## 1 Introduction  
_Salvadora_ is a small genus of evergreen trees or shrubs found mainly in hot and dry areas of Asia and tropical Africa. It belongs to the family _Salvadoraceae_, a small plant family comprising of about three genera and a total of 11 known species [1, 2]. Among these, _Salvadora persica_ L. is one of the most important plant species as it is famous for its applications in oral hygiene and various other ethnobotanical uses [3]. _S. persica_ L., native to middle East and Africa, is a small well branched shrub with greenish yellow flowers and small red fruits when rip. It grows up to a maximum height of 6-7 m with greenish small leaves (3.8-6.3 by 2-3.2 cm), having a robust aroma of creas or mustard [4, 5]. _S. persica_ L. roots/stems are known with various names like Mustard tree, Miswak, Koyojiji and Pilu in Gulf region, Japan and India, respectively and have been traditionally used for maintaining oral hygiene for centuries [3, 6]. In addition to extensive use in oral hygiene, other organs of the plants, such as the stems, leaves, flowers, roots, fruits and twigs have other wide spread traditional applications. For example, several organs of _S. persica_ L. have been used for the remedy of various illnesses, including piles, urinary disorder, rheumat, skin diseases, seasonal cough and cold, diabetes, asthma, gout, constipation and respiratory infections [3].  
Recently, aqueous extracts of _S. persica_ L. root have been effectively utilized for the synthesis of different nanoparticles (NPs) which have shown attractive microbicidal and catalytic activity for the Suzuki coupling reactions [7, 8]. Moreover, the pharmacological properties of _S. persica_ L. are widely known and several fascinating biological activities, including antimicrobial, antitumor, hypoglycemic, anti-osteoporosis, anti-inflammatory, analgesic, antiulcer, antioxidant, enzyme inhibitory, anticonvulsant and sedative activity have been reported for this plant [3].  
Although various pharmacological properties of _S. persica_ L., along with its applications in folk medicines for the remedy of several diseases, have been reported, _S. persica_ L. is most widely used and known for maintaining oral hygiene. One reason may be the remarkable antimicrobial activities of various compounds from _S. persica_ L. [9, 10, 11]. Many of these reports evaluated the microbicidal properties of _S. persica_ L. against different microorganisms. Yet, a detailed report on the drying effects on the chemical constituents of _S. persica_ L. essential oil (EO) and their effects on antimicrobial activity are lacking. Furthermore, oral pathogens are a matter of greater concern than previously thought as their role in other diseases is becoming clear [12]. Moreover, the development of drug resistance in bacteria requires minimizing the use of synthetic chemotherapeutic agents and promote the use of plant-based products as alternative antimicrobial agent [13, 14]. One of the most widely used chemotherapeutic agents for oral hygiene is chlorhexidine digluconate (CHX). However, it is also known to be associated with a number of disadvantages, such as bitter taste, taste alteration for at least 4 h and tooth discoloration [15]. Moreover, clove oil extracted from flower buds of _Syzygium aromaticum_, a plant species of the family _Myrtaceae_ has been popularly used for toothache and maintaining oral hygiene in traditional medicine. However, detailed comparative studies on the antimicrobial activities of clove oil, CHX and _S. persica_ L. oils are lacking. This paper reports on the chemical determination of EO components of fresh and dried _S. persica_ L. roots and undertakes a detailed comparative study on the antimicrobial activity of these EOs with those of commercially used antispeptic agents CHX and clove oil against a well-known oral pathogen _Streptococcus mutans_. Here, we report, for the first time, a comparative study of _S. persica_ L. essential oils, clove oil and CHX against _S. mutans_ and their possible mechanism of actions against the test organism.