in vitro study of the penetration of three bacterial strains into root dentine

In vitro sl:udy of the penetration of three bacterial strains into root dentine

Fabienne Perez, DCD,” Tarik Rochd,b Jean-Philippe Lodter, DSO,” Paul Calas, DSO,d and Georges Michel,e Toulouse, France UFR ODONTOLOGIE AND UFR SC. PHARMACEUTIQUES.

Thle purpose of this study was to assess in vitro migration of the following three bacterial species into dentinal tubules: Streptococcws sanguis, Actlnomyces naeslundii and Prevotella intermedia. Twenty-seven bovine incisors were randomly divided into three groups. Only the root dentin was used for the experiment. Within each group, the nine incisors were sectioned into 36 dentin blocks that were incubated with one of the tested bacterial suspensions. Twelve samples were taken after 10, 20, and 28 days. Half the samples were observed with scanning electron microscopy and the others with light microscopy after standard histologic procedures, and the depth of bacterial penetration was measured. Analysis of the examinations showed that whatever the technique used, only one strain on this experimental model migrated into dentinal tubules. Ssnguis was observed at a depth of 792 pm, but no migration was observed for either A.naes/uncfi or P.intermedia. The differences in migration of the bacteria tested appear to be related to their morphologic factors and cellular arrangement. (ORAL SURC ORAL MED ORAL PATHOL 1993;76:97-103)

The bacteria responsible for endodontic infections have been well known since the works of Sundqvist and Bystriim.le4 Even if the bacteria considered to be the most pathogenic are gram-negative bacilli, par- ticularly black-pigmented Bacteroides,5-8 other bacterial species such as Actinomyces seem to be found frequently in failed endodontic treatment cases.gy lo These failures are mainly linked to the persistence of bacteria inside the root canal system. Besides the main root canal, bacteria can lodge in cementum crypts, secondary canals, or dentinal tubules.

Previous studies have revealed bacterial penetration in the root dentin, but the bacterial species studied were different from the root canal flora, l1 or were only observed by a single method, light microscopy,r2’ I3 thus giving random results. Consequently, the aim of this study was to observe in vitro migration of the foIllowing three nonmotile bacterial species that belong to conventional root canal flora into root dentinal tubules: a gra-m-positive COCCUS, Streptococcus sanguis; a gram-positive bacillus, Actinomyces naeslundii; and a gram-negative bacillus, Prevotella

aAssociate Assistant, Endodontics Department, Laboratoire de Bi- omateriaux, UFR Odontologie. bLaboratoire de Virologie et Microbiologic Industrielle, UFR SC. Pharmaceutiques. “Professor, Head of Research Department, Laboratoire de Biolo- gie Buccale, UFR Odontologie. dProfessor, Head of Endodontics Department, Laboratoire de Bi- omattriaux, UFR Odontologie. eProfessor, Head of Research Department, Laboratoire de Virol- ogie et Microbiologic Industrielle, UFR SC. Pharmaceutiques. Copyright @ 1993 by Mosby-Year Book, Inc. 0030-4220/93/$1.00 + .lO 7/15/46623

intermedia. Two observation techniques were used: scanning electronic microscopy (SEM) and light microscopy.

MATERIAL AND METHODS The following bacterial strains were used for this

experiment: Streptococcus sanguis NCTC 7853 cultured in trypticase soy agar (TS) and incubated in TS broth; Actinomyces naeslundii ATCC 10039 cultured in chocolate-polyvitex agar and incubated in Schaedler broth with Vit K3; and Prevotella intermedia NCTC 93336 cultured in schaedler agar and incubated in Schaedler broth. P.intermedia is an obligate anaerobe, A.naeslundii, and S.sanguis are facultative anaerobes, and the three test organisms were cultured under anaerobic conditions.

Twenty-seven freshly extracted bovineincisors were used for these experiments. They were stored in a 0.5% sodium hypochlorite solution (NaOCl) (Prola- bo, Paris, France) for surface disinfection. In accor- dance with the technique described by Haapasalo and Orstavik,14 the apical 5 mm and the crown were sectioned with the use of a rotating diamond saw and water irrigation (Komet 925 P, Brasseler Gmbh, Lemgo, Germany) (Fig. 1). The root canal was wid- ened to a diameter of 2 mm with a tungsten carbide round bur (Komet HlS IS0 023, Brasseler Gmbh). Root cementum was completely removed by a tungsten carbide bur (Bisico 55 1 O-60, Lan9on/Provence, France) to obtain a 15 mm dentin cylinder. Four equally thick slices were sectioned with the use of the rotating diamond saw and water irrigation (Komet 925 P, Brasseler Gmbh). Each slice sample had a mean diameter of 6 mm and a height of 4 mm. The

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Fig. I. Preparation of bovine dentin specimens.

samples were then treated in an ultrasonic bath (Bransonic 2200 El-50 KHz) in a 17% disodium eth- ylenediamine tetraacetate (EDTA) solution (Prola- bo, Paris, France) for 10 minutes, followed by a 2.5% NaOCl solution for 10 minutes. They were finally rinsed in a phosphate-buffered saline solution (pH = 7.2) to remove all traces of EDTA and NaOCl. A few random samples were taken at this stage with a SEM to check the apertures of the dentinal tubules. The samples were then sterilized for 30 minutes in an autoclave at 120” C. Sterilization was checked by placing them in a TS broth (Bio-Merieux, Marcy l’ttoile, France) for 10 days at 37” C in anaerobic conditions. This also allowed the culture medium to penetrate the tubules, thus favoring bacterial growth. Such penetration was further increased by placing a 2 mm diameter sterile ultrasonic titanium probe in the medium for 10 minutes before inoculating (Vibracell, Bioblock scientific-600W, 20KHz-Sonics and mate- rials, Danbury, Conn.).

The first 36 blocks of dentin were incubated in tubes that contained a S.sanguis NCTC 7863 suspension in 2 ml TS (1 O9 cells/ml). Another 36 blocks were then incubated with a A.naeslundii suspension in Schae- dler broth and the last 36 samples with a suspension of P.intermedia in Schaedler broth. Each tube contained two samples. The medium was changed every 4 days. Culture purity was systematically checked whenever the medium and suspension were changed. Such handling necessitated highly rigorous asepsis and was performed in a laminar flow hood. Twelve blocks were removed for each experiment after 10 days, 20 days, and 28 days of incubation respec- tively. Development of infection, hence of the migration of the bacterial strain inside the tubules, was revealed by examining samples with the SEM and by using light microscopy after a Brown and Brenn stain.

July I993

For SEM examination, samples were fixed in 2.5% glutaraldehyde for 30 minutes, dehydrated in increas- ingly concentrated ethanol solutions, and longitudi- nally split. They were then gold plated to a thickness of 200 nm and observed with a SEM (CAMBRIDGE S 250 MK3, Cambridge, U.K.).

After fixing (10% neutral formol), demineraliza- tion (5% nitric acid), and dehydration in ethanol solutions, the preparations were embedded in paraffin for histologic examination. Five micron thick sections were then cut. Each section was stained using the Brown and Brenn technique for routine light micro- scopic examination. l5 On average; 11 sections were extracted from each dentin block in different areas after reorientation of the dentin piece. These sections were representative of the entire thickness of the dentin block. Observations were carried out using two magnification strengths (X40 and X100), and the migration distance from the pulpal surface to the outer dentin was estimated with a graduated, precal- ibrated microscope eyepiece (Leitz, Hamburg, Ger- many, X10).

RESULTS Over the three experiments, 27 bovine incisors pro-

vided 108 dentin blocks of which 54 were observed by SEM and 54 by light microscopy. Only light microscopy revealed maximum penetration of the bacteria. SEM observation was impossible as SEM examination did not display all of the bacteria migrating through the tubules with sufficient certainty. Indeed, the defects in the slices or the actual anatomy of the dentinal tubules and their three-dimensionally wind- ing paths restrict observation of the bacteria and hence the detection of their maximum depth of penetration. SEM gives less complete results but does confirm the data obtained with the use of a light microscope.

Migration of Streptococcus sang& SEM examination. After 10 days S.sangwis cells

are shown at a depth of 200 pm in samples. Many of the tubules are empty. Penetration increases after 20 then 28 days. The canal wall is entirely colonized by S.sanguis that assures a chain-like organization and bacteria are located more than 600 pm inside the tubules (Fig. 2).

Light microscopy examination. As with the SEM examination, it could be seen after 10 days that there were a great number of empty tubules among the sections. Bacteria was collected on the canal wall on tubule apertures and scattered throughout the circum- ference of the canal.

After 20 days, not all tubules were infected, but

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Fig. 2. SEM of’ Ssanguis at 28 days. S: Cells of S.sanguis 666 pm deep, many tubules are empty. (magnification X50008.)

migration had increased to reach a maximum of 792 Table I. Lengths of penetration of S. sanguis cells w. into root dentine

After 28 days, a fringe of dentinal infection of approximately 150 bbrn could be located around the canal and the slices observed revealed a major penetration of S.sanguis (Fig. 3). Data on penetration minimum and maximum are given in Table I. In light microscopy, one sample had to be discarded because the observations did not provide a sufficiently reliable reading and measurement of bacterial penetration.

Extreme Mean penetration values (minimum-

(s.4 PM maximum) pm

All specimens 458.8 (+ 109.3) 150-792 10 days

I 4”;::s’ I: Kj 150-648

20 days 180-792 28 days 478.6 (t 101.0) 300-737

Migration of Actinomyces naeslundii SEM examination. After 10 days, A.naeslundii

cells covered the canal wall but no tubules were infected. After 20 days, areas of intense bacterial concentration alternated with these where no cells were observed. After 28 days, bacteria were arranged in clusters, but there was no penetration into the dentinal tubules.

pletely empty without any traces of P.intermedia penetration.

Light microscopy examination. No bacterial migration was observed on histologic slices. Bacteria stained blue with the Brown and Brenn technique created a fringe concentrated on the canal periphery but had not penetrated the tubules. After 28 days, enormous bands of bacteria were occupying the canal wall and blocked tubule apertures (Fig. 4).

Light microscopy examination. The pink staining of the gram-negative bacteria made it difficult to ex- amine the tubules themselves, which were stained yellow. We could, however, observe that there were few bacteria visible in the lumen of the canal and that no P.intermedia cells had penetrated the dentinal tubules. Some clusters were located on the canal wall (Fig. 5).

DISCUSSION

Migration of Prevotella intermedia

The experiment revealed that ofithe three bacterial species tested, only one S.sanguis migrated into the tubules of bovine dentin.

SEMexamination. Bacterial colonization was very slight on the wall after 10 days, and no bacilli were detected in the dentin. Bacilli had accumulated on the wall over time, but the tubules had remained com-

However, can this be correlated to any classification? S.sanguis is a gram-positive coccus, but belong- ing to any particular gram category does not seem to affect migration because A.naeslundii, which is also

%I Perez et al. ORAL SURGERY ORAL MEDICINE ORAL PATHOLOGY July 1993

Fig. 3. Light microscopy of S.sanguis at 28 days. Chain of S.sanguis in dentinal tubules. (Brown and Brenn stain; original magnification X100.)

Fig. 4. Light microscopy of A.naeslundii at 28 days. Bacterial fringe of A.naeslundii cells: A, covers canal wall; D is dentin. (Brown and Brenn stain; original magnification ~25.)

a gram-positive bacillus, does not migrate any more than P.interPnedia, which is gram-negative.

If we consider the classification from the oxygen tolerance point of view, this experiment studied the migration of two facultative anaerobes: S.sanguis, A. naeslundii and of one obligate anaerobe: P.intermedia. According to the results, this factor doesn’t seem to be the cause of the different migratory behavior of the three bacterial strains. This corroborates the works of Akpata and Blechman12 who studied the migration of two facultative and two obligate anaerobes.

More logically, the intervening factor would appear to be morphologic both from a qualitative and quantitative viewpoint. It is a qualitative factor because cocci penetrate the tubules and the two bacilli do no% migrate, but it is also a quantitative factor because bacteria aggregate form clusters, and it is this morphologic structure that seems to condition bacteria! penetration into dentinal tubules.

Measurements of bovine dentinal tubules made in iight microscopy or in SEM have revealed varying di- ameters: 4 to 7 pm after dentinal pretreatment. It is obvious that S.sanguis, the size of which is about 1

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Fig. 5. Light microscopy of Pintermedia at 28 days. Clusters of P.intermedia cells: Pi located on canal surface, D is dentin. (Brown and Brenn stain; original magnification X100.)

w, l6 can penetrate the tubules. Yet, the mode of interbacterial grouping seems to be responsible for the behavior of each species.

S.sanguis cells grow by elongation along main cell axis and are organized in chain.“j This organization is like a “train” that slips into and slides around the tubules without the chain being a geometric obstacle to its progression. This was very clearly observed by SEM and light microscopy. Moreover, the S.sanguis strain is not mobile, and given the depth of penetration, it is remarkable that such a bacterial strain should travel such a distance. We can therefore ask how and why bacteria penetrate the dentin? Two phenomena may occur: first, bacteria that grow under favorable conditions multiply in the culture medium and as this medium penetrates dentinal tubules, S.sanguis can penetrate deeply by a passive process, and second the culture medium cannot be renewed in the tubules but is enough nutrition to permit growth and multiplication of bacteria inside dentinal tubules and to increase their penetration.

As for the A.naesZundii migration, tubule diameter is the same as in the first experiment, but in the results we have seen th[at A.naeslundii cells were only concentrated on the canal wall. Yet the size of these filamentous bacteria, approximately 1 pm X 5 pm, and their elongated forms would be propitious in making their way through the tubules.17 Conse- quently, their grouping mode should be examined to try to explain this lack of penetration.

After 24 hours of incubation, A.naeslundii micro- colonies had a dense center composed of diphteroidal cells and of partly fragmented filaments surrounded

by long, branched, irregularly curved threads, pro- jecting out in all directions.18 The morphologic structure of Actinomyces may vary considerably according to media, incubation conditions, and the age of inoculum. Consequently, curved filaments can be straighter and of varying length. Others have bulging or budding ends. A.naeslundii cells may produce long filamentous projections that radiate from the cell wall surface.

SEM was used to describe Actinomyces cells and mode of growth more precisely. Surface fibrils may play a major role in the attachment of these bacteria to epithelial cells, tooth surfaces, or other bacteria.” The works of Ellen et a1.20 have in fact shown that when pili or fimbriae are removed from A.naeslundii cells, their attachment is significantly reduced. In ad- dition to pili, A.naeslundii cells possess devices for aggregation through the production of dextrans and levans.21 Culture media that contains carbohydrates and consequently has low pHs also allow an accumu- lation of A.naeslundii through the synthesis of ele- ments required for their aggregation. So both their fibril structure and wall composition can agglutinate A.naesZundii cells and form the entangled clusters that we have clearly observed. Although the initial size of bacteria is much lower than tubule diameter, this standard grouping mode prevents all cells from penetrating dentinal tubules.

As we did not find any isolated bacteria whatever the observation techniques used, it is highly likely that aggregation be produced very quickly, and it is consequently unlikely that single cells can be observed in the tubules.

702 Perez et al. ORAL SURGERY ORAL MEDlClNE ORAL PATHOLOGY July 1993

In SEM, clusters of A.naesEundii frequenlly have a nonhomogeneous disposition. Perfectly clean dentin zones are close to zones covered with bacteria. This preferential location in some areas could be the result of constituent dentin differences. But, as all samples are taken from bovine dentin and have undergone the same dentinal treatment, it is difficult to explain this observation.

or of bacteria that are similar in cellular morphologic factors but differ in growth pattern,

REFERENCES

1.

2.

In light microscopy, histologic sections reveal a great cell density, but the canal wall is partially covered. We also found the same results in the SEM examination

3.

4.

As was seen earlier, the absence of intratubular P.intermedia migration was more diflicult to observe because of the pink tint of the cells after Brown and Brenn staining.

5.

6. The SEM study shows P.intermedia cells on the

canal wall, which is the sign of a bacterial growth, but without major penetration into the dentin. Bacteria have progressively accumulated on tubule apertures over time, which prevents any progression into the tubules. In this same experiment, the size of P,intermedia cells did not initially seem to be an obstacle to their penetration because the mean size was 0.7 pm wide by 2 pm long. 22, 23 But we are forced to admit that no bacteria were detected in the dentinal tubules.

I.

8.

9.

10. P. intermedia cells are little bacilli with central or

terminal swellings. Culture conditions may also mod- ify the morphologic aspect of the cells; they are longer when stained from broth cultures and larger when fermentable carbohydrates are present. The cell surface of P.intermedia also contains fibrillar structures or pili that allow coaggregation.24

11.

Another obstacle to penetration could also be the persistence of the odontoblastic prolongment inside the tubules that could act as a physical barrier. But SEM images did not show any traces of these structures, and we consider that they were totally elimi- nated from the samples before bacterial inoculating.

As there are a large number of bacteria on the wall after 28 days, it is plausible to think that if they could have migrated into the tubules, they would have already done so.

12.

13.

14.

15.

16.

ONCL~SION The differences in penetration observed in this

study among the three bacterial species show that the process of migration under in vitro conditions could depend not only on morphologic criteria but also on interbacterial aggregation criteria.

A possible evolution of this work will be to study the migratory behavior of isogenic strains of the same bacteria that differ only in intercellular arrangement

17.

18.

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2 1. Bourgeau G, McBride BC. Dextran-mediated interbacterial aggregation between dextran synthetizing streptococci and Actinomyces viscosus. Infect Immun 1976; 13: 1228-34.

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Reprint requests: Fabienne Perez, DCD Service d’Odontologie conservatrice-Endodontie Faculti: de Chirurgie Dentaire 3 Chemin des Maraichers 3 1062-Toulouse Cedex France

CORRECTION

In the article “Narcotic receptor blockade and its effect on the analgesic response to placebo and ibu- profen after oral surgery” by Hersch et al., which appeared in the May 1993 issue (Oral Surg Oral Med Oral Path01 1993;75:539-46), the name, Andrei Barasch, DMD, should have been included as a coau-

in vitro study of the penetration of three bacterial strains into root dentine

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