Specific Antibodies, Levofloxacin, and Modulation of Capsule-Associated Virulence in Streptococcus pneumoniae
Abstract
Amoxicillin subinhibitory concentrations produced 100% survival in passively immunized mice infected with non-amoxicillin-susceptible, poorly or highly encapsulated Streptococcus pneumoniae strains (2, 6), with negligible values of time that serum levels exceeded the MIC.
We explored this phenomenon and its modulation by capsular production with levofloxacin against the same serotype 6B Streptococcus pneumoniae strain (6) (MIC of levofloxacin = 32 μg/ml) with two types of infecting inocula. (i) For the poorly encapsulated (PE) phenotype, the microorganism was grown in Todd-Hewitt broth supplemented with 0.5% yeast extract (Difco, Detroit, Mich.) until an absorbance of 0.3 at 580 nm (UV-visible spectrophotometer, Shimadzu UV-1203, Japan) was reached. (ii) For the highly encapsulated (HE) phenotype (4), after serial passages in mice, the microorganism was grown three times in Todd-Hewitt broth supplemented with 0.5% yeast extract (Difco, Detroit, Mich.) and enriched with 5% fetal bovine serum until an absorbance of 0.3 at 580 nm (UV-visible spectrophotometer, Shimadzu UV-1203, Japan) was reached.
Eight-to 12-week old female BALB/c mice weighing 19 to 22 g were used. The challenge dose with the PE and the HE inocula (2, 6) was 4 × 108 CFU/ml. Previously described methods (6) were followed for hyperimmune serum production and determination of protection with and without levofloxacin doses decreasing on a twofold basis from 25 mg/kg of body weight. Groups of 10 animals per dose…
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