BioMed Research International

Volume 2013 (2013), Article ID 640682, 5 pages

http://dx.doi.org/10.1155/2013/640682

Research Article

Modulation of the Antibiotic Activity by Extracts from Amburana cearensis A. C. Smith and Anadenanthera macrocarpa (Benth.) Brenan

Fernando G. Figueredo,¹ Emerson O. Ferreira,¹ Bruno F. F. Lucena,¹ Cícero M. G. Torres,¹ Daniel L. Lucetti,1,2 Elaine C. P. Lucetti,1,2 João Marcos F. L. Silva,1,2 Francisco A. V. Santos,1,2 Cássio R. Medeiros,¹ Gardênia M. M. Oliveira,¹ Aracélio V. Colares,1,3 José G. M. Costa,1,3 Henrique D. M. Coutinho,4,5 Irwin R. A. Menezes,⁶ Júlio C. F. Silva,1,2 Marta R. Kerntopf,⁶ Patrícia R. L. Figueiredo,⁶ and Edinardo F. F. Matias1,4

¹Faculdade Leão Sampaio, 63100-000, Juazeiro do Norte, CE, Brazil

²Faculdade de Medicina Estácio de Juazeiro do Norte, 63100-000, Juazerio de Norte, CE, Brazil

³Laboratório de Pesquisa em Produtos Naturais, Universidade Regional do Cariri, 63105-000, Crato, CE, Brazil

⁴Laboratório de Microbiologia e Biologia Molecular, Universidade Regional do Cariri, 63105-000, Crato, CE, Brazil

⁵Departamento de Química Biológica, Universidade Regional do Cariri, 63105-000, Crato, CE, Brazil

⁶Laboratório de Farmacologia e Química Molecular, Universidade Regional do Cariri, 63105-000, Crato, CE, Brazil

Received 15 August 2012; Revised 15 October 2012; Accepted 15 October 2012

Academic Editor: Fabio Ferreira Perazzo

Copyright © 2013 Fernando G. Figueredo et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The aim of this study was to verify the possible interactions between ethanol extracts of Amburana cearensis A. C. Smith and Anadenanthera macrocarpa (Benth.) Brenan, combined with six antimicrobial drugs against multiresistant strains of Staphylococcus aureus and Escherichia coli isolated from humans. The antibacterial activity of the extracts was determined using the minimum inhibitory concentration (MIC). The microdilution assay was performed to verify the interactions between the natural products and the antibiotics using a subinhibitory concentration. The activity of amikacin associated with the extract of Anadenanthera macrocarpa against EC 27 was enhanced, demonstrating an MIC reduction from 128 to 4 μg/mL. Among the β-lactams, no potentiation on its activity was observed, with exception to the antagonism of the natural products with ampicillin against S. aureus 358.

1. Introduction

The research for new antibacterial substances becomes necessary due to increase of the antibiotic resistance of clinically important pathogens [1]. Due to this fact, substances derived from plants could be attractive alternatives [2, 3]. Natural products from plant may change or modulate the action of the antibiotic, enhancing or reducing the activity of this drug [4]. In recent years, many plants have been evaluated not only for the direct antibacterial action, but also as a modulator of the antibiotic activity [5, 6].

The use of natural products, mainly the chemical components of plants with antimicrobial properties have contributed to significant results in therapeutic treatments [7–9].

The Amburana cearensis (German) A. C. Smith, Fabaceae, is a tree which reaches 10–12 m of height [10]. Also known as “Cumaru”, it has been explored for use in fine furniture making, sculpture and carpentry, being listed as a threatened species [11]. Moreover, due to their medicinal properties, the bark and seeds are used to produce popular drugs for the treatment of cough, asthma, bronchitis, and pertussis. The species is still used in the perfume industry [12]. Medical trials have demonstrated preclinical anti-inflammatory, bronchodilator and analgesic activity for the hydroalcoholic extract, being possible to associate these effects of coumarin and flavonoidic fraction [13, 14].

The Anadenanthera macrocarpa is a species belonging to Mimosoideae [15]. This is a species of “angico” with larger geographic areas, occurring from southern Bolivia to northern Argentina, in Brazil, and is not only found in southern region [16]. Popular medicine has been used against several diseases through the preparation of syrups and lickers, it is used for the treatment of coughs, bronchitis, fads, external wounds and inflammation [17].

The objective of this study was to realize the phytochemical prospecting and assay of the in vitro ethanolic extracts of leaves of A. Cearensis and A. macrocarpa to determine the antibacterial activity and the modifying antibiotic activity of aminoglycosides and beta-lactams against the Escherichia coli and Staphylococcus aureus.

2. Material and Methods

2.1. Bacterial Material

The bacterial strains used were E. coli (EC-ATCC10536 and EC27) and S. aureus (SA-ATCC25923 e SA358) with a resistance profile identified in Table 1. All strains were maintained on heart infusion agar (HIA, Difco Laboratories Ltd.). Before the tests, the strains were grown for 18 h at 37°C in broth brain heart infusion (BHI, Difco Laboratories Ltd.).

Table 1: Origin of the bacterial strains and profile of resistance to antibiotics.

Bacteria
Origin
Profile of resistance

Escherichia coli 27
Surgical wound
Ast, Ax, Amp, Ami, Amox, Ca, Cfc, Cf,

Caz, Cip, Chlo, Im, Kan, Szt, Tet, Tob

Escherichia coli ATCC10536
ATCC
—

Staphylococcus aureus 358
Surgical wound
Oxa, Gen, Tob, Ami, Kan, Neo, Para,

But, Sis, Net

Staphylococcus aureus ATCC25923
ATCC
—

Ast: Aztreonam; Ax: Amoxicillin; Amp: Ampicillin; Ami: Amikacin; Amox: Amoxicillin; Ca: Cefadroxil; Cfc: Cefaclor; Cf: Cefalotin; Caz: Ceftazidime; Cip: Ciprofloxacin; Chlo: Chloramphenicol; Im: Imipenem; Kan: Kanamycin; Szt: Sulfametim; Tet: Tetracycline; Tob: Tobramycin; Oxa: Oxacillin; Gen: Gentamicin; Neo: Neomycin; Para: Paramomycin; But: Butirosin; Sis: Sisomicin; Net: Netilmicin; (—): sensitivity. ATCC: american type culture collection.

2.2. Plant Material

Leaves of Amburana cearensis and Anadenanthera macrocarpa were collected at Penaforte, Ceara, Brazil. The plant material was identified and a voucher specimen was placed in the respective herbal collections (Table 2).

Table 2: Botanical families, species, and number of the title of the plants used in this study.

Family
Species
Number HCDAL
Herbarium

Leguminosae
Amburana cearensis
5545
Vale do São Francisco-UNIVASF

Fabaceae

Anadenanthera macrocarpa
6490

Dárdano Andrade Lima-URCA

2.3. Preparation of Ethanol Extracts of Amburana Cearensis and Anadenanthera Macrocarpa

For the preparation of extracts, leaves were collected and weighed (Table 3). The material was powdered and wrapped in a container with an amount of solvent to submerge the plant material by 72 hours. After this time, the eluent was filtered and concentrated in a rotary vacuum condenser (model Q-344B-Quimis, Brazil) and in an ultrathermal bath (model Q-214 M2-Quimis, Brazil) [18]. For the tests, the solutions used were prepared from extracts in a concentration of 10 mg/mL dissolved in DMSO (dimethyl-sulfoxide), then diluted with distilled water to a concentration of 1024 μg/mL.

Table 3: Dry weight and yield of ethanolic extracts (g).

Biological species
Solvent
Leaves (mass)
Extract gross (yield)

Anadenanthera macrocarpa
Ethanol (EEAM)
50 g
9,24%

Amburana cearensis
Ethanol (EEAC)
50 g
8,15%

EEAM: ethanolic extract of Anadenanthera macrocarpa; EEAC: ethanolic extract of Amburana cearensis.

2.4. Phytochemical Prospecting

The phytochemicals tests to detect the presence of heterosides, tannins, flavonoids, steroids, triterpenes, coumarins, quinones, organic acids, and alkaloids were performed according to the method described by Matos [19]. The tests were based on visual observation of the change in color or formation of precipitate after the addition of specific reagents.

2.5. Antibacterial Activity Test

The MIC (minimal inhibitory concentration) was determined in a microdilution assay utilizing an inoculum of 100 μL of each strain, suspended in brain heart infusion (BHI) broth up to a final concentration of 10⁵ CFU/mL in 96-well microtiter plates, using twofold serial dilutions. Each well received 100 μL of each extract solution. The final concentrations of the extracts varied from 512 to 8 μg/mL. MICs were recorded as the lowest concentrations required to inhibit growth. The minimal inhibitory concentration for the antibiotics was determined in BHI by the microdilution assay utilizing suspensions of 10⁵ CFU/mL and a drug concentration range from 2.5 to 0.0012 mg/mL (twofold serial dilutions). MIC was defined as the lowest concentration at which no growth was observed. For the evaluation of the extracts as modulators of the resistance to the antibiotics, MIC of the antibiotics was determined in the presence or absence of EEAC and EEAM at subinhibitory concentrations (8 μg/mL) and the plates were incubated for 24 h at 37°C. Each antibacterial assay for MIC determination was carried out in triplicate.

2.6. Evaluation of the Modulation of Extracts on the Resistance to Aminoglycosides and -Lactams Antibiotics

To evaluate the extracts as modulators of antibiotic action, the MICs of antibiotics of the class aminoglycoside and beta-lactams, were evaluated in the presence and absence of the extracts in sterile microplates. The antibiotics were evaluated at concentrations ranging from 512 to 0.5 mg/mL. All antibiotics tested were obtained from Sigma. The extracts were mixed in BHI broth at 10% sub—inhibitory concentrations obtained and determined after the test evaluation of MIC, and for the modulation test concentration was used a concentration of extract referring the MIC diluted 8 times (MIC/8). The preparation of the antibiotic solutions was performed by adding sterile distilled water in a double concentration (1024 μg/mL) in relation to the initial concentration set volume of 100 μL and serially diluted 1 : 1 in 10% BHI broth. In each well with 100 μL of culture medium containing the bacterial suspension diluted (1 : 10). The same controls used in the evaluation of MIC for the extracts were used for the modulation [4]. The plates were filled and incubated at 35°C for 24 hours, and after that the reading was evidenced by the use of resazurin as previously mentioned in the test of determination of the MIC.

3. Results and Discussion

The extracts evaluated in this work showed the yields demonstrated in Table 3, where we observed a higher yield of extract A. macrocarpa compared to A. cearensis. To perform the microdilution assay, the extracts were diluted in DMSO obtaining a solution of concentration of 10 mg/mL. A pilot study was conducted using only DMSO, but no antibacterial or modulatory activity was observed, indicating nontoxic effect.

Table 4 shows the presence of various potentially bioactive compounds in the extracts evaluated, like phenols, tannin pyrogallatos, tannin phlobaphenes, anthocyanins, anthocyanidins, flavones, flavonols, xanthones, chalcones, aurones, flavononols, leucoanthocyanidins, catechins, flavonones, and alkaloids. Through phytochemical prospecting of extracts, it was possible to identify the presence of several classes of secondary metabolites that exhibit a wide variety of biological activities such as antimicrobial [20–22], antioxidant [23], antitumor and antiophidic [24].

Table 4: Phytochemical prospecting of the ethanolic extracts of Anadenanthera macrocarpa and Amburana cearensis.

Metabolites

Extracts
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

EEAC
−
+
−
+
+
+
+
+
+
+
+
+
+
+
+

EEAM
−
+
−
+
+
+
+
+
+
+
+
+
+
+
−

1: phenols; 2: tannin pyrogallates; 3: tannin phlobaphenes; 4: anthocyanins; 5: anthocyanidins; 6: flavones; 7: flavonols; 8: xanthones; 9: chalcones; 10: aurones; 11: flavonols; 12: leucoanthocyanidins; 13: catechins; 14: flavonones; 15: alkaloids; (+): presence; (−): absence. EEAC: etanolic extract of Amburana cearensis; EEAM: Ethanolic Extract of Anadenanthera macrocarpa.

Table 5 shows the determination of minimum inhibitory concentration (MIC) of ethanol extracts tested against E. coli and S. aureus of reference and multiresistant. Comparatively, the extracts EEAM and EEAC showed the same MIC with the exception of EEAC against SA-ATCC 25923 which showed a better MIC of 512 μg/mL.

Table 5: Minimal inhibitory concentration (MIC) of the ethanolic extracts of Anadenanthera macrocarpa and of Amburana cearensis (μg/mL).

Extracts and antimicrobials
EC 27
EC-ATCC 10536
AS 358
SA-ATCC 25923

EEAM
≥1024
≥1024
≥1024
≥1024

EEAC
≥1024
≥1024
≥1024
512

EEAM: ethanolic extract of Anadenanthera macrocarpa; EEAC: ethanolic extract of Amburana cearensis. EC: Escherichia coli, SA: Staphylococcus aureus.

Several medicinal plants were used as a source of many antimicrobial drugs used in the treatment of infectious diseases, including against bacteria multiresistant to antibiotics [25]. It is known that the synergistic action of the natural products with antimicrobial agents is commonly used in the therapeutic treatment [26, 27].

Table 6 shows the interference of the extracts on the activity of aminoglycosides, demonstrating a modulation in the activity of antibiotics, reducing the MICs. The more representative effect was observed with the association of EEAM and amikacin, an increase being observed in the antibiotic activity against EC27, reducing the MIC of the antibiotic from 128 to 4 μg/mL.

Table 6: MIC of the aminoglycosides in the presence and absence of ethanolic extracts of A.cearensis and A. macrocarpa at a concentration 128 μg/mL.

EC 27
SA 358

     Antibiotics
MIC
MIC

combined
MIC
MIC

combined

EEAC
EEAM

EEAC
EEAM

Gentamicin
64
4
4
16
16
4

Amikacin
128
8
4
64
64
16

EEAC: ethanolic extract of Amburana cearensis; EEAM: ethanolic extract of Anadenanthera macrocarpa. EC: Escherichia coli, SA: Staphylococcus aureus.

Due to absorption into the intracellular space, the cell toxicity is common to all aminoglycosides (except to streptomycin). Nephrotoxicity, ototoxicity, and neuromuscular blockade are the most important toxic effects of aminoglycosides [7, 28]. The reported frequency of these side effects is highly variable due to different criteria used for diagnosis [29]. The combination of the aminoglycosides with natural products can be an alternative to minimize the side effects of this class of antibiotics, since the association leads to a synergistic effect significantly reducing the MIC of these drugs, decreasing the dose needed for therapeutic usage.

Many β-lactam antibiotics can penetrate Gram-negative bacteria via protein channels present in the outer membrane. Through these channels, the drug can reach its receptor on the cell wall and exert its bactericidal action [30]. Although extracts have in its constitution secondary metabolites such as tannins and flavonoids, which are synthesized by plants in response to microbial infections [31, 32], modifying the cell wall or disrupting the bacterial cell membrane [33, 34]. However, Gram-negative strains were not susceptible to association between the extracts and the antibiotics (Table 7). This fact can be explained by other resistance mechanisms present in these bacteria as efflux pump, production of enzymes that cleave the beta-lactam ring (β-lactamases), changes in PBP, among other [35, 36].

Table 7: Minimal inhibitory concentration (MIC) of beta-lactam in the presence and absence of ethanolic extracts of Amburana cearensis and Anadenanthera macrocarpa and a concentration of MIC/8 (128 μg/mL).

EC 27
SA 358

     Antibiotics
MIC
MIC

combined
MIC
MIC

combined

EEAC
EEAM

EEAC
EEAM

Benzetacil
≥1024
≥1024
≥1024
512
512
512

Cephalothin
16
16
16
0.5
0.5
0.5

Ampicillin
≥1024
≥1024
≥1024
128
512
512

Oxacillin
≥1024
≥1024
≥1024
0.5
0.5
0.5

EEAC: ethanolic extract of Amburana cearensis; EEAM: ethanolic extract of Anadenanthera macrocarpa. EC: Escherichia coli, SA: Staphylococcus aureus.

A similar fact occurs with the combination of natural products with beta-lactams against the Gram-positive strains. No modulatory effect was observed against these strains, with exception to the combination with ampicillin, which resulted in an antagonism with the MIC enhancing from 128 to 512 μg/mL (Table 7).

4. Conclusion

The present results indicate that ethanolic extracts of A. cearensis and A. macrocarpa are an alternative source of natural products with antibacterial action, due to the presence of several antibacterial, which can be responsible for the observed modulatory effects, indicating the possibility of using natural products combined with aminoglycosides to increase the antimicrobial potential of these drugs against multiresistant microorganisms.

Acknowledgments

The authors are grateful to the Brazilian research agencies CNPq and FUNCAP for the funding of this work.

References

J. G. M. Costa, F. F. G. Rodrigues, E. C. Angélico et al., “Composição química e avaliação da atividade antibacteriana e toxicidade do óleo essencial de Croton zehntneri(variedade estragol),” Revista Brasileira de Farmacognosia, vol. 18, pp. 583–586, 2007.

F. Q. Oliveira, B. Gobira, C. Guimarães, J. Batista, M. Barreto, and M. Souza, “Espécies vegetais indicadas na odontologia,” Revista Brasileira de Farmacognosia, vol. 17, pp. 466–476, 2007.

J. G. Silva, I. A. Souza, J. S. Higino, J. P. Siqueira-Junior, J. V. Pereira, and M. S. V. Pereira, “Atividade antimicrobiana do extrato de Anacardium occidentale Linn em amostras multiresistentes de Staphylococcus aureus,” Revista Brasileira de Farmacognosia, vol. 17, pp. 572–577, 2007.

H. D. M. Coutinho, J. G. M. Costa, J. P. Siqueira-Júnior, and E. O. Lima, “In vitro anti-staphylococcal activity of Hyptis martiusii Benth against methicillin-resistant Staphylococcus aureus-MRSA strains,” Brazilian Journal of Pharmacognosy, vol. 18, pp. 670–675, 2008.

A. Gurib-Fakim, “Medicinal plants: traditions of yesterday and drugs of tomorrow,” Molecular Aspects of Medicine, vol. 27, no. 1, pp. 1–93, 2006.

S. Gibbons, “Anti-staphylococcal plant natural products,” Natural Product Reports, vol. 21, no. 2, pp. 263–277, 2004.

R. A. G. De Oliveira, E. D. O. Lima, E. L. De Souza et al., “Interference of Plectranthus amboinicus (Lour.) Spreng essential oil on the anti-Candida activity of some clinically used antifungals,” Brazilian Journal of Pharmacognosy, vol. 17, no. 2, pp. 186–190, 2007.

U. P. Albuquerque and N. Hanazaki, “As pesquisas etnodirigidas na descoberta de novos fármacos de interesse médico e farmacêutico: fragilidades e perspectivas,” Revista Brasileira de Farmacognosia, vol. 16, pp. 678–689, 2006.

S. Mesia-Vela, M. T. Santos, C. Souccar, M. T. R. Lima-Landman, and A. J. Lapa, “Solanum paniculatum L. (Jurubeba): potent inhibitor of gastric acid secretion in mice,” Phytomedicine, vol. 9, no. 6, pp. 508–514, 2002.

D. Andrade-Lima, Plantas da Caatinga, Academia Brasileira de Ciências, Rio de Janeiro, Brazil, 1989.

C. Hilton-Taylor, 2000 IUCN Red List of Threatened Species, IUCN, Cambridge, UK, 2000.

A. M. E. Bezerra, K. M. Canuto, and E. R. Silveira, “Estudo fitoquímico de espécimes jovens de Amburana cearensis A. C. Smith,” in Reunião Anual da Sociedade Brasileira de Química, SBQ, Águas de Lindóia, Brazil, 2005.

L. K. A. M. Leal, M. E. Matos, F. J. A. Matos, R. A. Ribeiro, F. V. Ferreira, and G. S. B. Viana, “Antinociceptive and antiedematogenic effects of the hydroalcoholic extract and coumarin from Torresea cearensis Fr. All,” Phytomedicine, vol. 4, no. 3, pp. 221–227, 1997.

L. K. A. M. Leal, M. Nechio, E. R. Silveira et al., “Anti-inflammatory and smooth muscle relaxant activities of the hydroalcoholic extract and chemical constituents from Amburana cearensis A. C. Smith,” Phytotherapy Research, vol. 17, no. 4, pp. 335–340, 2003.

T. S. Elias, “Leguminosae: mimosoideae,” in Advances in Legumes Systematic, R. M. Pollhill and P. H. Raven, Eds., pp. 143–152, Kew, Royal Botanic Garden, UK, 1981.

H. Lorenzi, rvores brasileiras: manual de identificação e cultivo de plantas arbóreas do Brasil, Instituto Plantarum, São Paulo, Brazil, 2002.

F. J. A. Matos, O Formulário fitoterápico do professor Dias da Rocha: informações sobre o emprego na medicina caseira, de plantas do Nordeste, especialmente do Ceará, EUFC, Fortaleza, Brazil, 1997.

B. G. Gonçalves Brasileiro, V. R. Ramos Pizziolo, D. S. Soares Raslan, C. M. Mashrouah Jamal, and D. Silveira, “Antimicrobial and cytotoxic activities screening of some Brazilian medicinal plants used in Governador Valadares district,” Revista Brasileira de Ciencias Farmaceuticas, vol. 42, no. 2, pp. 195–202, 2006.

F. J. A. Matos, Introdução à Fitoquímica Experimental, Edições UFC, Fortaleza, Brazil, 1997.

R. O’kennedy and R. D. Thomes, Coumarins: Biology Applications and Mode of Action, John Willey, New York, NY, USA, 1997.

C. D. Djipa, M. Delmée, and J. Quetin-Leclercq, “Antimicrobial activity of bark extracts of Syzygium jambos (L.) Alston (Myrtaceae),” Journal of Ethnopharmacology, vol. 71, no. 1-2, pp. 307–313, 2000.

D. Esquenazi, M. D. Wigg, M. M. F. S. Miranda et al., “Antimicrobial and antiviral activities of polyphenolics from Cocos nucifera Linn. (Palmae) husk fiber extract,” Research in Microbiology, vol. 153, no. 10, pp. 647–652, 2002.

A. L. B. S. Barreiros and J. M. David, “Estresse Oxidativo: relação entre geração de espécies reativas e a defesa do organismo,” Química Nova, vol. 29, pp. 113–123, 2006.

T. Okuda, T. Yoshida, and T. Hatano, “Ellagitannins as active constituents of medicinal plants,” Planta Medica, vol. 55, no. 2, pp. 117–122, 1989.

M. S. Butler and A. D. Buss, “Natural products—the future scaffolds for novel antibiotics?” Biochemical Pharmacology, vol. 71, no. 7, pp. 919–929, 2006.

E. F. F. Matias, K. K. A. Santos, T. S. Almeida, J. G. M. Costa, and H. D. M. Coutinho, “Enhancement of antibiotic activity by Cordia verbenacea DC,” Latin American Journal of Pharmacy, vol. 29, no. 6, pp. 1049–1052, 2010.

E. O. Sousa, F. S. Barreto, F. F. G. Rodrigues, and J. G. M. Costa, “Atividade antibacteriana e interferência de Lantana camara Linn e Lantana montevidensis Briq na resistência de aminoglicosídeos,” Revista Brasileira de Biociências, vol. 9, pp. 1–5, 2011.

J. C. Vallejo, M. N. Silva, J. A. A. Oliveira et al., “Detecção precoce de ototoxicidade usando emissões otoacústicas produtivas de distorção,” Revista Brasileira de Otorrinolaringologia, vol. 67, pp. 845–851, 2001.

D. N. Gilbert, “Aminoglycosides,” in Principles and Practice of Infectious Diseases, G. L. Mandell, J. E. Bennett, and R. Dolin, Eds., pp. 279–306, Churchill Livingstone, New York, NY, USA, 1995.

H. Nikaido, “Prevention of drug access to bacterial targets: permeability barriers and active efflux,” Science, vol. 264, no. 5157, pp. 382–388, 1994.

R. A. Dixon, P. M. Dey, and C. J. Lamb, “Phytoalexins: enzymology and molecular biology,” Advances in Enzymology and Related Areas of Molecular Biology, vol. 55, pp. 1–136, 1983.

K. Y. Ho, C. C. Tsai, J. S. Huang, C. P. Chen, T. C. Lin, and C. C. Lin, “Antimicrobial activity of tannin components from Vaccinium vitisidaea L,” Journal of Pharmacy and Pharmacology, vol. 53, no. 2, pp. 187–191, 2001.

H. Tsuchiya, M. Sato, T. Miyazaki et al., “Comparative study on the antibacterial activity of phytochemical flavanones against methicillin-resistant Staphylococcus aureus,” Journal of Ethnopharmacology, vol. 50, no. 1, pp. 27–34, 1996.

E. F. F. Matias, K. K. A. Santos, J. G. M. Costa, and H. D. M. Coutinho, “Atividade antibacteriana in vitro de Croton campestris A., Ocimum gratissimum L. e Cordia verbenacea DC,” Revista Brasileira de Biociências, vol. 8, no. 3, pp. 294–298, 2010.

K. Bush, “The impact of β-lactamases on the development of novel antimicrobial agents,” Current Opinion in Investigational Drugs, vol. 3, no. 9, pp. 1284–1290, 2002.

M. C. Enright, D. A. Robinson, G. Randle, E. J. Feil, H. Grundmann, and B. G. Spratt, “The evolutionary history of methicillin-resistant Staphylococcus aureus (MRSA),” Proceedings of the National Academy of Sciences of the United States of America, vol. 99, no. 11, pp. 7687–7692, 2002.