Transformational exchanges in the dihydropteroate synthase gene of Neisseria meningitidis: a novel mechanism for acquisition of sulfonamide resistance

Continuing genomic evolution of the Neisseria meningitidis cc11.2 urethritis clade, NmUC: a narrative review

Neisseria meningitidis (Nm) is a bacterial pathogen responsible for invasive meningococcal disease. Though typically colonizing the nasopharynx, multiple outbreaks of meningococcal urethritis were first reported in 2015-2016; outbreaks originally presumed to be caused by Neisseria gonorrhoeae (Ng). Genomic analysis revealed that the Nm isolates causing these outbreaks were a distinct clade, and had integrated gonococcal DNA at multiple genomic sites, including the gonococcal denitrification apparatus aniA-norB, a partial gonococcal operon of five genes containing ispD, and the acetylglutamate kinase gene argB with the adjacent gonococcal locus NGO0843. The urethritis isolates had also deleted the group C capsule biosynthesis genes cssA/B/C and csc, resulting in loss of capsule. Collectively, these isolates form the N. meningitidis urethritis clade (NmUC). Genomic analysis of recent (2016-2022) NmUC isolates revealed that the genomic features have been maintained in the clade, implying that they are important for NmUC's status as a urogenital pathogen. Furthermore, the analysis revealed the emergence of a sub-clade, designated NmUC-B, phylogenetically separated from the earlier NmUC-A. This sub-clade has integrated additional gonococcal alleles into the genome, including alleles associated with antimicrobial resistance. NmUC continues to adapt to a urethral niche and evolve as a urogenital pathogen.

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.

Origin of the Mobile Di-Hydro-Pteroate Synthase Gene Determining Sulfonamide Resistance in Clinical Isolates

Sulfonamides are synthetic chemotherapeutic agents that work as competitive inhibitors of the di-hydro-pteroate synthase (DHPS) enzyme, encoded by the folP gene. Resistance to sulfonamides is widespread in the clinical setting and predominantly mediated by plasmid- and integron-borne sul1-3 genes encoding mutant DHPS enzymes that do not bind sulfonamides. In spite of their clinical importance, the genetic origin of sul1-3 genes remains unknown. Here we analyze sul genes and their genetic neighborhoods to uncover sul signature elements that enable the elucidation of their genetic origin. We identify a protein sequence Sul motif associated with sul-encoded proteins, as well as consistent association of a phosphoglucosamine mutase gene (glmM) with the sul2 gene. We identify chromosomal folP genes bearing these genetic markers in two bacterial families: the Rhodobiaceae and the Leptospiraceae. Bayesian phylogenetic inference of FolP/Sul and GlmM protein sequences clearly establishes that sul1-2 and sul3 genes originated as a mobilization of folP genes present in, respectively, the Rhodobiaceae and the Leptospiraceae, and indicate that the Rhodobiaceae folP gene was transferred from the Leptospiraceae. Analysis of %GC content in folP/sul gene sequences supports the phylogenetic inference results and indicates that the emergence of the Sul motif in chromosomally encoded FolP proteins is ancient and considerably predates the clinical introduction of sulfonamides. In vitro assays reveal that both the Rhodobiaceae and the Leptospiraceae, but not other related chromosomally encoded FolP proteins confer resistance in a sulfonamide-sensitive Escherichia coli background, indicating that the Sul motif is associated with sulfonamide resistance. Given the absence of any known natural sulfonamides targeting DHPS, these results provide a novel perspective on the emergence of resistance to synthetic chemotherapeutic agents, whereby preexisting resistant variants in the vast bacterial pangenome may be rapidly selected for and disseminated upon the clinical introduction of novel chemotherapeuticals.

Antibiotic Resistance Mechanisms in Bacteria: Relationships Between Resistance Determinants of Antibiotic Producers, Environmental Bacteria, and Clinical Pathogens

Emergence of antibiotic resistant pathogenic bacteria poses a serious public health challenge worldwide. However, antibiotic resistance genes are not confined to the clinic; instead they are widely prevalent in different bacterial populations in the environment. Therefore, to understand development of antibiotic resistance in pathogens, we need to consider important reservoirs of resistance genes, which may include determinants that confer self-resistance in antibiotic producing soil bacteria and genes encoding intrinsic resistance mechanisms present in all or most non-producer environmental bacteria. While the presence of resistance determinants in soil and environmental bacteria does not pose a threat to human health, their mobilization to new hosts and their expression under different contexts, for example their transfer to plasmids and integrons in pathogenic bacteria, can translate into a problem of huge proportions, as discussed in this review. Selective pressure brought about by human activities further results in enrichment of such determinants in bacterial populations. Thus, there is an urgent need to understand distribution of resistance determinants in bacterial populations, elucidate resistance mechanisms, and determine environmental factors that promote their dissemination. This comprehensive review describes the major known self-resistance mechanisms found in producer soil bacteria of the genus Streptomyces and explores the relationships between resistance determinants found in producer soil bacteria, non-producer environmental bacteria, and clinical isolates. Specific examples highlighting potential pathways by which pathogenic clinical isolates might acquire these resistance determinants from soil and environmental bacteria are also discussed. Overall, this article provides a conceptual framework for understanding the complexity of the problem of emergence of antibiotic resistance in the clinic. Availability of such knowledge will allow researchers to build models for dissemination of resistance genes and for developing interventions to prevent recruitment of additional or novel genes into pathogens.

Antibiotic Resistance Mechanisms in Bacteria: Relationships Between Resistance Determinants of Antibiotic Producers, Environmental Bacteria, and Clinical Pathogens

Emergence of antibiotic resistant pathogenic bacteria poses a serious public health challenge worldwide. However, antibiotic resistance genes are not confined to the clinic; instead they are widely prevalent in different bacterial populations in the environment. Therefore, to understand development of antibiotic resistance in pathogens, we need to consider important reservoirs of resistance genes, which may include determinants that confer self-resistance in antibiotic producing soil bacteria and genes encoding intrinsic resistance mechanisms present in all or most non-producer environmental bacteria. While the presence of resistance determinants in soil and environmental bacteria does not pose a threat to human health, their mobilization to new hosts and their expression under different contexts, for example their transfer to plasmids and integrons in pathogenic bacteria, can translate into a problem of huge proportions, as discussed in this review. Selective pressure brought about by human activities further results in enrichment of such determinants in bacterial populations. Thus, there is an urgent need to understand distribution of resistance determinants in bacterial populations, elucidate resistance mechanisms, and determine environmental factors that promote their dissemination. This comprehensive review describes the major known self-resistance mechanisms found in producer soil bacteria of the genus Streptomyces and explores the relationships between resistance determinants found in producer soil bacteria, non-producer environmental bacteria, and clinical isolates. Specific examples highlighting potential pathways by which pathogenic clinical isolates might acquire these resistance determinants from soil and environmental bacteria are also discussed. Overall, this article provides a conceptual framework for understanding the complexity of the problem of emergence of antibiotic resistance in the clinic. Availability of such knowledge will allow researchers to build models for dissemination of resistance genes and for developing interventions to prevent recruitment of additional or novel genes into pathogens.

Various pathways leading to the acquisition of antibiotic resistance by natural transformation

Natural transformation can lead to exchange of DNA between taxonomically diverse bacteria. In the case of chromosomal DNA, homology-based recombination with the recipient genome is usually necessary for heritable stability. In our recent study, we have shown that natural transformation can promote the transfer of transposons, IS elements, and integrons and gene cassettes, largely independent of the genetic relationship between the donor and recipient bacteria. Additional results from our study suggest that natural transformation with species-foreign DNA might result in the uptake of a wide range of DNA fragments; leading to changes in the antimicrobial susceptibility profile and contributing to the generation of antimicrobial resistance in bacteria.

Surveillance of invasive Neisseria meningitidis with a serogroup Y update, Sweden 2010 to 2012

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A recently transferred cluster of bacterial genes in Trichomonas vaginalis--lateral gene transfer and the fate of acquired genes

Background

Lateral Gene Transfer (LGT) has recently gained recognition as an important contributor to some eukaryote proteomes, but the mechanisms of acquisition and fixation in eukaryotic genomes are still uncertain. A previously defined norm for LGTs in microbial eukaryotes states that the majority are genes involved in metabolism, the LGTs are typically localized one by one, surrounded by vertically inherited genes on the chromosome, and phylogenetics shows that a broad collection of bacterial lineages have contributed to the transferome.

Results

A unique 34 kbp long fragment with 27 clustered genes (TvLF) of prokaryote origin was identified in the sequenced genome of the protozoan parasite Trichomonas vaginalis. Using a PCR based approach we confirmed the presence of the orthologous fragment in four additional T. vaginalis strains. Detailed sequence analyses unambiguously suggest that TvLF is the result of one single, recent LGT event. The proposed donor is a close relative to the firmicute bacterium Peptoniphilus harei. High nucleotide sequence similarity between T. vaginalis strains, as well as to P. harei, and the absence of homologs in other Trichomonas species, suggests that the transfer event took place after the radiation of the genus Trichomonas. Some genes have undergone pseudogenization and degradation, indicating that they may not be retained in the future. Functional annotations reveal that genes involved in informational processes are particularly prone to degradation.

Conclusions

We conclude that, although the majority of eukaryote LGTs are single gene occurrences, they may be acquired in clusters of several genes that are subsequently cleansed of evolutionarily less advantageous genes.

Application of microarray and functional-based screening methods for the detection of antimicrobial resistance genes in the microbiomes of healthy humans

The aim of this study was to screen for the presence of antimicrobial resistance genes within the saliva and faecal microbiomes of healthy adult human volunteers from five European countries. Two non-culture based approaches were employed to obviate potential bias associated with difficult to culture members of the microbiota. In a gene target-based approach, a microarray was employed to screen for the presence of over 70 clinically important resistance genes in the saliva and faecal microbiomes. A total of 14 different resistance genes were detected encoding resistances to six antibiotic classes (aminoglycosides, β-lactams, macrolides, sulphonamides, tetracyclines and trimethoprim). The most commonly detected genes were erm(B), bla_TEM, and sul2. In a functional-based approach, DNA prepared from pooled saliva samples was cloned into Escherichia coli and screened for expression of resistance to ampicillin or sulphonamide, two of the most common resistances found by array. The functional ampicillin resistance screen recovered genes encoding components of a predicted AcrRAB efflux pump. In the functional sulphonamide resistance screen, folP genes were recovered encoding mutant dihydropteroate synthase, the target of sulphonamide action. The genes recovered from the functional screens were from the chromosomes of commensal species that are opportunistically pathogenic and capable of exchanging DNA with related pathogenic species. Genes identified by microarray were not recovered in the activity-based screen, indicating that these two methods can be complementary in facilitating the identification of a range of resistance mechanisms present within the human microbiome. It also provides further evidence of the diverse reservoir of resistance mechanisms present in bacterial populations in the human gut and saliva. In future the methods described in this study can be used to monitor changes in the resistome in response to antibiotic therapy.

Genetic basis for sulfonamide resistance in Bacillus anthracis

Natural resistance of field strains of Bacillus anthracis to drugs from the sulfonamide class of antimicrobials that act by inhibiting dihydropteroate synthase (DHPS) has been reported. Though the structure of B. anthracis DHPS has been determined, its connection to the apparent intrinsic sulfonamide resistance of the bacterium has not been established. The aim of this study was to determine if a connection exists between DHPS and the observed sulfonamide resistance of B. anthracis. Microdilution broth assays verified that B. anthracis Sterne is highly resistant to a variety of sulfonamides with minimum inhibitory concentrations (MICs) exceeding 1250 microg/ml. A putative gene encoding DHPS (folP) was amplified from B. anthracis Sterne chromosomal DNA by polymerase chain reaction (PCR) and cloned. Sequence comparisons showed 100% identity with DHPSs from published genome sequences for various strains of B. anthracis. Additionally, expression of folP in B. anthracis Sterne was confirmed. Functionality of the B. anthracis DHPS was confirmed by complementation of an Escherichia coli folP deletion mutant as well as a standard enzyme assay. Concomitant transfer of high level sulfonamide resistance to this mutant along with increased sulfonamide IC(50)values for purified B. anthracis DHPS links DHPS to sulfonamide resistance in B. anthracis. These findings lay the groundwork that will aid future development of antimicrobics that target DHPS to treat anthrax infections.

Variations in gene organization and DNA uptake signal sequence in the folP region between commensal and pathogenic Neisseria species

Background

Horizontal gene transfer is an important source of genetic variation among Neisseria species and has contributed to the spread of resistance to penicillin and sulfonamide drugs in the pathogen Neisseria meningitidis. Sulfonamide resistance in Neisseria meningitidis is mediated by altered chromosomal folP genes. At least some folP alleles conferring resistance have been horizontally acquired from other species, presumably from commensal Neisseriae. In this work, the DNA sequence surrounding folP in commensal Neisseria species was determined and compared to corresponding regions in pathogenic Neisseriae, in order to elucidate the potential for inter-species DNA transfer within this region.

Results

The upstream region of folP displayed differences in gene order between species, including an insertion of a complete Correia element in Neisseria lactamica and an inversion of a larger genomic segment in Neisseria sicca, Neisseria subflava and Neisseria mucosa. The latter species also had DNA uptake signal sequences (DUS) in this region that were one base different from the DUS in pathogenic Neisseriae. Another interesting finding was evidence of a horizontal transfer event from Neisseria lactamica or Neisseria cinerea that introduced a novel folP allele to the meningococcal population.

Conclusion

Genetic recombination events immediately upstream of folP and horizontal transfer have resulted in sequence differences in the folP region between the Neisseria species. This variability could be a consequence of the selective pressure on this region exerted by the use of sulfonamide drugs.

Mutations in folP associated with elevated sulfonamide MICs for Neisseria meningitidis clinical isolates from five continents

Sulfonamide resistance in meningococci is associated with mutations in the chromosomal gene folP, which encodes dihydropteroate synthase. Several mutations associated with resistance have been previously described, including amino acid substitutions at codons 31 and 194, a glycine-serine insertion at codons 195 and 196, and, recently, an additional mutation at nucleotide 682 (C682A). In this study, sulfisoxazole MICs were determined for 424 geographically diverse clinical isolates of Neisseria meningitidis, including all major subtypes. A subset of 134 isolates with MICs ranging from 0.5 to >64 microg/ml were assayed for the C682A mutation by real-time PCR, and 25 isolates were selected for folP gene sequencing. All isolates for which the sulfisoxazole MIC was >/=8 possessed the C682A mutation by real-time PCR or folP sequencing, and 34 of 35 isolates with a MIC of </=2 lacked this mutation. Of 16 sequenced isolates for which the sulfisoxazole MIC was >/=4, 15 possessed previously described mutations, including 10 at codon 31, 1 at codon 194, and 4 with the 2-amino-acid insertion codons 195 and 196; all 16 possessed the C682A mutation. The C682A mutation predicted elevated sulfonamides MICs for a large number of geographically diverse clinical isolates of meningococci. Detection of this mutation by real-time PCR or other methods may allow more wide-scale detection of meningococcal isolates with for which the sulfonamide MICs are elevated without resorting to multiple assays or folP gene sequencing, providing a simple, high-throughput screening method for use in public health and epidemiologic settings.

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.

PCR and restriction endonuclease assay for detection of a novel mutation associated with sulfonamide resistance in Neisseria meningitidis

We identified a previously undocumented mutation in the dihydropteroate synthase (folP) gene associated with Neisseria meningitidis sulfonamide resistance. A PCR-based assay to detect this mutation, which is 100% predictive of sulfonamide resistance, was developed.

Worldwide trends in antimicrobial resistance among common respiratory tract pathogens in children

BACKGROUND

Respiratory tract infections among children are a common reason for health care provider visits and the primary reason for antimicrobial prescribing in this population. The increased prevalence of resistance among Streptococcus pneumoniae and Haemophilus influenzae pathogens poses a serious challenge in the successful treatment of respiratory tract infections caused by these pathogens.

METHODS

This paper reviews worldwide trends in antimicrobial resistance among common respiratory tract pathogens, highlighting data obtained from the pediatric population where available.

RESULTS

S. pneumoniae resistance to beta-lactams is mediated through alterations in the penicillin-binding proteins and macrolide resistance to acquisition of efflux or methylation genes. The mechanisms of resistance to the fluoroquinolones include target enzyme alterations via genetic mutations and transport out of the bacterial cell via an efflux pump. Beta-lactamase production is the primary mechanism of resistance to penicillins among H. influenzae isolates. Although S. pneumoniae with reduced susceptibility to penicillin was first documented > 30 years ago, resistance has increased at an alarming rate worldwide in the past decade. According to recent surveillance data, the worldwide prevalence of S. pneumoniae with reduced susceptibility to penicillin is 18.2%. Beta-lactamase production among H. influenzae ranges from approximately 4% in Russia to 26% in the United States and to 31% in France. The prevalence of beta-lactamase-negative, ampicillin-resistant H. influenzae remains very low (< 1%) worldwide, except in Japan, where the incidence is higher. In general, the highest rates of resistance are observed in isolates obtained from children, and risk factors for infection with a resistant pathogen include young age, the site of infection, day-care center attendance and recent antimicrobial use.

CONCLUSIONS

Increased prevalence of antimicrobial resistance among respiratory tract pathogens isolated from children and adults is evident worldwide. Treatment of infections caused by S. pneumoniae and H. influenzae with older agents or ineffective dosing regimens may not eradicate infections and may contribute to the spread of resistance. These observations confirm the need for appropriate antimicrobial use to halt or at least limit the spread of resistance.

Successful management of disseminated Nocardia transvalensis infection in a heart transplant recipient after development of sulfonamide resistance: case report and review

Nocardia transvalensis is a rarely reported cause of clinically significant disease, and, to our knowledge, has not been reported previously as a cause of infection in the cardiac transplant population. We report a case of N transvalensis new taxon-2 pulmonary infection that disseminated to the brain and skin in a cardiac transplant recipient despite adequate sulfonamide serum levels. Subsequent isolates were resistant to sulfonamides, and molecular ribotyping of the primary and subsequent isolates confirmed that these were the same N transvalensis new taxon-2 strain. The taxonomic and diagnostic considerations, as well as the clinical significance of anti-microbial-resistant nocardia, are reviewed and discussed herein.

Sulphonamide resistant commensal Neisseria with alterations in the dihydropteroate synthase can be isolated from carriers not exposed to sulphonamides

BACKGROUND

Development of sulphonamide resistance in Neisseria meningitidis has been suggested to involve horizontal DNA-transfer from a commensal Neisseria species. In this study, we isolated commensal Neisseria from throat specimens and examined the isolates with respect to sulphonamide resistance.

RESULTS

Three resistant clones were identified and the resistance phenotype could be explained by amino acid variations in their dihydropteroate synthase, the target molecule for sulphonamides. Some of these variations occurred in positions corresponding to previously detected variations in resistant N. meningitidis.

CONCLUSIONS

Sulphonamide resistant commensal Neisseria were isolated from an environment not exposed to sulphonamides, suggesting that resistant Neisseria has become a natural part of the commensal throat flora.

Sulfonamide resistance in Haemophilus influenzae mediated by acquisition of sul2 or a short insertion in chromosomal folP

Determinants of sulfonamide resistance were investigated in clinical isolates of Haemophilus influenzae from the United Kingdom and Kenya. The mechanism of sulfonamide resistance in H. influenzae has not previously been reported. Eight isolates requiring at least 1,024 microg of sulfamethoxazole per ml for inhibition carried the sul2 gene, a common mediator of acquired sulfonamide resistance in enteric bacteria. In other isolates with similarly high levels of resistance, the chromosomal gene encoding dihydropteroate synthase, folP, was found to carry an insertion of 15 bp together with other missense mutations relative to folP of H. influenzae strain Rd RM118 (MIC, 8 microg/ml); the folP sequence was identical in all seven such isolates investigated, although they represented three different strains by restriction pattern analysis. The 15-bp insertion was absent in isolates inhibited by sulfamethoxazole at 2 to 64 microg/ml (although these exhibited considerable divergence in folP sequence) and in highly resistant isolates carrying sul2. Transformation with a 599-bp fragment of folP containing the insertion but no other differences conferred high-level resistance on a recipient strain, confirming the role of the insertion. Other amino acid substitutions in dihydropteroate synthase may modulate the level of sulfonamide inhibition in susceptible isolates and those with more moderate levels of resistance. The two mechanisms of resistance, mediated by sul2 and modified folP, were detected in isolates from both the United Kingdom and Kenya.

Resistance to tetracycline, macrolide-lincosamide-streptogramin, trimethoprim, and sulfonamide drug classes

The discovery and use of antimicrobial agents in the last 50 yr has been one of medicine's greatest achievements. These agents have reduced morbidity and mortality of humans and animals and have directly contributed to human's increased life span. However, bacteria are becoming increasingly resistant to these agents by mutations, which alter existing bacterial proteins, and/or acquisition of new genes, which provide new proteins. The latter are often associated with mobile elements that can be exchanged quickly across bacterial populations and may carry multiple antibiotic genes for resistance. In some case, virulence factors are also found on these same mobile elements. There is mounting evidence that antimicrobial use in agriculture, both plant and animal, and for environmental purposes does influence the antimicrobial resistant development in bacteria important in humans and in reverse. In this article, we will examine the genes which confer resistance to tetracycline, macrolide-lincosamide-streptogramin (MLS), trimethoprim, and sulfonamide.

Molecular techniques for the investigation of meningococcal disease epidemiology

Meningococcal disease remains a major cause of childhood morbidity and mortality world wide and no comprehensive vaccine is available against the causative organism, Neisseria meningitidis. Molecular studies of the diversity of this bacterium have provided a number of key insights into its biology, which have implications for control of meningococcal disease. These have included the identification of hyperinvasive lineages and the correlation of genetic type with antigenic type and disease epidemiology. In practical terms, such studies have enabled the application of DNA-based technologies in the development of improved methods for diagnosis and epidemiological monitoring. These data are of especial importance with the current, and ongoing, development and introduction of new meningococcal vaccines.

Rapid detection of dihydropteroate polymorphism in AIDS-related Pneumocystis carinii pneumonia by restriction fragment length polymorphism

Sulpha agents, which act by inhibiting the enzyme dihydropteroate synthase (DHPS), are used widely for the treatment and prophylaxis of Pneumocystis carinii pneumonia (PCP). Recently, we have shown that mutations in the dihydropteroate synthase (DHPS) gene of Pneumocystis carinii f.sp hominis are associated with failure of sulpha prophylaxis and increased mortality in HIV-1 positive patients with PCP, suggesting that DHPS mutations may cause sulpha resistance. To facilitate detection of DHPS mutations we developed a restriction fragment length polymorphism (RFLP) assay, detecting mutations at codon 55 and 57 of the P. carinii DHPS gene. The RFLP-assay was compared with direct DNA sequencing on 27 PCP isolates from HIV-1 positive patients with a mixture of wildtype and mutant DHPS types. In all samples the RFLP-assay correctly identified wildtype or DHPS mutation at codon 55 or 57. Combined with DNA extraction by a Chelex-based method, this method can be performed within 1 d and allows a fast, cost-efficient and reliable method of detection of DHPS mutations in P. carinii.

Sulfonamide resistance: mechanisms and trends

Sulfonamides were the first drugs acting selectively on bacteria which could be used systemically. Today they are infrequently used, in part due to widespread resistance. The target of sulfonamides, and the basis for their selectivity, is the enzyme dihydropteroate synthase (DHPS) in the folic acid pathway. Mammalian cells are not dependent on endogenous synthesis of folic acid and generally lack DHPS. Instead, they have a folate uptake system which most prokaryotes lack. Laboratory mutants in the dhps (folP) gene can be easily isolated and show a trade off between sulfonamide resistance and DHPS enzyme performance. Clinical resistant mutants, however, have additional compensatory mutations in DHPS that allow it to function normally. In many pathogenic bacteria sulfonamide resistance is mediated by the horizontal transfer of foreign folP or parts of it. Clinical resistance in gram-negative enteric bacteria is plasmid-borne and is effected by genes encoding alternative drug-resistance variants of the DHPS enzymes. Two such genes, sul1 and sul2, have been sequenced and are found at roughly the same frequency among clinical isolates. Remarkably, the corresponding DHPS enzymes show pronounced insensitivity to sulfonamides but normal binding to the p -aminobenzoic acid substrate, despite the close structural similarity between substrate and inhibitor. Copyright 2000 Harcourt Publishers Ltd.

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.

Cloning and expression of Mycobacterium tuberculosis and Mycobacterium leprae dihydropteroate synthase in Escherichia coli

The genes for dihydropteroate synthase of Mycobacterium tuberculosis and Mycobacterium leprae were isolated by hybridization with probes amplified from the genomic DNA libraries. DNA sequencing revealed an open reading frame of 840 bp encoding a protein of 280 amino acids for M. tuberculosis dihydropteroate synthase and an open reading frame of 852 bp encoding a protein of 284 amino acids for M. leprae dihydropteroate synthase. The dihydropteroate synthases were expressed under control of the T5 promoter in a dihydropteroate synthase-deficient strain of Escherichia coli. Using three chromatography steps, we purified both M. tuberculosis and M. leprae dihydropteroate synthases to >98% homogeneity. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed molecular masses of 29 kDa for M. tuberculosis dihydropteroate synthase and 30 kDa for M. leprae dihydropteroate synthase. Gel filtration of both enzymes showed a molecular mass of ca. 60 kDa, indicating that the native enzymes exist as dimers of two identical subunits. Steady-state kinetic parameters for dihydropteroate synthases from both M. tuberculosis and M. leprae were determined. Representative sulfonamides and dapsone were potent inhibitors of the mycobacterial dihydropteroate synthases, but the antimycobacterial agent p-aminosalicylate, a putative dihydropteroate synthase inhibitor, was a poor inhibitor of the enzymes.

Dihydropteroate synthase from Streptococcus pneumoniae: characterization of substrate binding order and sulfonamide inhibition

Dihydropteroate synthase (DHPS) catalyses a key step in the biosynthesis of folic acid and is the target for inhibition by the sulphonamide class of antimicrobial agents. Here we describe a study of the enzymatic mechanism and sulphonamide inhibition of DHPS from the pathogen Streptococcus pneumoniae. Equilibrium binding assays showed that binding of the substrate para-aminobenzoic acid (pABA) to DHPS was absolutely dependent on the presence of pyrophosphate, which acts as an analogue of the second substrate 6-hydroxymethyl-7, 8-dihydropterin pyrophosphate (DHPPP). The product of the reaction, dihydropteroate, was also able to bind to DHPS. Sulphonamides were capable of displacing pABA in a competitive manner, with equilibrium binding constants that were significantly higher than the equivalent Ki values deduced from steady state kinetic measurements. These results indicate that the target for sulphonamide inhibition of S. pneumoniae DHPS is the enzyme-DHPPP binary complex, rather than the apoprotein form of the enzyme.

High-level resistance to trimethoprim in clinical isolates of Campylobacter jejuni by acquisition of foreign genes (dfr1 and dfr9) expressing drug-insensitive dihydrofolate reductases

The pathogenic bacterium Campylobacter jejuni has been regarded as endogenously resistant to trimethoprim. The genetic basis of this resistance was characterized in two collections of clinical isolates of C. jejuni obtained from two different parts of Sweden. The majority of these isolates were found to carry foreign dfr genes coding for resistant variants of the dihydrofolate reductase enzyme, the target of trimethoprim. The resistance genes, found on the chromosome, were dfr1 and dfr9. In about 10% of the strains, the dfr1 and dfr9 genes occurred simultaneously. About 10% of the examined isolates were found to be negative for these dfr genes and showed a markedly lower trimethoprim resistance level than the other isolates. The dfr9 and dfr1 genes were located in the context of remnants of a transposon and an integron, respectively. Two different surroundings for the dfr9 gene were characterized. One was identical to the right-hand end of the transposon Tn5393, and in the other, the dfr9 gene was flanked by only a few nucleotides of a Tn5393 sequence. The insertion of the dfr9 gene into the C. jejuni chromosome could have been mediated by Tn5393. The frequent occurrence of high-level trimethoprim resistance in clinical isolates of C. jejuni could be related to the heavy exposure of food animals to antibacterial drugs, which could lead to the acquisition of foreign resistance genes in naturally transformable strains of C. jejuni.

Antimicrobial resistance of Neisseria gonorrhoeae in Liberia

The prevalence and molecular characteristics of penicillinase-producing Neisseria gonorrhoeae (PPNG) and tetracycline-resistant N. gonorrhoeae (TRNG) were determined in 10 clinics in Monrovia, Liberia, to assess the likely effectiveness of the current standard treatment with penicillin or tetracycline. One hundred gonococcal strains were isolated from 146 urethral swabs and 261 cervical swabs and screened for susceptibility to ceftriaxone, penicillin, spectinomycin and tetracycline by the disk diffusion method; 83% were resistant to penicillin and 63% to tetracycline. Twenty-one strains from 18 men and 3 women with uncomplicated gonorrhoea were subjected to more detailed characterization. These 21 strains belonged to 5 auxotype/serovar classes; 86% were PPNG/TRNG. Three PPNG harboured the 4.4 MDa penicillinase plasmid and 16 the 3.2 MDa plasmid. All TRNG harboured the 25.2 MDa plasmid and their MICs for tetracycline were > 32 mg/L. They gave a PCR product which, according to its restriction pattern, corresponded to the American type tetM gene. By the agar dilution method, all strains exhibited intermediate resistance to sulphamethoxazole-trimethoprim (19:1) (co-trimoxazole) with MICs of 8-32 mg/L. All strains were susceptible to spectinomycin and ciprofloxacin. The MICs for gentamicin were 4-8 mg/L. The use of effective and affordable antimicrobial chemotherapy with either 500 mg ciprofloxacin or a single dose of gentamicin is discussed, with consideration of molecular biological, pharmacological and public health aspects.

High-level chloramphenicol resistance in Neisseria meningitidis

BACKGROUND

Neisseria meningitidis is nearly always susceptible to the penicillins, the cephalosporins, and chloramphenicol. Between 1987 and 1996, however, chloramphenicol-resistant strains were isolated from 11 patients in Vietnam and 1 in France.

METHODS

The minimal inhibitory concentration of chloramphenicol was determined for the 12 isolates. The isolates were analyzed by monoclonal-antibody-based serotyping and subtyping, pulsed-field gel electrophoresis, and multilocus enzyme electrophoresis. Bacterial DNA was analyzed by hybridization, the polymerase chain reaction, and sequencing to identify the resistance gene and determine the origin of the resistance.

RESULTS

The isolates were resistant to chloramphenicol (minimal inhibitory concentration, > or =64 mg per liter) and produced an active chloramphenicol acetyltransferase. All 12 strains belonged to serogroup B but had a high degree of diversity, and 10 could not be typed with the use of monoclonal antibodies. The nucleotide sequence of the resistance gene and the flanking regions was identical to that of an internal portion of transposon Tn4451 that carries the catP gene in Clostridium perfringens. Moreover, this gene was located in the same genomic site in the chloramphenicol-resistant isolates.

CONCLUSIONS

The high-level chloramphenicol resistance that we describe in N. meningitidis isolates is of great concern, since in developing countries, chloramphenicol given intramuscularly is the standard therapy for meningococcal meningitis. The resistance to chloramphenicol is due to the presence of the catP gene on a truncated transposon that has lost mobility because of internal deletions, and the transformation of genetic material between strains of N. meningitidis probably played an important part in the dissemination of the gene.

Clonal distribution of invasive Neisseria meningitidis isolates from the Norwegian county of Telemark, 1987 to 1995

Forty-two Neisseria meningitidis isolates were obtained from patients with meningococcal disease in the Norwegian county of Telemark (January 1987 to March 1995), and all were compared by PCR amplicon restriction endonuclease analysis (PCR-AREA) of the dhps gene, chromosomal DNA fingerprinting, and serological analysis. PCR-AREA divided the isolates into 11 classes, of which 4, comprising 15, 8, 6, and 2 isolates, were clonal while the remaining 8 classes were genetically heterogeneous or contained only 1 isolate. Three of the four clonal classes could be tentatively equated with recognized epidemic clones (ET5, ET37, and cluster A4) on the basis of their phenotypic characteristics, while the remaining clone appears to be new. There were significant differences in the geographical distribution of clones, with class 1 (ET5-like) isolates significantly overrepresented in rural parts of Telemark. Class 1 (ET5-like) isolates occurred throughout the study period and were dominant in 1987. Class 2 (ET37-like) isolates occurred from 1988 to 1992, and class 3 isolates (with no recognizable ET affinities) were found only in 1991 and 1992.

Sulfonamide resistance in Streptococcus pyogenes is associated with differences in the amino acid sequence of its chromosomal dihydropteroate synthase

Sulfonamide resistance in recent isolates of Streptococcus pyogenes was found to be associated with alterations of the chromosomally encoded dihydropteroate synthase (DHPS). There were 111 different nucleotides (13.8%) in the genes found in susceptible and resistant isolates, respectively, resulting in 30 amino acid changes (11.3%). These substantial changes suggested the possibility of a foreign origin of the resistance gene, in parallel to what has already been found for sulfonamide resistance in Neisseria meningitidis. The gene encoding DHPS was linked to at least three other genes encoding enzymes of the folate pathway. These genes were in the order GTP cyclohydrolase, dihydropteroate synthase, dihydroneopterin aldolase, and hydroxymethyldihydropterin pyrophosphokinase. The nucleotide differences in genes from resistant and susceptible strains extended from the beginning of the GTP cyclohydrolase gene to the end of the gene encoding DHPS, an additional indication for gene transfer in the development of resistance. Kinetic measurements established different affinities for sulfathiazole for DHPS enzymes isolated from resistant and susceptible strains.

Characterization of mutations contributing to sulfathiazole resistance in Escherichia coli

A sulfathiazole-resistant dihydropteroate synthase (DHPS) present in two different laboratory strains of Escherichia coli repeatedly selected for sulfathiazole resistance was mapped to folP by P1 transduction. The folP mutation in each of the strains was shown to be identical by nucleotide sequence analysis. A single C-->T transition resulted in a Pro-->Ser substitution at amino acid position 64. Replacement of the mutant folP alleles with wild-type folP significantly reduced the level of resistance to sulfathiazole but did not abolish it, indicating the presence of an additional mutation(s) that contributes to sulfathiazole resistance in the two strains. Transfer of the mutant folP allele to a wild-type background resulted in a strain with only a low level of resistance to sulfathiazole, suggesting that the presence of the resistant DHPS was not in itself sufficient to account for the overall sulfathiazole resistance in these strains of E. coli. Additional characterization of an amplified secondary resistance determinant, sur, present in one of the strains, identified it as the previously identified bicyclomycin resistance determinant bcr, a member of a family of membrane-bound multidrug resistance antiporters. An additional mutation contributing to sulfathiazole resistance, sux, has also been identified and has been shown to affect the histidine response to adenine sensitivity displayed by these purU strains.

Mechanism of sulfonamide resistance in clinical isolates of Streptococcus pneumoniae

The genetic basis of sulfonamide resistance in six clinical isolates of Streptococcus pneumoniae was demonstrated to be 3- or 6-bp duplications within sulA, the chromosomal gene encoding dihydropteroate synthase. The duplications all result in repetition of one or two amino acids in the region from Arg58 to Tyr63, close to but distinct from the sul-d mutation, a duplication previously reported in a resistant laboratory strain (P. Lopez, M. Espinosa, B. Greenberg, and S. A. Lacks, J. Bacteriol. 169:4320-4326, 1987). Six sulfonamide-susceptible clinical isolates lacked such duplications. The role of the duplications in conferring sulfonamide resistance was confirmed by transforming 319- or 322-bp PCR fragments into the chromosome of a susceptible recipient. Two members of a clone of serotype 9V, one susceptible and one resistant to sulfonamide, which are highly related by other criteria, were shown to have sulA sequences that differ in 7.2% of nucleotides in addition to the duplication responsible for resistance. It is postulated that horizontal gene exchange has been involved in the acquisition (or loss) of resistance within this clone. However, five of the six resistant isolates have distinct duplications and other sequence polymorphisms, suggesting that resistance has arisen independently on many occasions.

Structure and function of the dihydropteroate synthase from Staphylococcus aureus

The gene encoding the dihydropteroate synthase of staphylococcus aureus has been cloned, sequenced and expressed in Escherichia coli. The protein has been purified for biochemical characterization and X-ray crystallographic studies. The enzyme is a dimer in solution, has a steady state kinetic mechanism that suggests random binding of the two substrates and half-site reactivity. The crystal structure of apo-enzyme and a binary complex with the substrate analogue hydroxymethylpterin pyrophosphate were determined at 2.2 A and 2.4 A resolution, respectively. The enzyme belongs to the group of "TIM-barrel" proteins and crystallizes as a non-crystallographic dimer. Only one molecule of the substrate analogue bound per dimer in the crystal. Sequencing of nine sulfonamide-resistant clinical isolates has shown that as many as 14 residues could be involved in resistance development. The residues are distributed over the surface of the protein, which defies a simple interpretation of their roles in resistance. Nevertheless, the three-dimensional structure of the substrate analogue binary complex could give important insight into the molecular mechanism of this enzyme.

Adaptation to sulfonamide resistance in Neisseria meningitidis may have required compensatory changes to retain enzyme function: kinetic analysis of dihydropteroate synthases from N. meningitidis expressed in a knockout mutant of Escherichia coli

Previously, the effects of three point mutations (at amino acid positions 31, 84, and 194) in the gene coding for a sulfonamide-resistant dihydropteroate synthase of Neisseria meningitidis were analyzed by site-directed mutagenesis. Changes at positions 31 and 194 abolished the phenotypic expression of sulfonamide resistance, while a change at position 84 appeared to be neutral. These studies are here extended to correlate the alterations in phenotype with effects on enzyme kinetics by expressing the cloned meningococcal genes in an Escherichia coli strain that had its dhps gene partially deleted and replaced by a resistance determinant. The most dramatic effects were produced by mutations at position 31. A change from the Leu found in the resistant isolate to a Phe (the residue found in sensitive isolates) led to a 10-fold decrease in the Km and a concomitant drop in the Ki. Changes at position 194 also affected both the Km and Ki but not to the same extent as mutations at position 31. Changing position 84 altered the Km only slightly but significantly. This latter change was interpreted as a compensatory change modulating the function of the enzyme. In another type of resistance gene, 2 amino acid residues, proposed to be an insertion, were deleted, resulting in a sensitive enzyme. However, the resulting Km was raised 10-fold, suggesting that compensatory changes have accumulated in this type of resistance determinant as well. This resistance gene differs by as much as 10% from the sensitive isolates, which makes identification of important mutations difficult.

Cloning and characterization of the Neisseria gonorrhoeae MS11 folC gene

The gene coding for folylpoly-(gamma)-glutamate synthetase (FPGS)-dihydrofolate synthetase (DHFS) of Neisseria gonorrhoeae (Ngo) has been cloned by functional complementation of an Escherichia coli folC mutant (SF4). The sequence encodes a 224-residue protein of 46.4 kDa. It shows 46% identity to the E. coli FPGS-DHFS and 29% identity to the FPGS of Lacto-bacillus casei. Sequence comparisons between the three genes reveal regions of high homology, including ATP binding sites required for bifunctionality, all of which may be important for FPGS activity. In contrast to L. casei FPGS, the E. coli and Ngo enzymes share some additional regions which may be essential for DHFS activity. The products of Ngo folC and flanking genes were monitored by T7 promoter expression. Interestingly, deletion of the upstream folI gene, which encodes a 16.5 kDa protein, abolishes the capacity of folC to complement E. coli SF4 to the wild-type phenotype. The ability to complement can be restored by folI provided in trans. Unlike folC mutants, gonococcal folI mutants are viable and display no apparent phenotype. Thus, in contrast to E. coli, Ngo folC is expressed at a sufficiently high level from its own promoter, in the absence of FolI This study provides the first insights into the genetic complexity of one-carbon metabolism in Nqo.

Identification of epidemiologic markers for Neisseria meningitidis using difference analysis

The feasibility of identifying epidemiologic markers based solely on the identification of DNA fragments present in outbreak-associated isolates was investigated using Neisseria meningitidis (Nm) as a model system. The clonal structure of Nm has been well characterized using multilocus electrophoresis. In Canada, electrophoretic types ET1, ET5, ET9 and ET21 are being displaced from the natural population by type ET15, and the latter type is associated with an increased prevalence of serogroup C meningococcal disease. Difference analysis, which uses subtractive hybridization and polymerase chain reaction (PCR) amplification, was employed to identify amplifiable DNA fragments (amplicons) that differ between the ET15 and the ET1, ET5, ET9 and ET21 genomes. 14 amplicons were cloned which were further characterized by Southern blot analysis to identify six amplicons that represent fragments either unique to or highly polymorphic in the ET15 genome. Oligodeoxyribonucleotide primer pairs were designed for each of the six amplicons, and PCR amplification was used to determine their prevalence across a panel of 167 Nm isolates representative of other serogroups and ETs. Among group C isolates only two of the six amplicons, designated as A and G, were effective in discriminating ET15 from non-ET15 isolates. Amplicon A detects a deletion in the dhps gene which effectively differentiates sulfonamide-sensitive and -resistant serogroup C isolates. The frequency of amplicon A and G detection in the other serogroups and ETs was too great to facilitate their direct use as diagnostic markers for the differentiation of virulent Nm isolates.

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.

PCR amplicon restriction endonuclease analysis of the chromosomal dhps gene of Neisseria meningitidis: a method for studying spread of the disease-causing strain in contacts of patients with meningococcal disease

We tested two sets of primers derived from the dhps gene of Neisseria meningitidis for the amplification of meningococcal DNA by PCR. Both the NM1-NM6 primers and the NM3-NM6 primers amplified dhps DNA from all of the meningococci included in the study, resulting, in most cases, in amplicons of 0.70 and 0.23 kb, respectively. Also, dhps DNAs of N. gonorrhoeae and some commensals were amplified but Haemophilus influenzae, Streptococcus pneumoniae, and Escherichia coli DNAs were not. By PCR amplicon restriction endonuclease analysis (AREA) of the larger amplicon, we could differentiate between individual strains of N. meningitidis. Following two cases of meningococcal disease, we used PCR AREA to identify healthy contacts carrying the disease-causing strain. We conclude that PCR AREA is a useful method for meningococcal strain differentiation and that it has potential as a method for studying the spread of a disease-causing strain in an affected population. The method is quicker and easier to perform and interpret than chromosomal DNA fingerprinting.

Interspecies recombination in nature: a meningococcus that has acquired a gonococcal PIB porin

A vaginal isolate of Neisseria has been reported to resemble Neisseria meningitidis in biochemical characteristics but to react with serological reagents that are specific to the PI porin from Neisseria gonorrhoeae. We have confirmed that this isolate has the biochemical attributes of a meningococcus and have shown that it clusters among meningococcal isolates on a dendrogram based on isoenzyme variation within housekeeping enzymes from populations of N. meningitidis and N. gonorrhoeae. Furthermore, the sequences of the fbp and adk genes were typical of those of N. meningitidis and were distinct from those of N. gonorrhoeae. However, the porB gene was very similar to the por genes of N. gonorrhoeae isolates that express the PIB class of outer-membrane porin (differing from one gonococcal por allele at only a single nucleotide site), and was clearly distinct from the porB genes of N. meningitidis. The isolate therefore appears to be a typical meningococcus, except that its porB gene has been replaced with the por gene from a gonococcus.

Detection of bacterial DNA in cerebrospinal fluid by an assay for simultaneous detection of Neisseria meningitidis, Haemophilus influenzae, and streptococci using a seminested PCR strategy

Primers specific to conserved and variable regions in the 16S rRNA sequence were selected from the partially sequenced 16S rRNA genes of Neisseria meningitidis, Haemophilus influenzae, Streptococcus pneumoniae, S. agalactiae, and Staphylococcus epidermidis. The PCR assay was divided into two DNA amplifications. The first resulted in a general bacterial amplicon, and the second resulted in a species-specific amplicon. The high specificity of the PCR assay was documented after testing bacteria of 28 different species (133 strains). A total of 304 clinical cerebrospinal fluid samples, including 125 samples from patients with bacterial meningitis, were assayed to investigate the diagnostic sensitivity and specificity for bacterial meningitis. The assay showed high sensitivity (0.94) and specificity (0.96) with the clinical samples, although some false results were obtained, the reasons for which are discussed. With agarose gel electrophoresis for detection of the PCR products, the detection limit for meningococci in cerebrospinal fluid was 3 x 10(2) CFU/ml.

Sequence variation of the hydroxymethyldihydropterin pyrophosphokinase: dihydropteroate synthase gene in lines of the human malaria parasite, Plasmodium falciparum, with differing resistance to sulfadoxine

Dihydropteroate synthase (H2Pte synthase) is the target of the sulfur-based antimalarial drugs, which are frequently used in synergistic combination with inhibitors of dihydrofolate reductase (H2folate reductase) to combat chloroquine-resistant malaria. We have isolated the H2Pte synthase coding sequence of the most pathogenic human parasite Plasmodium falciparum. It forms part of a longer coding sequence, located on chromosome 8, that also specifies 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase (CH2OH-H2pterinPP kinase) at its 5' proximal end. This domain is unusually large, with two long insertions relative to other CH2OH-H2pterinPP kinase molecules. To investigate a possible genetic basis for clinical resistance to sulfa drugs, we sequenced the complete H2Pte synthase domains from eleven isolates of P. falciparum with diverse geographical origins and levels of sulfadoxine resistance. Overall, point mutations in five positions were observed, affecting four codons. Parasite lines exhibiting high-level resistance were found to carry either a double mutation, altering both Ser436 and Ala613, or a single mutation affecting Ala581. The mutations at positions 436 and 581 have the same location relative to each of two degenerate repeated amino acid motifs that are conserved across all other known H2Pte synthase molecules. The amino acid alteration at residue 613 is identically positioned relative to a different conserved motif. The fourth amino acid residue (437) affected by mutation, though adjacent to the apparently crucial residue 436, shows no obvious correlation with resistance. Although these mutations have no exact counterparts in any other organism, that at position 581 falls within a region of three amino acids where H2Pte synthase is modified in various ways in a number of sulfonamide-resistant pathogenic bacteria. Copy-number analysis indicated that there was no amplification of the H2Pte synthase domain in resistant parasite lines of P. falciparum, compared to sensitive lines.

Primary structure and expression of the dihydropteroate synthetase gene of Plasmodium falciparum

The enzyme dihydropteroate synthetase (DHPS) from Plasmodium falciparum is involved in the mechanism of action of the sulfone/sulfonamide group of drugs. We describe the cloning and sequencing of the gene encoding the P. falciparum DHPS enzyme and show that it is a bifunctional enzyme that includes dihydro-6-hydroxymethylpterin pyrophosphokinase (PPPK) at the N terminus of DHPS. The gene encodes a putative protein of 83 kDa that contains two domains that are homologous with the DHPS and PPPK enzymes of other organisms. The PPPK-DHPS gene is encoded on chromosome 8 and has two introns. An antibody raised to the PPPK region of the protein was found to recognize a 68-kDa protein that is expressed throughout the asexual life cycle of the parasite. We have determined the sequence of the DHPS portion of the gene from sulfadoxine-sensitive and -resistant P. falciparum clones and identified sequence differences that may have a role in sulfone/sulfonamide resistance.

Resistance to antibiotics mediated by target alterations

The development of resistance to antibiotics by reductions in the affinities of their enzymatic targets occurs most rapidly for antibiotics that inactivate a single target and that are not analogs of substrate. In these cases of resistance (for example, resistance to rifampicin), numerous single amino acid substitutions may provide large decreases in the affinity of the target for the antibiotic, leading to clinically significant levels of resistance. Resistance due to target alterations should occur much more slowly for those antibiotics (penicillin, for example) that inactivate multiple targets irreversibly by acting as close analogs of substrate. Resistance to penicillin because of target changes has emerged, by unexpected mechanisms, only in a limited number of species. However, inactivating enzymes commonly provide resistance to antibiotics that, like penicillin, are derived from natural products, although such enzymes have not been found for synthetic antibiotics. Thus, the ideal antibiotic would be produced by rational design, rather than by the modification of a natural product.

Microevolution within a clonal population of pathogenic bacteria: recombination, gene duplication and horizontal genetic exchange in the opa gene family of Neisseria meningitidis

Opacity (Opa) proteins are a family of antigenically variable outer-membrane proteins of Neisseria meningitidis. Even among clonally related epidemic meningococcal isolates, there is greater variation of Opa protein expression than can be accounted for by the opa gene repertoire of any individual strain. We characterized the opa genes of eight closely related isolates of serogroup A N. meningitidis (subgroup IV-1) from a recent meningitis epidemic in West Africa. DNA sequence analysis and Southern blot experiments indicated that changes occurred in the opa genes of these bacteria as they spread through the human population, over a relatively short period of time. Such changes in one or a few loci within a clonal population are referred to as microevolution. The distribution of sequences present in hypervariable (HV) regions of the opa genes suggests that duplication of all or part of opa genes into other opa loci changed the repertoire of Opa proteins that could be expressed. Additional variability in this gene family appears to have been introduced by horizontal exchange of opa sequences from other meningococcal strains and from Neisseria gonorrhoeae. These results indicate that processes of recombination and genetic exchange contributed to variability in major surface antigens of this clonal population of pathogenic bacteria.

Population genetics of a transformable bacterium: the influence of horizontal genetic exchange on the biology of Neisseria meningitidis

Information of the biochemistry and genetics of bacterial species, usually obtained by the study of single isolates, is enhanced by studies of populations of bacteria. Recent advances in molecular technology, particularly polymerase chain reaction-based nucleotide sequence analysis, provide powerful tools for the study of population genetics. Data obtained by such techniques indicate that, while some bacterial species have a clonal population structure, others are non-clonal or panmictic. Clonal populations are a consequence of asexual reproduction by binary fission; panmictic population structures result from 'horizontal' exchange of genetic material between clones. A consequence of horizontal genetic exchange is mosaic gene structures, recognisable by comparisons of nucleotide sequences. In transformable bacteria, for example the human pathogen Neisseria meningitidis, several different genes, including the gene encoding the class 1 outer membrane protein, a major surface antigen, are mosaics. This genetic process has implications both for vaccine design and in the interpretation of epidemiological data.