SUPPLEMENT ARTICLE
Update on the Taxonomy and Clinical Aspects
of the Genus Fusobacterium
Diane M. Citron
R. M. Alden Research Laboratory, Santa Monica, California
The genus Fusobacterium currently includes 13 species. Fusobacterium nucleatum, the most frequently encountered species in humans, is heterogeneous and currently includes 5 subspecies. A potentially new subspecies
of F. nucleatum that is intrinsically quinolone-resistant and phylogenetically separate from the other 5 subspecies has been identified from dog and cat oral flora. Two subspecies have been described for Fusobacterium
necrophorum, and a new species, Fusobacterium equinum, which is related to F. necrophorum, has been
described from horse oral flora. Additional molecular studies have characterized Fusobacterium ulcerans as
separate from the phenotypically similar Fusobacterium mortiferum and Fusobacterium varium. Fusobacterium
sulci and Fusobacterium alocis have been reclassified as Eubacterium sulci and Filifactor alocis, respectively.
Fusobacterium prausnitzii is phylogenetically related to the Eubacterium-like organisms and will likely be
reclassified in the future. The status of the remaining species is unchanged.
The genus Fusobacterium has traditionally included a
variety of gram-negative bacilli that either had pointed
or fusiform ends or that produced major amounts of
butyric acid as an end product of metabolism [1–3].
The present review discusses various aspects of the extant species of Fusobacterium listed in table 1 and of
other species that have been reclassified.
Fusobacterium nucleatum is the species most frequently isolated from humans. Five subspecies have
been described on the basis of electrophoretic patterns
of whole-cell proteins, in addition to DNA homology
[4], and on the basis of the electrophoretic mobility of
2 enzymes and DNA homology [5]. Various other techniques have been used to confirm or challenge the validity of the type strains. Morris et al. [6] grouped these
subspecies by electrophoresis of 21 alloenzymes and
found overlapping identifications and entirely different
groupings. On the other hand, the arbitrarily primed
PCR technique, which uses 2 primers, has demon-
Reprints or correspondence: Diane M. Citron, R. M. Alden Research Laboratory,
1250 16th St., Santa Monica, CA 90404 ([email protected]).
Clinical Infectious Diseases 2002; 35(Suppl 1):S22–7
2002 by the Infectious Diseases Society of America. All rights reserved.
1058-4838/2002/3505S1-0006$15.00
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strated unique profiles for the 5 subspecies [7]. Our
group examined the 16S–23S internal transcribed
spacer regions (ITS) sequences of all currently defined
Fusobacterium spp., along with several related taxons,
to determine the inter- and intraspecies and subspecies
relationships [8]. The 5 subspecies of F. nucleatum were
differentiated both from each other and as a category
from the other closely related species (figures 1 and 2),
which confirms the validity of the earlier descriptions.
We noted in previous antimicrobial susceptibility
studies that some strains of F. nucleatum isolated from
cat- and dog-bite wounds in humans appeared to be
more resistant to quinolone antibiotics than were the
human isolates [9]. Although the colony and cellular
morphology were not in any way unusual, the ITS sequence clearly set these strains apart from the other
subspecies. As with the other F. nucleatum subspecies,
they fermented glucose and fructose very weakly in
prereduced anaerobically sterilized biochemicals (Anaerobe Systems), were indole-positive, and were unreactive in enzyme tests as measured in API-ZYM
(BioMerieux). We are proposing the name F. nucleatum
spp. canifelium; a full description has been submitted
elsewhere.
We have encountered only 2 clinical isolates of F.
Table 1.
Current status of the genus Fusobacterium.
Current name
Comment
Fusobacterium nucleatum subspecies
nucleatum
polymorphum
vincentii
fusiforme
animalis
“canifelium”
Proposed new subspecies
Fusobacterium simae
Fusobacterium periodonticum
Fusobacterium naviforme
Fusobacterium russii
Fusobacterium necrophorum
subspecies necrophorum
Biovar A of F. necrophorum
F. necrophorum subspecies
funduliformis
Biovar B of F. necrophorum
Fusobacterium equinum
New species
Fusobacterium gonidiaformans
Fusobacterium mortiferum
Fusobacterium ulcerans
Fusobacterium varium
Includes F. pseudonecrophorum
(biovar C of F. necrophorum)
Fusobacterium necrogenes
Fusobacterium perfoetans
Filifactor alocis
Fusobacterium sulci
Eubacterium sulci
F. sulci
Fusobacterium prausnitzii
“Eubacterium-like”
nucleatum that were b-lactamase producers out of hundreds of
F. nucleatum strains, cultured mostly from adults, and tested
in our laboratory over the past 18 years [10]. One strain was
isolated from a blood culture and further identified as subspecies polymorphum [11], whereas the other was from a mixed
culture and was of no consequence for the patient. There is
evidence, however, that F. nucleatum isolated from children
contain a greater proportion of b-lactamase producers. Kononen et al. [12] studied multiple isolates of F. nucleatum cultured from saliva samples collected from 20 healthy children.
Of the 123 isolates of F. nucleatum recovered from the 20 samples, 25 were b-lactamase producers, and 10 (50%) of the children harbored ⭓1 b-lactamase–producing strains. Subspecies,
identified by CFA analysis (Microbial Identification System),
showed that 16 (64%) of 25 strains were subspecies polymorphum, 7 (28%) of 25 were subspecies nucleatum, and 2 strains
(8%) were subspecies vincentii. Penicillin MICs ranged from 2
to 256 mg/mL. Of interest, there appeared to be no relationship
between previous antimicrobial therapy and the presence of blactamase–producing strains in these children. Whether the
subspecies polymorphum typically harbors additional virulence
factors remains to be determined. These authors also found an
association between F. nucleatum and otitis media in children.
Another pathogenic role for F. nucleatum in pregnancy outcome has been recently described in a study of 49 women with
idiopathic premature labor compared with 38-week term delivery controls [13]. Organisms associated with bacterial vaginosis were strongly associated with preterm delivery, and
Mobiluncus curtisii and F. nucleatum were recovered from amniotic fluid exclusively from women with preterm delivery.
Some of the strains associated with preterm labor may originate
in the oral cavity and also infect the chorioamnion during
episodes of transient bacteremia [14]. The pathogenic mechanisms of F. nucleatum have been further elucidated by Mikamo
et al. [15] in an in vitro study. They noted that phospholipase
A2 secreted by F. nucleatum stimulates liberation of arachadonic
acid from endometrial cells, accompanied by lysophospholipid
formation. By increasing production and release of prostaglandin E2, these events may induce the onset of labor associated
with intra-amniotic infection.
Other oral strains, listed in table 1, that are similar to F.
nucleatum have been mentioned in other references [1–3]. FuTaxonomy of the Genus Fusobacterium • CID 2002:35 (Suppl 1) • S23
Figure 1. Phylogram (neighbor-joining method) showing the genetic relationships among fusobacterial species as based on the DNA sequences of
their short 16S–23S rDNA spacer region. Fusobacterium prausnitzii American Type Culture Collection (ATCC) 27766 and ATCC 27768T were used as
an outlying group (from [8]).
sobacterium simae was first isolated from monkey oral flora and
differs phenotypically from F. nucleatum, Fusobacterium naviforme, and Fusobacterium periodonticum strains by being
strongly positive for butyrate esterase and moderately positive
for caprylate esterase lipase in API-ZYM (bioMerieux) tests,
whereas the others are very weakly positive or negative. These
species are rarely associated as pathogens in human infections.
Fusobacterium russii is found in canine and feline oral flora
and has been isolated from infected dog and cat bites in humans
[16, 17]. The nonoral strain, Fusobacterium gonidiaformans, has
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been isolated from urogenital and intestinal tracts in humans
and occasionally from infections at other sites.
Fusobacterium necrophorum is a major pathogen and is the
Fusobacterium spp. most frequently isolated in veterinary medicine [16]. It is the cause of necrotic infections in many species
of animals, ranging from ovine foot rot, bovine hepatic abscess,
stomatitis in calves and pigs, and gangrenous dermatitis in
horses and mules, to “jaw disease” in macropod marsupials
[18]. In humans, 150% of infections that contain F. necrophorum present as the classical “Lemierre’s syndrome” of postan-
Figure 2. Phylogenetic tree of Fusobacterium spp. as based on the 16S–23S rDNA spacer region sequences (from [8]). F. nu. ssp. animalis includes
3 strains. A, ATCC strains; F. nu., F. nucleatum; F. ne., necrophorum; fundulif., funduliformis; N, NCTC strains; R, R. M. Alden Research Lab Collection;
ssp. nuc., subspecies nucleatum; ssp. vin, vincentii; ssp. necroph, necrophorum; T, type strain.
ginal sepsis that typically occurs in young, previously healthy
persons [19]. During the preantibiotic era, this disease was
frequently fatal. Other types of infection in humans include
liver abscess, lung abscess, infections of the female genital tract,
intra-abdominal infections, and, occasionally, skin-structure infections. The typical virulent diseases in animals and humans
are caused by biovar A, now called F. necrophorum ssp. necrophorum, whereas biovar B strains, now called subspecies funduliformis, are less pathogenic [18, 20]. A new species that is
phenotypically similar to F. necrophorum has been described
from the normal oral cavity and oral-associated disease of
horses [21]. Phylogenetic analysis of 16S rRNA gene sequences
placed the novel strains as distinct members of the genus. The
proposed name is Fusobacterium equinum.
Another recently described species, isolated from tropical
ulcers, is Fusobacterium ulcerans [22]. Since the original description, additional isolates from different patients or localities
have not been reported. Phenotypically, the strains most closely
resemble the gastrointestinal species Fusobacterium varium, except that F. ulcerans strains reduce nitrate to nitrite. Many of
the preformed enzyme kits used for identification of anaerobes
do not include a test for nitrate reduction and generate a profile
Taxonomy of the Genus Fusobacterium • CID 2002:35 (Suppl 1) • S25
number for F. ulcerans that matches F. varium in the kit’s
database. Thus, unless the microbiologist was aware of this
difference, isolates of F. ulcerans during the past decade could
very easily have been overlooked. Claros et al. [23] characterized
the F. ulcerans strains using 2 universal primers, AP 3 and OPB
4, and demonstrated unique fragments of 1000 and 550 bp
that were not present in the comparator strains of F. varium
and Fusobacterium mortiferum. Differences among the 3 species
were also seen by use of self-designed primers that amplified
the spacer region between the 16S and 23S rRNA genes. This
group of fusobacteria are generally more resistant to antimicrobial agents compared with the previously discussed oral
strains. F. necrogenes is also phenotypically similar to the aforementioned 3 species. F. necrogenes was originally described from
a necrotic abscess in a chicken and from cecal contents of ducks
and is rarely isolated from animal or human infections [1, 16].
Fusobacterium perfoetans is another fecal isolate that has not
been isolated from clinical specimens. It is distantly related to
the other Fusobacterium spp. (figure 1).
Fusobacterium prausnitzii is purportedly a major constituent
of fecal flora in humans when fecal specimens are cultured on
media that contain rumen fluid [24]. Unlike the other Fusobacterium spp., this organism fails to grow on Brucella agar
supplemented with vitamin K1, hemin, and sheep blood unless
rumen fluid has been incorporated into the medium. The mol
% of G ⫹ C DNA is 52–57, which is much higher than that of
other Fusobacterium spp. [1], and phylogenetic studies have
demonstrated that it really belongs with Eubacterium-like organisms [8, 25]. Several studies have examined the microbial
composition of feces by detecting 16S rRNA gene sequences of
species described elsewhere [26] or by PCR procedures based
on unique primers [27] and have reported the presence of large
numbers of F. prausnitzii–like sequences. Suau et al. [26] found
that, on the basis of 16S rRNA gene sequences, F. prausnitzii
belonged to the Clostridium leptum subgroup, which suggests
that it is a gram-positive organism. In our study that used ITS
sequences, F. prausnitzii placed more distantly than Escherichia
coli on the phylogenetic tree from all other fusobacteria [8].
Clearly, this organism warrants reclassification into a different
genus.
Additional studies of organisms previously classified as fusobacteria discovered heat-resistant spores (e.g., Fusobacterium
symbiosum), motility (e.g., Fusobacterium praecutum), or that
the species were found to actually be gram-positive (e.g., Fusobacterium plauti), and they have thus been reclassified into
other genera [2]. In more recent years, Fusobacterium alocis
and F. sulci were described as new species from the human
gingival sulcus [28]. However, their 16S rRNA gene sequences
were shown to lack affinity with gene sequences of other members of the genus Fusobacterium; thus, they have been reclassified as Filifactor alocis and Eubacterium sulci, respectively [29].
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The molecular techniques available for research continue to
define and refine the taxonomy of anaerobes. Fusobacterium is
becoming a more concise genus as a result of these studies.
References
1. Holt JG, Krieg NR, eds. Bergey’s manual of determinative bacteriology.
Baltimore, MD: Williams and Wilkins, 1984:631–7.
2. Jousemies-Somer HR. Recently described clinically important anaerobic bacteria: taxonomic aspects and update. Clin Infect Dis 1997;
25(Suppl 2):S78–87.
3. Jousimies-Somer, HR, Summanen, PH, Citron, DM, et al. Wadsworth
anaerobic bacteriology manual Belmont, CA: Star Publishing, 2002.
4. Dzink JL, Sheehan MT, Socransky SS. Proposal of three subspecies of
Fusobacterium nucleatum Knorr 1922: Fusobacterium nucleatum subsp.
nucleatum subsp. nov., comb. nov., Fusobacterium nucleatum subsp.
polymorphum subsp. nov., nom. rev., comb. nov.; and Fusobacterium
nucleatum subsp. vincentii subsp. nov., nom. rev., comb. nov. Int J Syst
Bacteriol 1990; 40:74–8.
5. Gharbia SE, Shah HN. Fusobacterium nucleatum subsp. fusiforme subsp.
nov. and Fusobacterium nucleatum subsp. animalis subsp. nov. as additional subspecies within Fusobacterium nucleatum. Int J Syst Bacteriol
1992; 42:296–8.
6. Morris ML, Andrews RH, Rogers AH. Investigations on the taxonomy
and systematics of Fusobacterium nucleatum using alloenzyme electrophoresis. Int J Syst Bacteriol 1997; 47:103–10.
7. George KS, Reynolds MA, Falkler WA Jr. Arbitrarily primed polymerase
chain reaction fingerprinting and clonal analysis of oral Fusobacterium
nucleatum isolates. Oral Microbiol Immunol 1997; 12:219–26.
8. Conrads G, Claros MC, Citron DM, Tyrell KL, Merriam V, Goldstein
EJ. 16S-23S rDNA internal transcribed spacer sequences for analysis
of the phylogenetic relationships among species of the genus Fusobacterium. Int J Syst Evol Microbiol 2002; 52:493–9.
9. Goldstein EJC, Citron DM, Merriam CV, Warren Y, Tyrrell K. Comparative in vitro activities of GAR-936 against aerobic and anaerobic
animal and human bite wound pathogens. Antimicrob Agents Chemother 2000; 44:2747–51.
10. Citron DM, Gerardo SH, Hudspeth M, Perino V, Cross S, Goldstein
EJC. Distribution of Fusobacterium species and prevalence of betalactamase production at a community hospital. Anaerobe 1999; 5:
469–72.
11. Goldstein EJC, Summanen PH, Citron DM, Rosove MH, Finegold SM.
Fatal sepsis due to a b-lactamase producing strain of Fusobacterium
nucleatum ssp. polymorphum. Clin Infect Dis 1995; 20:797–800.
12. Kononen E, Kanervo A, Salminen K, Jousimies-Somer H. b-lactamase
production and antimicrobial susceptibility of oral heterogenous Fusobacterium nucleatum populations in young children. Antimicrob
Agents Chemother 1999; 43:1270–3.
13. Holst E, Goffeng AR, Andersch B. Bacterial vaginosis and vaginal microorganisms in idiopathic premature labor and association with pregnancy outcome. J Clin Microbiol 1994; 32:176–86.
14. Hill GB. Preterm birth: association with genital and possibly oral microflora. Ann Periodontol 1998; 3:222–32.
15. Mikamo H, Kawazoe K, Sato Y, Imai A, Tamaya T. Preterm labor and
bacterial intraamniotic infection: arachadonic acid liberation by phospholipase A2 of Fusobacterium nucleatum. Am J Obstet Gynecol
1998; 179:1579–82.
16. Jang SS, Hirsch DC. Characterization, distribution and microbiological
associations of Fusobacterium spp. in clinical specimens of animal origin. J Clin Microbiol 1994; 32:384–7.
17. Talan DA, Citron DM, Abrahamian FM, Moran GJ, Goldstein EJ.
Bacteriologic analysis of infected dog and cat bites. N Engl J Med
1999; 340:85–92.
18. Smith GR, Thornton EA. Pathogenicity of Fusobacterium necrophorum
strains from man and animals. Epidemiol Infect 1993; 110:499–506.
19. Kristensen LH, Prag J. Human necrobacillosis with an emphasis on
Lemierre’s syndrome. Clin Infect Dis 2000; 31:524–32.
20. Shinjo T, Fujisawa T, Mitsuoka T. Proposal of two subspecies of Fusobacterium necrophorum (Flugge) Moore and Holdeman: Fusobacterium necrophorum subsp. necrophorum subsp. nov., nom. Rev (ex Flugge
1886), and Fusobacterium necrophorum subsp. funduliformis subsp.
nov., nom. rev. (ex Halle 1898). Int J Syst Bacteriol 1991; 41:395–7.
21. Dorsch M, Love DN, Bailey GD. Fusobacterium equinum sp. nov. from
the oral cavity of horses. Int J Syst Evol Microbiol 2001; 51:1959–63.
22. Aadrians B, Shah H. Fusobacterium ulcerans sp. nov. from tropical
ulcers. Int J Syst Bacteriol 1988; 38:447–8.
23. Claros MC, Papke Y, Kleinkauf N, et al. Characteristics of Fusobacterium ulcerans, a new and unusual species compared with Fusobacterium
varium and Fusobacterium mortiferum. Anaerobe 1999; 5:137–40.
24. Wang RF, Cao WW, Cerniglia CE. Phylogenetic analysis of Fusobacterium prausnitzii based upon the 16S rRNA gene sequence and PCR
confirmation. Int J Syst Bacteriol 1996; 46:341–3.
25. Moore WEC, Moore LH. Intestinal floras of populations that have high
risk of colon cancer. Appl Environ Microbiol 1995; 61:3202–7.
26. Suau A, Bonnet R, Sutren M, et al. Direct analysis of genes encoding
16 S rRNA from complex communities reveals many novel molecular
species within the human gut. Appl Environ Microbiol 1999; 65:
4799–807.
27. Wang RF, Cao WW, Cerniglia CE. PCR detection and quantitation of
predominant anaerobic bacteria in human and animal fecal samples.
Appl Environ Microbiol 1996; 62:1242–7.
28. Cato EP, Moore LVH, Moore WEC. Fusobacterium alocis sp. nov. and
Fusobacterium sulci sp. nov. from the human gingival sulcus. Int J Syst
Bacteriol 1985; 35:475–7.
29. Jalava J, Eerola E. Phylogenetic analysis of Fusobacterium alocis and
Fusobacterium sulci based on 16S rRNA gene sequences: proposal of
Filicactor alocis (Cato Moore and Moore) comb. nov. and Eubacterium
sulci (Cato Moore and Moore) comb. nov. Int J Syst Bacteriol 1999;
49:1375–9.
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