Antimicrobial effects of essential oils on Streptococcus pneumoniae

Of 73 essential oils tested for antibacterial activity against Streptococcus pneumoniae R36A (an unencapsulated strain) with a paper disk diffusion assay, three oils were highly inhibitory, fifteen moderately inhibitory and the remainder weakly or non-inhibitory. Three oils from each group were selected and tested with a broth assay in which each oil was added to growing cultures of S. pneumoniae R36A and optical densities (OD) were measured over time. The oils with high antibacterial activity; oregano, thyme and rosewood, induced rapid lysis of R36A as indicated by a decrease in OD, and appearance of dechaining and considerable cell debris within 30 min of addition. The lytic response of R36A to the three oils with moderate activity was variable but all induced some lysis. Oils that were weak inhibitors generally caused slowing of growth but little or no lysis. Several oils were also tested against an encapsulated isolate, S. pneumoniae IC2. Both disk assay and broth results were similar to those obtained with R36A, except that the oils were slightly less effective. Disk assay results showed some correlation with the broth assay, but were not always predictive of an oil's ability to induce bacterial lysis. Essential oils that induce lysis in S. pneumoniae may have potential as an alternative treatment for infections caused by drug resistant pneumococci.