Characterization of different plasmid-borne dihydropteroate synthases mediating bacterial resistance to sulfonamides

Molecular mechanism of plasmid-borne resistance to sulfonamide antibiotics

The sulfonamides (sulfas) are the oldest class of antibacterial drugs and inhibit the bacterial dihydropteroate synthase (DHPS, encoded by folP), through chemical mimicry of its co-substrate p-aminobenzoic acid (pABA). Resistance to sulfa drugs is mediated either by mutations in folP or acquisition of sul genes, which code for sulfa-insensitive, divergent DHPS enzymes. While the molecular basis of resistance through folP mutations is well understood, the mechanisms mediating sul-based resistance have not been investigated in detail. Here, we determine crystal structures of the most common Sul enzyme types (Sul1, Sul2 and Sul3) in multiple ligand-bound states, revealing a substantial reorganization of their pABA-interaction region relative to the corresponding region of DHPS. We use biochemical and biophysical assays, mutational analysis, and in trans complementation of E. coli ΔfolP to show that a Phe-Gly sequence enables the Sul enzymes to discriminate against sulfas while retaining pABA binding and is necessary for broad resistance to sulfonamides. Experimental evolution of E. coli results in a strain harboring a sulfa-resistant DHPS variant that carries a Phe-Gly insertion in its active site, recapitulating this molecular mechanism. We also show that Sul enzymes possess increased active site conformational dynamics relative to DHPS, which could contribute to substrate discrimination. Our results reveal the molecular foundation for Sul-mediated drug resistance and facilitate the potential development of new sulfas less prone to resistance.

Antibacterial Properties and Computational Insights of Potent Novel Linezolid-Based Oxazolidinones

The mounting evidence of bacterial resistance against commonly prescribed antibiotics warrants the development of new antibacterial drugs on an urgent basis. Linezolid, an oxazolidinone antibiotic, is a lead molecule in designing new oxazolidinones as antibacterial agents. In this study, we report the antibacterial potential of the novel oxazolidinone-sulphonamide/amide conjugates that were recently reported by our research group. The antibacterial assays showed that, from the series, oxazolidinones 2 and 3a exhibited excellent potency (MIC of 1.17 μg/mL) against B. subtilis and P. aeruginosa strains, along with good antibiofilm activity. Docking studies revealed higher binding affinities of oxazolidinones 2 and 3a compared to linezolid, which were further validated by molecular dynamics simulations. In addition to this, other computational studies, one-descriptor (log P) analysis, ADME-T and drug likeness studies demonstrated the potential of these novel linezolid-based oxazolidinones to be taken forward for further studies.

Folic Acid Antagonists: Antimicrobial and Immunomodulating Mechanisms and Applications

: Bacterial, protozoan and other microbial infections share an accelerated metabolic rate. In order to ensure a proper functioning of cell replication and proteins and nucleic acids synthesis processes, folate metabolism rate is also increased in these cases. For this reason, folic acid antagonists have been used since their discovery to treat different kinds of microbial infections, taking advantage of this metabolic difference when compared with human cells. However, resistances to these compounds have emerged since then and only combined therapies are currently used in clinic. In addition, some of these compounds have been found to have an immunomodulatory behavior that allows clinicians using them as anti-inflammatory or immunosuppressive drugs. Therefore, the aim of this review is to provide an updated state-of-the-art on the use of antifolates as antibacterial and immunomodulating agents in the clinical setting, as well as to present their action mechanisms and currently investigated biomedical applications.

Revitalizing antifolates through understanding mechanisms that govern susceptibility and resistance

In prokaryotes and eukaryotes, folate (vitamin B9) is an essential metabolic cofactor required for all actively growing cells. Specifically, folate serves as a one-carbon carrier in the synthesis of amino acids (such as methionine, serine, and glycine), N-formylmethionyl-tRNA, coenzyme A, purines and thymidine. Many microbes are unable to acquire folates from their environment and rely on de novo folate biosynthesis. In contrast, mammals lack the de novo folate biosynthesis pathway and must obtain folate from commensal microbiota or the environment using proton-coupled folate transporters. The essentiality and dichotomy between mammalian and bacterial folate biosynthesis and utilization pathways make it an ideal drug target for the development of antimicrobial agents and cancer chemotherapeutics. In this minireview, we discuss general aspects of folate biosynthesis and the underlying mechanisms that govern susceptibility and resistance of organisms to antifolate drugs.

Mechanisms of Bacterial Resistance to Antimicrobial Agents

During the past decades resistance to virtually all antimicrobial agents has been observed in bacteria of animal origin. This chapter describes in detail the mechanisms so far encountered for the various classes of antimicrobial agents. The main mechanisms include enzymatic inactivation by either disintegration or chemical modification of antimicrobial agents, reduced intracellular accumulation by either decreased influx or increased efflux of antimicrobial agents, and modifications at the cellular target sites (i.e., mutational changes, chemical modification, protection, or even replacement of the target sites). Often several mechanisms interact to enhance bacterial resistance to antimicrobial agents. This is a completely revised version of the corresponding chapter in the book Antimicrobial Resistance in Bacteria of Animal Origin published in 2006. New sections have been added for oxazolidinones, polypeptides, mupirocin, ansamycins, fosfomycin, fusidic acid, and streptomycins, and the chapters for the remaining classes of antimicrobial agents have been completely updated to cover the advances in knowledge gained since 2006.

Folate Biosynthesis, Reduction, and Polyglutamylation and the Interconversion of Folate Derivatives

Many microorganisms and plants possess the ability to synthesize folic acid derivatives de novo, initially forming dihydrofolate. All the folic acid derivatives that serve as recipients and donors of one-carbon units are derivatives of tetrahydrofolate, which is formed from dihydrofolate by an NADPH-dependent reduction catalyzed by dihydrofolate reductase (FolA). This review discusses the biosynthesis of dihydrofolate monoglutamate, its reduction to tetrahydrofolate monoglutamate, and the addition of glutamyl residues to form folylpolyglutamates. Escherichia coli and Salmonella, like many microorganisms that can synthesize folate de novo, appear to lack the ability to transport folate into the cell and are thus highly susceptible to inhibitors of folate biosynthesis. The review includes a brief discussion of the inhibition of folate biosynthesis by sulfa drugs. The folate biosynthetic pathway can be divided into two sections. First, the aromatic precursor chorismate is converted to paminobenzoic acid (PABA) by the action of three proteins. Second, the pteridine portion of folate is made from GTP and coupled to PABA to generate dihydropteroate, and the bifunctional protein specified by folC, dihydrofolate synthetase, or folylpolyglutamate synthetase, adds the initial glutamate molecule to form dihydrofolate (H2PteGlu1, or dihydropteroylmonoglutamate). Bacteriophage T4 infection of E. coli has been shown to cause alterations in the metabolism of folate derivatives. Infection is associated with an increase in the chain lengths in folylpolyglutamates and particularly the accumulation of hexaglutamate derivatives.

Increased abundance and transferability of resistance genes after field application of manure from sulfadiazine-treated pigs

Spreading manure containing antibiotics in agriculture is assumed to stimulate the dissemination of antibiotic resistance in soil bacterial populations. Plant roots influencing the soil environment and its microflora by exudation of growth substrates might considerably increase this effect. In this study, the effects of manure from pigs treated with sulfadiazine (SDZ), here called SDZ manure, on the abundance and transferability of sulfonamide resistance genes sul1 and sul2 in the rhizosphere of maize and grass were compared to the effects in bulk soil in a field experiment. In plots that repeatedly received SDZ manure, a significantly higher abundance of both sul genes was detected compared to that in plots where manure from untreated pigs was applied. Significantly lower abundances of sul genes relative to bacterial ribosomal genes were encountered in the rhizosphere than in bulk soil. However, in contrast to results for bulk soil, the sul gene abundance in the SDZ manure-treated rhizosphere constantly deviated from control treatments over a period of 6 weeks after manuring, suggesting ongoing antibiotic selection over this period. Transferability of sulfonamide resistance was analyzed by capturing resistance plasmids from soil communities into Escherichia coli. Increased rates of plasmid capture were observed in samples from SDZ manure-treated bulk soil and the rhizosphere of maize and grass. More than 97% of the captured plasmids belonged to the LowGC type (having low G+C content), giving further evidence for their important contribution to the environmental spread of antibiotic resistance. In conclusion, differences between bulk soil and rhizosphere need to be considered when assessing the risks associated with the spreading of antibiotic resistance.

Mutations and horizontal transmission have contributed to sulfonamide resistance in Streptococcus pyogenes

Two variants of dihydropteroate synthase (DHPS) were found among sulfonamide-resistant Streptococcus pyogenes, one of which was characterized by a 2-amino-acid addition in a conserved part of the enzyme. The enzyme kinetics of both variants was compared with the kinetics of DHPS from a sulfonamide-susceptible S. pyogenes. The most striking difference was a substantially elevated Ki for both variants, but variations in Km for both of its substrates p-aminobenzoic acid (p-AB) and dihydropteridine-pyrophosphate (pteridine) were also found. In the resistance variant lacking additions, the amino acid at position 213 was changed by site-directed mutagenesis from a Gly to an Arg, which resulted in a lower Ki. The corresponding change from an Arg to a Gly in the DHPS from a susceptible isolate led to a substantially increased Ki, confirming the importance of this amino acid difference for the resistance. Nucleotide sequence determinations of the complete folate operon revealed in some isolates a mosaic pattern of differences compared to the wild type, not only in the genes coding for DHPS and GTP cyclohydrolase (GTPCH) noted earlier but also in genes coding for dihydroneopterin aldolase (DHNA) and hydroxymethylpterin pyrophosphokinase (HPPK). Regions of sequence differences were interspersed with regions of complete identity in a mosaic pattern, indicating a dispersed pattern of uptake of foreign DNA in the resistant isolates.

A new sulfonamide resistance gene (sul3) in Escherichia coli is widespread in the pig population of Switzerland

A new gene, sul3, which specifies a 263-amino-acid protein similar to a dihydropteroate synthase encoded by the 54-kb conjugative plasmid pVP440 from Escherichia coli was characterized. Expression of the cloned sul3 gene conferred resistance to sulfamethoxazole on E. coli. Two copies of the insertion element IS15Delta/26 flanked the region containing sul3. The sul3 gene was detected in one-third of the sulfonamide-resistant pathogenic E. coli isolates from pigs in Switzerland.

Large drug resistance virulence plasmids of clinical isolates of Salmonella enterica serovar Choleraesuis

Salmonella enterica serovar Choleraesuis generally causes systemic human salmonellosis without diarrhea, and therefore, antimicrobial treatment is essential for such patients. The drug resistance information on this organism is thus of high value. Serovar Choleraesuis usually harbors a virulence plasmid (pSCV) of 50 kb in size. Of the 16 clinical isolates identified to be serovar Choleraesuis, all except one harbored a pSCV and seven of them carried a pSCV of more than 125 kb in size. A pSCV was defined as a plasmid carrying spvC and characteristic deletions detected by PCR and by DNA-DNA hybridization (for the former criterion). The results of PCR, restriction fragment profiles, and Southern DNA-DNA hybridizations of the profiles all indicated that such larger pSCVs were derived from the 50-kb plasmid recombined with non-pSCVs found in some clinical isolates. Fifteen of the 17 strains, including a laboratory strain, were then tested for drug resistance against 16 antibiotics with E-test and the dilution method. The laboratory strain, which harbored a 50-kb pSCV and a 6-kb non-pSCV, was resistant only to sulfonamides (SUL), and its resistance gene, sulII, checked with PCR and DNA-DNA hybridization, was located on the 6-kb non-pSCV. All 14 clinical strains were resistant to multiple drugs. Of the 14, 7 were resistant to SUL, and the resistance gene was located on a plasmid. The sulII gene, but not bla(TEM-1), was carried only on the 6-kb non-pSCV. Of the remaining six large plasmids, three of 90 kb, two of 136 kb, and one of 140 kb, the last three were pSCVs and carried the other SUL gene (sulI) and the bla(TEM-1) gene. The six strains were also resistant to trimethoprim-sulfamethoxazole. None of the 50-kb pSCVs carried resistance genes. These drug resistance genes on the large pSCVs were apparently also acquired through recombination.

p-Aminobenzoic acid and chloramphenicol biosynthesis in Streptomyces venezuelae: gene sets for a key enzyme, 4-amino-4-deoxychorismate synthase

Amplification of sequences from Streptomyces venezuelae ISP5230 genomic DNA using PCR with primers based on conserved prokaryotic pabB sequences gave two main products. One matched pabAB, a locus previously identified in S. venezuelae. The second closely resembled the conserved pabB sequence consensus and hybridized with a 3.8 kb NcoI fragment of S. venezuelae ISP5230 genomic DNA. Cloning and sequence analysis of the 3.8 kb fragment detected three ORFs, and their deduced amino acid sequences were used in BLAST searches of the GenBank database. The ORF1 product was similar to PabB in other bacteria and to the PabB domain encoded by S. venezuelae pabAB. The ORF2 product resembled PabA of other bacteria. ORF3 was incomplete; its deduced partial amino acid sequence placed it in the MocR group of GntR-type transcriptional regulators. Introducing vectors containing the 3.8 kb NcoI fragment of S. venezuelae DNA into pabA and pabB mutants of Escherichia coli, or into the Streptomyces lividans pab mutant JG10, enhanced sulfanilamide resistance in the host strains. The increased resistance was attributed to expression of the pair of discrete translationally coupled p-aminobenzoic acid biosynthesis genes (designated pabB/pabA) cloned in the 3.8 kb fragment. These represent a second set of genes encoding 4-amino-4-deoxychorismate synthase in S. venezuelae ISP5230. In contrast to the fused pabAB set previously isolated from this species, they do not participate in chloramphenicol biosynthesis, but like pabAB they can be disrupted without affecting growth on minimal medium. The gene disruption results suggest that S. venezuelae may have a third set of genes encoding PABA synthase.

Amino acid repetitions in the dihydropteroate synthase of Streptococcus pneumoniae lead to sulfonamide resistance with limited effects on substrate K(m)

Sulfonamide resistance in Streptococcus pneumoniae is due to changes in the chromosomal folP (sulA) gene coding for dihydropteroate synthase (DHPS). The first reported laboratory-selected sulfonamide-resistant S. pneumoniae isolate had a 6-bp repetition, the sul-d mutation, leading to a repetition of the amino acids Ile(66) and Glu(67) in the gene product DHPS. More recently, clinical isolates showing this and other repetitions have been reported. WA-5, a clinical isolate from Washington State, contains a 6-bp repetition in the folP gene, identical to the sul-d mutation. The repetition was deleted by site-directed mutagenesis. Enzyme kinetic measurements showed that the deletion was associated with a 35-fold difference in K(i) for sulfathiazole but changed the K(m) for p-aminobenzoic acid only 2.5-fold and did not significantly change the K(m) for 2-amino-4-hydroxy-6-hydroxymethyl-7,8-dihydropteridine pyrophosphate. The enzyme characteristics of the deletion variant were identical to those of DHPS from a sulfonamide-susceptible strain. DHPS from clinical isolates with repetitions of Ser(61) had very similar enzyme characteristics to the DHPS from WA-5. The results confirm that the repetitions are sufficient for development of a resistant enzyme and suggest that the fitness cost to the organism of developing resistance may be very low.

Additive effects of a two-amino-acid insertion and a single-amino-acid substitution in dihydropteroate synthase for the development of sulphonamide-resistant Neisseria meningitidis

Sulphonamide resistance in some clinical isolates of Neisseria meningitidis is associated with an insertion in the chromosomal folP gene leading to the addition of two amino acids, serine and glycine, in the drug target enzyme dihydropteroate synthase (DHPS). Removal of the insertion resulted in a markedly higher Km for the substrate p-aminobenzoic acid and a markedly lower Km for 2-amino-4-hydroxy-6-(hydroxymethyl)-7,8-dihydropteridine pyrophosphate. In the same isolates an additional important difference, compared to wild-type enzymes, was found at amino acid position 68, which is a proline in most DHPS enzymes, but is serine in one and leucine in another clinical isolate of sulphonamide-resistant N. meningitidis. The alteration at position 68 was found to affect mainly the level of sulphonamide resistance and had only a minor effect on the Km for the substrates. Introduction of the serine-glycine dipeptide at position 194 and a proline to serine substitution at position 68 in DHPS from normal, susceptible N. meningitidis failed to produce a functional sulphonamide-resistant enzyme. The conclusion of this study is that it is not possible to change a normal chromosomally encoded DHPS of N. meningitidis to a sulphonamide-resistant one simply by an insertion of serine and glycine as seen in clinical isolates. It is likely that the resistance gene found in clinical isolates has evolved in another bacterial species where a combination of other amino acid changes may have contributed to produce a functionally resistant enzyme. This new resistance gene may have then been introduced into N. meningitidis by natural transformation.

Sulfonamide resistance in clinical isolates of Campylobacter jejuni: mutational changes in the chromosomal dihydropteroate synthase

The characterization of the genetic basis of sulfonamide resistance in Campylobacter jejuni was attempted. The resistance determinant from a sulfonamide-resistant strain of C. jejuni was cloned and was found to show 42% identity with the folP gene (which codes for dihydropteroate synthase, the target of sulfonamides) of the related bacterium Helicobacter pylori. The sequences of the areas surrounding the folP gene in C. jejuni showed similarity to those of the areas surrounding the corresponding gene in H. pylori. The folP gene of C. jejuni, which mediates the resistance, was observed to show particular features when it was compared to other known folP genes. One of these features is the presence of two pairs of direct repeats (15 and 27 bp) within the coding sequence of the gene. Comparison of the C. jejuni folP genes that mediate susceptibility and resistance revealed the occurrence of mutations that changed four amino acid residues. Resistance of C. jejuni to sulfonamides could be associated with one or several of these four mutational substitutions, which all occurred in the five different resistant isolates studied. The codon for one of these changed amino acids was found to be located in the second direct repeat within the coding sequence of the gene. The change made the repeat perfect. The transformation of both the resistance and the susceptibility variants of the gene into an Escherichia coli folP knockout mutant was found to complement the dihydropteroate synthase deficiency, confirming that the characterized sulfonamide resistance determinant codes for the C. jejuni dihydropteroate synthase enzyme. Kinetic measurements established different affinities of sulfonamide for the dihydropteroate synthase enzyme isolated from the resistant and susceptible strains. In conclusion, sulfonamide resistance in C. jejuni was shown to be associated with mutational changes in the chromosomally located gene for dihydropteroate synthase, the target of sulfonamides.

Sulfonamide resistance gene in a transferable R plasmid of Pasteurella piscicida

The sulfonamide resistance (SAr) determinant was cloned from a transferable R plasmid of Pasteurella piscicida, pSP9351, and the sequence was determined. The resistance gene (pp-sul) was localized to an approximately 1-kb region that includes the PstI-EcoRI site in the restriction map. An open reading frame coding a sul II-type gene composed of 810 nucleotides was identified. A direct repeat sequence was shown in the 5' flanking region of pp-sul, and a plasmid recombinational event may have occurred during the construction of pSP9351. In the 3' flanking region of the gene, a sequence homologous to the 5' noncoding sequence of the trimethoprim resistance gene, dhfr IX was found.

Sulfonamide resistance in Neisseria meningitidis as defined by site-directed mutagenesis could have its origin in other species

Sulfonamide resistance in Neisseria meningitidis is mediated by altered forms of the chromosomal gene for the drug target enzyme dihydropteroate synthase. Sulfonamides have been used for decades both for prophylaxis and the treatment of meningococcal disease, and resistance is common. Two types of resistance determinants have been identified, and regions important for drug insusceptibility to the corresponding enzyme have been defined by site-directed mutagenesis. Both types of resistance traits have spread among strains of N. meningitidis of different serogroups and serotypes, and the large differences at the nucleotide level in a comparison of the resistance genes with the dhps genes of susceptible meningococci indicate the origin of one or maybe both types in other Neisseria species. One sulfonamide-sensitive strain of N. meningitidis was found to have a mosaic dhps gene with a central part identical to the corresponding part of a gonococcal strain. This observation supports the idea of an interspecies transfer of genetic material in Neisseria species as a mechanism for the development of chromosomally mediated resistance.

Mechanisms of bacterial resistance

Antibiotic resistance is a growing problem in clinical pediatrics. Many of the agents traditionally used to treat pediatric pathogens are becoming less effective because of increasing bacterial resistance. In addition, many more children are immunocompromised because of primary or acquired immunodeficiencies and because of advances in cancer chemotherapy and transplantation. These children are being admitted to hospitals where they may be exposed to multiply resistant nosocomial pathogens. An improved understanding of the mechanisms of antibiotic resistance and the development of treatment strategies to prevent the emergence of resistance will be increasingly required in pediatrics.

Cloning, sequencing, and enhanced expression of the dihydropteroate synthase gene of Escherichia coli MC4100

The Escherichia coli gene coding for dihydropteroate synthase (DHPS) has been cloned and sequenced. The protein has 282 amino acids and a compositional molecular mass of 30,314 daltons. Increased expression of the enzyme was realized by using a T7 expression system. The enzyme was purified and crystallized. A temperature-sensitive mutant was isolated and found to express a DHPS with a lower specific activity and lower affinities for para-aminobenzoic acid and sulfathiazole. The allele had a point mutation that changed a phenylalanine codon to a leucine codon, and the mutation was in a codon that is conserved among published DHPS sequences.

Genetic analyses of sulfonamide resistance and its dissemination in gram-negative bacteria illustrate new aspects of R plasmid evolution

In contrast to what has been observed for many other antibiotic resistance mechanisms, there are only two known genes encoding plasmid-borne sulfonamide resistance. Both genes, sulI and sulII, encode a drug-resistant dihydropteroate synthase enzyme. In members of the family Enterobacteriaceae isolated from several worldwide sources, plasmid-mediated resistance to sulfonamides could be identified by colony hybridization as being encoded by sulI, sulII, or both. The sulI gene was in all cases found to be located in the newly defined, mobile genetic element, recently named an integron, which has been shown to contain a site-specific recombination system for the integration of various antibiotic resistance genes. The sulII gene was almost exclusively found as part of a variable resistance region on small, nonconjugative plasmids. Colony hybridization to an intragenic probe, restriction enzyme digestion, and nucleotide sequence analysis of small plasmids indicated that the sulII gene and contiguous sequences represent an independently occurring region disseminated in the bacterial population. The sulII resistance region was bordered by direct repeats, which in some plasmids were totally or partially deleted. The prevalence of sulI and sulII could thus be accounted for by their stable integration in transposons and in plasmids that are widely disseminated among gram-negative bacteria.

Characterization of sulfonamide resistance determinants and relatedness of Bordetella avium R plasmids

Five plasmids, varying in size from 16 to 51.5 kb, were isolated from virulent strains of Bordetella avium and compared by restriction endonuclease digestion and DNA-DNA hybridization. These plasmids confer resistance to streptomycin and sulfonamides, and three of the five also confer resistance to tetracycline, but they are not closely related. Four of the plasmids, pRL100, p4093, pCW, and pWAM, carried determinants related to the heat-labile type I plasmid-mediated dihydropteroate synthase of the plasmid R388, while one plasmid, p4168, carried a determinant related to the heat-stable type II dihydropteroate synthase of pGS05.

Gene amplification contributes to sulfonamide resistance in Escherichia coli

A sulfathiazole-resistant strain of Escherichia coli was isolated and shown to contain a fourfold tandemly amplified segment of DNA 18 kilobase pairs in length in addition to a mutationally altered dihydropteroate synthase, the target enzyme for sulfonamide inhibition. The amplified DNA contained a gene designated sur that contributed to sulfathiazole resistance when present in greater amounts than those in the wild type. Sulfathiazole resistance was markedly decreased upon loss of the amplified DNA after nonselective growth. Plasmids that contained sur also conferred only weak sulfathiazole resistance on wild-type strains. Comparison of the restriction maps of the amplified DNA, wild-type DNA, and sur-containing plasmids showed that a DNA rearrangement occurred before or concomitant with the DNA amplification event. The DNA rearrangement resulted from an IS5 insertion, which, in conjunction with an IS5 element residing near sur in the wild-type strain, resulted in an -IS5-sur-IS5- configuration. Homologous recombination could account for duplication and subsequent amplification of the sur region. High-copy-number plasmids containing the sur locus did not express a sulfathiazole-resistant dihydropteroate synthase, nor did they overexpress wild-type dihydropteroate synthase. These data suggest that the high level of sulfathiazole resistance in this strain results from a synergistic effect of two different mutations.

RSF1010 and a conjugative plasmid contain sulII, one of two known genes for plasmid-borne sulfonamide resistance dihydropteroate synthase

The nucleotide sequence of the type II sulfonamide resistance dihydropteroate synthase (sulII) gene was determined. The molecular weight determined by maxicells was 30,000, and the predicted molecular weight for the polypeptide was 28,469. Comparison with the sulI gene encoded by Tn21 showed 57% DNA similarity. The sulII-encoded polypeptide has 138 of 271 amino acids in common with the polypeptide encoded by sulI. The sulII gene is located on various IncQ (broad-host-range) plasmids and other small nonconjugative resistance plasmids. Detailed restriction maps were constructed to compare the different plasmids in which sulII is found. The large conjugative plasmid pGS05 and the IncQ plasmid RSF1010 contained identical nucleotide sequences for the sulII gene. This type of sulfonamide resistance is very frequently found among gram-negative bacteria because of its efficient spread to various plasmids.

Site-specific recombination promotes linkage between trimethoprim- and sulfonamide resistance genes. Sequence characterization of dhfrV and sulI and a recombination active locus of Tn21

A new gene for trimethoprim resistance, dhfrV, found in several plasmid isolates with different characteristics, was sequenced and found to correspond to a peptide of 157 amino acids showing 75% similarity with the previously characterized, drug resistant dihydrofolate reductase of type I. The sequenced surroundings of dhfrV in plasmid pLMO20, were found to be almost identical with genetic areas surrounding resistance genes in transposon Tn21 and in R plasmid R388. The trimethoprim resistance genes of pLMO20 and R388 and the spectinomycin resistance gene of Tn21 could be regarded as having been inserted, by recombination, into an evolutionary older structure containing the sulfonamide resistance gene, sulI. The latter gene was sequenced and found to correspond to a peptide of 279 amino acids and with a molecular weight of 30,126 daltons. The inserted genes were found to be governed by a promoter situated in the highly conserved structure and also controlling expression of sulI. The insertion points of the different resistance genes were precisely defined, and at the 3' ends of the inserted genes inverted repeats allowing the formation of stem and loop structures were found. Similar structures were found at the 3' ends of the antibiotic resistance genes in Tn7, which could indicate similar recombination mechanisms to be effective in the evolutionary construction of all these different resistance elements.

Sulfonamide resistance in Streptococcus pneumoniae: DNA sequence of the gene encoding dihydropteroate synthase and characterization of the enzyme

A chromosomal gene of Streptococcus pneumoniae carrying a spontaneous mutation to sulfonamide resistance was identified. Comparison of its DNA sequence with the wild-type sequence showed that the mutation, sul-d, consisted of an insert of 6 base pairs, a repeat of an adjacent 6-base-pair segment. The gene encoded a 34-kilodalton polypeptide, SulA, which as a dimer or trimer constituted the enzyme dihydropteroate synthase. This was shown by enzyme activity measurements, expression in minicells of Bacillus subtilis, and the amino-terminal sequence of the polypeptide product. Subcloning of the gene in an Escherichia coli expression vector allowed purification of the enzyme to 80% homogeneity in a single step and at high yield. Although a deleted plasmid, pLS83, produced the mutant dihydropteroate synthase, it did not confer sulfonamide resistance in vivo. It is suggested that the SulA polypeptide is also a component of an enzyme that acts in another step of folate biosynthesis and that this step is inhibited in vivo by either free or conjugated sulfonamides.

Antimicrobial resistance of Staphylococcus aureus: genetic basis

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Organization of two sulfonamide resistance genes on plasmids of gram-negative bacteria

The organization of two widely distributed sulfonamide resistance genes has been studied. The type I gene was linked to other resistance genes, like streptomycin resistance in R100 and trimethoprim resistance in R388 and other recently isolated plasmids from Sri Lanka. In R388, the sulfonamide resistance gene was transcribed from a promoter of its own, but in all other studied plasmids the linked genes were transcribed from a common promoter. This was especially established with a clone derived from plasmid R6-5, in which transposon mutagenesis showed that expression of sulfonamide resistance was completely dependent on the linked streptomycin resistance gene. The type II sulfonamide resistance gene was independently transcribed and found on two kinds of small resistance plasmids and also on large plasmids isolated from clinical material.

Characterization of transferable plasmids from Shigella flexneri 2a that confer resistance to trimethoprim, streptomycin, and sulfonamides

A set of plasmids conferring resistance to several antibiotics, including the combination of trimethoprim and sulfamethoxazole, has been isolated from Escherichia coli following conjugative cotransfer from a clinical isolate of Shigella flexneri 2a. One of the plasmids, pCN1, was shown by subcloning and DNA sequencing to carry a gene encoding a trimethoprim-insensitive dihydrofolate reductase identical to that found in E. coli transposon 7. This plasmid was also shown to confer resistance to both streptomycin and spectinomycin by production of an adenylyltransferase that inactivated the drugs and the gene encoding this enzyme has also been sequenced. A second plasmid from the set, pCN2, was shown to inactivate streptomycin by a phosphotransferase mechanism and also to confer resistance to sulfonamides. The third plasmid from the set could not be correlated with a drug-resistance phenotype, but does appear to play a crucial role in plasmid mobilization.

Plasmid-specified aminoglycoside-modifying enzymes in clinical isolates of Klebsiella pneumoniae

In 106 clinical isolates of multiresistant Klebsiella pneumoniae strains, we found that aminoglycoside-resistance was due mostly to two adenylating enzymes: AAD (2") (56.6%), that modifies gentamicin, kanamycin, tobramycin and sissomicin, and AAD (3") 9 (56.6% + 19.8%) that modifies streptomycin and spectinomycin. The identification of these enzymes was possible by MICs determination against a set of aminoglycosides antibiotics. AAD (2") + AAD (3")9 were coded by conjugative plasmid of about 120 Md.

Plasmid-borne sulfonamide resistance determinants studied by restriction enzyme analysis

The relationship between sulfonamide resistance genes carried on different plasmids was investigated by restriction enzyme analysis and DNA-DNA hybridization. The results showed that sulfonamide resistance mediated by different plasmids is determined by the production of at least two different types of drug-resistant dihydropteroate synthase. Plasmids pGS01, pGS02, and R22259, found in bacteria isolated from patients in Swedish hospitals, contained identical sulfonamide resistance genes, which were also identical to those of plasmids R1, R100, R6, and R388. These latter plasmids, which have been well studied in different laboratories, were originally from clinical isolates from different parts of the world. Two other clinically isolated plasmids, pGS04 and pGS05, were shown to contain sulfonamide resistance determinants of a completely different type.