Transfer of amikacin resistance by closely related plasmids in members of the family Enterobacteriaceae isolated in Chile

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.

Antibiotic Resistance in Long-Term Care Facilities

Long-term care facilities are comprised of a heterogeneous group of institutions caring for residential patients over prolonged periods of time. Included as long-term care facilities in this review are private and Veterans' Affairs (VA) nursing homes, rehabilitation centers, institutions for the developmentally disabled, and hospital wards for both long-term and intermediate care. Patients in long-term care facilities incur bacterial infections at a prevalence of 10% to 16%. These infections usually are caused by common bacterial pathogens that invade the compromised host residing within a complex physical environment. The high prevalence of institutional infections leads, in turn, to the need for multiple courses of antimicrobials or for hospitalization. This process selects strains more resistant to antibiotics, which are then available for repeated dispersal in the long-term care facility.

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.

Diversity of aminoglycoside resistance in Enterobacter cloacae in Greece

Ninety Enterobacter cloacae strains isolated from 12 Greek hospitals were examined in terms of epidemiological types and resistance mechanisms. Using O serotyping 69% of the strains were assigned to a specific serotype and overall 16 different serotypes were identified. The combination of serotyping, phagetyping and biotyping efficiently discriminated most of the strains, indicating that single epidemic strains were not prevalent, although serotypes 3, 7, and group II predominated. Eight representative strains, all resistant to gentamicin, tobramycin, amikacin and netilmicin, were further examined for transferability and mechanisms of resistance. Aminoglycoside resistance was found to be transferable in most strains, and 13 R plasmids of 40-120 MDa molecular weight were detected. The enzymes detected consisted of three enzymes active against gentamicin [ANT(2h'), AAC(3)-I and AAC(3)-V]; three active against tobramycin [ANT(2"), AAC(3)-V and AAC(6')-I]; two active against netilmicin [AAC(3)-V and AAC(6')-I]; and one active against amikacin [AAC(6')-I]. APH(3') and ANT (3"), which modify neomycin and streptomycin plus spectinomycin respectively, were also found. Overall up to five aminoglycoside modifying enzymes were detected on the same R plasmid, AAC(6')-I plus ANT(2") being the most prevalent. The high incidence of multiresistance in Enterobacter cloacae and the fact that resistance is due to enzymatic inactivation of the antibiotics, indicate that in Greece this species might act as a gene pool for the spread of resistance to other bacteria of clinical relevance.

Nosocomial spread of an amikacin resistance gene on both a mobilized, nonconjugative plasmid and a conjugative plasmid

Resistance to amikacin among members of the family Enterobacteriaceae at a hospital in Venezuela rose from 2% in 1979 to 5% in 1984 and 10% in 1985 as amikacin usage rose 20-fold to exceed gentamicin usage. Resistance to gentamicin remained at 25 to 27%. We examined the plasmids from 21 isolates obtained in 1984 and 1985. Nine of eleven in 1984 and three of ten in 1985 carried aacA and sul on a 3.8-kb BamHI fragment of pBWH300, a 10.4-kb nonconjugative plasmid that had been mobilized into strains of six species by at least two different coresident conjugative plasmids. Six 1985 isolates of two species carried these genes on a similar BamHI fragment of the 104-kb conjugative plasmid pBWH303. One isolate in 1984 and one in 1985 carried the 69-kb conjugative plasmid pBWH301, which had aacA as the promoter-proximal gene of an operon that also encompassed the cat and aadB resistance genes. Another conjugative plasmid, pBWH302, was found in a single isolate. It carried a different aacA allele on the functional transposon Tn654, which appeared to be closely related to Tn1331, a transposon previously isolated in Argentina and Chile. Increased selection may thus have led to dissemination of an endemic aacA allele on two endemic plasmids, one spread by mobilization, with occasional intrusion of additional aacA alleles from outside.

Antibiotic resistance in long-term care facilities

Long-term care facilities are comprised of a heterogeneous group of institutions caring for residential patients over prolonged periods of time. Included as long-term care facilities in this review are private and Veterans' Affairs (VA) nursing homes, rehabilitation centers, institutions for the developmentally disabled, and hospital wards for both long-term and intermediate care. Patients in long-term care facilities incur bacterial infections at a prevalence of 10% to 16%. These infections usually are caused by common bacterial pathogens that invade the compromised host residing within a complex physical environment. The high prevalence of institutional infections leads, in turn, to the need for multiple courses of antimicrobials or for hospitalization. This process selects strains more resistant to antibiotics, which are then available for repeated dispersal in the long-term care facility.

Molecular epidemiology of TEM-3 (CTX-1) beta-lactamase

A total of 33 clinical isolates encoding TEM-3 (CTX-1) from four French hospitals were studied. The strains belonged to seven species, Klebsiella pneumoniae (n = 24), Escherichia coli (n = 3), Serratia marcescens (n = 2), Citrobacter freundii (n = 1), Enterobacter aerogenes (n = 1), Enterobacter cloacae (n = 1), and Klebsiella oxytoca (n = 1). All the strains harbored an Inc7 or M self-transferable plasmid with a size of approximately 85 kilobases. The plasmids had closely related EcoRI, HincII, HindIII, and PvuII restriction endonuclease-generated patterns and conferred resistance to all beta-lactams, except cephamycins and imipenem; to tetracycline, because of the presence of the genes blatem-3 and tetC, respectively, as determined by hybridization with specific probes; and to sulfonamide. Depending on the presence or absence and level of expression of the genes aacA4, aadA, and dfrI and of insertion element IS15, four types of plasmids could be distinguished. Plasmid pCFF04, the prototype plasmid encoding TEM-3, was widespread and appeared, by Southern hybridization, as the progenitor of the other types of replicons. The plasmid epidemic responsible for dissemination of TEM-3 in clinical isolates of members of the family Enterobacteriaceae may have originated in S. marcescens since pCFF04 was first detected in this species.

Antimicrobial susceptibility of selected bacterial enteropathogens in Latin America and worldwide

We conducted an in vitro susceptibility study of bacterial pathogens to various antimicrobials. Strains of Shigella, Salmonella, Escherichia coli, and Klebsiella pneumoniae collected in the 1960s, 1970s, and 1980s at the Hospital Infantil de Mexico Federico Gomez were tested against ampicillin, chloramphenicol, tetracycline, trimethoprim-sulfamethoxazole, amikacin, gentamicin, and furazolidone. Over the 3-decade period, the resistance of enteropathogens to furazolidone showed the least overall increase. Klebsiella susceptibility to the aminoglycosides decreased during the same period. Worldwide reports of enteropathogenic resistance to antimicrobials are also reviewed. In comparing the results of these worldwide studies with our own, we conclude that there is a need for periodic surveillance and testing of bacterial resistance to antimicrobials.

Aminoglycosides: current role in antimicrobial therapy

Aminoglycosides remain the cornerstone of antibiotic therapy for nosocomial, gram-negative bacillary infections despite the recent introduction of broad-spectrum beta-lactam antibiotics and quinolones with antipseudomonal activity. Initially, aminoglycosides were used as antiaerobic gram-negative antimicrobial therapy. Currently, they have a key role in many types of infections, such as gram-negative urosepsis and in febrile granulocytopenic patients, because of their established antipseudomonal activity. Empiric treatment of febrile episodes in granulocytopenic cancer patients with an aminoglycoside, in combination with an anti-pseudomonal beta-lactam, accounts for much of the aminoglycoside use. Amikacin is emerging as one of the most effective aminoglycosides on the basis of resistance rates, pharmacokinetic factors likely to affect clinical efficacy, safety, and overall cost of therapy.

Transposon-mediated amikacin resistance in Klebsiella pneumoniae

A multiresistant Klebsiella pneumoniae strain isolated from neonates in Mendoza, Argentina, harbored a 48-kilobase-pair (kbp) plasmid, pMET1, with genetic determinants for resistance to amikacin and also ampicillin, kanamycin, streptomycin, and tobramycin. This plasmid was compared with pJHCMW1, a previously isolated 11-kbp plasmid carrying transposon Tn1331, which encodes resistance to amikacin, as well as ampicillin, kanamycin, streptomycin, and tobramycin, and which was originally present in a K. pneumoniae strain that caused an outbreak in a hospital in Buenos Aires, Argentina. The comparison demonstrated that the replication regions of the two plasmids are unrelated. However, in pMET1 an 11-kbp transposition element, Tn1331.2, was identified; it was closely related to Tn1331, with the difference that a 3-kbp BamHI DNA fragment carrying the aminoglycoside resistance genes was duplicated in tandem.

Heterogeneity of 6'-N-acetyltransferases of type 4 conferring resistance to amikacin and related aminoglycosides in members of the family Enterobacteriaceae

DNA-DNA hybridization and immunoblotting were used to assess the relatedness between the 6'-N-acetyltransferases of type 4 encoded by plasmid pAZ007 from a clinical isolate of Serratia marcescens and those encoded by NR79 and R5. The absence of detectable DNA-DNA homology and of immunological cross-reactivity suggests the existence of at least two distinct 6'-N-acetyltransferase type 4 genes that mediate amikacin resistance in gram-negative bacilli.

Use of plasmid profiles in epidemiologic surveillance of disease outbreaks and in tracing the transmission of antibiotic resistance

Plasmids are circular deoxyribonucleic acid molecules that exist in bacteria, usually independent of the chromosome. The study of plasmids is important to medical microbiology because plasmids can encode genes for antibiotic resistance or virulence factors. Plasmids can also serve as markers of various bacterial strains when a typing system referred to as plasmid profiling, or plasmid fingerprinting is used. In these methods partially purified plasma deoxyribonucleic acid species are separated according to molecular size by agarose gel electrophoresis. In a second procedure, plasmid deoxyribonucleic acid which has been cleaved by restriction endonucleases can be separated by agarose gel electrophoresis and the resulting pattern of fragments can be used to verify the identity of bacterial isolates. Because many species of bacteria contain plasmids, plasmid profile typing has been used to investigate outbreaks of many bacterial diseases and to trace inter- and intra-species spread of antibiotic resistance.

Primary structure of an aminoglycoside 6'-N-acetyltransferase AAC(6')-4, fused in vivo with the signal peptide of the Tn3-encoded beta-lactamase

The gene aacA4 encoding an aminoglycoside 6'-N-acetyltransferase, AAC(6')-4, was cloned from a natural multiresistance plasmid, and its nucleotide sequence was determined. The gene was 600 base pairs (bp) long, and the AAC(6')-4 had a calculated molecular size of 22.4 kilodaltons and an isoelectric point of 5.35. The sequence of the 17 N-terminal amino acids was determined from the purified enzyme. The AAC(6')-4 gene was part of a resistance gene cluster, and its expression was under the control of the regulatory sequences of the beta-lactamase encoded by Tn3. The five N-terminal amino acids were identical to those of the signal peptide of the Tn3-encoded beta-lactamase, and the entire 5' region of aacA4, as far as it was sequenced (354 bp, including the promoter and the ribosome-binding site sequences), was identical to that of the beta-lactamase gene. This led us to presume an in vivo fusion between the beta-lactamase and the acetyltransferase genes. The latter was followed, in a polycistronic arrangement, by an aminoglycoside 3",9-adenylyltransferase gene, aadA, with an intergenic region of 68 bp. At a distance of ca. 1.3 kilobases in the 3' direction, we found remnants of a second Tn3-like element specifying an active beta-lactamase. At their 5' extremities, the two incomplete copies of Tn3, which were in tandem orientation, were interrupted within the resolvase gene. We speculate that Tn3-related sequences have played a role in the process of selection and dissemination of the AAC(6')-4 gene, which specifies resistance to amikacin and related aminoglycosides.

Tn1331, a novel multiresistance transposon encoding resistance to amikacin and ampicillin in Klebsiella pneumoniae

A 7.5-kilobase-pair multiresistance transposon, Tn1331, harboring amikacin resistance was identified as part of Klebsiella pneumoniae plasmid pJHCMW1. Restriction mapping, hybridization, and transposition complementation experiments demonstrated that Tn1331 belongs to the Tn3 family. Its structure is similar to that of Tn3 with the insertion of a DNA fragment encoding resistance to amikacin, kanamycin, and tobramycin.