Noma is a ravaging orofacial gangrenous stomatitis which is characterized by acute necrotizing ulcerative lesions (1, 2). Noma is prevalent in developing countries where most of the victims are children (3–5). Epidemiological case study reports have established risk factors for the disease such as poor hygiene and nutritional status, measles and other eruptive fevers and immunocompromising diseases (6, 7). Although there are studies on Noma dated more than a century ago, the etiological organisms as well as the trigger agents are yet to be sufficiently detailed (8).

For decades, periodontal diseases were reported as infectious diseases caused by singular organisms, but more recent studies have established the host’s response to microbial dysbiosis as pivotal in the pathogenesis (9, 10). Dysbiosis is a microbial community shift or loss of homeostasis which is detrimental to human health (11, 12). Such detrimental effect is influenced by an alteration in ecological diversity, decrease in beneficial species, and an expansion of pathobionts (13, 14). Periodontal diseases are driven by complex dysbiosis of the oral microbiota (15, Deng et al. 2017b). Key players in the microbial community perform a transitory role from healthy state to dysbiosis (16, 17, Wang et al. 2012). Prevalence of these key players inflame the periodontal conditions such that commensal microorganisms are unable to thrive (18, 19). Although some research findings show that the products of metabolic activity in commensal bacteria play a role in periodontal diseases, the initiation of dysbiosis in Noma has not been thoroughly explored (20, 21).

The application of microbial and molecular methods in microbial ecology has elucidated the potential causative agents or trigger organisms in the pathogenesis of several diseases (22, 23). These breakthroughs have resulted in the prevention, diagnosis, and treatment interventions of diseases (24–27). This review of all primary etiological studies on Noma was carried out to establish the extent of consistency in the determination of the oral microbiome and key players of dysbiosis related to Noma disease, to highlight the constraints of the research studies, and to recommend improved strategies for future etiological studies.

This review was performed observing the Preferred Reporting Items for Systemic review and Meta-Analyses (PRISMA) statement (28). A search was conducted electronically in the following database for related papers: Web of Science; MEDLINE via PubMed; Cochrane Library; Scopus; Science Direct. This search used “Cancrum Oris” or “Noma” as context as well the following terms and their combinations: “Isolation” or “bacterial” or “clinical” or “microbiota”. Clinical trials which investigated the microbiome or characterized microorganisms from Noma patients either by culture dependent or molecular techniques were included. Clinical trials which characterized microorganisms from the oral cavity of patients with periodontal or gingival diseases which were not explicitly reported as Noma were excluded. Clinical trials with a generalized deficient description of microorganisms were excluded. Publications which were not available online were sourced directly from corresponding authors. The final search was performed on 29 September 2022.

Screening of the title and abstract was performed by the first and last author. The eligibility criteria used were: Primary study, clinical trials, Noma disease, and human patients. The following data items were extracted from searching the full text: article title, first author, year of publication, country of origin of sample population (Table 1). The analysis was conducted manually by reporting individual parameters of the included article in depth. These areas of interest were explored included the subject description, sampling details, methodical approach, method of identification, organisms identified & their relative prevalence, unique features of each clinical trial, bias & limitations of each experimental design.

Distinct ecological habitats of various parts of the oral cavity represent different microbiota and disease dynamics (29, 30). The area of the oral cavity where the sample is collected plays some roles in the microbiome identified (31, 32). Although gingival crevicular fluid (GCF) has been extensively studied for antibodies and immunological inflammatory biomarkers of periodontal diseases (33, 34), saliva, supragingival and subgingival plaque are unequivocally preferred in the microbial characterization of periodontal diseases. Most studies (excluding 35) in this review carried out sampling from GCF and pus which raises the question of whether the organisms characterized are trigger microbes or merely a generic proliferation of the gingival microbiota in response to Noma disease. The analysis of GCF in chronic periodontitis showed that the distinguishing microorganisms and metabolites in these samples between periodontitis patients and healthy individuals are biomarkers (36). Saliva and gingival plaque may provide more information on the microbiota of periodontal dysbiosis than gingival crevicular fluid (37, 38).

All studies in this review were performed before the stage categorization of Noma disease was standardized (WHO Regional Office for Africa, 2016). As a result, there is evident disparity in the stage(s) of Noma disease reported in the sampled patients and the organisms identified. The sample population in many of the reviewed studies were patients with advanced lesions. These advanced lesions can be classified as stage 3 (Gangrenous), stage 4 (scarring), and stage 5 (sequelae) of Noma disease. At these stages, Noma is comparable to acute necrotizing ulcerative gingivitis (ANUG) (39, 40, Huyghe et al. 2013b). The identification of a trigger organism in post-virulent stages of Noma is therefore challenging and complex.

Although colony morphology is useful in distinguishing potential pathogens from normal flora, additional molecular techniques are required for discrimination of similar virulent species of the same genus (41). Fusobacterium necrophorum which causes pharyngotonsillitis and peritonsillar abscess but has not previously been linked with periodontal disease was identified by the first morphological study in this review (42). However, the absence of age-matched control groups and exclusion criteria for patients in this study poses a risk of bias in its design.

On the other hand, molecular studies in this review identified F. nucleatum, a periodontal pathogen which is ubiquitous in the oral cavity, as the main species in the Fusobacterium genus associated with Noma (43). The severity of periodontal diseases increases with the prevalence of F. nucleatum since it facilitates the formation of dental plaque (44, Han, 2015b). F. nucleatum is not considered a trigger organism in periodontal dysbiosis, although it connects the initial and later bacterial colonizers such that when F. nucleatum is absent, the prevalence of late colonizers is reduced (45, 46). F. nucleatum and F. necrophorum are the most frequent species implicated in Fusobacterium species bacteremia but morphological classification in the absence of molecular techniques such as 16s rRNA gene sequencing may lead to the misidentification of one for the other (47, 48).

Except for Fusobacterium species, the microbiota observed in ANUG is analogous to that described by molecular studies in this review (49, 50). Although this supports the theory of Noma as a dysbiosis, it does not support P. intermedia as the etiological trigger pathogen due to the advanced stage of the disease in which this species was characterized in (Darveau, Tanner, and Page, 1997 51). Previous studies have established the role of P. intermedia in the initiation and development of periodontitis by promoting periodontal connective tissue and bone matrix destruction through upregulated matrix metalloprotease production (52, Socransky et al. 1998b). Therefore, longitudinal molecular studies on the progression of simple gingivitis and acute necrotizing gingivitis to Noma sequelae will give a clearer picture of both the dysbiosis and trigger microorganism of Noma.

Like P. intermedia, oral spirochetes have been linked to necrotizing ulcerative gingivitis and other periodontal diseases. Particularly, clinical research has shown a correlation between the prevalence of Treponema denticola in periodontal pockets and the progression of periodontal disease in patients (53). Likewise, the higher representation of spirochaetes in Noma lesions is suggestive of its possible role in the dysbiosis of Noma. However, this was only reported in the molecular studies (35, Huyghe et al. 2013c). Due to the difficult nature of culturing Spirochaetaceae in the laboratory, specific culture conditions and specialized media are essential to optimize their growth and isolation (54, 55). Studies have demonstrated that Spirochaetaceae growth is only noticeable 4 weeks after culture and reaches its maximum within 8–12 weeks (56, 57). This contrasts with the longest incubation time of 5 days reported in this review. Also, Spirochaetaceae showed optional growth in Barbour-Stonner-Kelly (BSK) medium with rabbit serum, BSK swine serum + 5 fluorouracil, Cystine Tellurite Blood (CTB) medium, and brain heart infusion broth (56, 57). This potentially accounts for the absence of spirochaetes in the Noma samples since the culture dependent studies used Brucella blood agar supplemented with hemin (0.05%) and menadione (0.1%) (BBHK) (58, Chidzonga and Mahomva, 2008b).

The choice of primers in DNA Amplification (PCR) influences the microbiota detected and quantified using molecular techniques in the studies examined here, especially at the phylum level (59). 16S rRNA gene sequencing which used universally conserved primers in PCR read did not show a high representation of Spirochaetaceae. Whereas using the same sample population, PCR which used spirochete selective primers indicated that 85% of the clones have spirochetal inserts. Spirochaete species have a guanine (G): cytosine (C) ratio ranging from 51% to 65% in DNA (60, 61). This phenomenon not only segments DNA into several linear pieces, but it also makes it a difficult target for universal primers in DNA-DNA Hybridization (62, 63). This further suggests that the amplification achieved by Real-Time qPCR might not be a true reflection of the triggering possibilities of Spirochaete species in the dysbiosis of Noma disease.

The last four studies in this review demonstrate the influence of different molecular techniques on the etiological findings of Noma studies. Although conducted by the Geneva Study Group on Noma (GESNOMA) using the same sample population, the individual study designs and the limitations of each identification approach were discriminating factors in the results. The first 16S rRNA gene sequencing (Bolivar et al. 2012) attributed the underrepresentation (K-mean) of Spirochetes in Noma lesions to possible experimental bias of PCR reverse primer. Nonetheless, 11 phylotypes of Spirochaetaceae accounted for 15 occurrences in diseased sites. The study also emphasized that it’s library cloning yielded ratios of species not absolute numbers. Therefore, an unexpected increase in the representation of non-related Bacteroidetes (83%), might have resulted in a matched decrease for some other related bacteria such as Fusobacteriaceae (44%).

The last study by GESNOMA amplified the V1-3 segment of the bacterial 16S rRNA gene prior to 454 pyrosequencing (44). V2-3 hypervariable regions are not suitable for distinguishing all bacteria at genus level. This is the only study that reported Treponema amylovorum as the Spirochaetaceae species for Noma instead of Treponema Denticola. High-throughput sequencing of the entire 16S gene more accurately discriminates bacterial species and copy variants (64, 65). The pyrosequencing analysis ranked fusobacterium as low Noma indicators while placing Sharpea as the highest indicator species of Noma. On the contrary, Sharpea are commonly associated with rumen samples from low-methane-emitting sheep (66). While the abundance of Prevotella was well described in the results, the exclusion of a significant 21% of initial sequence read might have biased the computational analysis.

Although this systematic review demonstrated that Noma is a periodontal dysbiosis with Spirochaetes and P. intermedia as putative trigger organisms, F. nucleatum also enhances the late colonization of biofilms in subgingival plaque of advancing lesions. However, the limitations and therefore the bias of the study designs in this review demands stronger evidence from further studies. Longitudinal studies on the acute necrotizing gingivitis and oedema stages of Noma in endemic communities with age-matched controls is needed in the future. Such studies should employ high throughput full genome sequencing techniques on gingival plaque and saliva samples with primers and probe sets that are comprehensive to all possible oral microbiota or the use of PCR-independent molecular deep sequencing techniques.