The Flexible Genome Pool

In contrast to meningococci, most clinical isolates of N. gonorrhoeae harbor a small (4.2kbp) plasmid of unknown function [17] (Table 11.1). In some gonococcal strains regions of this cryptic plasmid have also been found integrated into the gonococcal genome [18]. Another two plasmids frequently encountered in gono-cocci contain genes that code for b-lactamase [19] and can be cotransferred to N. meningitidis via conjugation in the presence of a large, 37-kbp conjugative plasmid [20, 21] but this has not happened yet under in vivo conditions [22]. Although less frequently, N. meningitidis was also found to harbor mostly cryptic plasmids encoding resistance against b-lactam antibiotics [23], sulfonamides [24], and tetracycline [25], which has raised some concern regarding effective antibiotic treatment of meningococcal meningitis [22]. Plasmids differentially distributed among clonal lineages of meningococci have also been demonstrated [26, 27]. However, in contrast to many gram-negative pathogens, no virulence plasmids have been detected in Neisseria species so far.

In addition to plasmids, a computational analysis based on the published genome sequences [12, 13] revealed three partially defective mosaic relatives of the Mu-like group of prophages in the genome of N. meningitidis Z2491 called Pnm1, Pnm2, and Pnm3, respectively (Table 11.1 and Fig. 11.1). The MC58 genome contains similar prophages at the Pnm2 and Pnm3 integration sites called NeisMu1 and NeisMu2, respectively, but has no prophage at the Pnm1 integration site [28, 29] (Fig. 11.1). In addition, a number of f CTX-like phage genes were found in both genomes, summing up to at least three (MC58) and five (Z2491) probably defective prophages in both genomes [30]. The finding that at least prophage NeisMu1 codes for membrane-associated antigenic proteins suggests that these proteins contribute to the variability in envelope structure and may thus influence virulence and pathogenicity [29].

In addition to prophages, larger entities of laterally transferred DNA are frequently found in the genomes of pathogenic bacteria. Amongst other criteria, these regions are generally characterized by atypical DNA composition [31]. Accordingly, base composition analysis of the Z2491 genome resulted in the identification of at least 60 coding regions (nearly 5% in total) with a significantly lower G+C content when compared to the mean G+C content, ranging in size from 224bp to 11.3 kbp and averaging 1.8 kbp [12] (Table 11.1 and Fig. 11.1). One-third of these regions encode proteins that are likely to be located on the surface of the cell or are responsible for the production of surface structures like the sero-group A capsule cassette [12]. In the MC58 genome three such regions were iden-

Table11.1 Basic features of Neisseria spp. genomes as given in the primary annotations [11-14] unless stated otherwise.
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