Antibacterial activity in spices and local medicinal plants against clinical isolates of Karachi, Pakistan

Abstract

Context: Development of resistance in human pathogens against conventional antibiotic necessitates searching indigenous medicinal plants having antibacterial property. Twenty-seven medicinal plants used actively in folklore, ayurvedic and traditional system of medicine were selected for the evaluation of their antimicrobial activity for this study. Eleven plants chosen from these 27 are used as spices in local cuisine.

Objective: Evaluation of the effectiveness of some medicinal plant extracts against clinical isolates.

Material and methods: Nonedible plant parts were extracted with methanol and evaporated in vacuo to obtain residue. Powdered edible parts were boiled three times and cooled in sterile distilled water for 2 min each and filtrate collected. The minimum inhibitory concentration (MIC) of plant extracts and filtrates/antibiotics was evaluated against clinical isolates by microbroth dilution method.

Results: Water extract of Syzygium aromaticum L. (Myrtaceae) buds, methanol extracts of Ficus carica L. (Moraceae) and Olea europaea L. (Oleaceae) leaves and Peganum harmala L. (Nitrariaceae) seeds had MIC ranges of 31.25–250 µg/ml. S. aromaticum inhibited growth of Staphylococcus aureus, Staphylococcus epidermidis, Streptococcus pyogenes, Salmonella enterica serovar Typhi and Pseudomonas aeruginosa. F. carica and O. europaea inhibited growth of S. aureus, S. epidermidis, and S. pyogenes whereas P. harmala was effective against S. aureus, Acinetobacter calcoaceticus and Candida albicans. Ampicillin, velosef, sulfamethoxazole, tetracycline and ceftazidime, cefotaxime, cefepime, which are used as control, had MIC ≥50 and 1.5 µg/ml, respectively, for organisms sensitive to extracts.

Discussion and conclusion: Mono/multiextract from identified plants will provide an array of safe antimicrobial agents to control infections by drug-resistant bacteria.

Keywords::

• Antibacterial activity
• hospital-acquired infections
• medicinal plants
• methanol extract
• opportunistic bacteria
• spices
• traditional medicine
• water extracts

Introduction

Infectious diseases are the leading cause of deaths worldwide causing mortality of more than 50,000 people every year. This situation has developed awareness among microbiologists and phytochemists that most of the available synthetic pharmaceutical products have limited ability to control infectious diseases (Cowan, 1999; Ahmad and Beg, 2001). Herbal remedies used in traditional medicine provide an interesting and still largely undiscovered source to develop potentially new chemotherapeutic drugs that might help to overcome the growing problem of resistance and toxicity of currently available antibiotics (Sheldon, 2005; Dhiman, 2006; Hota, 2007). It is a well-known fact that man derived medicinal benefit from plant since ancient times and today this knowledge of herbal medicine has accumulated into a vast bank of invaluable information constituting a traditional system of medicine in every inhabitant region of the world. It is estimated that two thirds of the earth’s 6.8 billion people rely on the healing power of plants for ailment ranging from common cold to cancer (Swerdlow, 2000). But it is strange that no plant-derived antibiotic that can be used clinically to cure various infectious diseases is available (Nishino et al., 1987; Gibbons, 2004; Ellsworth & Smith, 2005). For this study, by ethno-directed sampling approach, 27 medicinal plants belonging to 20 different families that are actively used by traditional healers and ayurvedic medicinal practitioners (Nadkarni, 1976; Cowan, 1999; Khafagi & Dewedar, 2000; Manisha, 2004; Bhattacharjee & De, 2005; Dhiman, 2006) were selected. Their minimum inhibitory concentrations (MICs) were determined in vitro against 500 local clinical isolates belonging to 12 different genera.

Traditional medical practitioners (TMPs) play a pivotal role in providing healthcare need in underdeveloped countries and rural population of the world. They have sound knowledge about plant species, its ecology and uses. They prescribe and provide effective plant parts for treatment to their patients (Manisha, 2004). In developed countries and cosmopolitan cities large numbers of people prefer herbal remedies, but instead of consulting TMPs they acquire the knowledge about beneficial uses of herbs and medicinal plants from books and media; they then buy the suggested plants conveniently from their neighboring shops. Plants undergo a lot of trauma and unfavorable circumstances from the time of their harvest to the time a consumer buys them. This study was aimed to ascertain whether spices and medicinal plants available in local markets and some of the plants growing in University of Karachi campus that are prescribed in indigenous system of medicine have any antimicrobial activity against potential human bacterial pathogens that have developed resistance to conventional antibiotics. For this purpose, water extracts of edible plant parts and methanol extracts of nonedible plant parts (Table 1) were screened for their antibacterial activity against potential human isolates obtained from different hospitals of Karachi. For authentication, the antimicrobial sensitivity profile of the bacterial isolates against seven antibiotics having different modes of action was also evaluated.

Table 1.   Plants, source, parts and extracts.

Botanical name	Family	Local name	Type and source	Parts used	Extract
Cuminum cyminum L.	Apiaceae	Zeera	Dry, purchased	Fruit	Water
Daucus carota L.	Apiaceae	Carrot	Fresh, purchased	Tap Root	Water
Anethum sowa Roxb.	Apiaceae	Soya	Dry, purchased	Fruit	Water
Trachyspermum ammi L.	Apiaceae	Ajwain	Dry, purchased	Seeds	Water
Foeniculum vulgare Mill.	Apiaceae	Sauf	Dry, purchased	Fruit	Water
Brassica rappa ssp. camprestris L.	Brassicaceae	Sarsoo	University of Karachi Campus	Fruit	Water
Beta vulgaris Linn.	Chenopodiaceae	Beet	Fresh, purchased	Root	Water
Cinnamomum tamala Buch.-Ham. T. Nees & Eberm.	Lauraceae	Tejpat	Dry, purchased	Leaves	Water
Cassia fistula Linn.	Fabaceae/Leguminosae	Amaltas	University of Karachi Campus	Fruit	Water
Myristica fragrans Houtt.	Myristicaceae	Jaiphal	Dry, purchased	Fruit	Water
Myristica fragrans Houtt.	Myristicaceae	Javantary	Dry, purchased	Fruit	Water
Syzygium aromaticum L.	Myrtaceae (Myrtle Family)	Clove	Dry, purchased	Buds	Water
Sesamum sesame L.	Pedaliaceae	Till	Dry, purchased	Pods	Water
Piper nigrum L.	Piperaceae	Black pepper	Dry, purchased	Seeds	Water
Nigella sativa L.	Ranunculaceae	Kalonchi	Dry, purchased	Seeds	Water
Rosa centifolia L.	Rosaceae	Rose	Dry, purchased	Petals	Water
Solanum tuberosum L.	Solanaceae	Potato	Fresh, purchased	Root	Water
Amomum subulatum Roxb.	Zingiberaceae	Black cardamom	Dry, purchased	Fruit	Water
Bombax ceiba L.	Malvaceae	Sumbul	University of Karachi Campus	Leaves	Methanol
Melia azedarach L.	Meliaceae	Bakayan	University of Karachi Campus	Leaves	Methanol
Ficus carica L.	Moraceae	Injeer	University of Karachi Campus	Leaves	Methanol
Ficus benghalensis L.	Moraceae	Bargad	University of Karachi Campus	Leaves	Methanol
Artocarpus heterophyllus Lam.	Moraceae	Jack fruit	Dry, purchased	Leaves	Methanol
Moringa oleifera Lam.	Moringaceae	Sanjna	University of Karachi Campus	Leaves	Methanol
Peganum harmala L.	Nitrariaceae	Harmal	Dry, purchased	Seeds	Methanol
Olea europaea L.	Oleaceae	Olive	Dry, purchased	Leaves	Methanol
Aegle marmelos L. Corr. Serr.	Rutaceae	Bael fruit	University of Karachi Campus	Leaves	Methanol

Materials and methods

Plant material

Plant materials collected from the campus of University of Karachi (Table 1) were identified by taxonomists working at the Department of Botany, University of Karachi, and voucher specimen of each plant was submitted in the herbarium of the same department. Aegle marmelos (L.) Corr. Serr. (Rutaceae) voucher specimen number 68509 KUH was collected in October 2005 and identified by taxonomist Dr. Sherwali. Cassia fistula L. (Fabaceae) voucher specimen number GH 68508 KUH collected in May 1999 was identified by taxonomist Dr. Jan Alam. Moringa oleifera Lam. (Moringaceae) voucher specimen number 66250 KUH was collected in November 1990 and identified by Abrar Hussain. Taxonomist Prof. Dr. Surriya Khatoon identified Melia azedarach L. (Meliaceae) voucher specimen number GH 63495 KUH collected in February 1997, Bombax ceiba L. (Malvaceae) voucher specimen number KUH 66854 collected in June 2003 and Brassica rapa ssp. campestris L. (Brassicaceae) voucher specimen number KUH GH 66947 collected in 1998. Plant parts used as spices, and vegetables as well as seeds of Peganum harmala L. (Nitrariaceae) were bought from the local market in Karachi, Pakistan (Table 1).

Preparation of the plant extract

Water extract of the dry plants

Dry plant parts and shade-dried fresh plants (Table 1) were ground into powder. Their 3% solutions were prepared aseptically in sterile distilled water by continuous stirring and boiling for 2 min and then cooling and leaving undisturbed for 2 min. This procedure was repeated three times and followed by filtration. Filtrates collected were divided into two portions; one portion was sterilized in an autoclave whereas the other portion was filter sterilized. Filtrates were tested for their antibacterial activity against the clinical isolates included in the study.

Water extract of the fresh plants

Beta vulgaris Linn. (Chenopodiaceae) and Solanum tuberosum L. (Solanaceae) were washed once with tap water and then with distilled water. Plants were kept at 4°C and peel was removed from B. vulgaris, and from S. tuberosum peel, subpeel and pulp were separated and cut into minute pieces; they were individually soaked in sterile distilled water and stored at 4°C. Solutions were aseptically stirred on a magnetic stirrer for 2 h a day for 3 days and aseptically filtered through sterile cheese cloth. Filtrates were collected and tested for their antibacterial activity against clinical isolates.

Methanol extracts

Parts of the plant to be tested were extracted with methanol three times (100 g plant material was dissolved in 500 ml of methanol). They were combined together and the solvent was completely removed under reduced pressure in vacua to give residue that did not have a trace of methanol.

Percentage yield of methanol extracts are: Bombax. ceiba (8.33%), M. azedarach (12.45%), Ficus carica (4.6%), Artocarpus heterophyllus (6.5%) M. oleifera (8.35%), P. harmala (5.67%), Olea europaea (5.89%.), and A. marmelos (7.35%).

Microorganisms

Clinical bacterial isolates were obtained from different hospitals of Karachi. Acinetobacter calcoaceticus was isolated from burned ward of a local hospital. Their morphological, cultural and biochemical characteristics were confirmed by the standard methods (Holt et al., 1994). Clinical isolates maintained on brain hearth infusion agar were Staphylococcus aureus (n = 30), Staphylococcus epidermidis (n = 30), Streptococcus pyogenes (n = 30), Streptococcus agalactiae (n = 30), Salmonella enterica serovar Typhi (n = 30), Pseudomonas aeruginosa (n = 30), A. calcoaceticus (n = 20), Candida albicans (n = 30), Proteus vulgaris (n = 30), Streptococcus faecalis (n = 30), Shigella dysenteriae (n = 30), Klebsiella pneumoniae (n = 30), Escherichia coli (n = 30), enteropathogenic E. coli (n = 30), enterohemolytic E. coli (n = 30), Vibrio cholerae (n = 30), and Corynebacterium xerosis (n = 30).

Antimicrobial agents

Ampicillin, velosef, sulfamethoxazole, ceftazidime, cefotaxime and cefepime were purchased from Squibb Pvt. Ltd. (Karachi, Pakistan) and tetracycline from Pfizer Laboratories Pvt. Ltd. (Karachi, Pakistan) These antibiotics were used as positive control.

Determination of minimum inhibitory concentration

Microbroth dilution method was used for the determination of MIC. Briefly overnight cultures were used to prepare 0.5 McFarland standard suspensions, which were further diluted into 1.5 (~2 × 10⁶ colony-forming units/ml) with MHB (BBL, Cockeysville, MD). In microtiter plate, twofold dilutions of plant extracts and antibiotics were prepared in Mueller–Hinton broth. The inoculum (20 µl) was added in each well. Final volume in each well was kept at 0.2 ml. Plates were incubated aerobically at 37°C overnight and observed for the development of turbidity. The lowest dilution giving no turbidity was recorded as the MIC (Murray et al., 2007; NCCLS, 2008).

Results

Minimal inhibitory concentration

Water extracts of Syzygium aromaticum L. showed promising broad-spectrum, heat-stable antibacterial activity with MIC values in the range of 31.25–250 µg/ml for S. aureus, S. epidermidis, S. pyogenes, S. enterica serovar Typhi, A. calcoaceticus and P. aeruginosa. Methanol extracts of F. carica L. and O. europaea L. showed strong antibacterial activity as they inhibited the growth of S. aureus, S. epidermidis and S. pyogenes at MIC range of 31.25–62.5 µg/ml. P. harmala L. seed had MIC 31.25–62.5, 250, 125–250 and 31.25–250 µg/ml, respectively for S. aureus, S. enterica serovar Typhi, A. calcoaceticus and C. albicans. It is the only plant that inhibited the C. albicans responsible for causing infection in immunocompromised patients (Table 2).

Table 2.   Minimal inhibitory concentration (µg/l) of plant extracts and commercially available antibiotics.

Extract/antibiotics	Part	Type	1	2	3	4	5	6	7
Syzygium aromaticum L.	B	W	31.25–62.5	31.25–62.5	62.5–125	NA	62.5–250	125–250	62.5–250
Ficus carica L.	L	M	125–250	31.25–62.5	62.5–125	NA	NA	NA	NA
Olea europaea L.	L	M	125–250	125–250	125–250	NA	NA	NA	NA
Peganum harmala L.	S	M	31.25–62.5	NA	NA	31.25–250	125–250	125–250	NA
Sulfamethoxazole	–	–	50	50	25	ND	50	12.5	50
Tetracycline	–	–	12.5	12.5	25	ND	12.5	25	50
Ampicillin	–	–	12.5	25	12.5	ND	25	12.5	25
Velosef	–	–	12.5	25	12.5	ND	12.5	25	50
Ceftazidme	–	–	3.0	3.0	3.0	ND	6.0	3.0	6.0
Cefotaxime	–	–	6.0	6.0	6.0	ND	3.0	3.0	6.0
Cefepime	–	–	1.5	1.5	1.5	ND	1.5	1.5	1.5

All the extracts tested were found to be inactive against Proteus vulgaris, Streptococcus faecalis, Shigella dysenteriae, Klebsiella pneumoniae, Escherichia coli, enteropathogenic Escherichia coli (EPEC), enterohemolytic Escherichia coli (EHEC), Vibrio cholerae, Corynebacterium xerosis, Streptococcus agalactiae.

Name of the pathogen: 1, Staphylococcus aureus; 2, Staphylococcus epidermidis; 3, Streptococcus pyogenes; 4, Candida albicans; 5, Salmonella enterica Serovar Typhi; 6, Acinetobacter calcoaceticus; 7, Pseudomonas aeruginosa.

B, bud; L, leaves, M, methanol; NA, not applicable (MIC in the range of 500–2000 µg/ml); ND, not done; S, seeds; W, water.

Antimicrobial potency profile of these extracts was compared with that of commercially available antibiotics. Most of the bacterial isolates sensitive to the plant extracts were resistant to conventional antimicrobial drugs. The MIC values of ampicillin, velosef, sulfamethoxazole and tetracycline was ≥50 µg/ml whereas MIC of ceftazidime, cefotaxime and cefepime was evaluated in the range of 1.5–12 µg/ml (Table 2).

Discussion

Most of the pathogenic bacteria have developed resistance to currently available antibiotics due to their misuse or overuse. This situation has led to an urgent need to explore different sources for development of efficient, less toxic and cost-effective antimicrobial agents (Russell, 1999; Sheldon, 2005). Medicinal plants play a major role and constitute the backbone of traditional medicine. According to the World Health Organization (WHO) estimate, 80% of populations in developing countries rely exclusively on traditional medicine for their healthcare need. Moreover, 20% of the available allopathic drugs have an active principal obtained from higher plants (Manisha & Tandon, 2004; Dhiman, 2006; Hota, 2007). Recognizing the significance of indigenous medicinal plants, WHO in its 1997 guideline states that “effective locally available plants, be used as substitutes for drugs. Research work on medicinal plants and exchange of information obtained will go long way in scientific exploration of medicinal plants for the benefit of man and is likely to decrease dependence on imported drugs” (Veerappan et al., 2007).

Plant synthesizes natural products as its chemical weapon that arrest the growth of environmental microbes (Gibbons, 2004) and some plants inhibit the growth of potential human pathogens too. In the current study, in vitro MIC of spices and edible and nonedible parts of medicinal plants prescribed in indigenous system of medicine that are available in the local market or growing in the campus of University of Karachi were evaluated against local clinical bacterial isolates and fungus C. albicans. Determination of MIC of title plants is important for choosing the best plant that eradicates infectious agents (Table 2). Clinicians also select the antibiotic on the basis of their MIC value to treat infectious disease. Plant extracts having MIC below 8000 µg/ml have been reported as therapeutically effective (Fabry et al., 1998). For this study, plants having MIC below or equal to 250 µg/ml was considered significant; however, plants having MIC in the range of 500–2000 µg/ml were recorded (their values are not included in Table 2). In recent years, many papers have been published on the plants mentioned in this study, but careful literature search revealed that, mostly, they have described antibacterial activity of volatile oils of the plants (Dorman & Deans, 2000; Jacobellis et al., 2005; Rossi et al., 2007; Shayegh et al., 2008; Pinto et al., 2009). Alcoholic extracts of these medicinal plants are reported to have MIC ≥500 µg/ml for antibiotic-sensitive and -resistant strains of clinical isolates, American Type Culture Collection (ATCC) and reference cultures (Jacobellis et al., 2005; Doughari et al., 2007; Parekh and Chanda, 2007; Jabeen et al., 2008; Khan et al., 2009). Moreover, a number of scientists have evaluated antibacterial activity of these plants against veterinary, poultry, plant pathogens and industrial and food spoilage bacteria (Duško et al., 2006; Sofia et al., 2007; Vimalraj et al., 2009).

The focus of this study was to explore antimicrobial property of water/methanol extracts of the medicinal plant against bacterial strains isolated from clinical specimen. The significance of our study is that we have identified antimicrobial plants that have arrested the growth of bacteria causing hospital-acquired and opportunistic infections. Water extracts were prepared from spices and edible plants, and methanol extract from nonedible plants (Table 1). Plant extracts could be prepared in harmony with different systems of medicine. In traditional Chinese medicine, drugs are pretreated by heating, addition of alcohol, alum or other substances to eliminate or neutralize the adverse effect of agent in the extract. In folkloric medicine, dry parts of medicinal plants are consumed as infusions dissolved in hot water, and in some cases vapors of heated water solutions are inhaled and rarely are they consumed as tincture or solution in alcohol (Eloff, 1998; Cowan, 1999).

S. aromaticum (clove) showed promising broad-spectrum heat-stable antibacterial activity. MIC of its filter-sterilized extract was similar to the MIC of its autoclaved extract. In the current study, MIC of water extract of its bud is in the range of 31.25–62.5 µg/ml against S. aureus and S. epidermidis. S. aureus, one of the most important and common cause of hospital-acquired disease (HAD), is highly antibiotic resistant and continues to develop resistance against emerging antibiotics. S. epidermidis has a well-developed glycocalyx and causes infection in artificial joints and shunts. Moreover, it is assumed to cause the skin disease acne vulgaris (pimples) (Kumar et al., 2007). MIC value of S. aromaticum for P. aeruginosa is 62.5–250 µg/ml, which is a common opportunistic pathogen causing around 10% of all HAD infections. It causes septicemia with 50–90% mortality specifically in immunocompromised patients. It can grow everywhere as it has no special nutritional requirement and it is highly resistant to most of the available antibiotics. It is very crucial to develop a cost-effective antibiotic for it. In traditional medicine, clove bud is applied on tooth with dental caries to give relief from pain and its paste is applied on small boils as household remedy. Dried buds are widely employed as an ingredient of a large number of medicinal preparations (single and compound formulation) that are classically prescribed in indigenous system of medicine (Dhiman, 2006). Ethanol extract of S. aromaticum is reported to have MIC 780 µg/ml against ATCC strains (Khan et al., 2009). R. centrifolia and C. fistula gave broad-spectrum antibacterial activity and their MIC value was in the range of 250–2000 µg/ml for S. aureus, S. epidermidis, S. enterica serovar Typhi, P. aeruginosa, S. pyogenes, A. calcoaciticus and C. albicans. Bark of C. fistula is reported to have MIC up to 6250 µg/ml for veterinary origin ATTC cultures (Vimalraj et al., 2009). Rose petals are considered as a mild laxative and carminative. Rose water distilled from flowers is employed as vehicle for lotion and collyriumms (Nadkarni, 1976). C. fistula is used to treat round worm infection, facial paralysis and rheumatism (Jayaweera, 1981; Bhavan, 1992). Water extract of its fruit is reported to have immunomodulating properties and its synergistic combination with amoxicillin is reported to have activity against multidrug-resistant (MDR) S. enterica serovar Typhi (Ali et al., 2007, 2008). Methanol extract of F. carica has MIC 31.25–62.5 µg/ml for clinical isolates of S. epidermidis and 62.5–125 µg/ml for S. pyogenes. F. carica is used to remove gravel in the kidney/bladder, obstruction of the liver and treatment of gout and piles. Its milky juice is a remedy for ulcers in the mouth (Nadkarni, 1976). O. europaea has MIC 125–250 µg/ml for human clinical isolates of S. pyogenes that causes toxic shock syndrome. MIC ≥250,000 µg/ml was reported for its leaves and fruit against antibiotic-sensitive clinical isolates and MIC ≥400,000 for S. aureus isolated from burn patients (Alsaimary, 2009). O. europaea is used to treat urinary infections. It acts as a diuretic, hypotensive, emollient, laxative and febrifuge (Cowan, 1999; Khan et al., 2007). P. harmala, reported to act as a narcotic and antiseptic (Nadkarni, 1976), is the only nonedible plant having promising bactericidal activity against causative agents of HAD and opportunistic infections. Methanol extract of its seeds has MIC of 31.25–62.5, 125–250 and 31.25–250 µg/ml for S. aureus, A. calcoaceticus and C. albicans, respectively. All these bacteria are responsible for causing nosocomial and opportunistic infections. A. calcoaceticus was isolated from burn ward of a local hospital.

Most of the bacterial species studied are resistant to commercially available antibiotics. It is reported that typical antibiotics produced from bacteria and fungi have MIC in the range of 0.01–10 µg/ml (Tegos et al., 2002); however, a few of the commercially available antibiotics included in the study could not arrest the growth of some bacterial species at the concentration of 50 µg/ml (Table 2). Photochemicals classified as antimicrobial usually have MIC 100–1000 µg/ml. On the contrary, in the current study it is observed that some of the extracts are inhibiting growth of drug-resistant mutant bacteria at MIC lower then 100 µg/ml. Our results suggest that Gram-negative bacteria are less susceptible to the spices and medicinal plant extracts than Gram-positive bacteria. It may be due to the significant differences between their cell wall structures. Outer membrane of Gram-negative bacteria contains thick murein layer and unique periplasmic space. Resistance of antibacterial substance is related to the hydrophilic surface of its outer membrane that is rich in lipopolysachharide molecule. It presents a barrier against the penetration of numerous antibiotic molecules. Besides, the enzyme in the periplasmic space has the ability to break down the molecules that are introduced from outside. A Gram-positive bacterium lacks all such structures. Its cell wall and cytoplasmic membrane are easily destroyed by antibacterial substances resulting in the leakage of cytoplasm and its coagulation (Russell, 1999).

This study showed that methanol extract of P. harmala has MIC 31.25–62.5 µg/ml against S. aureus, whereas Ahmad et al. (1992) evaluated MIC value 250 µg/ml and higher for alkaloidal fractions of methanol extract of P. harmala that consisted of harmine, harmaline, harmalol, tetrahydroharmine and tetrahydroharmal against S. aureus. The possible hypothesis for low MIC of P. harmala extracts as compared to reported high MIC of its pure compounds/fraction may be due to the fact that methanol extract of P. harmala have variety of compounds with diverse functional groups that act synergistically. Any one or a few of these compounds may have inhibited the action of MDR pump/efflux pump, which resulted in retention of sufficient concentration of active component that killed the bacterial cell (Sheldon, 2005). HADs and opportunistic infection can be treated empirically with broad-spectrum antibiotics but the toxicity of a single drug restricts its prescription at the concentration that could effectively eradicate all the microorganisms sensitive to it. This results in the development of resistance strains of certain surviving sensitive bacteria, which further complicates the infections.

Complex pathophysiological effects are observed on host tissue during an infectious process. Besides, most of the drugs available to cure the infection produce harmful side effects. A plant extract consists of large array of compounds that aid in counteracting against the symptoms associated with infectious diseases (Nadkarni, 1976; Dhiman, 2006; Wagner & Ulrich-Merzenich, 2009). Besides, its active component is combined with other components that help in neutralizing the malice effect produced by the drug (Adebiyi et al., 2009).

In the present decade, immense development in analytical high-tech and molecular biology methods coupled with the wisdom of ancient tried and tested therapies has opened unforeseen avenues in the field of modern medicine (Philipson, 2003). It has become possible to optimize the mono- and multiextract preparation, and major pharmaceutical companies are now showing interest in carrying out efficacy of complex herbal drug mixtures since there is an urgent need to develop a large collection of novel antibiotics to respond against the growing problem of existence of MDR bacteria. In the current study, extracts based on three edible plants, S. aromaticum, F. carica and O. europaea, and the nonedible plant P. harmala, emerged as promising antibacterial agents having very low MIC values (Table 2).

Bioassay-guided isolation work on these extracts may lead to the development of novel antimicrobial agents to respond against the growing problem of existence of MDR bacteria. Moreover, mono- and multiextract–based formulations of the extract may also be prepared, for their development as antimicrobial agents.

Conclusion

The current study has identified promising antibacterial activity of some edible and medicinal plants against drug-resistant human pathogens. They can emerge as a potential therapeutic source to treat hospital-acquired, opportunistic and wound infections. However, there is further need to carry out in vitro and in vivo studies to find their mode of action, development of the synergistic combinations of antimicrobial extracts and isolation of active components. All findings should be confirmed in controlled clinical trials.

Declaration of interest

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.