INTRODUCTION
Mollusks represent the second-largest invertebrate animal phylum after Arthropoda (Baldwin, 2003; Rusmore-Villaume, 2008). They are also considered one of the most important elements in food chains as a balanced source of protein (Haszprunar and Wanninger, 2012) and represent the most prominent members of marine faunal ecosystems. Some species have direct and indirect commercial importance and even medical benefits to humans (Rusmore-Villaume, 2008) and are utilized as prospective resources of bioactive products in numerous cultures worldwide (Khan et al., 2009).
Recently, the molluscan natural products (high content of proteins, vitamins, and trace elements) have become very valuable foodstuffs, and their synthetically formulated structural analogs have been recognized in clinical trials as promising therapeutic agents for many diseases (Alves and Diederich, 2021; Tabakaeva et al., 2018). Marine bivalves are considered a rich source of antimicrobial peptides that possess various biological activities such as antibacterial, antioxidant, anticoagulant, anti-inflammatory, and anticarcinogenic (Galdiero et al., 2015). The main cause of their bioactivities is the presence of biologically active peptides (Alves and Diederich, 2021). Bivalves occurring in Egypt represent a neglected animal group, and little is known about them or their diversity and importance (Temraz, 2016). Therefore, more light should be shed on their chemical and biological aspects.
In particular, Paratapes undulatus, common name undulate Venus, is a species of saltwater clams. These clams are a popular food in Egypt, inhabiting the inshore shallow sandy seabed in Timsah Lake, located near Ismailia City at the midpoint of the Suez Canal and about 80?km south of Port Said, Egypt (Loftus et al., 2015). Lately, a broad spectrum of naturally occurring bioactive molecules separated from the mollusks was utilized in the manufacture of several medications (Alves and Diederich, 2021). The strong impact of antimicrobial resistance on healthcare and the economy has led to a crucial demand to secure unique molecules with novel modes of action to reduce the harmful effects of infectious diseases (Ghareeb et al., 2015). Several antimicrobial compounds have been isolated from peptides subunits of marine bivalves due to their thorough capability to resist infection with different types of viruses (Khan et al., 2009). They also exhibit a wide range of antibacterial activities against Gram-positive and Gram-negative bacteria, as well as yeast (Burge et al., 2014; Schmitt et al., 2012). The rapid and increasing development of antimicrobial resistance is one of the current global health challenges. Pathogenic microorganisms have the capability to hinder the effect of antimicrobial agents, which leads to raising the accompanying hazards of infections via pathogenic microorganisms like bacteria, fungi, viruses, and parasites. Furthermore, the appearance of novel pathogens like SARS, H1N1, and various types of influenza has become a vital communal health risk. To overcome this phenomenon, scientists have worked to discover novel antimicrobial and antiviral pharmaceuticals from natural resources like plants, marine organisms, mollusks, and fungal extracts (Abdel-Aziz et al., 2018, 2021; Elkhouly et al., 2021a; Ghareeb et al., 2014a, 2019; Hamed et al., 2020).
On the other hand, overproduction of reactive species led to the initiation of oxidative stress that in turn initiates several dangerous disorders such as “cancer, cardiovascular, and inflammation.” Moreover, the accompanying destructive side effects can be reduced using naturally occurring antiradical ingredients that act as strong free radical scavengers (Ghareeb et al., 2018; Sobeh et al., 2018).
In the current study, we aimed to investigate for the first time the chemical and biological profiles of the soft parts of the marine clams P. undulatus and the scientific evidence regarding their chemical composition and associated antibacterial, antibiofilm, and antioxidant activities.
MATERIALS AND METHODS
Chemicals, reagents, and instruments
All solvents, standards, and reagents are of high analytical grade. Methanol and sulfuric acid were obtained from El-Nasr Pharmaceutical Chemicals Company (Cairo, Egypt). Nutrient agar and Nutrient Broth media were purchased from HiMedia Laboratories Pvt. Ltd. (Mumbai, India). 2,2-Diphenyl-1-picrylhydrazyl (DPPH) radical, ascorbic acid, gallic acid, Folin–Ciocalteu’s reagent, sodium carbonate, sodium phosphate, and ammonium molybdate were purchased from Sigma-Aldrich (Steinheim, Germany). For antioxidant assays, the absorbance was measured using a spectrophotometer (UV-VS spectrophotometer, Milton Roy 601, CO). The obtained extract was concentrated under vacuum using a rotary evaporator, Buchi R-300 (Flawil, Switzerland).
Collection of P. undulatus
Venus clams (P. undulatus) were collected from Timsah Lake which is located close to Ismailia City at the midpoint of the Suez Canal and about 80?km south of Port Said, Egypt (Fig. 1) (El-Serehy et al., 2018). The collected samples were cleaned with distilled water to eliminate any sands or contaminations and then kept in a refrigerator at −18°C till being used.
Preparation of the methanolic extract of P. undulatus clams (Born, 1778)
Paratapes undulatus clams are a popular food in Egypt, so after being washed with distilled water, their shells were opened with stainless steel knives and their soft bodies were separated from these shells by forceps. The collected soft bodies (250 g) were homogenized and then were extracted four times with methanol (2 l) at room temperature (25°C ± 2°C). The combined extracts were concentrated to afford 19.0 g methanol extract.
Antibacterial activity assay
The antibacterial activity of the tested extract was evaluated against some pathogenic microbial strains including four Gram-negative bacteria (Escherichia coli ATCC 25955, Pseudomonas aeruginosa ATCC 10145, Proteus vulgaris, and Klebsiella pneumoniae), one Gram-positive bacterium (Staphylococcus aureus NRRL B-767), and one yeast (Candida albicans ATCC 10231) according to the reported procedures (Elkhouly et al., 2021b). Ciprofloxacin and nystatin were utilized as a control.
Antibiofilm activity assay
The biofilm inhibitory potential was determined against four pathogenic microbial strains, including S. aureus and Bacillus subtilis as Gram-positive bacteria as well as P. aeruginosa and E. coli as Gram-negative bacteria, according to the reported procedures (El-Shazly et al., 2021; Hamed et al., 2020).
Total phenolic content (TPC) using Folin–Ciocalteu’s assay
The TPC was evaluated using Folin–Ciocalteu’s reagent according to the reported procedures (El-Neekety et al., 2016; Hathout et al., 2016). Briefly, 100 μl of the tested sample (500 μg/ml) was mixed with 500 μl of Folin–Ciocalteu’s reagent and 1.5 ml of sodium carbonate (20%). The reaction mixture was shaken and completed to 10 ml using distilled water. The mixture was allowed to stand for 120 minutes. Afterward, the absorbance was recorded at 765 nm. All measurements were performed in triplicate. The TPC was presented as mg gallic acid equivalent (GAE) per g extract.
Antioxidant activity evaluation
Free radical scavenging activity using DPPH
Free radical scavenging activity was evaluated using the DPPH assay according to the reported procedures (Ghareeb et al., 2016; Shirwaikar et al., 2006). Briefly, various serial concentrations from the tested sample (1.5 ml) were added to a (1.5 ml) solution of 0.1 mmol/l DPPH. Equivalent volumes of methanol and DPPH acted as the control. After incubation for 20 minutes at 37°C in the absence of light, the absorbance was recorded at 517 nm. The test was carried out in triplicate. The free radical scavenging activity was expressed in IC50 value (concentration from tested sample required to scavenge 50% of the radical).
| Figure 1. A map of Timsah Lake with the inset showing the position of the lake on the Suez Canal.
[Click here to view] |
Total antioxidant capacity (TAC) using phosphomolybdenum assay
The TAC was evaluated using the phosphomolybdenum assay based on the reported procedures (Prieto et al., 1999). Basically, this assay is based on the reduction of molybdenum (VI) to molybdenum (V) by the tested sample and consequent development of a green color [phosphate = molybdenum (V)] complex at low pH with maximum absorption at 695 nm. Briefly, 0.5 ml of the tested sample (500 µg /ml) in methanol was mixed with 5 ml from the test reagent [0.6 M H2SO4, 28 mM NaH2PO4, and 4 mM (NH4)6Mo7O24]. The tubes containing the tested samples and reagents were capped and incubated in a water bath at 95°C for 1.5 hours. After cooling, the absorbance was recorded at 695 nm against a blank. The blank comprised all reagents and solvents without the tested sample. All experiments were carried out in triplicate. The TAC was presented as the number of ascorbic acid equivalent (AAE) (Elkhouly et al., 2021; Prieto et al., 1999).
Chemical characterization using GC-MS analysis
GC-MS examination was conducted according to the reported procedures (Khalaf et al., 2020), using a Thermo Scientific, Trace GC Ultra/ISQ Single Quadrupole MS, TG-5MS fused silica capillary column (30 m, 0.251 mm, and 0.1 mm film thickness). An electron ionization system with ionization energy of 70 eV was utilized for GC/MS recognition. Helium gas was utilized as the carrier gas at a regular flow rate of 1 ml/minute. The injector and MS transfer line temperature was set at 280°C. The oven temperature was instructed to an initial temperature of 50°C (hold 2 minutes) to 150°C at an increasing rate of 7°C/minute, then to 270°C at an increasing rate of 5°C/minute (hold 2 minutes), and subsequently to 310°C as the definitive temperature at a growing level of 3.5°C/minute (hold 10 minutes). The quantitative determination of all the identified compounds was examined using a percent relative peak area. A tentative recognition of the components was accomplished based on the comparison of the irrelative retention time and mass spectra with those of the National Institute of Standards and Technology (NIST), Wiley library data of the GC-MS technique.
Statistical analysis
All investigations were carried out thrice, and the obtained results are recorded as the average and standard deviation (SD). Significant variations were explored using the one-way analysis of variance. Variances at p < 0.05 were considered significant.
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