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In Vitro antibacterial and antibiotic-potentiation activities of four edible plants against multidrug-resistant gram-negative species

  • Jaurès AK Noumedem1, 2,
  • Marius Mihasan2,
  • Jules R Kuiate1Email author,
  • Marius Stefan2,
  • Dumitru Cojocaru2,
  • Jean P Dzoyem1 and
  • Victor Kuete1Email author
BMC Complementary and Alternative MedicineThe official journal of the International Society for Complementary Medicine Research (ISCMR)201313:190

DOI: 10.1186/1472-6882-13-190

Received: 7 March 2013

Accepted: 24 July 2013

Published: 25 July 2013

Abstract

Background

The present study was designed to investigate the antibacterial activities of the methanol extracts of four Cameroonian edible plants, locally used to treat microbial infections, and their synergistic effects with antibiotics against a panel of twenty nine Gram-negative bacteria including Multi-drug resistant (MDR) phenotypes expressing active efflux pumps.

Methods

The broth microdilution method was used to determine the minimum inhibitory concentrations (MICs) of the extracts [alone and in the presence of the efflux pumps inhibitor (EPI) Phenylalanine-Arginine β- Naphtylamide (PAβN)], and those of antibiotics in association with the two of the most active ones, Piper nigrum and Telfairia occidentalis. The preliminary phytochemical screening of the extracts was conducted according to the standard phytochemical methods.

Results

Phytochemical analysis showed the presence of alkaloids and flavonoids in all studied extracts. Other chemical classes of secondary metabolites were selectively present in the extracts. The results of the MIC determination indicated that the crude extracts from P. nigrum and V. amygdalina were able to inhibit the growth of all the twenty nine studied bacteria within a concentration range of 32 to 1024 μg/mL. At a similar concentration range (32 to 1024 μg/mL) the extract from T. occidentalis inhibited the growth of 93.1% of the tested microorganisms. At MIC/2 and MIC/5, synergistic effects were noted between the extracts from P. nigrum and T. occidentalis and seven of the tested antibiotics on more than 70% of the tested bacteria.

Conclusion

The overall results of the present study provide information for the possible use of the studied edible plants extracts in the control of bacterial infections including MDR phenotypes.

Keywords

Antibacterial activities Edible plants Gram-negative bacteria Multidrug resistance Efflux pumps

Background

Despite the impressive scientific progress in vaccination and chemotherapy, infectious diseases remain a serious health issue. Following the massive and inappropriate use of antibiotics, bacteria have developed various mechanism of resistance; consequently, infectious diseases remain one of the leading causes of morbidity worldwide [1]. Microbial infections constitute a major public health problem in developing countries [2] where the high cost of antibiotics makes them unaffordable to the majority of the population. Therefore, the discovery of new antimicrobial agents is still relevant nowadays. Among the bacterial resistance mechanisms, efflux of antibiotics plays an important role; In fact it is widely recognized that the expression of efflux pumps encoded by house-keeping genes in bacteria is largely responsible for the phenomenon of intrinsic antibiotic resistance [3]. Also, the shortcomings of the drugs available today and the scarcity of novel antibiotics propel the discovery of new chemotherapeutic agents from medicinal plants [4]. The medicinal properties of many phytochemicals have been demonstrated [5]. In addition, promising new concepts such as the efflux pump inhibitors [6, 7], and synergy between antibiotics and phytochemicals are now been explored.

The present work was therefore designed to investigate the antibacterial potential of four Cameroonian edible plants used traditionally in the treatment of bacterial infections, namely the fruits of Piper nigrum L (Piperaceae), the leaves of Telfairia occidentalis Hook. F. (Cucurbitaceae) and Vernonia amygdalina Del. (Asteraceae) and the fruits of Syzygium aromaticum [L.] Merr & Perry (Myrtaceae) against MDR bacteria expressing active efflux via the Resistance-Nodulation Cell Division (RND)-type pumps.

Methods

Plant material and extraction

The four edible plants used in this work were purchased from Dschang local market, West Region of Cameroon in June 2010. The collected plants material were the fruits of Piper nigrum, the fruits of Syzygium aromaticum, the leaves of Telfairia occidentalis and the leaves of Vernonia amygdalina. These plants were identified by M. Victor Nana of the National Herbarium (Yaounde-Cameroon) where all the voucher specimens were available under the reference numbers (see Table 1). The air dried and powdered sample (1 kg) from each plant was extracted with methanol (MeOH) for 48 h at room temperature. The extracts were then filtered and concentrated under reduced pressure to give the crude extracts. All extracts were kept at 4°C until further investigations.
Table 1

Plants used in the present study and evidence of their bioactivities

Plant (family); and voucher numbera

Traditional uses

Parts used

Bioactive or potentially bioactive components

Bioactivities of extracts and/or compounds

Piper nigrum L. (Piperaceae); 25818/SFRcam

Cardiovascular diseases, intoxication, inflammation, bacterial, fungal and parasitic infections, respiratory diseases, asthma [8]

Seeds, bark, leaves

Piperine, pipene [9], piperamides, piperamine [10], pellitorine [11]

Anti-apoptotic [12, 13], antibacterial [8, 14], antidepressant [15] antifungal [16], analgesic, anti-inflammatory [17], antidiarrhoeal [18], antimutagenic, antioxidative, increase plasma [19], antipyretic [17], immuno-modulatory, antispasmodic [20, 21], asthma, obesity, sinus antispermatogenic, antithyroid, antitumor Ciprofloxacin potentiator, transcription inhibitor, insecticidal, hepatoprotective, increase pancreatic enzymes, Cytochrome Inhibitor [8]

Syzygium aromaticum (Myrtaceae) 28524/HNC

Aphrodisiac, used to treat male sexual disorders [22, 23], anti-inflammatory, bacterial infections [24], microbial infections [25, 26]

fruits

Eugenol (2-methoxy-4-(2-propenyl) phenol), glycosides, flavonoids, saponins and tannins [23], essential oils

Antipyretic, antispasmodic [27], anticarcinogenic [28], inhibition of 5-Lox enzyme activity in human polymorphonuclear leukocytes cells [29], antioxidant, protection against peroxynitrite-mediated tyrosine nitration and lipid peroxidation [29], antifungal activity of essential oil [26] and antimicrobial [25], antibacterial [30]

Telfairia occidentalis ( Curcubitaceae); 33423/HNC

Microbial infections, cholesterolemia, liver problems and impaired defense immune systems [31]

Leaves, seeds, roots

Phenols, alkaloids and tannins [32]

Antimicrobial, antioxidant and free radical-scavenger [32, 33], antiplasmodial, cure lactating properties, hypoglycemic and antidiabetic [31]

Vernonia amygdalina Del. ( Asteraceae ); 31149/SRFK

Microbial infections [34], hiccups, kidney and stomach problems, discomfort [35], stomach-ache and gastrointestinal infections, malarial fever , cough remedy [36], anti-malarial, purgative, anti-parasitic, eczema blood glucose levels control [37], treatment of eczema [37]

Leaves, roots

Flavonoids, saponins and alkaloids [36], vernodalin, vernomygdin, vernonioside B1 and vernoniol B1 [38]

Active anticancer [39], antimalarial and antiparasitic agents [40], Hypoglycaemic [41], antimicrobial, antibacterial [35, 40, 42], antihelminthic, anti-shitosomal, tumor inhibitor [38], hypolipidaemic and antioxidant properties [43]

a(HNC): Cameroon National Herbarium; (SRFC): Société des réserves forestières du Cameroun.

Preliminary phytochemical investigations

The major classes of secondary metabolites such as alkaloids, anthocyanins, anthraquinones, flavonoids, phenols, saponins, tannins, sterols and triterpenes were screened according to the common phytochemical methods described by Harbone [44].

Bacterial strains and culture media

The studied microorganisms included the reference (from the American Type Culture Collection) and clinical (Laboratory collection) strains of Providencia stuartii, Pseudomonas aeruginosa, K. pneumoniae, Escherichia coli, Enterobacter aerogenes and Enterobacter cloacae (See supporting information Additional file 1: Table S1 for their features). They were maintained in a Nutrient Broth at 4°C and activated on a fresh appropriate Mueller Hinton Agar plates 24 h prior to antimicrobial test. The Mueller Hinton Broth (MHB) was also used for all the antibacterial assays.

Chemicals for antimicrobial assays

Tetracycline (TET), cefepime (FEP), cloxacillin (CLX), streptomycine (STR), ciprofloxacine (CIP), norfloxacine (NOR), chloramphenicol (CHL), cloxacillin (CLX), ampicillin (AMP), erythromycin (ERY), kanamycin (KAN) and streptomycin (STR) (Sigma-Aldrich, St Quentin Fallavier, France) were used as reference antibiotics. p-Iodonitrotetrazolium chloride (INT) and Phenylalanine Arginine β-naphthylamide (PAβN) were used as microbial growth indicator and efflux pumps inhibitor (EPI) respectively.

Bacterial susceptibility determination

The MICs were determined using the rapid INT colorimetric assay [45, 46]. Briefly, the test samples were first dissolved in DMSO/MHB. The solution obtained was then added to MHB, and serially diluted two fold (in a 96- wells microplate). One hundred microlitres (100 μL) of inoculum (1.5 × 106 CFU/mL) prepared in MHB was then added. The plates were covered with a sterile plate sealer, then agitated to mix the contents of the wells using a shaker and incubated at 37°C for 18 h. The final concentration of DMSO was set at 2.5% (a concentration at which DMSO does not affect the microbial growth). Wells containing MHB, 100 μL of inoculum and DMSO at a final concentration of 2.5% served as negative control (this internal control was systematically added). Chloramphenicol was used as reference antibiotic. The MICs of samples were detected after 18 h incubation at 37°C, following addition of 40 μL of a 0.2 mg/mL INT solution and incubation at 37°C for 30 minutes. Viable reduce the yellow dye to pink. MIC was defined as the lowest sample concentration that exhibited complete inhibition of microbial growth and then prevented this change [47].

Samples were tested alone and the best three extracts (those from the seeds of P. nigrum, T. occidentalis and V. amygdalina) were also tested in the presence of PAβN at 30 mg/L final concentration. After a preliminary assay on one of the MDR bacteria, P. aeruginosa PA124 (See supporting information Additional file 1: Table S2), the two best extracts were those from P. nigrum and T. occidentalis. They were then selected and tested at MIC/2 and MIC/5 in association with antibiotics. Fractional inhibitory concentration (FIC) was calculated as the ratio of MICAntibiotic in combination/MICAntibiotic alone and the results were discussed as follows: synergy ( 0.5), indifferent (0.5 to 4), or antagonism (> 4) [48, 49]. All assays were performed in triplicate.

Results

Phytochemical composition and antibacterial activity of the extracts

The results of the qualitative phytochemical analysis showed that each of the tested plant extract contains at least 3 classes of secondary metabolites (Table 2). The antibacterial activities of the extracts alone and in some cases in combination with PAβN on a panel of 29 Gram-negative bacteria are depicted in Table 3. It appears that extracts from P. nigrum and V. amygdalina inhibited the growth of all the twenty nine tested bacterial strains within a concentration range from 32 to 1024 μg/mL. A good spectrum of antibacterial activity was also recorded with the extract of T. occidentalis, its inhibitory effects being observed against 27/29 (93.1%) of the tested microorganisms. The lowest MIC value (32 μg/mL) was obtained with the extract of P. nigrum on P. aeruginosa PA01.
Table 2

Extraction yields, aspects and phytochemical composition of the plant extracts

Scientific names

Part used

Yield* (%)

Physical aspect

Secondary metabolites

 
    

Alkaloids

Phenols

Tannins

Triterpenes

Steroids

Flavonoids

Anthraquinones

Anthocyanins

Saponins

Piper nigrum

Fruits

13.18

Brown sticky paste

+

+

+

-

-

+

+

-

-

Syzygium aromaticum

Fruits

9.49

Dark brown paste

+

-

-

-

-

+

-

-

-

Telfairia occidentalis

Leaves

11.58

Brown sticky paste

+

+

+

-

-

+

-

+

-

Vernonia amygdalina

Leaves

7.16

Green dark paste

+

+

+

-

+

+

-

-

+

(+): Present; (−): Absent; *the yield was calculated as the ratio of the mass of the obtained methanol extract/mass of the plant powder.

Table 3

Minimal Inhibitory Concentration (MIC) in μg/mL of methanol extracts from the studied plants and chloramphenicol

Bacteria strains

Plant extracts and MICs (μg/mL)

 

Piper nigrum

Syzygium aromaticum

Telfairia occidentalis

Vernonia amygdalina

Chloramphenicol

E. coli

     

 ATCC8739

128

1024

512

512

4

 ATCC10536

256

-

512

512

4

 W3110

256

512

256

512

8

 MC4100

256

1024

512

512

16

 AG100A

128 (64)

-

512 (512)

512 (512)

< 2 (< 2)

 AG100Atet

256 (256)

-

512 (512)

1024 (1024)

64 (< 2)

 AG102

512 (128)

-

1024 (1024)

256 (256)

64

 AG100

256 (32)

-

512 (512)

512 (32)

8 (< 2)

E. aerogenes

     

 ATCC13048

512

1024

512

512

8

 EA294

512

-

512

1024

16

 CM64

512 (256)

1024

256 (64)

256 (32)

256 (8)

 EA3

512 (256)

-

1024 (1024)

512 (512)

256 (128)

 EA298

256 (128)

-

512 (512)

256 (256)

64 (< 2)

 EA27

512 (256)

-

-

1024 (1024)

≥ 256 (< 2)

 EA289

512 (256)

1024

512 (256)

256 (256)

≥ 256 (64)

K. pneumoniae

     

 ATCC11296

256

1024

1024

512

8

 KP55

256 (64)

-

512 (512)

512 (512)

32 (4)

 KP63

1024 (256)

-

256 (256)

1024 (1024)

64 (< 2)

 K2

512

-

-

1024

32

 K24

512

-

512

512

16

P. aeruginosa

     

 PA01

32

-

512

512

16

 PA124

256 (128)

512

512 (512)

1024 (1024)

32 (< 2)

P. stuartii

     

 ATCC29916

128

-

128

256

16

 NAE16

512

1024

512

256

64

 PS2636

256

-

512

512

32

 PS299645

1024 (1024)

-

1024

512

32

E. cloacae

 

-

   

 BM47

128

-

512

1024

≥ 256

 ECCI69

128

-

512

512

≥ 256

 BM67

128 (32)

-

512 (16)

1024 (1024)

128 (32)

(−) MIC > 1024 μg/mL.

Role of efflux pumps in the susceptibility of Gram-negative bacteria to the tested plant extracts

Fourteen of the studied MDR bacteria were also tested for their susceptibility to the most active plant extracts (P. nigrum, V. amygdalina and T. occidentalis) in the presence PAβN at 30 μg/mL. When combined with extracts, PAβN improved the activity (decrease of MIC values) of P. nigrum on almost all of the tested MDR strains [13/14 (92.9%)]. The EPI also improved the activity of T. occidentalis against E. aerogenes CM64, EA 289 and E. cloacae BM67 as well as that of V. amygdalina against E. coli AG100 and E. aerogenes EA 289 (Table 3).

Effect of the association of extracts with antibiotics

A preliminary study (See supporting information; Additional file 1: Table S2) was performed against P. aeruginosa PA124 using the three most active plant extracts. The results permitted the selection of the extracts from P. nigrum and T. occidentalis with the appropriate sub-inhibitory concentrations of MIC/2 and MIC/5 for further studies. Therefore, the extracts from P. nigrum and T. occidentalis were combined with eleven antibiotics [TET, DOX, CIP, NOR, STR, KAN, CHL, ERY, FEP, CLX and AMP] separately to evaluate their possible synergistic effects. As results, synergistic effects were observed with the two extracts and most of the tested antibiotics except ß-lactams (AMP, FEP and CLX) (Tables 4 and 5). At MIC/2 and MIC/5 of the extract from T. occidentalis, synergistic effects were observed with 7 of the 11 antibiotics (TET, DOX, CIP, NFX, KAN, CHL, ERY) against the tested MDR bacteria (Table 5).
Table 4

MIC of different antibiotics after the association of the extract of Piper nigrum at MIC/2, MIC/5 against eleven MDR bacteria strains

 

Bacterial strains, MIC (μg/mL ) of antibiotics in the absence and presence of the extract

Antibiotics

Extract concentration

PA124

AG100

AG102

AG100Atet

CM64

EA3

EA27

EA289

BM67

KP55

NEA16

PBSS (%)

TET

0

8

16

256

64

8

16

64

16

8

> 256

4

 

MIC/2

4(2)S

16(1)I

≤ 2(≥ 128)S

16(4)S

≤ 2(> 4)S

4(4)S

16(4)S

16(1)I

8(1)I

> 256

≤2(> 2)S

63.63

MIC/5

4(2)S

16(1)I

≤ 2(≥ 128)S

16(4)S

≤ 2(> 4)S

4(4)S

8(8)S

16(1)I

8(1)I

> 256

≤ 2(> 2)S

63.63

DOX

0

16

8

32

32

32

32

32

32

8

32

64

na

MIC/2

8(2)S

≤ 2(> 4)S

≤ 2(> 16)S

≤ 2(> 16)S

≤ 2(> 16)S

4(8)S

8(4)S

32(1)I

≤ 2(> 4)S

32(1)I

4(16)S

81.81

MIC/5

8(2)S

≤ 2(> 4)S

≤ 2(> 16)S

≤ 2(> 16)S

4(8)S

≤ 2(> 16)S

8(4)S

16(1)I

≤ 2(> 4)S

32(1)I

≤ 2(> 32)S

81.81

CIP

0

32

4

128

64

64

128

≤ 2

8

128

64

128

na

MIC/2

32(1)I

≤ 2(>  2)S

16(8)S

4(16)S

8(8)S

64(2)S

≤ 2

4(2)S

64(2)S

≤ 2(> 32)S

64(2)S

90

MIC/5

32(1)I

≤ 2(> 2)S

64(2)S

16(4)S

4(16)S

64(2)S

≤ 2

4(2)S

128(1)I

≤ 2(> 32)S

128(1)I

70

NFX

0

128

128

64

128

≤ 2

128

32

64

128

128

256

na

MIC/2

64(2)S

4(32)S

≤2(> 32)S

16(8)S

≤ 2

32(4)S

8(4)S

16(4)S

128(1)I

16(8)S

128(2)S

90

MIC/5

128(1)I

≤ 2(> 64)S

64(1)I

16(8)S

≤ 2

32(4)S

16(2)S

16(4)S

128(1)I

16(8)S

128(2)S

80

STR

0

256

≤ 2

256

64

8

32

16

64

≤ 2

4

64

na

MIC/2

256(1)I

≤ 2

64(4)S

≤ 2(> 32)S

≤ 2(> 4)S

≤ 2(> 16)S

32(0.5)I

4(16)S

≤ 2

≤ 2(> 2)S

≤ 2(> 32)S

77.78

MIC/5

256(1)I

≤ 2

64(4)S

≤ 2(> 32)S

≤ 2(> 4)S

4(8)S

16(1)I

4(16)S

≤ 2

≤ 2(> 2)S

≤ 2(> 32)S

77.78

KAN

0

ND

8

128

32

≤2

16

16

32

≤ 2

8

≤ 2

na

MIC/2

ND

≤ 2(> 4)S

≤ 2(> 64)S

16(2)S

≤2

≤ 2(> 8)S

4(4)S

4(8)S

≤ 2

16(0.5)

≤ 2

75

MIC/5

ND

≤ 2(> 4)S

≤ 2(> 64)S

4(8)S

≤2

≤ 2(> 8)S

4(4)S

4(8)S

≤ 2

8(1)I

≤ 2

75

CHL

0

32

64

> 256

64

256

32

> 256

> 256

64

16

128

na

MIC/2

16(2)S

4(16)S

16(> 16)S

64(1)I

32(8)S

4(8)S

32(> 8)S

> 256

32(2)S

8(2)S

8(16)S

81.81

MIC/5

32(1)I

16(4)S

16(> 16)S

64(1)I

128(2)S

4(8)S

64(> 4)S

> 256

32(2)S

16

128(1)I

54.54

ERY

0

128

32

256

128

> 256

256

64

256

256

256

256

na

MIC/2

128(1) I

8(4)S

64(4)S

256(0.5)I

32(> 8)S

64(4)S

32(2)S

256(1)I

64(4)S

16(16)S

64(4)S

72.72

MIC/5

128(1)I

16(2)S

32(4)S

128(1)I

8(> 32)S

256(1)I

8(8)S

256(1)I

128(2)S

16(16)S

64(4)S

63.63

AMP

0

128

128

> 256

> 256

> 256

> 256

> 256

>256

> 256

> 256

> 256

na

MIC/2

128(1)I

128(1)I

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

na

MIC/5

128(1)I

128(1)I

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

na

FEP

0

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

na

MIC/2

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

na

MIC/5

> 256

> 256

> 256

> 256

> 256

> 256

> 256

>256

> 256

> 256

> 256

na

CLX

0

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

na

MIC/2

> 256

16(> 16)S

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

1.1

MIC/5

> 256

32(> 8)S

> 256

> 256

64(> 4)S

> 256

> 256

> 256

> 256

> 256

> 256

18.2

aAntibotics [TET : tetracycline, DOX: doxycyclin, CIP : ciprofloxacin, NOR : norfloxacin, STR : streptomycin, KAN : kanamycin, CHL: chloramphenicol, ERY : erythromycin, AMP : ampicillin, FEP: cefepime, CLX : cloxacillin]. bBacterial strains: Escherichia coli [AG100, AG100A, AG100Atet], Pseudomonas aeruginosa [PA124], Enterobacter aerogenes [CM64, EA3, EA27, EA289], Enterobacter cloacae [BM67], Klebsiella pneumoniae [KP55], Providencia stuartii [NAE16]. cPBSS: percentage of bacteria strain on which synergism has been observed; (): fold increase in MIC values of the antibiotics after association with plants extract; S: Synergy, I: Indifference; na: not applicable.

Table 5

MIC of different antibiotics after the association of the extract of Telfairia occidentalis at MIC/2, MIC/5 against eleven MDR bacteria

Antibioticsa

Bacterial strainsb, MIC (μg/mL ) of antibiotics in the absence and presence of the extract

 

PBSSc(%)

Extract concentration

PA124

AG100

AG102

AG100Atet

CM64

EA3

EA27

EA289

BM67

KP55

NEA16

TET

0

8

16

256

64

8

16

64

16

8

> 256

4

 

MIC/2

4(2)S

8(2)S

256(1)I

32(2)S

≤ 2(> 4)S

≤ 2(> 8)S

64(1)I

8(2)S

4(2)S

256(> 1)s

≤ 2(> 2)S

81.8

MIC/5

4(2)S

8(2)S

256(1)I

32(2)S

≤ 2(> 4)S

≤ 2(> 8)S

64(1)I

8(2)S

4(2)S

256(> 1)s

4(1)I

72.7

DOX

0

16

8

32

32

32

32

32

32

8

32

64

na

MIC/2

2(8)S

≤ 2(> 4)S

16(2)S

4(8)S

4(8)S

≤ 2(> 16)S

4(8)S

16(2)S

4(2)S

≤ 2(> 16)S

32(2)S

100

MIC/5

8(2)S

≤ 2(> 4)S

32(1)I

8(4)S

≤ 2(> 16)S

≤ 2(> 16)S

8(4)S

16(2)S

≤ 2(> 4)S

64(< 0.5)

64(1)I

72.7

CIP

0

32

4

128

64

64

128

≤ 2

8

128

64

128

na

MIC/2

32(1)I

≤ 2(> 2)S

64(2)S

≤ 2(> 32)S

16(4)S

≤ 2(> 64)S

≤ 2

≤ 2(> 4)S

128(1)I

≤ 2(> 32)S

128(1)I

70

MIC/5

32(1)I

≤ 2(> 2)S

64(2)S

≤ 2(> 32)S

16(4)S

≤ 2(> 64)S

≤ 2

≤ 2(> 4)S

128(1)I

≤ 2(> 32)S

128(1)I

70

NFX

0

128

128

64

128

≤ 2

128

32

64

128

128

256

na

MIC/2

64(2)s

4(32)S

32(2)S

4(32)S

≤ 2

4(32)S

4(8)S

32(2)S

128(1)I

≤ 2(> 64)S

128(2)S

90

MIC/5

32(4)s

4(32)S

32(2)S

4(32)S

≤ 2

4(32)S

4(8)S

16(4)S

128(1)I

8(16)S

256(1)I

80

STR

0

256

≤ 2

256

64

8

32

16

64

≤ 2

4

64

na

MIC/2

256(1)I

≤ 2

256(1)I

16(4)S

≤ 2(> 4)S

≤ 2(> 16)S

16(1)I

≤ 2(> 32)S

≤ 2

≤ 2(> 2)S

16(4)S

66.7

MIC/5

256(1)I

≤ 2

256(1)I

8(8)S

≤ 2(> 4)S

≤ 2(> 16)S

16(1)I

≤ 2(> 32)S

≤ 2

≤ 2(> 2)S

16(4)S

66.7

KAN

0

ND

8

128

32

≤ 2

16

16

32

≤ 2

8

≤ 2

na

MIC/2

ND

4(2)S

128(1)I

8(4)S

≤ 2

≤ 2(> 8)S

≤ 2(> 8)S

≤ 2(> 16)S

≤ 2

8(1)I

≤ 2

71.4

MIC/5

ND

4(2)S

128 (1)I

≤ 2(> 16)S

≤ 2

≤ 2(> 8)S

≤ 2(> 8)S

≤ 2(> 16)S

≤ 2

8(1)I

≤ 2

71.4

CHL

0

32

64

> 256

64

256

32

> 256

64

16

128

128

na

MIC/2

16(2)s

32(2)S

> 256

64(1)I

≤ 2(> 128)S

4(8)S

256(> 1)s

16(4)S

≤ 2(> 8)S

16(8)S

16(8)S

90

MIC/5

32(1)I

32(2)S

> 256

64(1)I

≤ 2(> 128)S

8(4)S

256(> 1)s

32(2)S

16(1)I

64(2)S

64(2)S

70

ERY

0

128

32

256

128

> 256

256

64

256

256

256

256

na

MIC/2

64(2)s

16(2)S

> 256

32(4)S

≤ 2

4(64)S

32(2)S

64(4)S

32(8)S

8(32)S

128(2)S

72.7

MIC/5

64(2)s

32(1)I

> 256

32(4)S

64(> 4)S

4(64)s

64(1)I

256(1)I

32(8)S

16(16)S

128(2)S

45.5

AMP

0

128

128

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

na

MIC/2

256(0.5) I

128(1)I

> 256

> 256

> 256

256(> 1)s

> 256

> 256

> 256

> 256

> 256

na

MIC/5

256(0.5) I

128(1)I

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

na

FEP

0

ND

> 256

> 256

> 256

> 256

> 256

> 256

> 256

256

> 256

> 256

na

MIC/2

ND

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256(< 1)

8(> 32)S

> 256

1.1

MIC/5

ND

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256(< 1)

> 256

> 256

na

CLX

0

> 256

256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

> 256

na

MIC/2

> 256

64(> 4)S

> 256

> 256

256(≥2) S

64(> 4)S

> 256

> 256

> 256

> 256

> 256

27.3

 

MIC/5

> 256

64(> 4)S

> 256

> 256

256(≥2) S

128(> 2)S

> 256

> 256

> 256

> 256

> 256

27.3

aAntibotics [TET : tetracycline, DOX: doxycyclin, CIP: ciprofloxacin, NOR : norfloxacin, STR : streptomycin, KAN : kanamycin, CHL: chloramphenicol, ERY : erythromycin, AMP : ampicillin, FEP: cefepime, CLX: cloxacillin]. bBacterial strains: Escherichia coli [AG100, AG100A, AG100Atet], Pseudomonas aeruginosa [PA124], Enterobacter aerogenes [CM64, EA3, EA27, EA289], Enterobacter cloacae [BM67], Klebsiella pneumoniae [KP55], Providencia stuartii [NAE16]. cPBSS: percentage of bacteria strain on which synergism has been observed; (): fold increase in MIC values of the antibiotics after association with plants extract; S: Synergy, I: Indifference; na: not applicable tract; S: Synergy, I: Indifference; na: not applicable.

Discussion

Antibacterial activities and chemical composition of the tested extracts

Many secondary metabolites belonging to alkaloids, anthocyanins, anthraquinons, flavonoids, phenols, saponins, sterols, tannins and triterpenes were detected in the tested plant extracts. Several compounds from the investigated classes of phytochemicals were reported for their antibacterial activities [50, 51], and their presence in the tested extracts could explain their antibacterial effects. The differences in bacterial susceptibility to the extracts may be either due to the differences in cell wall composition and/or genetic content of their plasmids [52] or to the differences in the composition and the mechanism of action of the bioactive compounds [53]. As shown in Table 3, the three most active plants (P. nigrum, T. occidentalis and V. amygdalina) possess more classes of phytochemicals than the extract from S. aromaticum. Each of the three most active plant extracts contains at least four classes of secondary metabolites namely alkaloids, phenols, flavonoids and tannins. However, it should be noted that the activity does not depend on the number of classes of detected bioactive compounds, but mostly on their concentration. The inhibitory activity of P. nigrum was previously reported against some bacteria such as Staphylococcus aureus, Bacillus cereus, Streptococcus faecalis, Pseudomonas aeruginosa, Salmonella typhi and Escherichia coli[54], and the data reported in this study confirms the anti-infective potential of this plant. It has also been demonstrated that the acetone-ethanol extract of the leaves from V. amygdalina was weakly active against K. pneumoniae, E. coli, S. aureus, B. cereus, S. dysentriae and S. typhimurium[35] with MIC values ranged from 7.5 mg/mL to 25 mg/mL [42]. These activities are in accordance with the results obtained in the present work, but we observed higher antibacterial activity of this plant on all 29 bacteria including MDR phenotypes (with MIC values ranging between 256 and 1024 μg/mL).

Role of efflux pumps in the susceptibility of Gram-negative bacteria to the tested extracts and effects of the association of some extracts with antibiotics

All the bacterial strains tested with a combination of plant extract and PAβN were proven to possess multidrug resistance efflux pumps [5559]. Tripartite efflux systems, mainly those clinically described such as AcrAB-TolC in Enterobacteriaceae or MexAB-OprM in P. aeruginosa play a central role in multidrug resistance of pathogenic Gram-negative bacteria [55, 56]. PAßN, a potent inhibitor of the RND efflux systems is especially active on AcrAB-TolC and MexAB-OprM [57, 58] and does not present any intrinsic effect on the bacteria at the concentration of 30 μg/mL used in this work [59]. In the presence of PAßN at this concentration, significant increase of the activity of the extract from P. nigrum was noted against 13/14 of the tested MDR bacteria. This shows that at least one active compound from this plant, acting inside the bacteria cell could be the substrate of efflux pumps. From this observation, it can be suggested that the association of the extract of P. nigrum and efflux pump inhibitors could be helpful in the fight against infections due to MDR bacteria [5].

Moreover, we demonstrated in this study that the beneficial effect of the combination of two of the tested plant extracts namely those from P. nigrum and T. occidentalis, with the first line antibiotics could be achieved. Their synergistic effects with antibiotics were noted on more than 70% of the tested MDR bacteria (with seven antibiotics), also suggesting that some of their constituents can act as efflux pump inhibitor [49]. This hypothesis is emphasized by the fact that these extracts were more synergistic with antibiotics acting inside the bacteria cells. Besides, it has already been proved that the extract from P. nigrum can also act by improving the penetration of antibiotics in cells via membrane alteration [54]. However, further phytochemical investigations will be done to isolate the active constituents of P. nigrum, T. occidentalis and V. amygdalina. Besides, toxicological studies will be carried out to evaluate their safety.

Conclusion

The overall results of the present study provide baseline information for the possible use of the tested plants and mostly P. nigrum, T. occidentalis and V. amygdalina in the control of infections due to MDR Gram-negative bacteria. In addition, the extracts from P. nigrum and T. occidentalis could be used in association with antibiotics to combat multidrug resistant pathogens.

Abbreviations

AMP: 

Ampicillin

ATCC: 

American type culture collection

CEF: 

Cefepime

CFU: 

Colony forming unit

CHL: 

Chloramphenicol

CIP: 

Ciprofloxacin

DMSO: 

Dimethylsulfoxyde

EPI: 

Efflux pump inhibitor

ERY: 

Erythromycin

FIC: 

Fractional inhibitory concentration

INT: 

p-iodonitrotetrazolium chloride

KAN: 

Kanamycin

MDR: 

Multidrug resistant

MHB: 

Mueller hinton broth

MIC: 

Minimal inhibitory concentration

NOR: 

Norfloxacin

PAßN: 

Phenylalanine arginine ß-Naphthylamide

RND: 

Resistance nodulation-cell division

STR: 

Streptomycin

TET: 

Tetracycline

Declarations

Acknowledgements

Authors are thankful to Romanian Government and The Agence Universitaire de la Francophonie for travel grant to JAKN, and also to Professor Jean-Marie Pagès, Chair of the UMR-MD1 Unit, Université de la Mediterranée, France for providing us with some MDR bacteria. Authors are also thankful to Dr Gerald Ngo Teke for the language editing.

Authors’ Affiliations

(1)
Department of Biochemistry, Faculty of Science, University of Dschang
(2)
Department of Biochemistry and Molecular Biology, Faculty of Biology, University ALI Cuza

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  60. Pre-publication history

    1. The pre-publication history for this paper can be accessed here:http://www.biomedcentral.com/1472-6882/13/190/prepub

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