"L. reuteri persisted at reduced but detectable levels 4 weeks after the end of intake"
Probiotics dont do a permanent colonization... the "theory behind probiotics" is that they stimulate the growth of your own bacteria by creating a good environment: compiting with bad bacteria for room and food, lowering the ph of the gut by creating lactic acid, creating natural Antibiotics , etc... so your own bacteria have a chance to growth. For this reason is highly recommended to take also prebiotic fiber to feed your own flora.
You are not "replacing" the bacteria killed by ABx with probiotics. Probiotics helps to restore normal levels of your own bacteria in the gut.