Essential Oils, an Answer to the Dilemma of Multiply Resistant Bacteria?

Antibiotic resistance is an increasing worldwide phenomenon. Recently, there has been an interest in using essential oils (EO) both in and out of the hospital. Most current research concentrates on the antimicrobial activity of Tea tree. The purpose of this study is to evaluate the activity of 10 EO, (Tea tree, Thyme, Lavender, German chamomile, Peppermint, Palma rosa, Eucalyptus globulus, Eucalyptus citriodora, Lemongrass and Rose) against multiply resistant organisms. Twenty five strains of Methicillin Resistant Staphylococcus aureus (MRSA), ten strains of vancomycin resistant Enterococcus sp. (VRE), and 5 strains of extended spectrum beta-lactamase (ESBL) or Amp C resistant gram negative rods were tested. An aromatogram (BauerKirby [KB]), killing curve, and synergy tests were performed against the isolates. Organisms were classified as multiply resistant using NCCLS approved methods. The bacteria were obtained from the Pinnacle Health microbiology repository. The ten EO were tested in three phases. Antimicrobial activity occurred between several EO and the multiply resistant bacteria. Lemongrass, Thyme and Palma rosa oils were the most active compounds overall. The Lemongrass and Thyme were the most active compounds versus MRSA. Lemongrass and Palma rosa showed the greatest activity versus VRE and Tea Tree and Thyme showed the most activity against resistant gram negative rods. EO shows antimicrobial activity versus multiply resistant bacteria in vitro. More studies are needed to confirm this activity in vivo.


Introduction
Antibiotic resistance is an increasing worldwide phenomenon [1][2][3][4]. It is at least partially driven by the overuse of antibiotics, particularly broad-spectrum antibiotics and has been occurring for many years without resolution [5][6][7][8]. Physicians, hospitals, drug companies, and government agencies have been looking for new and innovative ways to deal with these resistant strains of bacteria. Over the last few decades, a decrease in the number of new antibiotics have been realized [1]. Between 1983 and1987 16 new antibiotics were approved while only 2 new antibiotics were introduced between 2008 and 2012. In addition, antibiotic resistance is increasing worldwide [1]. Essential oils (EO) have been used since ancient times for healing and the treatment of patients [9], recently, there is an interest in them as an alternative to antimicrobial therapy as a replacement or as an adjunctive therapy source [10]. There has been an interest in using essential oils (EO) both in and out of the hospital [10].  [10][11][12].
Organisms were collected from distinct geographical regions (Illinois, Michigan, North Carolina and Pennsylvania) and stored in the Pinnacle Health microbiology repository. This was done in an attempt to test separate clones of organisms and prevent testing multiples of the same organisms. Organisms were classified as multiply resistant using NCCLS approved methods [13,14]. The ten EO (Table 1) were tested in three phases. The antimicrobial activity of the essential oils was studied using three phases as described below.  Figure 1 & 2).

Conclusions
In Phase I the results are consistent with other studies looking at ranges of zone diameters [11]. In the current study the most inhibitory essential oil was Lemongrass against VRE and MRSA. The most inhibitory essential oil against ESBL/AmpC strains was Tea Tree. The least inhibitory essential oil against all the bacteria tested was German chamomile. Consistent results were observed when measuring the zone diameters of inhibition. Some limitations of this testing may be the activity of the solvent as recognized in the current study, the volume of EO on the susceptibility disks and the thickness of the agar layer [11].
In Phase II , of the essential oils tested, Lemongrass and Tea Tree had the most significant killing effect. A control test tube of almond oil (without any essential oil added) also killed the bacteria.
Since the almond oil appeared to kill the bacteria faster alone than mixed with essential oil we decided a third phase of the study was necessary.
In Phase III we discovered that the carrier oil often used by aromatherapists in conjunction with essential oils had varying antagonistic effects on the essential oils' antimicrobial activity. In this phase the almond oil alone did not produce zones of inhibition.
We believe this happened because the almond oil was too viscous to diffuse through the agar medium.
In the current study, the solvent (Sweet Almond Oil) showed antagonism with several of the EO studied and showed difficulty in diffusion in the test method. Different parameters in this test could affect the result, such as the volume of EO on the paper discs, the thickness of the agar layer and the solvent. As previously seen, this could result in difficulty in reproducibility of screening methods [11,[15][16][17]. An important parameter in phase I is that the zone diameters seen are reproducible in each of the EOtested and are consistent with those seen in other studies [11]. As demonstrated in the current study, a disk test is useful in screening for antibacterial activity of EO, however further studies maybe needed to determine the best EO and solvent to be used for medicinal therapy.
As evidenced in other studies [18], Lemongrass and Tea Tree oil were effective in killing organisms in vitro and in our study did it faster than the other oils. Natural products are interesting alternative or supplemental to antibiotics as a treatment moiety for patients inside and outside the hospital.