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Jundishapur J Microbiol. 2015 May; 8(5): e14814. DOI: 10.5812/jjm.14814
Published online 2015 May 31. Research Article
Fresh Garlic Extract Enhances the Antimicrobial Activities of Antibiotics on
Resistant Strains in Vitro
Guoliang Li 1; Xudong Ma 1; Lisha Deng 2; Xixi Zhao 2; Yuejiao Wei 1; Zhongyang Gao 2; Jing Jia 1;
Jiru Xu 2,*; Chaofeng Sun 1
1The First Affiliated Hospital, Xi’an Jiaotong University Health Science Center, Xi’an, China
2Department of Pathogen Biology and Immunology, College of Basic Medicine, Xi’an Jiaotong University Health Science Center, Xi’an, China
*Corresponding author: Jiru Xu, Department of Pathogen Biology and Immunology, College of Basic Medicine, Xi’an Jiaotong University Health Science Center, Xi’an, China.
Tel/Fax: +86-2985323805, E-mail: xujiru@mail.xjtu.edu.cn
Received: September 13, 2013; Revised: April 13, 2014; Accepted: April 20, 2014
Background: Infections caused by strains with multi-drug resistance are difficult to treat with standard antibiotics. Garlic is a powerful
remedy to protect against infections of many bacteria, fungi and viruses. However, little is known about the potentials of fresh garlic
extract (FGE) to improve the sensitivity of multi-drug resistant strains to antibiotics.
Objectives: In this study, we used the disk diffusion method to investigate the antimicrobial activities of FGE and the combination of
antibiotics with FGE, on methicillin-resistant Staphylococcus aureus (MRSA), Pseudomonas aeruginosa and Candida albicans, to evaluate the
interactions between antibiotics and FGE.
Materials and Methods: Clinical isolates were isolated from clinical specimens obtained from the inpatients at the First Affiliated
Hospital of Xi’an Jiaotong University Health Science Center. The isolates consisted of MRSA, (n = 30), C. albicans (n = 30) and P. aeruginosa (n
= 30). Quality control for CLSI (Clinical and Laboratory Standards Institute) disk diffusion was performed using S. aureus ATCC®25923, C.
albicans ATCC®90028 and P. aeruginosa ATCC®27853. The 93 microorganisms were divided into four groups in a factorial design: control
(deionized water), FGE, antibiotics without FGE, and antibiotics with FGE. Next, antibacterial activity was evaluated by measuring the
diameter of inhibition zones according to performance standards for antimicrobial susceptibility testing of the Clinical and Laboratory
Standards Institute (CLSI, formerly NCCLS).
Results: Fresh garlic extract displayed evident inhibition properties against C. albicans and MRSA, yet weak inhibition properties against
P. aeruginosa. Additionally, FGE showed the potential to improve the effect of antibiotics on antibiotic resistant pathogens. The synergism
of fluconazole and itraconazole with FGE on C. albicans yielded larger sized inhibition zones compared with fluconazole and itraconazole
without FGE (P < 0.01). The factorial analysis represents intense positive interaction effects (P < 0.01). The synergism of cefotaxime and
ceftriaxone with FGE on P. aeruginosa yielded larger sized inhibition zones than cefotaxime and ceftriaxone without FGE (P < 0.01). The
factorial analysis represents intense positive interaction effects (P < 0.01).
Conclusions: The results suggest that FGE can improve the antibiotic sensitivity of these pathogens to some antibiotics.
Keywords: Garlic; Drug Resistance, Microbial; Candida albicans; Methicillin-Resistant Staphylococcus aureus; Pseudomonas aeruginosa
Copyright © 2015, Ahvaz Jundishapur University of Medical Sciences. This is an open-access article distributed under the terms of the Creative Commons Attribu-
tion-NonCommercial 4.0 International License (http://creativecommons.org/licenses/by-nc/4.0/) which permits copy and redistribute the material just in noncom-
mercial usages, provided the original work is properly cited.
1. Background
With antibacterial drugs being widely used in clinical
settings, many microorganisms, especially methicillin-
resistant Staphylococcus aureus (MRSA), Pseudomonas ae-
ruginosa and Candida albicans, have adapted to synthetic
antibiotics and become highly resistant to these drugs
over time (1-4). Microorganisms with multi-drug resis-
tance now cause thousands of deaths throughout the
world each year (1, 3). Although some of these organisms
can live harmlessly in humans and are carried in the na-
sal passage and on the skin, they can cause fatal infection
in hospitals and nursing homes, where patients with
open wounds, invasive devices and immunodeficiency
are at higher risk of infection than healthy people (5, 6).
Furthermore, resistance does make the infection more
difficult to treat with standard antibiotics and thus more
dangerous (7, 8). Therefore, the continuing spread of
multi-drug resistant strains and the increased abuse of
antibiotics highlight the need for alternative agents.
Garlic has been found to help prevent many diseases.
Numerous modern studies confirm that garlic has defi-
nite antibiotic properties and is effective against a wide
spectrum of bacteria, fungi and viruses (9, 10). In addi-
tion, the antimicrobial activities of garlic are linked to
the presence of some bioactive compounds (11). More-
over, many studies have demonstrated that garlic can be
more effective as a broad-spectrum antibiotic compared
Li G et al.
Jundishapur J Microbiol. 2015;8(5):e148142
with conventional antibiotics. However, most previous
studies have only focused on the antimicrobial activities
of garlic and garlic-derived organ sulfur compounds or
the difference between garlic or garlic-derived organ sul-
fur compounds and standard antibiotics, while little is
known about the potential of fresh garlic extract (FGE) to
improve the susceptibility of multi-drug resistant strains
to conventional antibiotics.
2. Objectives
The present study aimed to investigate the antimicro-
bial activities of FGE and the combination of FGE and
conventional antibiotics on common clinical strains,
including MRSA, multi-drug resistant P. aeruginosa and C.
albicans, to evaluate the interactions between antibiotics
and FGE.
3. Materials and Methods
3.1. Fresh Garlic Extract
Fresh garlic bulbs (Chinese white garlic) were pur-
chased from the Northwest Agriculture Forestry Univer-
sity (Yangling, China). Peeled garlic bulbs (100 g) were
blended in 50 mL sterile distilled water. The mixture was
crushed finely using a juicer. The resulting paste was cen-
trifuged at 3000 rmp for 30 minutes and the supernatant
was then sterilized by a lter (0.2 μm pore size, Steriip,
Millipore). The final concentration of FGE in aqueous so-
lution was determined to be 40.7% (w/v) by subtracting
the weight of the precipitate from the weight of the origi-
nal peeled garlic bulbs. The FGE was stored in 1.5 mL mi-
cro test tubes at -20°C until used.
3.2. Microbial Strains
A total of 90 clinical isolates and three control strains
were used. The clinical isolates were isolated from clini-
cal specimens obtained from the inpatients at the First
Affiliated Hospital of Xi’an Jiaotong University Health Sci-
ence Center. The isolates consisted of MRSA, (n = 30), C.
albicans (n = 30) and P. aeruginosa (n = 30). Quality control
for CLSI (Clinical and Laboratory Standards Institute) disk
diffusion was performed using S. aureus ATCC®25923, C.
albicans ATCC® 90028 and P. aeruginosa ATCC® 27853. All
isolates were identified at the strain level using the Vitek
2 automatic system (Bio Merieux Company, Marcy l'Etoile,
France) and showed multiple antibiotic resistances, and
had no apparent epidemiological connection. The isolat-
ed strains were then transported to nutrient agar slants
and stored at 2-8 °C until use. Purity of the organisms was
checked at regular intervals by plating and staining. The
93 microorganisms were divided into four groups in a
factorial design: control (deionized water), FGE, antibiot-
ics without FGE, and antibiotics with FGE.
3.3. Antimicrobial Susceptibility Test
The following antibiotic-containing paper disks (Oxoid,
UK) were used: 8 μg itraconazole (I), 15 μg uconazole (F),
30 µg cefoxitin (FOX), 1 µg oxacillin (OX), 100 µg piperacil-
lin (PRL), 30 µg cefotaxime (CTX), 5 µg levofloxacin (LEV),
30 µg ceftriaxone (CRO), 10 µg ampicillin (AMP) and 30 µg
cefazolin (KZ). The 0.5 McFarland standard suspension of
germ solution was inoculated onto Mueller Hinton agar
(Becton Dickinson, Sparks, MD, USA) supplemented with
2% glucose and 0.5 g/ml methylene blue. Fifteen minutes
after the agar absorbed the bacterial suspension; the
antibiotic-containing paper disk on Muller-Hinton agar
plates was placed onto the surface of the inoculated plate
(90 mm) with sterile forceps to investigate the antibiotic
activity. Next, 50 μL of FGE was pipetted on the Whatman
No. 1 filter paper discs (6 mm in diameter) and antibiotic-
containing paper disk to evaluate the activities of FGE
and the combinative activities of antibiotics and FGE.
Meanwhile, 50 μL of sterile distilled water was also pipet-
ted onto Whatman No. 1 filter paper discs as the control.
The plates were left on a flat bench for one hour after the
paper disk absorbed the solution. Inoculated plates were
then incubated at 37°C for 24 hours. The antibacterial ac-
tivity was then evaluated by measuring the diameter of
inhibition zones according to Performance Standards for
Antimicrobial Susceptibility Testing of The Clinical and
Laboratory Standards Institute (CLSI, formerly NCCLS).
3.4. Reproducibility and Statistics
All measurements were repeated three times and each
strain was examined at least three separate times. Statisti-
cal analysis was conducted using the SPSS software version
18.0 (SPSS Inc, Chicago IL). Means that were significantly dif-
ferent were separated using the one-way ANOVA. The facto-
rial experiment was analyzed to evaluate the main effects
and interactions between FGE and antibiotics; data were
analyzed by two-way ANOVAs using the same program. P
values of 0.05 were considered to be significant.
4. Results
4.1. Candida albicans
The antifungal susceptibility of C. albicans to F and I
with FGE and without FGE is summarized in Figure 1. The
ranking was as follows, I with FGE ＞ FGE ＞ I without FGE
＞ control; and F with FGE ＞ FGE ＞ F without FGE ＞ con-
trol. The data showed that I and F with FGE could lead to
an increase in the size of the inhibition zones against C.
albicans compared with I and F without FGE (P < 0.01). The
factorial analysis represents intense positive interaction
effects (P < 0.01) (data is not shown). The fungistatic ac-
tivity of I and F was dramatically enhanced by addition
of FGE, and the synergism of I and F with FGE yielded an
obvious increase in the size of inhibition zones, 29.0 mm
and 30.5 mm, respectively (Figure 1).
Li G et al.
3
Jundishapur J Microbiol. 2015;8(5):e14814
Figure 1. In Vitro Antifungal Activity of Fluconazole and Itraconazole
With and Without Fresh Garlic Extract Against Candida albicans
(A) Representative zones of antibacterial activity of F and I with and with-
out FGE against C. albicans in vitro. 1, control; 2, I; 3, F; 4, FGE; 5, I + FGE; 6, F
+ FGE. (B) The analysis of antibacterial activity of F and I with and without
FGE against C. albicans in vitro. * P value of < 0.05 indicates a significant
difference from the respective antibiotic without FGE.
4.2. Methicillin-Resistant Staphylococcus aureus
The antibacterial susceptibility of MRSA to FOX, OX and
PRL with and without FGE is summarized in Figure 2. The
ranking was as follows: FGE with FOX ＞ FGE ＞ FOX with-
out FGE ＞ control; and FGE with OX ＞ FGE ＞ OX without
FGE ＞ control; FGE with PRL ＞ FGE ＞ PRL without FGE
＞ control. Although FOX, OX and PRL with FGE produced
larger sized inhibition zones against MRSA compared
with FOX, OX and PRL without FGE (P < 0.01), the factorial
analysis indicated no intense positive interaction effects
(P > 0.05) (data is not shown).
4.3. Pseudomonas aeruginosa
The antibacterial susceptibility of P. aeruginosa to CTX,
LEV, CRO, KZ and AMP with and without FGE is summa-
rized in Figure 3. The ranking was as follows: FGE with
CTX ＞ FGE ＞ CTX without FGE ＞ control; FGE with LEV
Figure 2. In Vitro Antifungal Activity of Cefoxitin, Oxacillin and Piperacil-
lin With and Without Fresh Garlic Extract Against Methicillin Resistant
Staphylococcus aureus
(A) Representative zones of antibacterial activity of FOX, OX and PRL with
and without FGE against MRSA in vitro; 1, FGE; 2, OX; 3, FOX; 4, PRL; 5, OX +
FGE; 6, FOX + FGE; 7, PRL+FGE. (B) The analysis of antibacterial activity of
FOX, OX and PRL with and without FGE against MRSA in vitro. * P values of
< 0.05 indicate significant difference from the respective antibiotic with-
out FGE.
＞ FGE ＞ LEV without FGE ＞ control; FGE with CRO ＞
FGE ＞ CRO without FGE ＞ control; FGE with KZ ＞ FGE ＞
KZ without FGE ＞ control; FGE with AMP ＞ FGE ＞ AMP
without FGE ＞ control. The data showed that CTX, LEV,
CRO, KZ and AMP with FGE could produce larger sized in-
hibition zones against P. aeruginosa compared with CTX,
LEV, CRO, KZ and AMP without FGE. The anti-microbial ac-
tivity of CTX and CRO was dramatically enhanced by ad-
dition of FGE, CTX and CRO with FGE and yielded larger
sized inhibition zones compared with CTX and CRO with-
out FGE (Figure 3). The factorial analysis represents an
intense positive interaction effect between FGE and CTX
and between FGE and CRO (P < 0.0 1) (data is not shown).
Furthermore, LEV, KZ and AMP with FGE could also pro-
duce larger sized inhibition zones against P. aeruginosa
compared with LEV, KZ and AMP without FGE. However,
the factorial analysis indicated no intense positive inter-
action effects (P > 0.05) (data is not shown).
Li G et al.
Jundishapur J Microbiol. 2015;8(5):e148144
Figure 3. In Vitro Antifungal Activity of Cefoxitin, Levofloxacin, Ceftri-
axone, Cefazolin and Ampicillin With and Without Fresh Garlic Extract
Against Pseudomonas aeruginosa
(A) Representative zones of antibacterial activity of FGE against P. aerugi-
nosa in vitro; 1, FGE; 2, CRO; 3, AMP; 4, KZ; 5, LEV; 6, CTX; 7, CTX; 8, AMP; 9, KZ;
10, LEV; 11, CRO. (B) The analysis of antibacterial activity in vitro of FOX, LEV,
CRO, KZ and AMP with and without FGE against P. aeruginosa. * P values
of < 0.05 indicate significant difference from the respective antibiotic
without FGE.
5. Discussion
In this study, we found that FGE displayed inhibition
properties against C. albicans and MRSA and weak inhi-
bition properties against P. aeruginosa. In addition, FGE
could help improve the antibiotic susceptibility of these
strains to some traditional antibiotics. Fresh Garlic Ex-
tract may be a candidate for the treatment of infections
by multi-drug resistant strains. Our results suggest that
FGE helps improve the antibiotic resistance of pathogens
to some antibiotics. With the increased abuse of synthet-
ic antibiotics and the continuing spread of strains with
multi-drug resistance, the need for alternative agents is
urgent. Garlic has been demonstrated to be a powerful
remedy to protect against infections of many bacteria,
fungi and viruses (12-14). Of all its reputed benefits, one
significant advantage of garlic is its effectiveness against
nosocomial strains that frequently display above average
resistance to many antibiotics. Garlic contains various
active components that work in complex ways. Some of
these components can work together in the body to pro-
tect against infections. Of all the biotical ingredients, al-
licin, an organ sulfur compound, is regarded as the para-
mount antibacterial agent in crushed garlic extracts and
exhibits protective effects against attacks by pests (14, 15).
However, allicin is rapidly oxidized, unstable and vola-
tile, meaning it rapidly breaks down after raw garlic is
cracked. It has been reported that garlic extract has more
potent anti-staphylococcal activity than an equal amount
of allicin. This may be because a water-based extract of
garlic stabilizes allicin, at least partially, due to the hy-
drogen bonding between water and the reactive oxygen
atom in illicit that lessens its instability and/or there may
be water-soluble ingredients in cracked garlic that desta-
bilize the molecule.
Systemic fungal infections induced by C. albicans have
emerged as the main criminal of morbidity and mortal-
ity in immunocompromised patients (1). Some research-
ers reported on the antifungal activity of garlic in vitro
against C. albicans (9, 10). In this study, C. albicans was
resistant to F and I without FGE but susceptible to FGE.
Fresh garlic extracts showed a powerful inhibitory effect
against C. albicans compared with F and I; the fungistatic
activity of F and I was dramatically enhanced by addition
of FGE. The factorial analysis of F or I and FGE indicated in-
tense positive interaction effects (P < 0.01). Thus it can be
suggested that FGE can distinctly improve the sensitivity
of C. albicans to F or I. An et al. (16) suggested that allicin
could enhance the activity of AmB against C. albicans in
vitro and in vivo. Another study showed that a combina-
tion of F and allicin exhibited a good synergism against
C. albicans. Some underlying mechanisms have been sug-
gested by previous studies. Low et al. (10) found that gar-
lic and its bioactive ingredients could suppress hyphae
generation and affect the expression level of SIR2 gene.
Yousuf et al. (17) affirmed that both diallyl sulfide (DAS)
and diallyl disulfide (DADS) in garlic significantly inhibit-
ed proteinase, phospholipase secretion and dimorphism
in C. albicans.
Methicillin-resistant Staphylococcus aureus is often
considered to be as a "superbug" (12). It was estimated
that the number of MRSA infections in hospitals has in-
creased significantly and the annual deaths from MRSA
infections are even more than AIDS (18, 19). Garlic has
been scientifically proven to be a powerful natural antibi-
otic against MRSA infections (12, 14). Ingredients in fresh
garlic, other than illicit, have strong natural antibiotic ef-
fects (12). Garlic extract, DAS and DADS provide powerful
protective activity against MRSA by affecting the patho-
gen distribution and plasma levels of pro-inflammatory
cytokines, endothelial injury-associated proteins, and
coagulation and anti-coagulation factors as well as lipid
oxidation levels, and by boosting the immune system. In
this study, FGE produced a strong antibacterial effect on
all MRSA resistant to standard antibiotics, FOX, OX and
PRL. However, the factorial analysis of FOX, OX or PRL and
FGE indicated no positive interaction effects (P > 0.05);
there exists no FGE antibiotic resistance-modifying activ-
Li G et al.
5
Jundishapur J Microbiol. 2015;8(5):e14814
ity against MRSA. The antibacterial effect of the combina-
tion of FOX, OX or PRL and FGE is only attributed to FGE.
Pseudomonas aeruginosa contributes to chronic oppor-
tunistic infections, which can be fatal for immunocom-
promised patients and the elderly (5, 20, 21). Biofilms of
P. aeruginosa protect these strains from adverse environ-
mental factors and enable the unique ability of P. aeru-
ginosa to evade host innate immune defenses and the
intrinsic resistance to many antibiotics (20, 22, 23). The
antimicrobial activity of garlic against P. aeruginosa has
been widely recognized. Garlic-treated biofilms were
susceptible to both tobramycin and polymorphonuclear
leukocytes (PMNs) grazing. Furthermore, the PMNs incu-
bated with garlic-treated biofilms showed an increase in
respiratory burst activation. The garlic treatment initial-
ly provoked a higher degree of inflammation and signifi-
cantly improved clearing of the infecting bacteria (24).
In our experiment, the interaction effect of CTX, LEV,
CRO, KZ and AMP with FGE was evaluated (Figure 3). The
data showed that CTX, LEV, CRO, KZ and AMP with FGE
could produce larger sized inhibition zones against P.
aeruginosa compared with CTX, LEV, CRO, KZ and AMP
without FGE. The factorial analysis indicated an intense
positive interaction effect between FGE and CTX or CRO
(P < 0.01) (data is not shown). Although the LEV, KZ and
AMP with FGE could produce larger sized inhibition
zones against P. aeruginosa compared with LEV, KZ and
AMP without FGE, the factorial analysis indicated no in-
tense positive interaction effects (P > 0.05) (dates is not
shown). The combination failed to efficiently inhibit the
bacteria and the factorial analysis indicated no intense
positive interaction effects (P > 0.05). These findings are
not in accordance with previous studies (24). This may be
due to the variability of the strains or differences among
species of garlic.
In this study, the results indicated that FGE has inhibi-
tion properties against C. albicans and MRSA but weak
inhibition properties against P. aeruginosa, while it had
the potential to improve the effect of antibiotics on an-
tibiotic resistant pathogens. Fresh Garlic Extract may be
used to aid the treatment of infections from multi-drug
resistant strains. In addition, further efforts are needed
to elucidate the molecular mechanisms underlying the
synergistic effect between antibiotics and FGE in vitro.
Acknowledgements
The authors acknowledge Ning Zhang, the First Affili-
ated Hospital, Xi’an Jiaotong University Health Science
Center, Long Mei and the Department of Pathogen Biolo-
gy and Immunology. The Xi’an Jiaotong University Health
Science Center is further acknowledged for the generous
supply of tested strains.
Authors’ Contributions
Study concept and design: Guoliang Li, Xudong Ma, Li-
sha Deng, Xixi Zhao, Yuejiao Wei, Zhongyang Gao, Jing Jia,
Chaofeng Sun and Jiru Xu. Analysis and interpretation of
data: Guoliang Li, Xudong Ma, Lisha Deng, Xixi Zhao, Yue-
jiao Wei, Zhongyang Gao, Jing Jia, Chaofeng Sun and Jiru
Xu. Drafting of the manuscript: Xudong Ma, Chaofeng
Sun, and Jiru Xu. Critical revision of the manuscript for
important intellectual content: Chaofeng Sun, and Jiru
Xu. Statistical analysis: Xudong Ma.
Funding/Support
This research was supported by a faculty grant from the
National College Student Innovative Experiment Project
of China (610734).
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