Abstract
Background:
The increasing prevalence of antibiotic resistance and associated adverse reactions has prompted the search for alternative antimicrobial agents, including medicinal plant extracts, to be developed.

Aim:
In this study, we evaluated the in vitro antibacterial activity and minimum inhibitory concentration (MIC) of Ocimum gratissimum (scent leaf) extracts against selected gastrointestinal bacterial isolates.

Methods:
Fresh scent leaves were collected from the Rivers State University farm, dried for 2 weeks, and ground into powder. Methanolic and acetonic extracts were prepared from dried leaves, whereas aqueous extracts were obtained from fresh leaves. The extracts were tested against E. coli, Enterococcus spp., Klebsiella spp. and Proteus spp. (n = 3 isolates per species). The antibacterial activity was assessed using the agar well diffusion method at concentrations of 250, 187.5, 125, and 62.5 mg/ml. Dimethyl sulfoxide (DMSO) and sterile distilled water were used as negative controls, and ciprofloxacin (10 µg) was used as a positive control. All assays were performed in triplicate, and the results are expressed as mean ± standard deviation. MIC values were determined using the broth microdilution method. Data were analyzed using two-way ANOVA followed by Tukey’s HSD post hoc test, with significance set at p < 0.05.

Results:
Results on the antibacterial activity of O. gratissimum methanolic extracts at various concentrations (250, 187.5, 125, and 62.5 mg/ml) revealed the highest zone of inhibition (mm) in Klebsiella species (29 ± 1.00), followed by Enterococcus species (21 ± 0.00), while Escherichia coli was (19 ± 0.00) and Proteus species (19 ± 0.50). O. gratissimum acetonic extracts at different concentrations (250, 187.5, 125, and 62.5 mg/ml) revealed the highest zone of inhibition (mm) in Enterococcus species (22 ± 1.00), followed by Proteus species (20 ± 0.50), Klebsiella species was (18 ± 1.00), and Escherichia coli (15 ± 0.50). In contrast, O. gratissimum aqueous extracts at different concentrations (250, 187.5, 125, and 62.5 mg/ml) showed no zones of inhibition (0 ± 0.00). The MICs of the methanolic and acetonic extracts ranged from 62.5–187.5 mg/ml, depending on the bacterial species, whereas aqueous extracts showed no inhibition. A significant interaction between extract concentration and bacterial species was observed (p < 0.05).

Conclusion:
Methanolic and acetonic Ocimum gratissimum extracts exhibit in vitro antibacterial activity against selected gastrointestinal bacteria, whereas the aqueous extract showed no activity. However, these results are preliminary and are based on crude extracts tested at relatively high concentrations. Further studies are required to isolate active compounds, evaluate their safety, and assess their in vivo efficacy before clinical application.

Key words: Antibacterial activity; Antibiotic resistance; Gastrointestinal bacteria; Ocimum gratissimum; Plant extracts.