Of 73 essential oils tested for antibacterial activity against Streptococcu
s pneumoniae R36A (an unencapsulated strain) with a paper disk diffusion as
say, three oils were highly inhibitory, fifteen moderately inhibitory and t
he remainder weakly or non-inhibitory. Three oils from each group were sele
cted and tested with a broth assay in which each oil was added to growing c
ultures of S. pneumoniae R36A and optical densities (OD) were measured over
time. The oils with high antibacterial activity; oregano, thyme and rosewo
od, induced rapid lysis of R36A as indicated by a decrease in OD, and appea
rance of dechaining and considerable cell debris within 30 min of addition.
The lytic response of R36A to the three oils with moderate activity was va
riable but all induced some lysis. Oils that were weak inhibitors generally
caused slowing of growth but little or no lysis. Several oils were also te
sted against an encapsulated isolate, S. pneumoniae IC2. Both disk assay an
d broth results were similar to those obtained with R36A, except that the o
ils were slightly less effective. Disk assay results showed some correlatio
n 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. pneumonia
e may have potential as an alternative treatment for infections caused by d
rug resistant pneumococci.