Author: Chimezie Chinenye Chidimma
Department: Pharmacy
Affiliation: Nnamdi Azikiwe University Awka

This study was undertaken to evaluate the antimicrobial activity and possible synergistic antimicrobial activity of combinations of Hyptissuaveolens, Platostomaafricanum and two varieties of Psidiumguajava(red and white varieties) on multidrug-resistant bacteria. Three (3) strains of ESBL-producing E. coli, 2 strains of MRSA and one susceptible strain each of E. coli and S. aureus as control organisms were used in this study. Preliminary antimicrobial screening of both the aqueous and ethanol extracts of the three plants was carried out on the test isolates by the agar well diffusion method. The minimum inhibitory concentrations (MICs) of the plant extracts showing antimicrobial activity were determined against the test isolates. Synergistic interactions of the plant combinations against the test isolates were evaluated by the checkerboard method and their fractional inhibitory concentration (FIC) indices were used as indicators of synergistic activities of the antimicrobial combinations. The ethanolic extracts of all the tested plants had better inhibition activity on the test isolatescompared to the aqueous extracts. The antimicrobial screening of the ethanolic extracts of the red and white varieties of P. guajava revealed the red variety to be more active than the white variety, though they both had recordable activity against both groups of resistant organisms. Also, the red variety had relatively lower MICs against the test isolates compared to the white variety and the difference between their MICs against test isolates was statistically significant (P=0.03). Consequently, the ethanolic extracts of the leaves of the red variety of P. guajava was considered for use in the in vitro combination studies with each of P. africanumand H. suaveolens. P. africanum showed lower antimicrobial activity against the Gram-negative organisms compared to the Gram-positive strains; while H. suaveolens showed no activity against the Gram-negative organisms (both ESBL-producing E. coli strains and the E. coli strain), but had activity against the Gram-positive test organisms. The ethanolic extracts of the three plants showed killing activity against the various test organisms during the time-kill assay. The combinations [P. guajava(red) + P. africanum] showed more synergistic than indifferent or antagonistic effects, against the test isolates. The 8: 2 combination of the two plants recorded synergistic activities against all the test isolates (both resistant and susceptible Gram-negative and Gram-positive isolates), with FIC indices ranging from 0.106 to 0.825. Also, only the 9: 1 combination of P. guajava(red)and H. suaveolensrecorded a synergistic activity against S. aureus (control) with an FIC index of 0.925. The present study has shown the combinations of Platostomaafricanum and Psidiumguajava (red variety)ethanolic leaf extract to possess synergistic antibacterial activity against multidrug resistant strains and thus provide the initial steps for further isolation and identification of antibacterial agents from these plants.

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