only in isolates harboring the bla(IMP-4) and arr-2 cassettes, respectively. The aminoglycoside phosphotransferase [APH(3')-VIa] gene aph(3')-VIa was detected in all 21 isolates for which the MIC of amikacin was >/=8 micro g/ml, with or without aacA4, whereas aacA4 alone was found in isolates for which the MIC of amikacin was 0.5 to 2 micro g/ml. Significant differences between the 17 intI1-positive and 47 intI1-negative isolates belonging to genomic DNA group 3 from 1997 to 1998 in the MICs of amikacin, gentamicin, imipenem, sulfamethoxazole, and ceftazidime were observed (Mann-Whitney test, P < 0.001 to 0.01).

The relationships and susceptibilities of some industrial, laboratory and clinical isolates of Pseudomonas aeruginosa to some antibiotics and biocides

AIMS

To provide evidence to support or refute the hypothesis that cross-resistance between antibiotics and biocides can occur.

METHODS AND RESULTS

Fifty-five strains of Pseudomonas aeruginosa were tested for their resistance to anti-pseudomonal antibacterials. Twenty clinical, 19 industrial and 16 culture collection isolates were used. The MIC was found for the antibiotics amikacin, ceftazidime, ciprofloxacin, gentamycin, ticarcillin, tobramycin, imipenem and polymyxin B. The MIC was also found for the biocides benzalkonium chloride and chlorhexidine. The analysis of the data was based on the production of a normal distribution of the log (MIC) plots for each antimicrobial. Strains were then labelled as resistant, intermediate or sensitive based on the mean and standard deviation of the distributions.

CONCLUSIONS

In general the clinical isolates were the most recalcitrant organisms, with the industrial isolates being the most sensitive.

SIGNIFICANCE AND IMPACT OF THE STUDY

The work shows that antibiotic/biocide correlations do occur, especially with clinical strains. That such correlations were not found with industrial isolates suggests that the clinical environment is responsible for the correlation. We could infer that it is the selective pressure of antibiotic usage that differentiates the clinical environment from the industrial.

Aminoglycoside resistance genes aph(2")-Ib and aac(6')-Im detected together in strains of both Escherichia coli and Enterococcus faecium

Escherichia coli SCH92111602 expresses an aminoglycoside resistance profile similar to that conferred by the aac(6')-Ie-aph(2")-Ia gene found in gram-positive cocci and was found to contain the aminoglycoside resistance genes aph(2")-Ib and aac(6')-Im (only 44 nucleotides apart). aph(2")-Ib had been reported previously in Enterococcus faecium SF11770. aac(6')-Im had not been detected previously in enterococci and was found to be present also 44 nucleotides downstream from aph(2")-Ib in E. faecium SF11770. aph(2")-Ib and aac(6')-Im are separate open reading frames, each with its own putative ribosome binding site, whereas aac(6')-Ie-aph(2")-Ia appears to be a fusion of two genes with just one start and one stop codon. The deduced AAC(6')-Im protein exhibits 56% identity and 80% similarity to the AAC(6')-Ie domain of the bifunctional enzyme AAC(6')-APH(2"). Our results document the existence of a member of the aph(2") family of genes in gram-negative bacteria and provide evidence suggesting the horizontal transfer of aph(2")-Ib and aac(6')-Im as a unit between gram-positive and gram-negative bacteria.

The changing nature of aminoglycoside resistance mechanisms and prevalence of newly recognized resistance mechanisms in Turkey

OBJECTIVE

To determine the most frequently occurring individual and combined resistance mechanisms in Gram-negative bacteria resistant to any of the clinically available aminoglycosides in Turkey, and to compare these mechanisms with those found in smaller, earlier studies.

METHODS

Aminoglycoside resistance mechanisms in Gram-negative isolates resistant to either gentamicin, tobramycin, netilmicin or amikacin collected in different regions of Turkey were evaluated both phenotypically and genotypically using 12 aminoglycosides and up to 22 aminoglycoside resistance gene probes.

RESULTS

Among 696 aminoglycoside-resistant Gram-negative bacteria, resistance rates were very high for gentamicin (94.5%), tobramycin (82.4%), netilmicin (53.6%), and amikacin (49.7%). Although isepamicin was the most active aminoglycoside against Gram-negative bacteria, increased resistance (29.7%) was found and resistance rates were higher than those in most of the other countries surveyed in earlier studies. The most common aminoglycoside resistance mechanisms (AAC(3)-II (GTN), AAC(6')-I (TNA), and ANT(2")-I (GT)) in the earlier studies were also found in the present isolates of Klebsiella spp., Enterobacter spp. and Escherichia coli, with increased complexity. In addition to these old mechanisms, two new aminoglycoside resistance mechanisms, namely AAC(6')-III (TNAI) and AAC(6')-IV (GTNA), were also found at significant frequencies (11.9% and 26.9%, respectively) in these isolates of Enterobacteriaceae (n = 435). Among the isolates of Pseudomonas spp. (n = 150), in addition to the increased complexity of enzymatic resistance mechanisms (AAC(3)-I (16.6%), AAC(6')-II (29.3%), AAC(6')-III (19.3%), ANT(2")-I (40%)), permeability resistance seemed to be responsible for the high rates of resistance to aminoglycosides.

CONCLUSION

The results of this study indicated increased resistance to clinically available aminoglycosides, including isepamicin, even though it was the most active, as a result of both the presence of new aminoglycoside resistance mechanisms and the increased complexity of all mechanisms, including permeability resistance, particularly in Pseudomonas in Turkey.

Characterization of aminoglycoside resistance genes and class 1 integrons in porcine and bovine gentamicin-resistant Escherichia coli

A total of 78 gentamicin-resistant Escherichia coli strains from swine (27) and cattle (51) were characterized by phenotypic resistance, presence of selected aminoglycoside resistance genes, class 1 integrons and gene cassettes, and pulsed-field gel electrophoresis. Gentamicin resistance was mainly encoded by the ant(2'')-I gene that was found in 76% of all the strains investigated, whereas the aac(3)-IIa gene was found in 14%. The ant(2'')-I gene was predominant in strains from cattle, whereas the porcine strains contained both ant(2'')-I, aac(3)-IIa, and the aac(3)-IVa genes. The pulsed-field gel electrophoresis (PFGE) investigation indicated a close clonal relationship in half of the bovine strains whereas the remaining E. coli were unrelated. Among the E. coli investigated, 20% contained class 1 integrons. Genes encoding resistance to trimethoprim (dhfrI, dhfrIb, and dhfrVII), gentamicin, tobramycin, and kanamycin (ant(2'')-Ia streptomycin and spectinomycin (ant(3'')-Ia) and streptothricin (sat1) were identified as gene cassettes. The most prevalent gene cassettes were ant(3'')-Ia (11 isolates) and the dhfrI (nine isolates).

Distribution and content of class 1 integrons in different Vibrio cholerae O-serotype strains isolated in Thailand

In this study, 176 clinical and environmental Vibrio cholerae strains of different O serotypes isolated in Thailand from 1982 to 1995 were selected and studied for the presence of class 1 integrons, a new group of genetic elements which carry antibiotic resistance genes. Using PCR and DNA sequencing, we found that 44 isolates contained class 1 integrons harboring the aadB, aadA2, blaP1, dfrA1, and dfrA15 gene cassettes, which encode resistance to gentamicin, kanamycin, and tobramycin; streptomycin and spectinomycin; beta-lactams; and trimethoprim, respectively. Each cassette array contained only a single antibiotic resistance gene. Although resistance genes in class 1 integrons were found in strains from the same epidemic, as well as in unrelated non-O1, non-O139 strains isolated from children with diarrhea, they were found to encode only some of the antibiotic resistance expressed by the strains. Serotype O139 strains did not contain class 1 integrons. However, the appearance and disappearance of the O139 serotype in the coastal city Samutsakorn in 1992 and 1993 were associated with the emergence of a distinct V. cholerae O1 strain which contained the aadA2 resistance gene cassette. A 150-kb self-transmissible plasmid found in three O1 strains isolated in 1982 contained the aadB gene cassette. Surprisingly, several strains harbored two integrons containing different cassettes. Thus, class 1 integrons containing various resistance gene cassettes are distributed among different V. cholerae O serotypes of mainly clinical origin in Thailand.

Molecular epidemiology of integron-associated antibiotic resistance genes in clinical isolates of enterobacteriaceae

The epidemiology of integron-mediated antibiotic-resistant genes in clinical enterobacteria from a single location was investigated. Forty-nine isolates (kindly provided by Dr. D. Sirot, Clermont-Ferrand, France) were selected for transferable resistance to aminoglycosides or to other antibiotics. Total DNA prepared from these strains was screened for the presence of conserved segments of integrons by PCR. The nature and frequency of inserted resistance gene cassettes were determined by direct nucleotide sequencing and were related to the resistances expressed by the strain. Integron hot-spots were present in 59% of the strains from 6 species, in either one or two copies. For amplicons sequenced, one or two antibiotic-resistant genes were found in various combinations, and were always expressed at the phenotypic level. They included the aminoglycoside resistance genes ant(3")-Ia and aac(6')-Ib (75%), as well as dhfr-I,-VII (21.4%) and blaOXA-1 (3.6%). Almost half of the transferable resistance to aminoglycosides (53%) was mediated by integron hot-spots in strains characterized at the nucleotide level. The proportion rose to 100% for the AAC(6')-I resistance profile. This study emphasizes the important contribution of integrons to aminoglycoside resistance within enterobacteria from a clinical setting.

Detection of genes encoding aminoglycoside-modifying enzymes in staphylococci by polymerase chain reaction and dot blot hybridization

Dot blot hybridization and the polymerase chain reaction (PCR) were used to study aminoglycoside-modifying enzymes in aminoglycoside-resistant staphylococci isolated in hospitals in Kuwait. DNA encoding the acetyltransferase (AAC) (6')-phosphotransferase (APH) (2"), nucleotidyltransferase (ANT) (4') and APH (3') enzymes were detected in Staphylococcus aureus and coagulase negative staphylococci. ANT (4') was the most common enzyme detected. The majority of isolates contained genes for all three modifying enzymes, AAC (6')-APH (2"), ANT (4') and APH (3'); only few isolates carried genes for a single modifying enzyme. Genes encoding the AAC (6')-APH (2") were detected in all except two gentamicin-resistant isolates. In these isolates the genes for the AAC (6')-APH (2") enzyme could not be detected by PCR and dot blot hybridization. Whereas antibiotic resistance testing could be used to predict the presence of the AAC (6')-APH (2") enzyme it was not useful in predicting the presence of the ANT (4') or APH (3') enzymes in gentamicin-resistant isolates. Results obtained with dot blot hybridization were comparable to those obtained with PCR. However, PCR was fast and results were obtained within the same day. Therefore PCR would be preferred for the detection and confirmation of the presence of aminoglycoside-modifying enzymes in clinical microbiology laboratories.

Aminoglycoside resistance mechanisms in clinical isolates of Pseudomonas aeruginosa from the Canary Islands

Strains of Pseudomonas aeruginosa resistant to aminoglycoside antibiotics were selected from 152 clinical isolates. We identified two patterns of resistance correlating with the resistance mechanism characterized by changes in permeability, enzymatic modification due to the acetylating enzyme, AAC(6')-II, or a combination of both. We detected enzymatic activity of the phosphorylase enzyme, APH(3'), in all the isolates. We compared the mechanisms of resistance detected by three methods i.e., radioenzymatic assay, phenotype of resistance and DNA probes. The phenotype of resistance was tested using a kit developed by Schering-Plough Corp. Hybridization was made with 18 DNA probes for the most frequent genes encoding for aminoglycoside-modifying enzymes. All isolates with AAC(6') activity hybridized with the aac(6')-Ib probes and to a minor degree, with the aac(6')-IIb probe. None of the isolates showed hybridization with aph(3')-I, aph(3')-II, or aph(3')-III DNA probes. Serotyping of the strains showed that the O:11 serotype was the most frequent one in strains whose resistance was due to the AAC(6') enzyme. The O:6 serotype was associated with changes in permeability. Encoding of the resistance mechanism seemed to be chromosomal in all the strains.

Activities of tobramycin and six other antibiotics against Pseudomonas aeruginosa isolates from patients with cystic fibrosis

The in vitro activity of tobramycin was compared with those of six other antimicrobial agents against 1,240 Pseudomonas aeruginosa isolates collected from 508 patients with cystic fibrosis during pretreatment visits as part of the phase III clinical trials of tobramycin solution for inhalation. The tobramycin MIC at which 50% of isolates are inhibited (MIC(50)) and MIC(90) were 1 and 8 microg/ml, respectively. Tobramycin was the most active drug tested and also showed good activity against isolates resistant to multiple antibiotics. The isolates were less frequently resistant to tobramycin (5.4%) than to ceftazidime (11.1%), aztreonam (11.9%), amikacin (13.1%), ticarcillin (16.7%), gentamicin (19.3%), or ciprofloxacin (20.7%). For all antibiotics tested, nonmucoid isolates were more resistant than mucoid isolates. Of 56 isolates for which the tobramycin MIC was > or = 16 microg/ml and that were investigated for resistance mechanisms, only 7 (12.5%) were shown to possess known aminoglycoside-modifying enzymes; the remaining were presumably resistant by an incompletely understood mechanism often referred to as "impermeability."

Characterization of a Pseudomonas aeruginosa efflux pump contributing to aminoglycoside impermeability

Pseudomonas aeruginosa can employ many distinct mechanisms of resistance to aminoglycoside antibiotics; however, in cystic fibrosis patients, more than 90% of aminoglycoside-resistant P. aeruginosa isolates are of the impermeability phenotype. The precise molecular mechanisms that produce aminoglycoside impermeability-type resistance are yet to be elucidated. A subtractive hybridization technique was used to reveal gene expression differences between PAO1 and isogenic, spontaneous aminoglycoside-resistant mutants of the impermeability phenotype. Among the many genes found to be up-regulated in these laboratory mutants were the amrAB genes encoding a recently discovered efflux system. The amrAB genes appear to be the same as the recently described mexXY genes; however, the resistance profile that we see in P. aeruginosa is very different from that described for Escherichia coli with mexXY. Direct evidence for AmrAB involvement in aminoglycoside resistance was provided by the deletion of amrB in the PAO1-derived laboratory mutant, which resulted in the restoration of aminoglycoside sensitivity to a level nearly identical to that of the parent strain. Furthermore, transcription of the amrAB genes was shown to be up-regulated in P. aeruginosa clinical isolates displaying the impermeability phenotype compared to a genotypically matched sensitive clinical isolate from the same patient. This suggests the possibility that AmrAB-mediated efflux is a clinically relevant mechanism of aminoglycoside resistance. Although it is unlikely that hyperexpression of AmrAB is the sole mechanism conferring the impermeability phenotype, we believe that the Amr efflux system can contribute to a complex interaction of molecular events resulting in the aminoglycoside impermeability-type resistance phenotype.

Aminoglycoside resistance in Gram-negative blood isolates from various hospitals in Belgium and the Grand Duchy of Luxembourg. Aminoglycoside Resistance Study Group

A total of 1102 consecutive clinical blood isolates, including 897 Enterobacteriaceae and 205 non-fermenting bacilli, were obtained from 13 university and university-affiliated hospitals, which were divided into a Northern and a Southern group. Resistance to gentamicin, tobramycin, netilmicin, amikacin and isepamicin was determined using a microdilution technique according to NCCLS procedures. The overall mean resistance level was 5.9% for gentamicin, 7.7% for tobramycin, 7.5% for netilmicin, 2.8% for amikacin and 1.2% for isepamicin. Resistance to amikacin and isepamicin was significantly higher in the Northern hospitals than in the Southern hospitals. In total, 157 isolates were found not to be susceptible to aminoglycosides. By PCR, 179 aminoglycoside resistance mechanisms, i.e. 150 genes encoding modifying enzymes and 29 permeability mechanisms, were detected in 148 isolates. A resistance mechanism could not be detected in nine isolates. Moreover, in a further 14 isolates the resistance profile was not fully explained by the detected genes. The aac(6')-I genes were found to be the most predominant resistance mechanism in both the Northern and Southern isolates, followed by aac(3) genes and permeability resistance. A total of 29 non-susceptible isolates harboured a combination of genes, 72.4% of which were a combination with the aac(6')-lb gene. The majority of these combinations were broad-spectrum combinations which represented 9.0% of the resistance mechanisms in non-susceptible Enterobacteriaceae and 19.3% in the non-fermenting bacilli.

Aminoglycoside 6'-N-acetyltransferase variants of the Ib type with altered substrate profile in clinical isolates of Enterobacter cloacae and Citrobacter freundii

Three clinical isolates, Enterobacter cloacae EC1562 and EC1563 and Citrobacter freundii CFr564, displayed an aminoglycoside resistance profile evocative of low-level 6'-N acetyltransferase type II [AAC(6')-II] production, which conferred reduced susceptibility to gentamicin but not to amikacin or isepamicin. Aminoglycoside acetyltransferase assays suggested the synthesis in the three strains of an AAC(6') which acetylated amikacin practically as well as it acetylated gentamicin in vitro. Both compounds, however, as well as isepamicin, retained good bactericidal activity against the three strains. The aac genes were borne by conjugative plasmids (pLMM562 and pLMM564 of ca. 100 kb and pLMM563 of ca. 20 kb). By PCR mapping and nucleotide sequence analysis, an aac(6')-Ib gene was found in each strain upstream of an ant(3")-I gene in a sulI-type integron. The size of the AAC(6')-Ib variant encoded by pLMM562 and pLMM564, AAC(6')-Ib7, was deduced to be 184 (or 177) amino acids long, whereas in pLMM563 a 21-bp duplication allowing the recruitment of a start codon resulted in the translation of a variant, AAC(6')-Ib8, of 196 amino acids, in agreement with size estimates obtained by Western blot analysis. Both variants had at position 119 a serine instead of the leucine typical for the AAC(6')-Ib variants conferring resistance to amikacin. By using methods that predict the secondary structure, these two amino acids appear to condition an alpha-helical structure within a putative aminoglycoside binding domain of AAC(6')-Ib variants.

Cloning and characterization of an aminoglycoside 6'-N-acetyltransferase gene from Citrobacter freundii which confers an altered resistance profile

A novel gene encoding a 6'-N-aminoglycoside acetyltransferase, aac(6')-In, has been cloned and sequenced from Citrobacter freundii 13996-19, a clinical isolate from Venezuela. This gene mediates resistance to amikacin, 2'-N-ethylnetilmicin, isepamicin, kanamycin, netilmicin, and tobramycin. The aac(6')-In gene is 573 nucleotides in length and encodes a putative protein of 190 amino acids. AAC(6')-In is most closely related to AAC(6')-Im and AAC(6')-Ie, demonstrating 64.4% and 62.3% similarity, respectively, at the protein level, suggesting these proteins share a common ancestor. The aac(6')-In flanking sequences demonstrated homology to integron- and transposon-related elements which are often found associated with resistance determinants. Hybridization studies performed with an intragenic probe specific for aac(6')-In indicate that this gene is prevalent within Venezuela but has not been observed outside of the country. Furthermore, the aac(6)-In gene was found in 10 different species of gram-negative bacteria.

Resistance to amikacin and isepamicin in rabbits with experimental endocarditis of an aac(6')-Ib-bearing strain of Klebsiella pneumoniae susceptible in vitro

The effect of production of the aminoglycoside 6'-N-acetyltransferase [AAC(6')-IB] in Klebsiella pneumoniae on the outcome of amikacin and isepamicin treatment of rabbits with experimental endocarditis was assessed. Isogenic high-level (Hi) and low-level (Lo) AAC(6')-Ib-producing transconjugants (T) were constructed from clinical isolates with plasmid-borne resistance determinants. The MICs of amikacin and isepamicin, their bactericidal effects, and AAC(6')-Ib production appeared to be well correlated among the clinical isolates and the transconjugants. The susceptibility data determined in vitro, with MICs (in micrograms per milliliter) of amikacin and isepamicin for LoT and HiT of 4 and 0.5 and 32 and 8, respectively, were, however, not predictive of the in vivo efficacies of the drugs. While amikacin and isepamicin caused reductions in bacterial densities (log10 CFU per gram of cardiac vegetation) of 5.1 and 4.8 of the fully susceptible recipient strain (MICs of amikacin and isepamicin, 0.5 and 0.25, respectively), the reductions in density of both LoT and HiT caused by the two drugs (2.7 and 2.4 and 2.9 and 2.2, respectively) were only marginally significant, if at all. There was no significant difference (P > 0.05) when the reductions in density of LoT and HiT by either drug were compared or when the efficacies of the two drugs in reducing the density of any strain [non-AAC(6')-producing, LoT, or HiT] were compared (P > 0.5). It is concluded that AAC(6')-Ib in K.pneumoniae, even when produced at a low level and not conferring resistance to amikacin and isepamicin in vitro, compromises the efficacies of both drugs in vivo and possibly does so beyond the experimental model studied here.

Mechanism of amikacin resistance in Pseudomonas aeruginosa isolates from patients with cystic fibrosis

We studied 27 amikacin-resistant isolates of Pseudomonas aeruginosa from patients with cystic fibrosis to determine the mechanism of antibiotic resistance. The absence of aminoglycoside-modifying enzymes (AMEs) in these isolates was inferred from the failure of DNA probes for 16 candidate AMEs to hybridize with DNA harvested from these isolates and, in addition, the uniform reduction in susceptibility to a panel of aminoglycosides. In eight of the 27 isolates that were resistant to amikacin at high levels (minimum inhibitory concentration > or = 250 micrograms/ml), plasmids were not detected. The ribosomes of these isolates were sensitive to amikacin in studies of protein synthesis by cell "ghosts." These data suggest that impermeability is the mechanism of amikacin resistance in isolates of P. aeruginosa from patients with cystic fibrosis. Recognition of this mode of resistance may be difficult, as some isolates appeared to be borderline susceptible when tested against aminoglycosides other than amikacin, or had zone diameters that overlapped those obtained with amikacin-susceptible isolates.

Cloning and characterization of a 3-N-aminoglycoside acetyltransferase gene, aac(3)-Ib, from Pseudomonas aeruginosa

A novel gene encoding an aminoglycoside 3-N-acetyltransferase, which confers resistance to gentamicin, astromicin, and sisomicin, was cloned from Pseudomonas aeruginosa Stone 130. Its sequence was determined and found to show considerable similarity to an aac(3)-I gene previously cloned from R plasmids from Enterobacter, Pseudomonas, and Serratia spp. We have designated the genes from the R plasmids and this work aac(3)-Ia and aac(3)-Ib, respectively. The two aac(3)-I genes share 74% nucleotide identity, and their deduced protein products are 88% similar. These data suggest that the genes derive from a common ancestor. Homology between the flanking sequences of both aac(3)-I genes and other resistance determinants known to reside in integron environments was also observed. Intragenic probes specific for either aac(3)-Ia or aac(3)-Ib were used in hybridization studies with a series of gentamicin-, astromicin-, and sisomicin-resistant clinical isolates. Of 59 clinical isolates tested, no isolates hybridized with both probes, 30 (51%) hybridized with the aac(3)-Ia probe, 12 (20%) hybridized with the aac(3)-Ib probe, and 17 (29%) did not hybridize with either probe. These data suggest the existence of at least one other aac(3)-I gene.

Detection of aac(6')-I genes in amikacin-resistant Acinetobacter spp. by PCR

The distribution of aac(6')-I genes in 62 strains of Acinetobacter spp. resistant to amikacin, netilmicin, and tobramycin and susceptible to gentamicin, a phenotype compatible with synthesis of an AAC(6')-I enzyme, was studied by PCR and by DNA hybridization. Both methods gave similar results. Among the 51 Acinetobacter baumannii strains, aac(6')-Ib was found in 19 isolates and aac(6')-Ih was found in the remaining strains. The aac(6')-Ig gene was present in all 10 A. haemolyticus strains studied and was detected only in this species. A pair of degenerate oligonucleotides complementary to conserved regions of aac(6')-Ic, -Id, -If, -Ig, and -Ih enabled detection of these genes and also of aac(6')-Ij, recently recognized in Acinetobacter sp. strain 13.

Detection and surveillance of antimicrobial resistance

The clinical microbiology laboratory is strategically positioned to recognize changing patterns in bacterial resistance to antimicrobials. This requires the application of accurate testing methods and a methodological survey of drug-resistance patterns among clinically important bacteria. This information can be assembled into comprehensive international databases, using a common format to facilitate monitoring.

Characterization of transposon Tn1528, which confers amikacin resistance by synthesis of aminoglycoside 3'-O-phosphotransferase type VI

Providencia stuartii BM2667, which was isolated from an abdominal abscess, was resistant to amikacin by synthesis of aminoglycoside 3'-O-phosphotransferase type VI. The corresponding gene, aph(3')-VIa, was carried by a 30-kb self-transferable plasmid of incompatibility group IncN. The resistance gene was cloned into pUC18, and the recombinant plasmid, pAT246, was transformed into Escherichia coli DH1 (recA) harboring pOX38Gm. The resulting clones were mixed with E. coli HB101 (recA), and transconjugants were used to transfer pAT246 by plasmid conduction to E. coli K802N (rec+). Analysis of plasmid DNAs from the transconjugants of K802N by agarose gel electrophoresis and Southern hybridization indicated the presence of a transposon, designated Tn1528, in various sites of pOX38Gm. This 5.2-kb composite element consisted of aph(3')-VIa flanked by two direct copies of IS15-delta and transposed at a frequency of 4 x 10(-5). It therefore appears that IS15-delta, an insertion sequence widely spread in gram-negative bacteria, is likely responsible for dissemination to members of the family Enterobacteriaceae of aph(3')-VIa, a gene previously confined to Acinetobacter spp.

Aminoglycoside resistance patterns of Serratia marcescens strains of clinical origin

Aminoglycoside resistance patterns of 147 Serratia marcescens strains of clinical origin were studied. All strains analysed belonged to three different bacterial populations. The periods of study and the institutions the strains were isolated from correlated significantly with the resistance patterns shown by the strains. The most frequent resistance patterns found were the following: ACC (6')-I at the Hospital Infantil de México (Children's Hospital of México), and ANT (2'') + AAC(6')-I at the Instituto Nacional de Pediatría (INPed or National Institute of Pediatrics) in Mexico City. Furthermore, the isolation frequency of aminoglycoside-sensitive strains decreased remarkably at the INPed over a 12-year period. These results suggest that there has been a selection of Serratia marcescens strains that are very resistant to aminoglycosides.

A spontaneous point mutation in the aac(6')-Ib' gene results in altered substrate specificity of aminoglycoside 6'-N-acetyltransferase of a Pseudomonas fluorescens strain

The aac(6')-Ib' gene from Pseudomonas fluorescens BM2687, encoding an aminoglycoside 6'-N-acetyltransferase type II which confers resistance to gentamicin but not to amikacin, was characterized. Nucleotide sequence determination indicated total identity between aac(6')-Ib' and the aac(6')-Ib gene from Pseudomonas aeruginosa BM2656 [1] with the exception of a C-to-T transition that results in a serine to leucine substitution at position 83 of the deduced polypeptide. The aac(6')-Ib gene specifies a type I enzyme which confers resistance to amikacin but not to gentamicin [2]. It thus appears that the point mutation detected is responsible for enzymic altered substrate specificity.

Molecular epidemiology of two genes encoding 3-N-aminoglycoside acetyltransferases AAC(3)I and AAC(3)II among gram-negative bacteria from a Spanish hospital

The molecular epidemiology of the aacC1 and aacC2 genes, encoding 3-N-aminoglycoside acetyltransferases AAC(3)I and AAC(3)II, respectively, was studied by DNA-DNA hybridization. The sample included 315 gentamicin-resistant Gram-negative bacilli collected over a six-month period from patients attending a Spanish Hospital. The aminoglycoside resistance phenotype of these strains was also determined. The aacC1 probe hybridized with 39 strains, the aacC2 probe with 146 strains and both probes hybridized with 26 strains. The aacC1 gene was most frequently detected in Pseudomonas aeruginosa whereas the aacC2 gene was most frequently detected in enterobacteria and Acinetobacter spp. Strains harbouring aacC genes were isolated from both in- and outpatients with different infectious diseases, mainly urinary tract infections. As inferred from the results of Southern hybridization, both genes showed a wide horizontal dispersion among plasmids and bacteria.

Characterization of Acinetobacter haemolyticus aac(6')-Ig gene encoding an aminoglycoside 6'-N-acetyltransferase which modifies amikacin

The amikacin resistance gene acc(6')-Ig of Acinetobacter haemolyticus BM2685 encoding an aminoglycoside 6'-N-acetyltransferase was characterized. The gene was identified as a coding sequence of 438 bp corresponding to a protein with a calculated mass of 16,522 Da. Analysis of the deduced amino acid sequence suggested that it was the fourth member of a subfamily of aminoglycoside 6'-N-acetyltransferases. The resistance gene was not transferable either by conjugation to Escherichia coli or to Acinetobacter baumannii or by transformation into Acinetobacter calcoaceticus. Plasmid DNA from strain BM2685 did not hybridize with an intragenic aac(6')-Ig probe. These results suggest a chromosomal location for this gene. The gene was detected by DNA hybridization in all 20 strains of A. haemolyticus tested but not in 179 other Acinetobacter strains, including A. baumannii, A. lwoffii, A. junii, and A. johnsonii and genospecies 3, 6, 11, 13, 14, 15, 16, and 17, of which 162 were amikacin resistant. The probe did not hybridize in dot blot assays with DNAs purified from members of the families Enterobacteriaceae and Pseudomonadaceae that encode 6'-N-acetyltransferases. These data suggest that the aac(6')-Ig gene is species specific and may be used to identify A. haemolyticus.

Nucleotide sequence analysis and DNA hybridization studies of the ant(4')-IIa gene from Pseudomonas aeruginosa

The ant(4')-IIa gene was previously cloned from Pseudomonas aeruginosa on a 1.6-kb DNA fragment (G. A. Jacoby, M. J. Blaser, P. Santanam, H. Hächler, F. H. Kayser, R. S. Hare, and G. H. Miller, Antimicrob. Agents Chemother. 34:2381-2386, 1990). In the current study, the ant(4')-IIa gene was localized by gamma-delta mutagenesis. A region of approximately 600 nucleotides which contained the ant(4')-IIa gene was identified, and DNA sequence analysis revealed two overlapping open reading frames (ORFs) within this region. Northern (RNA) blot analysis demonstrated expression of both ORFs in P. aeruginosa; therefore, site-directed mutagenesis was used to identify the ORF which encodes the ant(4')-IIa gene. No homology was found between ant(4')-IIa and ant(4')-Ia DNA sequences. Hybridization experiments confirmed that the ant(4')-Ia probe hybridized only to gram-positive presumptive ANT(4')-I strains and that the ant(4')-IIa probe hybridized only to gram-negative strains presumed to carry ANT(4')-II. Seven gram-negative strains which had been classified as having ANT(4')-II resistance profiles did not hybridize with probes for either ant(4')-Ia or ant(4')-IIa, suggesting that at least one additional ant(4') gene may exist. The predicted amino-terminal sequences of the ANT(4')-Ia and ANT(4')-IIa proteins showed significant sequence similarity between residues 38 and 63 of the ANT(4')-Ia protein and residues 26 and 51 of the ANT(4')-IIa protein.

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.

Cloning and DNA sequence analysis of an aac(3)-Vb gene from Serratia marcescens

The AAC(3)-V resistance mechanism is characterized by high-level resistance to the aminoglycosides gentamicin, netilmicin, 2'-N-ethylnetilmicin, and 6'-N-ethylnetilmicin and moderate resistance levels to tobramycin. Serratia marcescens 82041944 contains an AA(3)-V resistance mechanism as determined from aminoglycoside resistance profiles. This strain, however, does not exhibit hybridization with a probe derived from the previously cloned aac(3)-Va gene, (R. Allmansberger, B. Bräu, and W. Piepersberg, Mol. Gen. Genet. 198:514-520, 1985). High-pressure liquid chromatography analysis of the acetylation products of sisomicin carried out by extracts of S. marcescens 82041944 have demonstrated the presence of an AAC(3) enzyme. We have cloned the gene encoding this acetyltransferase and have designated it aac(3)-Vb. Nucleotide sequence comparisons show that the aac(3)-Va and aac(3)-Vb genes are 72% identical. The predicted AAC(3)-Vb protein is 28,782 Da. Comparisons of the deduced amino acid sequences show 75% identity and 84% similarity between the AAC(3)-Va and AAC(3)-Vb proteins. The use of a DNA fragment internal to the aac(3)-Vb as a hybridization probe demonstrated that the aac(3)-Vb gene is very rare in clinical isolates possessing an AAC(3)-V mechanism.

Characterization of the chromosomal aac(6')-Ic gene from Serratia marcescens

The DNA sequence of the chromosomal aac(6')-Ic gene from Serratia marcescens, which had been previously cloned (H. M. Champion, P. M. Bennett, D. A. Lewis, and D. S. Reeves, J. Antimicrob. Chemother. 22:587-596, 1988) was determined. High-pressure liquid chromatographic analysis of extracts prepared from Escherichia coli carrying the chromosomal aac(6')-Ic gene on a plasmid confirmed the presence of 6'-N-acetyltransferase activity in this strain, which was suggested by the aminoglycoside resistance profile. DNA sequence analysis of the cloned 2,057-bp PstI fragment revealed several regions of homology to previously characterized sequences from GenBank, including the rpoD and tRNA-2 genes of E. coli. Subcloning experiments confirmed the coding sequence of the aac(6')-Ic gene to be at positions 1554 to 1992. The predicted amino acid sequence of the AAC(6')-Ic protein suggested that it was the third member of a family of AAC(6') proteins which included a coding region identified between the aadB and aadA genes of Tn4000 and an AAC(6') protein encoded by pUO490, which was isolated from Enterobacter cloacae. Primer extension analysis suggested that the -35 region of the aac(6')-Ic promoter overlapped a large palindromic sequence which may be involved in the regulation of the aac(6')-Ic gene. Hybridization experiments utilizing a restriction fragment from the aac(6')-Ic gene showed that all S. marcescens organisms carried this gene whether or not the AAC(6')-I resistance profile was expressed. Organisms other than Serratia spp. did not hybridize to this probe.