Genomic Content ofNeisseriaSpecies

The physical properties of most bacterial genomes are largely unexplored. We have previously demonstrated that the strict human pathogenNeisseria gonorrhoeae is polyploid, carrying an average of three chromosome copies per cell and only maintaining one pair of replication forks per chromosome (D. M. Tobiason and H. S. Seifert, PLos Biol. 4:1069-1078, 2006). We are following up this initial report to test several predictions of the polyploidy model of gonococcal chromosome organization. We demonstrate that theN. gonorrhoeae chromosomes exist solely as monomers and not covalently linked dimers, and in agreement with the monomer status, we show that distinct nucleoid regions can be detected by electron microscopy. Two different approaches to isolate heterozygousN. gonorrhoeae resulted in the formation of merodiploids, showing that even with more than one chromosome copy, these bacteria are genetically haploid. We show that the closely related bacteriumNeisseria meningitidis is also polyploid, while the commensal organismNeisseria lactamica maintains chromosomes in single copy. We conclude that the pathogenicNeisseria strains are homozygous diploids.