Introduction

Microbial populations in seawater and sediments may be as high as 106 and 109 per milliliter, respectively (Austin 1988). Marine invertebrates are therefore constantly exposed to high concentrations of bacteria, fungi and viruses, many of which may be pathogenic. The survival of these organisms depends on efficient antimicrobial mechanisms to protect themselves against microbial infections. During the last decade, there has been an increment in research on marine crustaceans, sponges, jellyfish, sea anemones, corals, bryozoans, molluscs, tunicates and echinoderms with particular interest on their secondary metabolites containing desirable antimicrobial properties (Bhakuni and Rawat 2005; Casas et al. 2011; Haug et al. 2002).

Echinoderms are benthic organisms, which are constantly exposed to relatively high concentrations of the pathogens which can be harmful to the organism (Haug et al. 2002). Sea urchins as an echinoderm belong to the class Echinoidea of the phylum Echinodermata, which also include starfish, sea cucumbers, sea lilies, and brittle stars. All sea urchins have a hard calcareous shell called “test”, which is covered with a thin epithelium and is usually armed with spines. The test contains the various polyhydroxylated naphthoquinone pigments such as spinochromes and echinochrome A that their bactericidal effect has been reported too (Shankarlal et al. 2011). The mouth, located on the underside, consists of five calcareous plates called “Aristotle’s lantern” in honor of the Greek naturalist and philosopher. The Aristotle’s lantern leads to the digestive tract, which empties through the anus located on the top of the test (Kato and Scheroeter 1985).

The human oral cavity contains a number of different habitats, including the teeth, gingival sulcus, tongue, cheeks, hard and soft palates, and tonsils, which are colonized by bacteria (Dewhirst et al. 2001). Oral diseases caused by aerobic and anaerobic bacteria are of the most important problems in the entire world. Dental caries is one of the most common chronic infectious diseases in the world (Aas et al. 2008; Beighton 2005; Chhour et al. 2005; Anusavice 2002; Becker et al. 2002). The streptococcal species are the major causative agents of the dental caries. They are the most common pathogens isolated from human dental plaque and their prevalence has been reported in epidemiological studies which suggest they have an important role in the development and progression of dental caries (Nishikawara et al. 2006; Okada et al. 2005).

The two species responsible for the initiation of dental caries are Streptococcus mutans and Streptococcus sobrinus (Okada et al. 2005; Ghasempour et al. 2013; Nascimento et al. 2004). They are a group of bacterial species characterized by their ability to produce extracellular glucans from sucrose and by their acid production in animal and human studies (Nishikawara et al. 2006; Okada et al. 2005; Ghasempour et al. 2013).

Antimicrobial activity in several species of echinoderms collected from the Gulf of California, Mexico, Caribbean and Coast of Norway has been reported (Haug et al. 2002; Rinehart et al. 1981; Bryan et al. 1994). Also, a variety of antimicrobial factors, including steroidal glycosides (Andersson et al. 1989), polyhydroxylated sterols (Iorizzi et al. 1995), lysozymes (Canicatti and Roch 1989; Stabili and Pagliara 1994), complement-like substances (Leonard et al. 1990) and antimicrobial peptides (Beauregard et al. 2001) have also been isolated from echinoderms. Unfortunately, in most of these studies, the whole bodies or body walls have been tested for the antimicrobial activity in the echinoderms (Abubakar et al. 2012).

The recent findings suggest that marine echinoderms are a potential source of new types of antibiotics for pharmaceutical development. The pathogenic bacteria resistance to the conventional antibiotics calls for a more focus on new antimicrobials metabolites. On the other hand, there is a high potential of marine biological reserves in the Persian Gulf. Hence, this study focused on the screening and comparing the antibacterial effects of various extracts from different tissues of sea urchin from the Persian Gulf.

Materials and methods

Sampling and preparation of specimens

Live specimens of the sea urchins (N = 10 specimens) were collected from an intertidal zone of Qeshm Island in the Persian Gulf. The samples were transferred on ice to the Marine Biology Laboratory of the University of Guilan. After dissection, all tissues of animals, including spines, test, gonads and the Aristotle’s lantern (Fig. 1) were separately frozen in liquid nitrogen for 5 min and kept at −70 °C until extraction.

Fig. 1
figure 1

The different tissues of sea urchin that were used for extract processing

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Specimen and sex determination

The specimens were identified using the test and spines of the sea urchin. After taking digital photographs of test and spines through a stereo microscope, identification of species was done using the diagnostic keys (Price 1981). The sex determination of samples was carried out by the histological method with routine hematoxylin and eosin staining.

Preparation of extracts

Phosphate-buffered saline (PBS) extract

All tissues were separately homogenized (Misonix Sonicator 3000, USA) with PBS (NaH2PO4.2H2O, 0.1 M; NaOH, 0.2 M) (1:5 w/v) for 5 min. The samples were shaken (Infros, RFI-150) on ice for 2 h (120 rpm) and centrifuged at 4000 rpm for 10 min at 4 °C. The supernatant was collected, stored at 4 °C, and the residue was again centrifuged in two volumes of the PBS. Then, the combined supernatants were centrifuged and lyophilized by freeze-dryer (Christ Alpha 1-2/LD, Osterodeam Harz, Germany) for 9 h (−55 °C and 0.02 mbar), and finally were kept at −70 °C until antibacterial test.

Ethanol (Et) and Acetonitrile (ACN) extracts

The organic extracts were prepared with 96 % Et and 80 % ACN (Merck-Schuchardt, Germany). As above, all tissues were separately homogenized in Et and ACN as solvents (1:5 w/v) for 5 min. Subsequently, the solutions were shaken at 40 °C for 24 h (120 rpm) and centrifuged at 4000 rpm for 10 min at 4 °C. The supernatants were collected and the residues were again centrifuged in two volumes of the solvents. After centrifuging the combined supernatants, they were lyophilized by freeze-dryer for 9 h (−55 °C and 0.02 mbar) and kept at −70 °C until antibacterial test.

Bacterial strains

The Gram-positive bacteria S. mutans (ATCC 35668) and S. sobrinus (ATCC 27607) purchased from the Persian Type Culture Collection (PTCC, Tehran, Iran). The strains were grown on blood agar (Biolife, Italy) containing 7 % fresh horse blood and maintained at 37 °C for 48 h.

Antibacterial test

Antibacterial activities of the aqueous (PBS) and organic extractions were assessed by the well diffusion method. S. mutans and S. sobrinus were cultured on blood agar for 24 h. Based on Kiani et al. (2014), the crude aqueous and organic extracts with two concentrations, 600 and 1500 µg, were dissolved in 40 µl of d.d. H2O and dimethyl sulfoxide (DMSO) (Merck, Germany) as solvents, respectively. Afterwards, the dissolved extracts were transferred into the wells with diameter of 5 mm punched out on plates and incubated at 37 °C for 24 h. The antibacterial activities were determined by measuring the diameter of the inhibitory zone (mm). Antibiotics, such as vancomycin (30 µg disc−1), tetracycline (30 µg disc−1) and bacitracin (15 µg disc−1) were used as positive controls. DMSO and d.d.H2O were also tested as negative control to ensure that they do not interfere with the tests.

Hemolytic assay

To test whether the sea urchin contains factors that display a toxic effect on eukaryotic cells, the hemolytic activity in the aqueous and organic extracts of the test, gonad, Aristotle’s lantern and spines were determined. The assessment was performed using the method described by Haug et al. (2002). Briefly, heparinized horse red blood cells (RBCs) washed three times with phosphate-buffered saline (PBS; pH = 7) and suspended in PBS to a hematocrit value of 10 %. Samples were diluted to a protein concentration of 500 µg.ml−1. The test mixture consisted of 40 µl PBS, 50 µl extract, and 10 µl of the RBC suspension. After incubation at 37 °C for 1 h, the mixture was centrifuged at 3000 rpm for 10 min. Finally, absorbance of the supernatants (60 µl) was measured at 550 nm. Baseline hemolysis and 100 % hemolysis were defined as the amount of hemoglobin released in the presence of PBS and 0.1 % Triton X-100 (Sigma), respectively.

Statistical analysis

The raw data were initially tested against the assumptions of normality and homogeneity of variance. The differences between various extracts were assessed using one-way ANOVA followed by Duncan test with a confidence level of 95 % using IBM SPSS Statistics 19 in Windows 7. Statistical independent T test was used to determine the significant differences between the two concentrations of 600 and 1500 µg well−1. All experiments (treatments and controls) were carried out triplicate and all data were expressed as the mean ± SD.

Results

Specimen and sex determination

The biometric data of samples were brought in Table 1. Based on the diagnostic keys of Price (1983), the photographs of test and spines belong to Echinometra mathaei (de Blainville 1825) (Figs. 2, 3). Also, histological sections of the gonads showed that the studied samples were female gender (Fig. 4).

Table 1 Biometric data of the sea urchin Echinometra mathaei
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Fig. 2
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The inner (a) and outer (b) surfaces of sea urchin test that were used to determine the species

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Fig. 3
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The spines of sea urchin Echinometra mathaei. a Long and short conical spines in aboral side. b Long and short paddle-shaped spines in oral side

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Fig. 4
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Histological section of the sea urchin ovary. O mature eggs, L lumen (×400)

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Antibacterial activity of the extracts

The results showed that antibacterial activities were found in some tissues of the echinoderm species Echinometra mathaei tested against streptococci. Tables 2 and 3 represent the detailed antibacterial activities of the different extracts, positive and negative controls on S. mutans and S. sobrinus.

Table 2 Antibacterial activity of aqueous and organic extracts of various tissues of E. mathaei against S. mutans and S. sobrinus
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Table 3 Antibiogram tests on S. mutans and S. sobrinus
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Streptococcus sobrinus

Both in a concentration of 600 and 1500 µg well−1, the extracts of Aristotle’s lantern and spines had no antibacterial effects (Table 2) while the ACN and Et extracts of the gonads and the test showed inhibitory effects against S. sobrinus (Fig. 5). The highest antibacterial activity belongs to an ACN extract of the gonad and an Et extract of the test with diameter of inhibitory zone 11 ± 1.73 and 10.33 ± 2.52 mm, respectively. The comparison of two concentrations of ACN extracts showed that the extracts with concentration of 1500 µg well−1 had more significant effect than of 600 µg well−1 (p = 0.016, F = 16.00).

Fig. 5
figure 5

The mean diameter of inhibitory zone for S. sobrinus under the effect of different extracts of Echinometra mathaei. Mean values bearing different superscripts are significantly different (p < 0.05)

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Streptococcus mutans

Both in a concentration of 600 and 1500 µg well−1, the extracts of Aristotle’s lantern, spines and test had no antibacterial effects (Table 2). The ACN and Et extracts of the gonad with concentration of 600 and 1500 µg well−1 had significantly antibacterial different effect in which ACN extract showed the maximum inhibitory zone in both concentrations (p = 0.018, F = 6.19, df = 11) (Fig. 6). The T-test analysis showed that the extracts with concentration of 1500 µg well−1 had more significant effect than of 600 µg well−1 (p = 0.047, F = 2.571).

Fig. 6
figure 6

The mean diameter of inhibitory zone for S. mutans under the effect of different extracts of Echinometra mathaei. Mean values bearing different superscripts are significantly different (p < 0.05)

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Hemolytic activity

The hemolytic activity of the PBS, 96 % ethanol and 80 % ACN extracts from the Aristotle’s lantern, spines, test and gonad of sea urchin were evaluated using horse red blood cells. The ACN and Et extracts of the gonads and an Et extract of the test showed hemolytic activity while the other extracts had no hemolytic effects (Table 4).

Table 4 Hemolytic activity in extracts from sea urchin, E. mathaei, against red blood cells
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Controls

The negative controls (DMSO and d.d.H2O) did not show any activity against streptococci while the antibiotics showed the different antibacterial effects (Table 3).

Discussion

In this study, it was trying to evaluate the antibacterial effects of various extracts in the different tissues of sea urchin E. mathaei on oral streptococci.

It has been discovered that echinoderms have stronger antibacterial effects than Porifera, Mollusca, Bryozoa or Annelida (Ridzwan et al. 1995). Antibacterial activity has previously been described in some species of echinoderms (Haug et al. 2002; Kiani et al. 2014; Ridzwan et al. 1995). Haug et al. (2002) studied the antibacterial activity in different parts of the green sea urchin Strongylocentrotus droebachiensis, the common starfish Asterias rubens, and the sea cucumber Cucumaria frondosa against Gram-positive and negative bacteria. They showed antibacterial activities in the extracts of several tissues from A. rubens and C. frondosa. The coelomocytes of the sea urchin Paracentrotus lividus showed antibacterial activity against Vibrio alginolyticus (Stabili et al. 1996). Rinehart et al. (1981) examined 83 unidentified species of echinoderms from the west coast of Baja California and the Gulf of California and found 43 % of them had antimicrobial activity. In the same study, 58 % out of 36 unidentified Caribbean species showed antimicrobial activity. Out of 22 species of echinoderms collected from the northern Gulf of Mexico, 80 % had antimicrobial activity (Bryan et al. 1994). This study demonstrated the presence of antibacterial factors in several tissues such as gonads and test of E. mathaei. Whether the same antibacterial factors are responsible for the activity in all organs, is unknown. However, it seems that the antibacterial factors have an important function as a first line of defense against pathogenic microorganisms.

A variety of antimicrobial compounds, including steroidal glycosides (Prokof’eva et al. 2003; Iorizzi et al. 2001), polyhydroxylated sterols (Iorizzi et al. 1995), naphthoquinone pigments Service and Wardlaw (1984), lysozymes (Canicatti and Roch 1989; Stabili and Pagliara 1994), complement-like substances (Leonard et al. 1990), and antimicrobial peptides (Li et al. 2010) have been isolated from echinoderms. In this study, ethanol and acetonitrile (organic) as well as PBS (aqueous) extracts from different tissues of the sea urchin were examined for antibacterial activity. These extracts with different solvents allowed us to determine the presence of antibacterial substances potentially present in the lipid or the water-soluble fraction. Since the antibacterial effects with organic solvents has been found, so protein factors could be responsible for such effects. Antimicrobial peptides (AMPs) are evolutionarily conserved small molecular weight proteins of the innate immune response, with a broad spectrum of antimicrobial activities against bacteria, viruses, and fungi (Mookherjee and Hancock 2007). In the green sea urchin, S. droebachiensis, the purification and characterization of two antibacterial peptides from coelomocyte extracts has been done (Li et al. 2008). Also, Ridzwan et al. (2012) studied the body wall’s proteins of three species of sea cucumber and at least 11 types of proteins (20–125 kDa) were identified.

The organic and PBS extracts of Aristotle’s lantern and spines in E. mathaei had no absolutely antibacterial effects on streptococci. It seems that these tissues have just a physical role against environmental stresses including prey and predator and have no chemical function. The ethanol extract from the test showed antibacterial activity on S. sobrinus which might be due to the innate immunity parameters and also related to the presence of bacterial symbionts (Strahl et al. 2002) living on the organisms.

In the sea urchin, E. mathaei, the highest antibacterial activity against two streptococcus strains was detected in the organic extract of the gonads. Due to the exposure of the sea urchin eggs against the environmental pathogens, maternal transfer of immune parameters, especially antimicrobial peptides is not out of mind. Hence, the high level of antibacterial effects in gonadal tissue of E. mathaei is inevitable.

Just as antibacterial effects aqueous extract of the test and gonads, the studied species showed the hemolytic activity of course with lower activity. Several hemolytic factors have been isolated from echinoderms, including proteins (Canicatti and D’Ancona 1990), lectins (Hatakeyama et al. 1994), complement-like factors (Bertheussen 1983), and a variety of saponins (Iorizzi et al. 2001). From a pharmaceutical point of view, it is an advantage when antibacterial drugs have no side effects such as hemolytic activity (Haug et al. 2002). In this study, although some antibiotics showed more antibacterial effects than the tested extracts, but cannot be confirmed the superiority of antibiotics compared to the extracts because of purifying bioactive compounds of the extracts was not carried out.

Conclusions

This study indicated that the sea urchin, E. mathaei, as an echinoderm, can be a source of novel antibiotics. The growth inhibition of S. sobrinus, and S. mutans, generally was controlled by the organic extracts of gonads, whereas the ethanol extract of test showed antibacterial effect against S. sobrinus. The extracts of the sea urchin should be further analyzed to isolate and purify as well as determine the chemical structure of the antibacterial compounds for their application in the medicine and dentistry sciences.