The effect of pH on the sensitivity of species of Lactobacillus to chlorhexidine and the antibiotics minocycline and spiramycin

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.