@donasmrs-- I'm not an expert but my doctor told me that it's very difficult for syphilis to spread through saliva, even if it's during the infectious stage.
Syphilis mainly passes through intercourse and I think it can pass from a pregnant mother to the child but that's it.
My doctor also said that sometimes people develop anxiety about STDs and are reluctant to believe medical testing that gives negative results. Medical testing for syphilis is conclusive, there is no reason not to trust it.
Skin to saliva contact is low to no risk for spreading STDs.