Plasmid-mediated chromosomal gene transfer in Neisseria gonorrhoeae

Even therapeutic antimicrobial use in animal husbandry may generate environmental hazards to human health

The potential negative impact for human health of veterinary use of antimicrobials in prophylaxis, metaphylaxis and growth promotion in animal husbandry was first established in the 1960s and 1970s. Determination of the molecular structure of antimicrobial resistance plasmids at that time explained the ability of antimicrobial resistance genes to disseminate among bacterial populations and elucidated the reasons for the negative effects of antimicrobials used in food animals for human health. In this issue of Environmental Microbiology, Liu et al. (2016) show that even therapeutic use of antimicrobials in dairy calves has an appreciable environmental microbiological footprint. We discuss the negative implications of this footprint for human health and the possibility they may lead to calls for increased regulation of veterinary antimicrobial use in terrestrial and aquatic environments.

Comparative overview of the genomic and genetic differences between the pathogenic Neisseria strains and species

The availability of complete genome sequences from multiple pathogenic Neisseria strains and species has enabled a comprehensive survey of the genomic and genetic differences occurring within these species. In this review, we describe the chromosomal rearrangements that have occurred, and the genomic islands and prophages that have been identified in the various genomes. We also describe instances where specific genes are present or absent, other instances where specific genes have been inactivated, and situations where there is variation in the version of a gene that is present. We also provide an overview of mosaic genes present in these genomes, and describe the variation systems that allow the expression of particular genes to be switched ON or OFF. We have also described the presence and location of mobile non-coding elements in the various genomes. Finally, we have reviewed the incidence and properties of various extra-chromosomal elements found within these species. The overall impression is one of genomic variability and instability, resulting in increased functional flexibility within these species.

The virgin population of Neisseria gonorrhoeae in Stockholm has decreased and antimicrobial resistance is increasing

AIMS

To investigate the evolution of chromosomal and plasmid mediated resistance for ampicillin and tetracycline of N gonorrhoeae strains in Stockholm during 1982-1993.

METHODS

A total of 404 gonococcal strains isolated in 1982, 1987, 1990, 1992, 1993 were analysed for minimal inhibitory concentrations (MIC) of ampicillin and tetracycline and for plasmid content. MIC values were determined by the agar dilution method and plasmid preparations were performed using alkaline lysis. To detect additional gonococcal strains with tet(M) plasmids all strains isolated in 1988-1989 and 1991, in all 234 isolates, were analysed retrospectively for MIC values of tetracycline. If an MIC value of > or = 4.0 mg/l was recorded plasmid analysis was performed.

RESULTS

Increased proportions of chromosomally mediated resistance to tetracycline (p < 0.001) as well as plasmid mediated resistance to both ampicillin (p < 0.02) and tetracycline were found in the later part of the study. In 1991 the first gonococcus with tet(M) plasmid was isolated in Sweden. The proportion of strains with chromosomally mediated resistance for ampicillin did not change during the study period. The proportion of gonococcal strains with the 39 kb conjugative plasmid was increased in the later part of the study.

CONCLUSIONS

The increased proportion of N gonorrhoeae strains with resistance to ampicillin and tetracycline is most likely due to importation of strains from areas with high prevalence of antibiotic resistant gonococci. The proportion of N gonorrhoeae strains with tet(M) plasmids is low in Sweden, but might increase in the same way as the proportion of PPNG strains has increased during 1982-1993.

Characterization of a catalase-deficient strain of Neisseria gonorrhoeae: evidence for the significance of catalase in the biology of N. gonorrhoeae

We obtained a catalase-deficient (Kat-) strain of Neisseria gonorrhoeae isolated from a patient who had been unsuccessfully treated with penicillin. Quantitative enzyme assays and electrophoresis of cell extracts on native polyacrylamide gels subsequently stained for catalase and peroxidase activities failed to detect both enzymes. The strain exhibited no growth anomalies or unusual requirements when grown under ordinary laboratory conditions. However, the Kat- strain proved extremely sensitive to exogenous hydrogen peroxide, and analysis of the bacterial DNA after such exposure showed extensive single-strand breakage in both chromosomal and plasmid DNAs. Partial characterization of the gonococcal catalase from a Kat+ laboratory strain revealed that the enzyme had the physical and chemical properties of both catalase and peroxidase.

Arginine and pyrimidine biosynthetic defects in Neisseria gonorrhoeae strains isolated from patients

Neisseria gonorrhoeae strains with nutritional requirements that include arginine (Arg-), uracil (Ura-), and hypoxanthine have attracted attention because of their tendency to cause disseminated infections, as a basis for genetic studies of arginine and pyrimidine biosynthesis, we examined the activities of four enzymes of these pathways in cell-free extracts of both prototrophic and Arg- Ura- strains. Activities of glutamate acetyltransferase, aspartate transcarbamylase, and orotate phosphoribosyltransferase, encoded respectively by argE, pyrB, and pyrE, were absent in some Arg- Ura- isolates. Gonococci that were unable to utilize ornithine for growth in place of citrulline lacked activity of carbamyl phosphate synthetase (encoded by car). Defects of car imposed requirements for both citrulline (or arginine) and a pyrimidine because of the dual role of carbamyl phosphate in the two pathways. Defects of argE, car, pyrB, and pyrE were separately introduced by genetic transformation into representatives of a gonococcal strain which initially was prototrophic. Results of enzyme assays of these isogenic auxotrophic transformants confirmed the gene-enzyme relationships.

High-frequency conjugal transfer of a gonococcal penicillinase plasmid

Gonococci containing a 24 X 10(6)-dalton conjugal plasmid were able to mobilize for transfer a smaller, non-self-transmissible penicillinase (Pcr) plasmid with high frequency under appropriate conditions. In some strains, over 10% of donor colony-forming units transferred the Pcr plasmid in a mating of less than 2 h, which suggests that the conjugal system was naturally derepressed. Colony-opacity variants containing different quantities of an approximately 28,000-dalton outer membrane protein were altered in their ability to act as conjugal donors and recipients. Maximal transfer of the Pcr plasmid was observed between transparent donors and recipients, lacking appreciable amounts of the 28,000-dalton protein. Under conditions of high-frequency Pcr plasmid mobilization, no conjugal mobilization of chromosomal markers could be discerned.

Physical map of the conjugal plasmid of Neisseria gonorrhoeae

The 24.5-megadalton plasmid of Neisseria gonorrhoeae is required for transfer of R-factors and possibly chromosomal markers during conjugal matings between gonococcal strains. We constructed a physical map of one such plasmid, pLE2451, using EcoRI, BglII, and HincII site-specific restriction endonucleases. The patterns of deoxyribonucleic acid digestion obtained with this plasmid were identical to those obtained with three other plasmids of similar size.

On the question of chromosomal gene transfer via conjugation in Neisseria gonorrhoeae

Transfer of chromosomal markers between cells of Neisseria gonorrhoeae does not require the presence of a 24.5-megadalton conjgal plasmid in the donor. Apparent conjugal transfer of chromosomal markers may be the result of leakage of deoxyribonucleic acid by some cells in the mating mixture and subsequent uptake of this deoxyribonucleic acid by others.

Genetic basis for colonial variation in Neisseria gonorrhoeae

When the piliated colony types of Neisseria gonorrhoeae, which predominate in recent isolates, were nonselectively subcultured in vitro, they gave rise to large numbers of nonpiliated, avirulent colonial variants. Evidence is presented to show that most of this variation occurs after active growth has ceased and that the variation is sensitive to the action of deoxyribonuclease. We suggest that this variation is a result of transformation. A second variation in colonial morphology involved differing levels of "colony opacity-associated proteins" in the outer membrane. This variation was also inhibited by the presence of deoxyribonuclease, but the genetic basis for it is not as yet clear.

Genetic exchange mechanisms in Neisseria gonorrhoeae

There are two mechanisms for genetic exchange in Neisseria gonorrhoeae. Plasmid deoxyribonucleic acid can be transferred by conjugation, which is dependent on the presence of a 24.5-megadalton plasmid in the donor cell. We have shown that chromosomal deoxyribonucleic acid can be exchanged between all colonial variants by transformation, but not by conjugation. In the nonpiliated variants, however, this exchange was dependent on the presence of the 24.5-megadalton plasmid in the recipient cell.

NgoII, a restriction endonuclease from Neisseria gonorrhoeae

EndoR . NgoII, a class II restriction endonuclease isolated from Neisseria gonorrhoeae, was purified to electrophoretic homogeneity. We were able to separate it from another restriction endonuclease of N. gonorrhoeae, NgoI, by phosphocellulose chromatography. NgoII is an isoschizomer of HaeIII, a restriction endonuclease of Haemophilus aegyptius, and was found to recognize the deoxyribonucleic acid nucleotide base sequence GGCC. NgoII was able to digest phage lambda deoxyribonucleic acid over a wide pH range, with optimal activity at pH 8.5. The enzyme has an absolute requirement for Mg2+; maximal enzyme activity was observed at 1 mM Mg2+. The active enzyme has a molecular weight of 65,000 and appears to be composed of six subunits of identical molecular weight (11,000). No methylase activity could be detected in the purified enzyme preparation.