The present study identified the most commonly used monofloral honey based on the honey harvesting calendar and geographical origin from Ethiopia. each honey type was characterized into pollen grain frequency class based on Melissopalynological (botanical) analysis, sensorial, color, its distinctive harvesting season, and regions. Hence a monofloral honey namely: Guzotia from Nedjo, C. arabica from Haro Sebu, Vernonia from Gedo, S. abyssinica from Bore, C. macrostachyus from Haro Sebu and Eucalyptus honey from Holota was harvested through December, February, February, April, May, and June respectively.
This finding is inconvenient to different investigators. For instance, a study by Bareke and Addi [20], have indicated that honey from S. abyssinica is harvested in April-May and is whitesh in color with an aroma and very pleasant characteristics, whereas Vernonia honey is harvested in February with a dark to a black color and has a very strong flavor and bitter test [21]. Guizotia honey is mostly harvested from November through December and its honey is very thick with a yellow color. The predominant pollen source in honey samples is due to their abundance and nectar potentiality of that area [22].
The current study screened the antibacterial activities of honey samples from different botanical origins. The study indicated that Chloramphenicol (used as a control) had shown statistically (p < 0.0001) more diameter of inhibition zone than all the honey types against tested pathogenic strain. However, C. macrostachyus, Vernonia, Guizotia, and C. arabica honey had statistically more effective and comparable antibacterial activity (p > 0.05) against all the tested pathogenic strains whilst S. abyssinica had shown statistically (p < 0.0001) less effective against all the tested strain compared to other honey types. The grand mean inhibition zone ranged from 17.0 ± 0.9 mm by C. macrostachyus to 10.5 ± 0.9 mm by S. abyssinica.
The overall mean diameter of the inhibition zone of this study was higher than that by Ofijan et al [23] who recorded 10.80 ± 2.63 mm from A. mellifera honey purchased from the market to 14.83 ± 2.45 mm from A. mellifera honey directly harvested from the hive at 75% w/v honey concentration. However, a study by Yalemwork et al [24] reported a higher mean diameter of inhibition zone (18.07 to 21 mm) at 50% v/v honey concentration by A. mellifera yellow and A. mellifera white honey respectively with the same bee species but different plant species and honey concentration. Moreover, higher than the present result was obtained from Cameroonian A. mellifera honey from the market which exhibited 29.3 ± 0.6 mm against E. coli to 37.0 ± 0.0 mm against P. aeruginosa at 75% (w/v) concentration by using agar well diffusion method [19].
Regarding MIC, the current studied honey types had inhibited the growth of the tested pathogenic strains which averagely varied from 11.7 ± 3.3–37.5 ± 1% w/v. The overall mean MIC of honey samples against all the examined bacterial pathogens ranged from 15.8 ± 2.9 (C. macrostachyus) to 31.3 ± 2.4% w/v (S. abyssinica). Gram-positive bacteria were less susceptible and inhibited at low honey concentrations relatively by all the examined honey types than gram-negative bacteria.
The average MIC result of this study is closer or within the values reported by Adgaba et al [11] who obtained an average range from 10.5 ± 0.5–28.6 ± 0.9 of MIC (% v/v), and they also indicated C. macrostachyus and V. amygdalina as a strong inhibitor at low honey concentration against the tested pathogens than other monofloral honey which is comparable with this finding. However, AL-Waili et al [25] recorded relatively higher MIC (32 ± 8.3% to 58 ± 13% v/v) against drug-resistant bacteria than the current result.
The MIC of the current study results was relatively higher than the findings of Alvarez-Suarez et al [26] who reported MIC 2.5–10.4%v/v from Cuban monofloral honey. Less MIC than the current result was reported [27] who obtained 11–20% v/v for P. aeruginosa whilst very closer was recorded [11] which is 10.5 ± 0.9 and 15.5 ± 1.0 by C. macrostachyus and V. amygdalina respectively against S. aureus.
The variation in results could be due to the methods and materials used. For instance, the turbidity of the inoculum suspension, incubation time and temperature, depth of poured medium, the medium used for susceptibility test, the diameter of well formation, and measurement of inhibition zone diameter [28]. Besides, honey qualities and freshness, botanical and entomological origin, geographical sources, and the plant bio and chemo-types of the honey bring a disparity in its antimicrobial and biological activities [29].
In general, the present finding indicated relatively strong antibacterial activities of C. macrostachyus, V. amygdalina, G. scabra, and C. arabica against all the tested pathogenic strains which could be probably due to their medicinal properties. In Ethiopia, different plant part extracts of these plants have been traditionally used in curing different diseases. For example, the bark part of C. macrostachyus is used for treating splenomegaly and gonorrhea [30], whereas its leaves, stem, and fruits extracts are for the treatment of a wide range of human and animal diseases [31]; V. amygdalina leave extract for preventing headache and intestinal worm, and for treating tumor/cancer in general [32], the leaves of G. scabra used as wound treatment [33], seeds of C. arabica for curing diarrhea by pounding and mixing with honey [33]. In Ethiopia, the leaves of V. amygdalina is used to wash materials that are used in preparation of homemade beverage such as “Tella and Tej” and food processing, and preparation.
The susceptibility of S. aureus from the present study to all monofloral honey types compared to all the gram -ve bacteria was comparison with the finding by Adgaba et al [11] and Ofijan et al [23]. The difference in cell wall structure between gram + ve and gram-ve bacteria might be made S. aureus to be inhibited at less honey concentration. The complex cell wall with an additional outer membrane protects the Gram -ve bacteria from external influence by excluding toxic molecules and providing an additional stabilizing layer around the cell [34].