Transfer of penicillin resistance between Neisseriae in microcosm

NMR solution structure and DNA-binding model of the DNA-binding domain of competence protein A

Competence protein A (ComA) is a response regulator protein involved in the development of genetic competence in the Gram-positive spore-forming bacterium Bacillus subtilis, as well as the regulation of the production of degradative enzymes and antibiotic synthesis. ComA belongs to the NarL family of proteins, which are characterized by a C-terminal transcriptional activator domain that consists of a bundle of four helices, where the second and third helices (alpha 8 and alpha 9) form a helix-turn-helix DNA-binding domain. Using NMR spectroscopy, the high-resolution 3D solution structure of the C-terminal DNA-binding domain of ComA (ComAC) has been determined. In addition, surface plasmon resonance and NMR protein-DNA titration experiments allowed for the analysis of the interaction of ComAC with its target DNA sequences. Combining the solution structure and biochemical data, a model of ComAC bound to the ComA recognition sequences on the srfA promoter has been developed. The model shows that for DNA binding, ComA uses the conserved helix-turn-helix motif present in other NarL family members. However, the model reveals also that ComA might use a slightly different part of the helix-turn-helix motif and there appears to be some associated domain re-orientation. These observations suggest a basis for DNA binding specificity within the NarL family.

Plasmid diversity in neisseriae

Horizontal gene transfer constitutes an important force in prokaryotic genome evolution, and it is well-known that plasmids are vehicles for DNA transfer. Chromosomal DNA is frequently exchanged between pathogenic and commensal neisseriae, but relatively little is known about plasmid diversity and prevalence among these nasopharyngeal inhabitants. We investigated the plasmid contents of 18 Neisseria lactamica isolates and 20 nasopharyngeal Neisseria meningitidis isolates. Of 18 N. lactamica strains, 9 harbored one or more plasmids, whereas only one N. meningitidis isolate contained a plasmid. Twelve plasmids were completely sequenced, while five plasmid sequences from the public databases were also included in the analyses. On the basis of nucleic acid sequences, mobilization, and replicase protein alignments, we distinguish six different plasmid groups (I to VI). Three plasmids from N. lactamica appeared to be highly similar on the nucleotide level to the meningococcal plasmids pJS-A (>99%) and pJS-B (>75%). The genetic organizations of two plasmids show a striking resemblance with that of the recently identified meningococcal disease-associated (MDA) phage, while four putative proteins encoded by these plasmids show 25% to 39% protein identity to those encoded by the MDA phage. The putative promoter of the gene encoding the replicase on these plasmids contains a polycytidine tract, suggesting that replication is subjected to phase variation. In conclusion, extensive plasmid diversity is encountered among commensal neisseriae. Members of three plasmid groups are found in both pathogenic and commensal neisseriae, indicating plasmid exchange between these species. Resemblance between plasmids and MDA phage may be indicative of dissemination of phage-related sequences among pathogenic and commensal neisseriae.

Natural transformation and phase variation modulation in Neisseria meningitidis

Neisseria meningitidis has evolved the ability to control the expression-state of numerous genes by phase variation. It has been proposed that the process aids this human pathogen in coping with the diversity of microenvironments and host immune systems. Therefore, increased frequencies of phase variation may augment the organism's adaptability and virulence. In this study, we found that DNA derived from various neisserial co-colonizers of the human nasopharynx increased N. meningitidis switching frequencies, indicating that heterologous neisserial DNA modulates phase variation in a transformation-dependent manner. In order to determine whether the effect of heterologous DNA was specific to the Hb receptor, HmbR, we constructed a Universal Rates of Switching cassette (UROS). With this cassette, we demonstrated that heterologous DNA positively affects phase variation throughout the meningococcal genome, as UROS phase variation frequencies were also increased in the presence of neisserial DNA. Overexpressing components of the neisserial mismatch repair system partially alleviated DNA-induced changes in phase variation frequencies, thus implicating mismatch repair titration as a cause of these transformation-dependent increases in switching. The DNA-dependent effect on phase variation was transient and may serve as a mechanism for meningococcal genetic variability that avoids the fitness costs encountered by global mutators.

Serotype distribution, antibiotic susceptibility, and genetic relatedness of Neisseria meningitidis strains recently isolated in Italy

The availability of new polysaccharide-protein conjugate vaccines against Neisseria meningitidis serogroup C prompted European National Health authorities to carefully monitor isolate characteristics. In Italy, during 1999-2001, the average incidence was 0.4 cases per 100,000 inhabitants. Serogroup B was predominant and accounted for 75% of the isolates, followed by serogroup C with 24%. Serogroup C was isolated almost twice as frequently in cases of septicemia than in cases of meningitis, and the most common phenotypes were C:2a:P1.5 and C:2b:P1.5. Among serogroup B meningococci, the trend of predominant phenotypes has changed from year to year, with a recent increase in the frequency of B:15:P1.4. Only a few meningococci had decreased susceptibility to penicillin, and, in the penA gene, all of these strains had exogenous DNA blocks deriving from the DNA of commensal Neisseria flavescens, Neisseria cinerea, and Neisseria perflava/sicca. Fluorescent amplified fragment-length polymorphism analysis revealed the nonclonal nature of the strains with decreased susceptibility to penicillin.

Mechanisms other than penicillin-binding protein-2 alterations may contribute to moderate penicillin resistance in Neisseria meningitidis

Penicillin resistance in Neisseria spp is thought to be generated by the interspecies transfer of genetic material from naturally penicillin-resistant, commensal species. We examined a series of successive transformants with increasing levels of penicillin resistance, obtained by co-cultivation of Neisseria meningitidis derivatives with Neisseria polysaccharea. Our results suggest that, in addition to the well-known decrease in penicillin affinity of penicillin-binding protein-2 (PBP-2), decreased expression of the class 3 porin as well as decreased affinity of PBP-1 may contribute to higher level resistance of N. meningitidis to penicillin G and other beta-lactam compounds.