In Vitro Antibacterial Activity of Aqueous Clove (Syzygium aromaticum) Extract against Multidrug-Resistant Enteric Bacterial Isolates from Al-Shomali General Hospital: A Comparative Study with Standard Antibiotics

Abstract

Background: The global rise of antimicrobial resistance (AMR) among Gram-negative and Gram-positive bacteria has renewed interest in plant-derived antibacterial agents. Clove (Syzygium aromaticum) is rich in eugenol, a phenolic compound with documented antibacterial properties, and has been proposed as a candidate adjunct or alternative to conventional antibiotics.

Objective: To evaluate the in vitro antibacterial activity of a 5% w/v aqueous clove extract against 100 clinical bacterial isolates collected from patients attending Al-Shomali General Hospital, and to compare this activity with two reference antibiotics.

Methods: In this cross-sectional laboratory study, 100 non-duplicate clinical isolates were cultured, identified by standard morphological and biochemical methods, and classified as Escherichia coli (n = 40), Enterococcus faecium (n = 7), Enterococcus faecalis (n = 9), Klebsiella pneumoniae (n = 5), Klebsiella oxytoca (n = 4), Proteus vulgaris (n = 6), Proteus mirabilis (n = 19) and Pseudomonas spp. (n = 10). A 5% w/v aqueous clove extract was tested by the agar-well diffusion method on Mueller–Hinton agar against a 0.5 McFarland inoculum of each isolate, and inhibition zone diameters were compared with those produced by two disk-diffusion antibiotics, labelled in the laboratory records as meropenem (MRP) and tetracycline (TE). Data were analysed descriptively (mean ± standard deviation, median, range) and by non-parametric inferential tests (Friedman, Kruskal–Wallis, Wilcoxon signed-rank and Spearman correlation).

Results: The aqueous clove extract produced inhibition zones ranging from 6 to 36 mm (mean 21.9 ± 9.7 mm), compared with 17–40 mm (mean 28.8 ± 6.4 mm) for MRP and 6–32 mm (mean 16.9 ± 6.0 mm) for TE. Clove activity was species-dependent (Kruskal–Wallis, p < 0.001), with the largest mean zones against Proteus mirabilis (30.8 ± 3.0 mm), Proteus vulgaris (30.0 ± 1.3 mm) and Klebsiella pneumoniae (30.0 ± 1.4 mm), and the smallest against E. coli (14.7 ± 7.9 mm). Clove zones equalled or exceeded MRP zones in 38% of isolates overall, rising to 94.7–100% among Proteus and P. vulgaris isolates, and equalled or exceeded TE zones in 72% of isolates overall. Paired comparisons (Wilcoxon) confirmed significant differences between clove and both antibiotics (p < 0.001).

Conclusion: Aqueous clove extract showed variable, species-dependent antibacterial activity, with particularly strong activity against Proteus spp., Klebsiella spp. and Pseudomonas spp. that approached or exceeded that of meropenem, but comparatively weaker activity against E. coli. These findings support further standardized investigation of clove as a candidate natural antibacterial agent, pending confirmation of extract standardization, minimum inhibitory/bactericidal concentrations, and testing against a fully characterised antibiotic panel under current CLSI breakpoints.