Journal of Dental Research, Vol 68, 1146-1150, Copyright © 1989 by International & American Associations for Dental Research Online Journals
The effect of pH on the sensitivity of species of Lactobacillus to chlorhexidine and the antibiotics minocycline and spiramycin
B. Cleghorn and G. H. Bowden
Department of Rehabilitative Dental Science, Faculty of Dentistry, University of Manitoba, Winnipeg, Canada.
The purpose of this study was to determine the sensitivity of a range of
Lactobacillus species to chlorhexidine, Minocycline, and Spiramycin, at a
range of pH from 5.0 to 7.4. Strains of Streptococcus were also tested for
their sensitivity to chlorhexidine, as a comparison between the genera.
There were both inter- and intra-species variations in the sensitivity of
these strains to chlorhexidine. The strains tested were sensitive at pH 6.7
to the following levels of chlorhexidine (micrograms/mL): L. casei (6
strains), 10-60; L. plantarum (4 strains), 40; L. fermentum (13 strains),
2-20; L. brevis (1 strain), 10; and L. acidophilus (3 strains), 10-60. The
Streptococcus species were sensitive to 1-4 micrograms/mL (13 strains);
4-10 micrograms/mL (3 strains); and 10-20 micrograms/mL (2 strains). One
strain was able to survive 20 micrograms/mL. Chlorhexidine was found to be
less effective at lower pH levels. The following examples show sensitivity
(micrograms/mL) at pH 6.5 followed by sensitivity at pH 5.0: L. casei (ATCC
15008), 40,60; L. plantarum (CH 374), 40,100; L. fermentum (CH 324) 10,40;
L. acidophilus (ATCC 4356), 10,40; and S. mutans (BM 52), 2,2. All of the
strains of Lactobacillus tested with Spiramycin were resistant at pH 5.0.
Minocycline was less affected by changes in pH, but at pH 7.4 Lactobacillus
strains were more resistant to Minocycline as compared with Spiramycin.
Both of these antibiotics are bacteriostatic, and therefore may have a more
limited effect than a bactericidal agent such as chlorhexidine. The
Lactobacillus strains tested required higher concentrations of
chlorhexidine than did the strains of Streptococcus for a killing effect in
vitro.
