GbpB expression in Streptococcus mutans strains exposed to penicillin G

Background: The virulence of Streptococcus mutans (SM) depends on their ability to adhere and accumulate in the dental biofilm. These processes are modulated by the synthesis of extracellular glucans and their interaction with glucan binding proteins (Gbps). One of them, GbpB, is a protein essential for bacteria that seems to be involved in the cell wall biosynthesis.

Objective: The aim of this study was to evaluate the effect of sublethal levels of penicillin G (PenG) in the expression of gbpB, in four SM strains (UA159, LT11, SJ32 and 5SM4).

Methods: Levels of gbpB transcripts were determined in planktonic growth curves of strains exposed or not to sublethal levels of penG, which were determined based in the minimal inhibitory concentrations of PenG. To this end, 100 µl of overnight cultures of each strain were inoculated in BHI containing or not sublethal levels of PenG and incubated anaerobically at 37oC. Culture absorbances (A550nm) were measured at each 1 to 2h of growth and cells harvested for extraction of RNA. cDNA were obtained from RNA samples in reverse transcription reactions performed with arbitrary primers. Transcript levels of gbpB were measured in semi-quantitative-PCR-assays using specific primers for gbpB and 16S (reference-gene). Two independent experiments were performed in triplicate per each strain.

Results: There were significant reductions in the growth rates of all strains exposed to PenG, when compared to strains not exposed to this antibiotic. The expression of gbpB significantly increased (Mann-Whitney, p≤ 0.05,) in the presence of PenG in several strains at late-log and stationary phases of growth, which corresponded to 6 to 10h of incubation.

Conclusion: Expression of gbpB may be upregulated in response to exposure to PenG. Further studies will be necessary to better investigate the effect of beta-lactam antibiotics in the expression of this gene. FAPESP (proc. 02/07156-1 and 05/55775-0).