10.22376/ijpbs.2019.10.1.p1-12 Volume 6 Issue 4 2015 (October - December) In the present investigation, a series of schiff base (1-20), carbohydrazide (31-43) and amides/anilides (21-30/44-56) of nalidixic acid was synthesized in appreciable yield and characterized by physicochemical as well as spectral means. The synthesized compounds were evaluated lessThan i greaterThan in vitro lessThan /i greaterThan for their antimicrobial activity against Gram-positive bacteria lessThan i greaterThan S. aureus, B. subtilis, lessThan /i greaterThan Gram negative bacteria lessThan i greaterThan E. coli lessThan /i greaterThan & fungal strains lessThan i greaterThan C. albicans lessThan /i greaterThan & lessThan i greaterThan A. niger lessThan /i greaterThan by tube dilution method. Antimicrobial activity results indicated that compounds 16, 24, 36, 53 (MIC = 6.25 μg/mL) were the most potent antimicrobial agents. The results of antimicrobial screening also indicated that compounds having NO lessThan sub greaterThan 2 lessThan /sub greaterThan , Br, F, Cl, OCH lessThan sub greaterThan 3 lessThan /sub greaterThan substituents were the most active one. lessThan i greaterThan mt- lessThan /i greaterThan QSAR investigation used Hansch analysis was applied to find out correlation between antimicrobial activities with physicochemical properties of synthesized compounds. The lessThan i greaterThan mt lessThan /i greaterThan -QSAR revealed that the antibacterial and antifungal activity of these synthesized derivatives against microorganisms under test was mainly governed by second order molecular connectivity index ( lessThan sup greaterThan 2 lessThan /sup greaterThan c)and zero order molecular connectivity index ( lessThan sup greaterThan 0 lessThan /sup greaterThan c lessThan sup greaterThan v lessThan /sup greaterThan ). It also indicated the importance of zero order molecular connectivity index ( lessThan sup greaterThan 0 lessThan /sup greaterThan c lessThan sup greaterThan v lessThan /sup greaterThan )in describing the antimicrobial activity of synthesized compounds. MONIKA1, SUCHETA, RUCHITA, SHILPA JAIN MEENU PALIWAL AND HIMANSHU Nalidixic acid, Schiff bases, Antimicrobial and QSAR. 591-627