Received 11 March 2016; accepted 23 April 2016; published 26 April 2016
The increasing global resistance of pathogenic bacteria has become a public health problem and, in attempts to overcome it, research efforts are now addressing the discovery of novel and efficient antibacterial compounds  . Furthermore, the emergence of multidrug-resistant bacteria has created a situation in which there are few or no treatment options for infections with certain microorganisms  . In this context, unusual sources, such as natural products from marine organisms, are important for antibacterial drug discovery, since they represent a high potential source of new drugs with diverse and often unique structures  -  .
Corals (phylum Cnidaria) represent a dominant group of benthic marine invertebrates that inhabit all oceans, including Brazilian waters. These organisms are of the great interest to the scientific community, since they are source of marine natural products that possess high potential in terms of chemical novelty and as drug leads, yielding structures with novel modes of action  -  .
Tubastraea coccinea Lesson, 1829 (Dendrophylliidae) is a yellow-orange non-indigenous coral found in some regions of Brazil  , probably first introduced to the Brazilian coast by oil platforms and ships by fouling  . A recent report suggests that T. coccinea produces chemical defenses by competing against native octocorals  , and a few data in relation to its ecological significance  , chemical composition  as well as pharmacological properties are reported.
Thus, the aim of this work was to evaluate the antimicrobial potential of extract and related fractions (HF, BF and AF) from the invasive stony coral T. coccinea, against several microorganism strains as well as multi-drug resistant bacteria by the disk diffusion, microdilution assay and by bioautography.
2. Material and Methods
2.1. Collection and Extractions
Coral collection and extraction: Specimens of T. coccinea were collected by SCUBA at Arvoredo Island, in the state of Santa Catarina, Brazil, at depths of 3 to 10 m, in February 2012. The coral material was manually cleaned to remove associated organisms, and immediately frozen and kept at −20˚C until processed. A voucher specimen (CC UFSC 0351) was deposited at the invertebrate collection of the Department of Ecology and Zoology of Universidade Federal de Santa Catarina. The material (160 g, wet weight) was blended in EtOH 92% (7 days), filtered and dried under reduced pressure, providing the ethanol crude extract (ECE). The ECE was partitioned with n-hexane/H2O and n-BuOH/H2O, yielding the n-hexane (HF), n-butanol (BF) and the aqueous residue (ARF) fractions, respectively. Then, ARF was submitted to XAD-4, yielding the AF (aqueous fraction).
2.2. Microorganism Strains
Twenty one microbial species were analyzed, including 14 ATCC strains [Candida albicans (ATCC 10231); Candida tropicalis (ATCC 13803); Clostridium sporogenes (ATCC 11437); Enterobacter cloacae (ATCC 13047); Enterococcus faecalis (ATCC 29212); Escherichia coli (ATCC 25922); Klebsiella pneumoniae (ATCC 13883); Pseudomonas aeruginosa (ATCC 27853); Salmonella typhimurium (ATCC 14028); Shigella flexneri (ATCC 12022); Staphylococcus aureus (ATCC 25923); Staphylococcus epidermidis (ATCC 12228); Streptococcus pneumoniae (ATCC 49619); Streptococcus pyogenes (ATCC 19615)], and 7 clinical strains isolated from patients in São José Hospital, Criciúma city/Brazil [Acinetobacter baumannii, Enterococcus faecium, Klebsiella pneumoniae, Klebsiella pneumoniae carbapenemase (KPC), Staphylococcus aureus, Methicillin- resistant Staphylococcus aureus (MRSA) and Vancomycin-resistant Enterococcus faecalis (VRE)].
2.3. Disk Diffusion Method
The antimicrobial activities were evaluated by the disk diffusion method, as previously described by De Oliveira et al.  with minor modifications. Standard antibiotic disks were selected according to the sensitivity of the microorganism tested. Thus, ampicillin (10 μg), ceftazidime (30 μg), chloramphenicol (30 μg), doxycycline (30 μg), fluconazole (25 μg), imipenem (10 μg), levofloxacin (5 μg), oxacilline (1 μg), polymyxin b (300 μg) and vancomycin (30 μg) were used  .
2.4. Determination of the Clinical Strains Sensitivity to Antibiotics
Antibiotic sensitivity testing of the clinical isolates was performed using disc diffusion method  . Antibacterial agents of different classes were used, including amikacin; ampicillin; ampicillin/sulbactam; ciprofloxacin; levofloxacin; cephepime; chloramphenicol; erytromycin; gentamicin; imipenem; oxacillin; piperacillin/tazo- bactam; sulphazotrim; vancomycin and clindamycin.
2.5. Micro Broth Dilution
The microdilution assay was performed according to the CLSI guidelines  . Briefly, each well of a 96-well microplate was coated with 100 μL of Muller Hinton broth together 100 μL of active fractions in serial dilutions (2000 μg/mL - 3.9 μg/mL). After 5 μL suspension, the test strain (equivalent to 0.5 McFarland standard) was replaced into wells. The microplates were incubated at 35˚C for 18 h and the MIC was defined as the lowest concentration of fractions in which the microorganism tested did not show visible growth. The MBC was performed on nutrient agar plates, and was defined as the lowest concentration that showed negative growth.
Thin-layer chromatography (TLC) plates (10 × 10 cm) were loaded with 20 μL of the extract (BF) solutions at a concentration of 100 mg/mL. The plates were developed using n-BuOH/AcOH/H2O (6:2:1) as mobile phase. After the solvent had evaporated from the TLC plates (24 h), Muller Hinton agar (HIMEDIA®) was deposited over the plates, and after solidification, a microorganism suspension at 1.5 × 108 CFU/mL was added over the culture medium. The plates were incubated at 35˚C ± 1˚C, and after 18 h, the bioautogram was sprayed with an aqueous solution of 2,3,5-triphenyltetrazolium chloride (TTC) (Vetec®) and incubated at 35˚C ± 1˚C for 4 h. Inhibition zones indicated the presence of bioactive compounds   .
3. Results and Discussion
Among the fractions assayed, BF and AF showed moderate activity against ATCC strains of S. aureus (10.5 mm and 9 mm of inhibition zones, respectively), and weak activity against S. typhimurium (8 mm and 8 mm), E. coli (8.5 mm and 7.5 mm), and P. aeruginosa (7 mm and 7 mm). The HF fraction did not show activity against all bacteria or fungi assayed. Concerning the clinical strains, only BF fraction showed moderate activity against S. aureus (12 mm), KPC (11 mm), MRSA (10 mm) and VRE (10 mm) (Table 1).
The fractions that showed antibacterial activity by the disk diffusion method were further tested by the microplate assay, to determine the minimum inhibitory concentration (MIC). In addition, the wells that showed negative visible growth after 18 h of incubation were replated on agar nutrient plates, to obtain the minimum bactericidal concentration (MBC) of the fractions. The MIC values of the BF and AF fractions were, respectively: 31.25 and 250 μg/mL (S. aureus), 125 and 500 μg/mL (S. typhimurium), 62.5 and 500 μg/mL (E. coli), and 62.5 and 500 μg/mL (P. aeruginosa) (Table 2). On the other hand, BF and AF fractions showed MBC values 62.5 and 500 μg/mL (S. aureus), 250 and 1000 μg/mL (S. typhimurium), 125 and 1000 μg/mL (E. coli), and 125 and 1000 μg/mL (P. aeruginosa).
It is important to emphasize that the BF fraction showed activity against P. aeruginosa, which has, in clinical cases, a high degree of resistant against multiple classes of antibiotics  . Moreover, in contrast to the weak
Table 1. Antibacterial and antifungal activity of fractions of the coral Tubastraea coccinea by the disk diffusion method.
(−): not active; (+): 7 - 8.5 mm; (++): 9 - 12 mm; (+++): 13 - 16 mm; (++++): 17 - 20 mm. Positive controls: for ATCC strains were used ampicillin (10 μg), ceftazidime (30 μg), chloramphenicol (30 μg), doxycycline (30 μg), fluconazole (25 μg) and oxacilline (1 μg); for clinical strains were used imipenem (10 μg), levofloxacin (5 μg), oxacilline (1 μg), polymyxin b (300 μg) and vancomycin (30 μg). HF: n-hexane fraction; BF: n-BuOH fraction; AF: aqueous fraction; KPC: Klebsiella pneumoniae carbapenemase; MRSA: Methicillin-resistant Staphylococcus aureus; VRE: Vancomycin-resistant Enterococcus faecalis.
Table 2. Results of MIC (μg/mL), MBC (μg/mL) and MBC/MIC ratio of BF fraction of the coral Tubastrea coccinea.
Samples with MIC > 1000 μg/mL were considered inactive; MIC from 500 - 1000 μg/mL antibacterial activity was considered weak; MIC from 100 to 500 μg/mL the antibacterial activity was considered moderate; MIC is equal or smaller than 100 μg/mL, the antibacterial activity was considered significant. KPC: Klebsiella pneumoniae carbapenemase; MRSA: Methicillin-resistant Staphylococcus aureus; VRE: Vancomycin-resistant Enterococcus faecalis.
activity detected by the disk diffusion assay, the MIC of the BF fraction against P. aeruginosa, was higher when compared among all the samples tested (62.5 μg/mL), except against S. aureus, which showed MIC 31.25 μg/mL.
The BF fraction also exhibited significant growth inhibitory activities against the four clinical strains with MICs ranging from 62.5 to 125 μg/mL. Further, this fraction showed the same value of MBC (125 μg/mL) against S. aureus, KPC and MRSA. The VRE was susceptible to BF fraction with MBC of 62.5 μg/mL (Table 2).
The minimum bactericidal concentrations (MBC) for each sample were compared with the MIC values. Low MBC/MIC ratios (≤4) demonstrate bactericidal activity, while ratios higher than four indicate the bacteriostatic mode of action  . Our results reinforce the bactericidal activity detected for BF fraction in the MBC assay for the all strains available (Table 2).
These findings demonstrated that the BF fraction has a higher potential antimicrobial activity, including clinical multi resistant strains, suggesting that this activity should be higher when the compounds responsible for the activity are isolated.
In addition, the most active fraction (BF) was submitted to the bioautography assay for S. aureus, S. typhimurium, E. coli, P. aeruginosa and four clinical strains (S. aureus, KPC, MRSA and VRE). In all bioautography analyses, the inhibition zone of the bioactive compounds was detected at Rf. 0.55 [TLC developed with n-BuOH/ AcOH/H2O (6:2:1)] (data not shown). Since the bioactive compound showed inhibition zone at Rf. 0.55, we decide to develop a TLC chemical characterization of this compound using the same chromatographic conditions. The TLC fingerprint of the bioactive fraction showed the presence of compounds with alkaloids profile after Dragendorff nebulization (data not shown). This result is in agreement to literature data that reports stony scleratinian corals, Tubastraea (Dendrophylliidae) genus as a source of biologically active alkaloid derivatives including aplysinopsin and (bis)-indole alkaloids  -  .
Regarding the pharmacological properties of this coral genus, there is only one report of antimicrobial activity from methanol extract of Tubastraea faulkneri, against Vibrio alginolyticus, Vibrio harveyi, Photobacterium damselae, Alteromonas rubra, Staphylococcus aureus and Synechococcus sp.  . Additionally, from the same species, aplysinopsin, 6-bromoaplysinopsin, 6-bromo-29-de-N-methylaplysinopsin and its dimer have also been reported  -  . Moreover, Zoraghi and colleagues  , reported that MRSA pyruvate kinase is a potential target for (bis)-indole alkaloids with antibacterial activity.
In this work we described the antimicrobial potential of the extract and fractions obtained from Scleractinia coral T. coccinea. Our results showed that the n-butanol fraction (BF fraction) was effective against Gram-positive and Gram-negative bacterial, including multi-resistant clinical strains. Moreover, it is relevant to emphasize the importance of marine biodiversity as sources of secondary metabolites with potential pharmacological activities, which could be used on development of new drugs and/or as lead compound. Further studies are in progress in our laboratory, to isolate these bioactive compounds.
The authors would like to thank CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico, Brazil), CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior) and FAPESC (Fundação de Amparo à Pesquisa e Inovação do Estado de Santa Catarina) for their financial support and their research fellowships. Hence, we are grateful to clinical laboratory of São José Hospital (Criciúma city/Brazil) for furnished clinical bacterial strains.
Conflicts of Interest
The authors declare no conflict of interest.
*These authors contributed equally to this work.