Guest Editorial

Of mice and men: animal modelsof human periodontal disease

Fine DH. Of mice and men: animal models of human periodontal disease. J ClinPeriodontol 2009; 36: 913–914. doi: 10.1111/j.1600-051X.2009.01456.x.

Key words: bone loss; micro-computedtomography; periodontal disease; P. gingivalis

Accepted for publication 21 June 2009

Animal models can provide criticallyimportant information regarding perio-dontal disease pathogenesis (Graveset al. 2008). This benefit results fromthe fact that these models can assessdisease in a longitudinal manner on acellular and molecular level. As such,animal models can provide otherwiseunattainable information at the hostlevel in spite of shortcomings that areknown to exist (Graves et al. 2008). Tooptimize their utility, animal modelsneed to be approached with the samerigour that is applied to clinical modelsof human disease (Fine et al. 2001).Moreover, information obtained fromanimal models should be interpretedwith the same caution used to analysehuman models of disease. One criticalissue related to the article by Wilenskyand colleagues rests in the area ofcomparison of Porphyromonas gingiva-lis-induced host responsiveness or thelack thereof. All infections, withoutexception, are dependent on the doseand form of the infectious agent as wellas the route of infection (Gibson et al.2004). Further, the challenge with aforeign antigen should be put in thecontext of the time of exposure. Whileanimal models of periodontal diseaseare still in their early stages of explora-tion these factors need to be considered.

With respect to P. gingivalis-inducedperiodontal disease in rodents, at first, itwas difficult to understand how P. gin-givalis could either colonize or survivein the oral cavity of rodents in spite ofantibiotic suppression of the flora andthe use of carboxymethyl cellulose toadd to the ability of P. gingivalis toadhere (Evans et al. 1992). This wasparticularly true because P. gingivalis isdifficult to grow in the laboratory due to

its mandatory requirement for heminand menadione-like substances andanaerobic growth conditions, whichwould not appear to be available in themouths of healthy mice (Gibbons &Macdonald 1960). The question aroseas to how this bacterium could survivein a supragingival environment in adisease-free mouse (Kesavalu et al.1997). It was assumed that the use oflavage, gavage and the copraphagicnature of rodents led to re-innoculationand ultimate colonization (Chang et al.1994). Genetic studies have shown thatP. gingivalis possesses a variety ofgenes or segments of genes that allowit to attach to many surfaces such asteeth, soft tissue, etc. (Lamont & Jen-kinson 1998). P. gingivalis also has agroup of proteinases, known as gingi-pains, that allow it to degrade connec-tive tissue and activate bone-resorbingcells (Lamont & Jenkinson 1998).Taken together, it would seem that thisbacterium has the machinery required tocolonize and cause disease in humans,and as more and more animal studieshave accumulated over time, it appearsthat P. gingivalis can colonize and causedisease in mice (Baker et al. 2000,Baker & Roopenian 2002). We canconclude from this mass of data thatthe adaptability of this microbe has beenseverely underestimated (Hart et al.2004).

The present study of Drs. Wilenskyand colleagues is commendable. Theiruse of micro-computed tomography hasadded to the accuracy and reproducibil-ity of interpretation of bone loss in themouse model of periodontal disease andthus moved the model to a new level ofcompetence (Wilensky et al. 2005).Further, because of the sensitivity of

the methodology, the authors havebeen able to reduce the number ofmice in each of the study groups toassess differences in bone loss ininfected mice as compared with theiruninfected controls. This of coursemakes for a more economical andhumane model because of the limitednumber of mice required. Further, thisreport provides support and extendsobservations indicating that differentstrains of P. gingivalis cause differentlevels of disease (Baker & Roopenian2002). Similar data have also beenshown recently for Aggregatibacteractinomycetemcomitans-infected rodents(Fine et al. 2009). Moreover, it has beenshown that different strains of rodentshave different susceptibilities to achallenge from the same strain ofP. gingivalis (Baker et al. 2000, Baker &Roopenian 2002). This holds truefor A. actinomycetemcomitans as well(Schreiner et al. 2008). These observationsparallel what is seen in human disease,particularly in the case of A. actinomyce-temcomitans-related disease where theJP2 strain of A. actinomycetemcomitansappears to be more virulent, and whereindividuals of African heritage appear tobe more susceptible to A. actinomyce-temcomitans infection (Haubek et al.2008). Taken together, these similaritiessupport the utility of an animal model ofperiodontal disease.

However, with these considerationstaken into account, it is still a concernthat most reports of P. gingivalis-induced periodontal disease in mice,including this one, fail to report theinfecting dose and the direct recoveryof the organism from experimentallyinfected animals (Gibson & Genco2001). This is especially important in

Daniel H. Fine

Department of Oral Biology, University of

Medicine and Dentistry of New Jersey,

Newark, NJ, USA

J Clin Periodontol 2009; 36: 913–914 doi: 10.1111/j.1600-051X.2009.01456.x

913r 2009 John Wiley & Sons A/S

studies where one strain of P. gingivalisis compared with another with the goalof assessing strain-dependent pathogen-esis. Moreover, while it is well knownthat growing P. gingivalis in the labo-ratory can be fraught with difficulty,current DNA methodologies, such asreal-time PCR, allow for recovery andestimation of the infecting dose in aquantitative manner.

In this study by Wilensky and collea-gues antibody titres were used to indi-cate exposure to P. gingivalis as well asan indicator of the host response. Canwe assume that a low IgG titre means alower level of host responsiveness toP. gingivalis 53977, or, is it an indicationof a lower infecting dose of P. gingivalis53977? Either could be true; however,with respect to antibody titres, antibodylevels represent a past history of expo-sure to the challenging antigen. Withoutdemonstrating some equivalence of theinfecting dose among the strains ofP. gingivalis used, and the time-depen-dent changes in antibody titres resultingfrom that infecting dose, it is difficult toconclude that one strain is more patho-genic than the other. With this said, theperiodontal literature has taught us thatthe assessment of the bacterial challengeand the host response to that challenge isbest accomplished at the site of theinfection (Assuma et al. 1998). Thereis no doubt that periodontal diseaseconsists of a local infection that isdirected at the surrounding tissues thatsupport the tooth in its alveolus. Theadvantage of an animal model is that wecan examine the local response in atime-dependent and site-specific andthus tissue-specific manner (Assuma etal. 1998). The models available for bothP. gingivalis and A. actinomycetemco-mitans have improved to the pointwhere it is possible to advance to thenext level, a level that permits us toexamine the bacterial challenge at thelocal site as well as the host response tothat challenge at the local level in aquantitative manner.

As we begin to gain a greater under-standing of the periodontal disease pro-cess, it becomes more and moreapparent that we need to consider boththe bacterial initiator and the hostresponse to these so-called ‘‘patho-

genic’’ oral microbes if we hope togain a better understanding of disease.The statements made herein are notintended to detract from the importanceof the manuscript by Wilensky andcolleagues but are aimed at pointingout that both the bacterial challengeas well as the host response to thatchallenge need to be considered in aquantitative, time-dependent, site- andtissue-specific manner so that the con-clusions drawn from the data obtainedcan be extrapolated to the broader bio-logical questions that we seek to answer.

References

Assuma, R., Oates, T., Cochran, D., Amar, S. &

Graves, D. T. (1998) IL-1 and TNF antago-

nists inhibit the inflammatory response and

bone loss in experimental periodontitis. Jour-

nal of Immunology 160, 403–409.

Baker, P. J., Dixon, M., Evans, R. T. & Roope-

nian, D. C. (2000) Heterogeneity of Porhpyr-

omonas gingivalis strains in the induction of

bone loss in mice. Oral Microbiology and

Immunology 15, 27–32.

Baker, P. J. & Roopenian, D. C. (2002) Genetic

susceptibility to chronic periodontal disease.

Microbes and Infection 4, 1157–1167.

Chang, K. M., Ramamurthy, N., McNamara, T.,

Evans, R., Klausen, B., Murray, P. & Golub,

L. (1994) Tetracyclines inhibit Porphyromo-

nas gingivalis-induced alveolar bone loss in

rats by a non-anti-microbial mechanism.

Journal of Periodontal Research 29, 242–

249.

Evans, R. K., Sojar, B., Bedi, H. T., Sfinrwau,

G. S., Ramamurthy, N. S., Golub, L. M.

& Genco, R. J. (1992) Immunization with

Porphyromonas (Bacteroides) gingivalis fim-

briae protects against periodontal destruction.

Infection and Immunity 60, 2926–2935.

Fine, D. H., Goncharoff, P., Schreiner, H. C.,

Chang, K. M., Furgang, D. & Figurski, D.

(2001) Colonization and persistence of rough

and smooth colony variants of Actinobacillus

actinomycetemcomitans in the mouths of rats.

Archives of Oral Biology 46, 1065–1078.

Fine, D. H., Schreiner, H., Nasri-Heir, C.,

Greenberg, B., Jiang, S., Markowitz, K. &

Furgang, D. (2009) An improved cost-effec-

tive, reproducible method for evaluation of

bone loss in a rodent model. Journal of

Clinical Periodontology 36, 106–113.

Gibbons, R. J. & Macdonald, J. B. (1960)

Hemin and vitamin K compounds as required

factors for the cultivation of certain strains of

Bacteriodes melaninogenicus. Journal of

Bacteriology 80, 200–206.

Gibson, F. C. III & Genco, C. A. (2001)

Prevention of Porphyromonas gingivalis-

induced oral bone loss following immuniza-

tion with gingipain R1. Infection and Immu-

nity 69, 7959–7963.

Gibson, F. C. III, Gonzalez, D. A., Wong, J. &

Genco, C. A. (2004) Porphyromonas gingi-

valis-specific immunoglobulin G prevents

P. gingivalis-elicited oral bone loss in a

murine model. Infection and Immunity 72,

2408–2411.

Graves, D. T., Fine, D., Tang, Y.-T., Van Dyke,

T. E. & Hajishengalls, G. (2008) The use of

rodent models to investigate host-bacteria

interactions related to periodontal diseases.

Journal of Clinical Periodontolog 35, 89–

105.

Hart, G., Shaffer, D. J., Akilesh, S., Brown, A.

C., Moran, L., Roopenian, D. C. & Baker, P.

J. (2004) Quantitative gene expression profil-

ing implicates genes for susceptibility and

resistance to alveolar bone loss. Infection and

Immunity 72, 4471–4479.

Haubek, D., Ennibi, O.-K., Poulsen, P., Vaeth,

M. & Kilian, M. (2008) Risk of aggressive

periodontitis in adolescent carriers of the JP2

clone of Aggregatibacter (Actinobacillus)

actinomycetemcomitans in Morocco: a pro-

spective longitudinal cohort study. Lancet

371, 237–242.

Kesavalu, L., Holt, S. C. & Ebersole, J. L. (1997)

Porphyrmonas gingivalis virulence in a mur-

ine lesion model: effects of immune altera-

tions. Microbial Pathogenesis 23, 317–326.

Lamont, R. J. & Jenkinson, H. F. (1998) Life

below the gum line: pathogenic mechanisms

of Porphyromonas gingivalis. Microbiology

and Molecular Biology Reviews 62, 1244–

1263.

Schreiner, H. C., Markowitz, K., Moore, D.,

Miryalkar, . M., Diehl, S. R. & Fine, D. H.

(2008) A. actinomycetemcomitans coloniza-

tion and induced bone loss in three rat strains.

Journal of Dental Research 87, (Spec Iss B):

#2170, www.dentalresearch.org.

Wilensky, A., Gabet, Y., Yumoto, H., Houri-

Haddad, Y. & Shapira, L. (2005) Three-

dimensional quantification of alveolar bone

loss in Porhpyromonas gingivalis-infected

mice using micro-computed tomography.

Journal of Periodontology 76, 1282–1286.

Address:

Daniel H. Fine

Department of Oral Biology

University of Medicine and Dentistry of New

Jersey

185 South Orange Ave

Newark

NJ 07103

USA

E-mail: Finedh@umdnj.edu

914 Fine

r 2009 John Wiley & Sons A/S