Garlic is one of the most researched medicinal plants in the world and has been used in traditional medicine for many centuries for the treatment of numerous diseases . According to Majewski , garlic’s properties result from the combination of several biologically active substances with antimicrobial properties responsible for its healing effect. That aroused our interest in studying their antimicrobial activities. To that end, we evaluated their susceptibility and drug interaction when associated with the antibiotic penicillin G and ampicillin, both of which belong to the class of β-lactams, against clinical isolates of S. agalactiae.
Penicillin G and ampicillin were used as they are antibiotics of first choice used in the intrapartum prophylaxis of pregnant women . In addition, no studies were found that evaluated the interaction of these antibiotics with garlic or the prospect of finding ways to enhance the action of these antibiotics, which could aid in the prophylaxis and/or treatment of this infection. It is worth noting that our study is the first one of its kind to be published in the literature to date.
The chromatography profile was performed using reverse-phase ultrafast liquid chromatography with UV (RP-UFLC-UV) system, and the fractions with antimicrobial activity found are F18, F19, F20 and F42. The complete phytochemical analyses will be carried out to identify the bioactive compounds presented in these fractions and will be presented in future works.
The SP80 was used instead of the isolated fractions as mentioned above, according to Majewski , garlic’s properties result from the combination of several biologically active substances with antimicrobial properties.
The methods most used for assessing antimicrobial activity are disk diffusion and broth microdilution [20–24]. The disk diffusion method was used because it is a quantitative method that also enables a synergistic assessment between SP80, and the antibiotics used in our study.
The Minimum inhibitory concentration (MIC) of SP80 obtained by purifying the raw garlic extract (RGE) on a Sep-Pak® C18 cartridge column was determined by the broth microdilution, in triplicate. The MIC used in our tests was the lowest value that completely inhibited microbial growth and exhibited a bacteriostatic action . As our work is pioneering, different extraction methods could lead to distinct biological responses. For this reason, in our study, the MIC established for SP80 was 2.40 mg/mL. This concentration was used in the disk diffusion tests.
When determining the MIC value by using the microdilution method, it was demonstrated that this garlic fraction has antimicrobial activity against S. agalactiae. This is an important finding that corroborates all other studies described in the literature regarding this plant’s antimicrobial activity [20, 26–29].
According to the CLSI , inhibition halos ≥ 24 mm formed by penicillin G and ampicillin demonstrate that the strain tested against these antibiotics, respectively, is sensitive. We compared the antimicrobial effect of penicillin G and ampicillin, isolated, respectively, against S. agalactiae strains. Of the 56 strains tested, 25 strains were resistant to penicillin G and another 2 were resistant to ampicillin, thus demonstrating that the strains tested are more resistant to penicillin G than they are to ampicillin. There is a dearth of information regarding the mechanism behind reduced penicillin g susceptibility in GBS. However, Chu et al.  believe that modification of penicillin-blinding proteins is a possible explanation for the reduced penicillin G susceptibility seen with their GBS isolates. Nevertheless, further molecular work is needed for confirmation.
Garlic has a variety of bioactive compounds, including organosulfur compounds, saponins, phenolic compounds, and polysaccharides. The main ones are organosulfur compounds, especially allicin [32, 33]. According to Choo et al. , garlic’s antibiotic activity is mainly attributable to allicin, which acts on the destruction and inhibition of Gram-positive and negative bacteria.
Cuttler et al.  evaluated the in vitro antibacterial activity of an allicin aqueous extract and a gel formulation incorporating allicin against 76 S. agalactiae strains. By the results, the allicin in gel produced inhibition halos of 23.00 ± 6 mm, whereas allicin in water, 21.00 ± 6 mm. In our study, a combination of garlic compounds was used instead of an isolated substance as allicin.
Arzanlou  assessed the antimicrobial activity of allicin against Group A Streptococcus (GAS) by the microdilution method. The results showed that allicin strongly inhibits the maturation of SpeBz and proteolytic activity of SpeBm in a concentration-dependent manner. This study differs from ours in its methodology and in using the compounds allicin isolated from garlic, but both studies corroborate the antimicrobial action attributed to garlic.
In a North American research cited by Bontempo  the author presented the results of microbial growth inhibition for 14 bacterial species using a dilution of fresh garlic extract. Marchese et al.  carried out a review based on the antifungal and antibacterial activities of A. sativum L. and its main compound, allicin. Our findings also corroborate these studies, to confirm the antimicrobial activity of A. sativum L.
Hayes et al.  carried out checkerboard and time-kill assays in vitro for determining the synergistic activity of erythromycin and nisin against clinical isolates of Group B Streptococcus against invasive and colonizing GBS strains. Their results suggest that erythromycin and nisin can act synergistically to inhibit the growth of GBS. This study differs from ours in relation to the antibiotics used and the methodology.
From this research, the fraction SP80 from A. sativum L. has in vitro antimicrobial activity against the bacteria S. agalactiae, with a MIC of 2.40 mg/mL; its association with the antibiotic ampicillin showed a synergistic effect, which did not occur when in association with penicillin G.
Based on our results, we believe that further studies should be carried out to pave the way for the development of translational research, given that this drug interaction in humans can prove beneficial in the therapy for S. agalactiae infections.