Neisseria species and their complicated relationships with human health

In 1879 Albert Ludwig Neisser observed diplococci found within neutrophils present in urethral exudates of men and women suffering from gonorrhoea and gonorrhoeal conjunctivitis. This organism was later named Neisseria gonorrhoeae and marks the first ever description of a member of the genus Neisseria¹. The genus Neisseria belongs to the family Neisseriaceae within the phylum β-Proteobacteria². Other genera of the family Neisseriaceae of medical importance include Kingella and Eikinella².

The Neisseria genus contains diverse species inhabiting mammals, reptiles and environmental sites³. Members of the genus are Gram-negative, generally diplococci. Some Neisseria species such as N.weaveri, N. elongata and N. bacilliformis do not conform to the general diplococcus morphology, instead existing as chains of bacilli or filaments⁴. Other classical characteristics of the genus Neisseria include lack of motility, absence of flagella, aerobic fermentation of sugars and oxidase production. Neisseria speciation is continuously being revised and so far there are 10 established species associated with humans (Table 1) with a further seven recently identified from a nasopharyngeal carriage study in an African population⁵. The current robust phylogeny of this species has been developed by applying multi-locus sequence typing (MLST)^6,7. The MLST scheme uses the single nucleotide polymorphisms in each gene to create a unique sequence type (ST) for every isolate. STs can be grouped into larger clusters based on their similarity to one another. The schemes use different numbers of genes with the basic approach using seven housekeeping genes, ribosomal MLST (rMLST) using 53 ribosomal genes⁸ and a core genome MLST (cgMLST) using 246 conserved loci⁹. This has resulted in the condensation of older isolates classified as N. subflava biovar subflava, perflava, flava and flavescens into a single species, N. subflava⁹. Isolates previously termed N. sicca are now variants of N. mucosa⁹ and those previously termed N. mucosa var heidelbergenisis are now called N. oralis¹⁰. Genomic approaches have been more robust than matrix-assisted laser desorption ionisation-time of flight mass spectroscopy (MALDI-ToF) at discriminating these species due to their close relatedness¹¹. In the case of laboratory diagnostic identification, whole genome sequencing is the best approach to identify an unknown Neisseria sp.

Table 1.  Summary of characteristics of human commensal Neisseria species.

Neisseria spp. have multiple modes of interfacing with the human host. N. gonorrhoeae is considered to be a true pathogen¹² as it elicits an inflammatory response upon urethral infection of the human male and causes a delayed inflammatory response, pelvic inflammatory disease, in women. Interestingly, although classified as a pathogen it can asymptomatically colonise the oral mucosa and anorectal sites that self-resolve over 4–12 months¹³. N. meningitidis, the causative agent of invasive meningococcal disease (IMD), is considered an opportunistic pathogen. Whereas N. gonorrhoeae is highly clonal⁷, N. meningitidis has diversified into at least 11 clonal complexes that are highly associated with the risk of IMD¹⁴. A much wider array of genetic lineages are colonisers of the human host but act as commensals as they are infrequently associated with IMD. These two groups are broadly distinguished by the possession of a capsule polysaccharide synthesis (cps) operon. Among many virulence factors¹⁵, the possession of a capsule by N. meningitidis is a key factor enabling survival of IMD-causing bacteria within the blood stream to cause bacteraemia and meningitis. This feature is the basis of genogrouping isolates by quantitative real-time PCR in meningococcal carriage studies. Isolates that are non-disease-causing and disease-causing isolates are stratified by the presence of a capsule null locus (cnl) and capsule transporter A (ctrA), respectively¹⁶. Meningococcal carriage studies have shown that the prevalence of nasopharyngeal carriage of the meningococcus ranges from 10–30% dependent upon a variety of community and behavioural factors¹⁴. However, since the incidence of IMD is much lower than this, other factors are involved in the risk of progressing to IMD after colonisation. This fulcrum rests on the virulence of the isolate and the underlying health of the host^17,18. Until recently, N. meningitidis was not associated with urogenital disease and was considered to be a transient asymptomatic coloniser of the urogenital compartment. This concept was dramatically revised with the report in 2017 of an outbreak of urogenital urethritis attributed to meningococci closely related to an IMD outbreak clade¹⁹. A retrospective review of published case reports of meningococcal disease has uncovered consistent reporting of sporadic cases of horizontal mother to child transmission in pregnancy resulting in rare cases of sepsis, anorectal infection and conjunctivitis²⁰.

In comparison to the two pathogenic species, the remaining eight species are atypical infectious disease agents^{3, 21}. Collectively they are sporadically associated with a wide variety of conditions usually in immunocompromised patients²¹. Since they are not widely known as infectious disease agents, it is also possible that the reports of their involvement in these disease manifestations is under-reported. Nevertheless, genomic comparisons of these commensal species with the pathogenic N. meningitidis shows that they lack multiple virulence determinants²² supporting the conclusion that they are naturally commensal and act as opportunistic pathogens in a disregulated host immune environment. Prevalence studies have typically examined pharyngeal carriage and have shown that all of these species are transient low abundance (<2% abundance) members of the human microbiome. N. lactamica has the highest prevalence of all species and with the highest incidence in children under the age of 4 (14%) before declining in young adults²³. N. polysaccharea also showed a similar distribution as N. lactamica but at a much lower incidence of 2%. In this study N. bergeri and N. subflava had very low prevalence and showed no age-related variation in incidence. Co-colonisation studies have not been performed recently, but an older study from the 1980s that used culture as the means of detection, found multiple Neisseria spp. occurred in 57% of people while 41% of carriage was with N. subflava alone²⁴. The high prevalence of N. subflava appears to be due to its role as a contributor to periodontal disease. Although multiple Neisseria spp. are present in both healthy teeth and dental caries samples, an increase in the abundance of N. subflava is a key signal as the microbial community changes in composition to become acid-secreting, resulting in tooth enamel erosion²⁵.

Human microbiome studies have begun to unravel some relationships of the Neisseria spp. within their relevant mucosal microbiome communities. Unfortunately, Neisseria spp. are typically reported at the genus level as variation in the 16S rRNA alone is insufficient to speciate them. Nevertheless, some generalities can be gained from the current literature. Numerous studies have shown that Neisseria spp. are absent from normal flora in the vulvovaginal mucosal surfaces of women²⁶. This suggests that the isolation of any Neisseria spp. from this compartment should be investigated as a potential pathogen related to an infection particularly urethritis^3,21. Commensal Neisseria spp. are transient, low abundance residents of the rhinopharynx and oropharynx²⁷ that are not associated with any known disease-state²⁸.

There are hints that there are complex interference patterns at both intra- and inter-species levels that influence colonisation by Neisseria spp. Many of these interactions have been examined through the lens of preventing or interfering with colonisation by the pathogens. Exposure to N. gonorrhoeae does not necessarily result in human infection. In surveys of human disease, the risk of contracting gonorrhoea has been linked to a syndrome termed bacterial vaginosis, in which the microbiome has a reduced abundance of Lactobacillus sp.²⁹. Although co-culture of the two species confirms Lactobacillus sp. will inhibit N. gonorrhoeae growth, probiotic treatment of mice with Lactobacillus shows no efficacy in mouse models of gonorrhoea infection³⁰. Streptococcus pneumoniae has been shown to inhibit N. meningitidis using two mechanisms: the secretion of hydrogen peroxide³¹ and a neuraminidase³². Inter-species antagonism is also a feature of the commensal Neisseria spp. against both N. gonorrhoeae and N. meningitidis. N. cinerea and N. lactamica impair early colonisation steps and reduce meningococcal invasion into host cells^33,34 while N. mucosa secretes a small molecule secondary metabolite that inhibits N. gonorrhoeae³⁵. However, all commensal Neisseria spp. could kill N. gonorrhoeae through a DNA-dependent mechanism³⁶. This mechanism is dependent on the expression of type IV pili, which enable the uptake of DNA into the bacterial cell. The DNA from the commensal bacteria have a different methylation pattern and this appears to poison the gonococcal and meningococcal bacteria³³. Direct synergism between Neisseria spp. and other species has not been extensively reported. However, a recent innovative model of meningococcal colonisation conducted by Audry et al.³⁷ showed that meningococcal colonisation of the human oropharyngeal site may not elicit an immediate inflammatory response as the bacteria can be trapped in the mucus layer, preventing invasion of the mucosal epithelium. This state of homeostasis can be perturbed by co-colonisation with other bacteria, and in this model, Streptococcus mitis but not Moraxella catarrhalis triggered the escape of the meningococcus from the mucus layer and invasion into the host cells. S. mitis potentiated growth of the meningococcus by degrading the mucins.

In summary, the taxonomy of the genus Neisseria is continually being redefined by modern molecular typing tools and the recent observation that the diversity of this group remains largely unexplored. This genus contains species that are either pathogenic or commensal with humans, whereas N. meningitidis contains clonal complexes that are pathogenic or commensal. Since its discovery 142 years ago, the interest in this genus has been driven by the medical interest in devising preventative measures against gonorrhoea and meningitis. Other members of this genus, such as N. lactamica have been investigated as a probiotic intervention strategy against IMD³⁴, while the recent observation that commensal Neisseria spp. may kill N. gonorrhoeae via a DNA-dependent mechanism has been recently patented (International Patent Application No. PCT/US2015/048114). Future work is likely to focus on whether commensal Neisseria spp. have a benefit to human health and are necessary for development of a healthy immune system.

The author declares no conflicts of interest.

This research did not receive any specific funding.