Bacteria themselves do not have noticeable effect on concrete. However, sulfate-reducing bacteria (SRB) in untreated sewage water tend to produce hydrogen sulfide (HS), which is then oxidized  in sulfuric acid (HSO) by atmospheric oxygen (abiotic reaction) and by aerobic bacteria present in biofilm (biotic reaction) on the concrete surface above the water level. The sulfuric acid dissolves the carbonates in the hardened cement paste (HCP), and also calcium hydroxide (portlandite: Ca(OH)) and calcium silicate hydrate (CaO·SiO·nHO), and causes strength loss, as well as producing sulfates which are harmful to concrete.
:HSO + Ca(OH) → CaSO + 2 HO
:HSO + CaO·SiO·n HO → CaSO + HSiO + n HO
In each case the soft expansive and water-soluble corrosion product of gypsum (CaSO) is formed. Gypsum is easily washed away in wastewater causing a loss of concrete aggregate and exposing fresh material to acid attack.
Concrete floors lying on ground that contains pyrite (iron(II) disulfide) are also at risk. 
As a preventive measure sewage may be pretreated to increase pH or oxidize or precipitate the sulfides in order to minimize the activity of sulfide-reducing bacteria.
As bacteria often prefer to adhere to the surfaces of solids than to remain into suspension in water (planktonic bacteria), the biofilms formed by sessile (i.e., fixed) bacteria are often the place where they are the most active. Biofilms made of multiple layers (like an onion) of dead and living bacteria protect the living ones from the harsh conditions often prevailing in water outside biofilm. Biofilms developing on the already exposed surface of metallic elements encased in concrete can also contribute to accelerate their corrosion (differential aeration and formation of anodic zones at the surface of the metal). Sulfides produced by the SRB bacteria can also induce stress corrosion cracking in steel and other metals.