These organic acids are used as growth substrates to obtain ATP by substrate level phosphorylation and by anaerobic respiration.
The bacteria get some ATP from substrate level phsophorylation using lactate which is metabolised to produce acetate. Lactate is also used to produce propionate in a sequence that regenerates NAD (so enabling the continued metabolism of fresh supplies of lactate) and this pathway incorporates also the succinate taken up from the growth environment. Propionate is the product of a short metabolic pathway that incorporates the succinate and involves the decarboxylation of methylmalonyl CoA derived from it.
Interestingly this decarboxylation reaction releases free energy sufficient to support an electrogenic sodium pump which generates a "sodium motive force" which drives ATP synthesis by anaerobic respiration.
This scheme is shown in the next diagram.