In vitro susceptibility testing of essential oils against carbapenem-resistant Enterobacteriaceae and selected ATCC strains.

Background:

In the era of antibiotic resistance, alternative anti-infectives must be explored. The National Action Plan for Combating Antibiotic-Resistant Bacteria calls for developing non-traditional therapeutics, including natural compounds such as essential oils (Goal 4.4). To that end, the in vitro activity of essential oils and the oil blend, Thieves® (cinnamon, clove, lemon, eucalyptus, rosemary) were studied against CRE from our institution. Selected ATCC isolates were also tested to explore the potential spectrum of essential oils.

Methods:

Oils were obtained from Young Living Essential Oils & lot #s recorded. CRE were defined as isolates resistant to imipenem, meropenem AND resistance to ceftriaxone, cefotaxime, or ceftazidime using CLSI methods. 23 CRE were identified from 2014-15 at university-affiliated hospitals. Isolates were grown overnight on TSA; 0.5 McFarland suspensions in sterile water were swabbed over Mueller-Hinton agar using the Kirby- Bauer method. 20 ml of full-strength oils were pipetted onto blank paper disks in a sterile dish. Disks were placed aseptically onto the plates immediately after inoculating disks. Plates were incubated overnight at 37C in ambient air. Zone diameters are shown.

Results:

	Table 1. Zone Diameters (mm for Essential Oils against CRE and ATCC strains)
CRE isolate	Lavender	Lemongrass	Eucalyptus	Oregano	Rosemary	Thieves®	Thyme
C. freundii	0	11	10	34	13	16.5	38
E. aerogenes	8	10	7	21	0	15	20
E. cloacae	8	9	0	19	9	17	18
E. cloacae	0	0	0	12	0	11	11
E. coli (n=4; median)	0	10	0	18.5	4	15.5	16.5
E. coli ATCC	10	17	10	28	9	16	30
K. pneumo (n=14;median)	0	9.5	0	19.5	8	13	16.5
S. marcescens	8	9	0	15	8	11	14
P. aeruginosa ATCC	0	0	0	7	0	11	8
S. pneumo ATCC	26	54	13	52	18	34	56
S. pyogenes ATCC	30	>50	18	50	30	36	>50
MRSA ATCC	13	42	10	37	0	24	42
S. aureus ATCC	11	36	0	29	0	16	34

Conclusion:

Essential oils oregano and thyme had the largest zones of inhibition against CRE organisms, followed by lemongrass and Thieves®. Oregano and Thieves® had the largest zones of inhibition for P. aeruginosa. Lemongrass had very large zones of inhibition against gram-positive ATCC strains, followed by oregano, thyme, Thieves®, and lavender. The in vitro and in vivo activity of essential oils against clinical pathogens deserves further study.