Carbothioamides 4a,b, obtained from nalidixic acid, were converted to the corresponding 1,3-thiazolidine derivatives 5a,b by cyclocondensation with 2-bromo-1-(4-chlorophenyl) ethanone. Treatment of 4a,b with base afforded 1,2,4-triazoles 6a,b. The synthesis of 1,3-oxazolidine 7 was performed by the reaction of compound 4a with ethyl bromoacetate. Treatment of 4a with acid produced 1,3,4-thiadiazole 8. The reaction of compounds 6a and 6b with several heterocyclic amines in the presence of formaldehyde gave the corresponding Mannich bases 9-15 containing various pharmacophore groups. Conventional and microwave-assisted methods were used for the synthesis. The effect of an acid catalyst on Mannich reactions was investigated. The structures of the newly synthesized compounds were elucidated on the basis of H-1 NMR, C-13 NMR, FTIR, EIMS techniques, and elemental analysis. All compounds were screened for their antimicrobial activity.