SummaryThe process of DNA donation for natural transformation of bacteria is poorly understood and has been assumed to involve bacterial cell death. Recently in Neisseria gonorrhoeae we found that mutations in three genes in the gonococcal genetic island (GGI) reduced the ability of a strain to act as a donor in transformation and to release DNA into the culture. To better characterize the GGI and the process of DNA donation, the 57 kb genetic island was cloned, sequenced and subjected to insertional mutagenesis. DNA sequencing revealed that the GGI has characteristics of a horizontally acquired genomic island and encodes homologues of type IV secretion system proteins. The GGI was found to be incorporated near the chromosomal replication terminus at the dif site, a sequence targeted by the site-specific recombinase XerCD. Using a plasmid carrying a small region of the GGI and the associated dif site, we demonstrated that this model island could be integrated at the dif site in strains not carrying the GGI and was spontaneously excised from that site. Also, we were able to delete the entire 57 kb region by transformation with DNA from a strain lacking the GGI. Thus the GGI was likely acquired and integrated into the gonococcal chromosome by site-specific recombination and may be lost by site-specific recombination or natural transformation. We made mutations in six putative type IV secretion system genes and assayed these strains for the ability to secrete DNA. Five of the mutations greatly reduced or completely eliminated DNA secretion. Our data indicate that N. gonorrhoeae secretes DNA via a specific process. Donated DNA may be used in natural transformation, contributing to antigenic variation and the spread of antibiotic resistance, and it may modulate the host immune response.

Summary Symptomatic infection by Neisseria gonorrhoeae (Gc) produces a potent inflammatory response, resulting in a neutrophil-rich exudate. A population of Gc can survive the killing activities of neutrophils for reasons not completely understood. Unlike other Gram-negative bacteria, Gc releases monomeric peptidoglycan (PG) extracellularly, dependent on two nonessential, nonredundant lytic transglycosylases (LTs), LtgA and LtgD. PG released by LtgA and LtgD can stimulate host immune responses. We report that ΔltgAΔltgD Gc were decreased in survival in the presence of primary human neutrophils but otherwise grew equally to wild-type Gc. Adding PG monomer failed to alter ΔltgAΔltgD Gc survival. Thus, LTs protect Gc from neutrophils independently of monomer release. We found two reasons to explain decreased survival of the double LT mutant. First, ΔltgAΔltgD Gc was more sensitive to the neutrophil antimicrobial proteins lysozyme and neutrophil elastase, but not others. Sensitivity to lysozyme correlated with decreased Gc envelope integrity. Second, exposure of neutrophils to ΔltgAΔltgD Gc increased the release of neutrophil granule contents extracellularly and into Gc phagosomes. We conclude that LtgA and LtgD protect Gc from neutrophils by contributing to envelope integrity and limiting bacterial exposure to select granule-localized antimicrobial proteins. These observations are the first to link bacterial degradation by lysozyme to increased neutrophil activation.

Peptidoglycan fragments released by Neisseria gonorrhoeae contribute to the inflammation and ciliated cell death associated with gonorrhea and pelvic inflammatory disease. However, little is known about the production and release of these fragments during bacterial growth. Previous studies demonstrated that one lytic transglycosylase, LtgA, was responsible for the production of approximately half of the released peptidoglycan monomers. Systematic mutational analysis of other putative lytic transglycosylase genes identified lytic transglycosylase D (LtgD) as responsible for release of peptidoglycan monomers from gonococci. An ltgA ltgD double mutant was found not to release peptidoglycan monomers and instead released large, soluble peptidoglycan fragments. In pulse-chase experiments, recycled peptidoglycan was not found in cytoplasmic extracts from the ltgA ltgD mutant as it was for the wild-type strain, indicating that generation of anhydro peptidoglycan monomers by lytic transglycosylases facilitates peptidoglycan recycling. The ltgA ltgD double mutant showed no growth abnormalities or cell separation defects, suggesting that these enzymes are involved in pathogenesis but not necessary for normal growth.Peptidoglycan (PG) fragments released during growth contribute to the pathogenesis of multiple bacterial infections, including those of Bordetella pertussis, Helicobacter pylori, and Neisseria gonorrhoeae (6,23,34). PG fragments induce the production of inflammatory cytokines, cause ciliated cell damage and fluid efflux, and trigger the Nod signaling cascade (reviewed in reference 5). Although PG fragments have been studied biochemically and for immunologic effects in multiple systems, the repertoire of genes and enzymes involved in PG fragment production and release from growing bacteria is unknown.PG fragments released from gram-negative bacterial pathogens are predicted to be produced by the action of lytic transglycosylases. Lytic transglycosylases cleave the N-acetylmuramic acid-␤-1,4-N-acetylglucosamine linkage in PG and catalyze the formation of a 1,6-anhydro bond on the N-acetylmuramic acid (16). PG monomers released from N. gonorrhoeae and B. pertussis were shown to have the 1,6-anhydro bond, indicating that they were generated by lytic transglycosylases (26, 30). To identify genes for PG monomer production, we systematically mutated the genes for lytic transglycosylase homologues in N. gonorrhoeae. Mutation of lytic transglycosylase A (ltgA) resulted in a substantial decrease in PG monomers released (3). Mutations in lytic transglycosylase B (ltgB) or lytic transglycosylase C (ltgC) genes had no effect on PG monomer release (4, 19), although the ltgC mutant showed a severe defect in cell separation. These findings suggested the presence of other lytic transglycosylases in N. gonorrhoeae involved in release of PG monomers.Here we show that lytic transglycosylase LtgD is involved in the release of PG monomers. Additionally, we generated and characterized an N. gonorrhoeae strain deleted for both ltgA and...

Neisseria gonorrhoeae acetylates its cell wall peptidoglycan (PG) at the C-6 position on N-acetylmuramic acid. To understand the effects of PG acetylation on PG metabolism and release of PG fragments, we have made mutations in the genes responsible for PG acetylation. An insertion mutation in a putative PG acetylase gene (designated pacA) resulted in loss of PG acetylation as detected by a high-performance liquid chromatographybased assay. Sequence analysis of a naturally occurring nonacetylating strain revealed the presence of a 26-bp deletion in pacA. Introduction of the deletion mutation into wild-type gonococci resulted in lack of acetylation, and the phenotype was complemented by the addition of a wild-type copy of pacA at a distant location on the chromosome. Mutations were also introduced into three genes downstream of pacA. The gene directly downstream of pacA was required for acetylation and was designated pacB, whereas the next two genes were not required. Sequences highly similar to pacA and pacB were also found in N. meningitidis and N. lactamica strains, and an insertion in the meningococcal pacA eliminated PG acetylation. Phenotypic analyses of an N. gonorrhoeae pacA mutant did not show any decrease in lysozyme resistance or serum resistance, and the release of PG fragments during growth was unchanged. However, purified PG from the wild-type strain was significantly more resistant to the action of human lysozyme than was PG purified from the pacA mutant. Interestingly, the pacA mutant was more sensitive to EDTA, a compound known to trigger autolysis.Gonococci acetylate their peptidoglycan (PG) at the C-6 position on N-acetylmuramic acid. The extent of acetylation varies by strain, with most strains acetylating 40 to 60% of the PG (35). One strain, RD 5 , was reported to have little or no PG acetylation (3,30). Studies performed with PG purified from RD 5 and acetylated PG purified from strain FA19 showed that the FA19 peptidoglycan was more resistant to digestion by lysozyme. Also, the acetylated PG showed greater activity in stimulating inflammation when administered to rats (11).Toxic PG fragments are released by N. gonorrhoeae during growth (23, 32), and PG acetylation could affect release of these PG fragments. During bacterial growth and division, PG strands are degraded and removed from the cell wall in order to expand it or to change its shape for forming the septum. In Escherichia coli, liberated PG fragments are efficiently taken up into the cytoplasm and recycled for new cell wall synthesis (25). However, in gonococci a significant portion of the fragments are not recycled but are instead released by the bacteria (29). In the fallopian tube organ culture model of gonococcal pelvic inflammatory disease, monomeric PG fragments and lipooligosaccharide were shown to be responsible for causing ciliated cell death (23). The same types of PG fragments have been shown to elicit an inflammatory immune response in several other infection models (8,14,18,22). The major fragments released by N. gonor...

Neisseria meningitidis (meningococcus) is a symbiont of the human nasopharynx. On occasion, meningococci disseminate from the nasopharynx to cause invasive disease. Previous work showed that purified meningococcal peptidoglycan (PG) stimulates human Nod1, which leads to activation of NF-B and production of inflammatory cytokines. No studies have determined if meningococci release PG or activate Nod1 during infection. The closely related pathogen Neisseria gonorrhoeae releases PG fragments during normal growth. These fragments induce inflammatory cytokine production and ciliated cell death in human fallopian tubes. We determined that meningococci also release PG fragments during growth, including fragments known to induce inflammation. We found that N. meningitidis recycles PG fragments via the selective permease AmpG and that meningococcal PG recycling is more efficient than gonococcal PG recycling. Comparison of PG fragment release from N. meningitidis and N. gonorrhoeae showed that meningococci release less of the proinflammatory PG monomers than gonococci and degrade PG to smaller fragments. The decreased release of PG monomers by N. meningitidis relative to N. gonorrhoeae is partly due to ampG, since replacement of gonococcal ampG with the meningococcal allele reduced PG monomer release. Released PG fragments in meningococcal supernatants induced significantly less Nod1-dependent NF-B activity than released fragments in gonococcal supernatants and tended to induce less interleukin-8 (IL-8) secretion in primary human fallopian tube explants. These results support a model in which efficient PG recycling and extensive degradation of PG fragments lessen inflammatory responses and may be advantageous for maintaining meningococcal carriage in the nasopharynx.