Interaction of strains of non-toxigenic Aspergillus flavus with Aspergillus parasiticus on aflatoxin production

Interacções de estirpes de Aspergillus flavus não-toxígenas com Aspergillus parasiticus na produção de aflatoxinas

Interaction of strains of non-toxigenic Aspergillus flavus with Aspergillus parasiticus on aflatoxin production

Interacções de estirpes de Aspergillus flavus não-toxígenas com Aspergillus parasiticus na produção de aflatoxinas

Hermínia Marina MARTINS¹; Maria Lígia MARTINS¹; Fernando Almeida BERNARDO²

CORRESPONDENCE TO:

Hermínia Marina Martins

Laboratório Nacional de Investigação Veterinária

Serviço de Micologia

Estrada de Benfica, 701

1549-011 – Lisboa – Portugal

e-mail: [e-mail]_agricultura.pt

SUMMARY

Aflatoxins production is affected by abiotic, biotic and genetic parameters. Eugenesic and dysgenetic condition for aflatoxinogenesis are relatively well studied, such as: the influence of temperature, pH, Aw, Oxigen tension, osmotic pression, organic and inorganic nutrients, substance with fungistatic effects: however there are relatively few knowledge about endogenous regulator mechanisms, the kinetic of the anabolism and the interaction between the productive moulds and remaining microflora holding in eutrophying substrates. The main objective of this study is to evaluate biotic interaction between five indigenous strains of A. flavus, confirmed non-toxigenic, and a productive strain (A. parasiticus ATCC 15 517), cultured simultaneously on two substrates: one natural (cracked corn) and a synthetic one, modified Czapeck-Dox broth. Aflatoxins quantifications were performed by HPLC at 8th and 12th days. Results of those interactive cultures showed that all strains were synergic, increasing aflatoxins production in a range between 5.6 and 106.5% over the control values, with an exception registered on one of the strains cultured in cracked corn at the 12th day, where the production decreased (-7.6%).

UNITERMS: Plant interaction; Aflatoxins; Vegetal production; Aspergillus flavus; Aspergillus parasiticus.

INTRODUCTION

Aspergillus flavus and A. parasiticus, producers of aflatoxins (AFs), are closely related fungi that contaminate seeds and plant debris of many crops in the field, during harvest, in storage, and during processing. A. flavus seems adapted to the aerial and foliar environment being dominant in corn, cottonseed and tree nuts whereas A. parasiticus appears to be adapted to a soil (peanuts). Many strains of A. flavus are not toxigenic, so the presence of mouldiness is not, of itself, indicative of toxin production².

The capacity for aflatoxins production depends on the individual metabolic systems, essential to the primary metabolism of lipids and specified enzymes (synthetases) able to produce this secondary metabolite.

Levels of aflatoxins production are affected by many abiotic parameters like temperature, water activity, pH, osmotic pressure, substrate nature and also by biotic factors. Many of these parameters have been investigated, most of the time, separately, but it must be realised that they all interact in natural conditions. The full knowledge of biosynthesis pathway will only became better understood when interactive and multi-factorial studies were performed. Based on its potent genotoxicity and hepatotoxicity and its widespread occurrence in food commodities, aflatoxin B₁ is most a significant initiator and promoter of human primary hepatocellular carcinoma^9,11. Aflatoxin B₁ has induced liver cancer in all the species of laboratory animals tested²⁵ and is an extremely potent carcinogen to the rats and rainbow trout. According to Diener et al.⁵, toxinogenic strains of Aspergillus flavus usually produce higher levels of aflatoxin B₁ and aflatoxin B₂, whereas with A. parasiticus more equitative amounts of aflatoxins B₁, B₂, G₁, and G₂ are obtained.

The main objective of this study was to evaluate biotic interactions between non-toxigenic wild strains of A. flavus, and a toxigenic strain (Aspergillus parasiticus ATCC 15517), cultured on two substrates: one natural (cracked corn) and another available in Czapeck Dox Broth (Oxoid), modified by Martins et al.¹⁵.

MATERIAL AND METHOD

Strains: A. parasiticus ATCC 15517 and non-toxigenic A. flavus (five wild strains) obtained from mixed feed were used to aflatoxin production. Colonies of A. flavus (wild) were picked to Czapek agar (Difco) plates, and incubated at 25ºC for 8 days. These isolates were identified, considering macroscopic and microscopic morphological aspects, compared to descriptions given by Raper and Fennel²², and Domsch et al ⁶, and previously confirmed as non-toxigenic¹⁸.

In vitro aflatoxin production: The studies on aflatoxin production by A. parasiticus ATCC 15517, were carried out in duplicate on Erlenmeyer flasks containing the following substrates:

a) 50 ml of Czapeck Dox Broth base (MCD) (Oxoid) modified with adding: 20 g of Saccharose (Merck) (instead of glucose); 20 g of Yeast extract (Difco); 0.005 g Zinc Sulphate (Merck); 4 g Citric acid (Merck); 1,000 ml of bi-distilled water with 30% of corn infusion^8,15;

b) 50 g of sterilised cracked corn, adding 20 ml of distilled water and adjusting Aw to 0.98¹⁸.

Autoclaved substrates were inoculated separately with spore suspensions: Blanks were inoculated with 2 ml of spore suspension of A. parasiticus ATCC 15517; Interactive cultures (A. parasiticus ATCC 15517 and each of the non-toxigenic A. flavus) were inoculated with 1 ml of each spore suspension according the following procedure: 5 ml of sterile distilled water was added to each slant of five days old culture and gently scraping the agar surface to give a turbid suspension, till an absorption density between 0.300 and 0.400 at 450 nm wavelength on Spectrophotometer (Model Junior III, Coleman 6/8). Two ml of this suspension were added to the MCD broth and to the cracked corn.

Inoculated flasks were shaken daily for the first three days. Incubations for in vitro AFs production were performed at 28ºC for 8 days and another series was incubated at the same temperature for 12 days.

Aflatoxin quantification by High Pressure Liquid Chromatografhy (HPLC):

The quantification of aflatoxins (AFs) was processed according to the method described by van Egmond²⁴. The aflatoxins were extracted with chloroform, filtered and purified of an aliquot portion over a Florisil cartridge (Sep-Pak, Waters), subsequently followed by a C18cartridge (Sep-Pak; Waters). Determination of aflatoxins (levels were carried out by isocratic reverse-phase liquid chromatography (HPLC) using a LiChrospher 100 RP-18, 5 µm column 25 x 4.6 mm i.d.) EcoPack (Merck, Portugal), with post-column derivatisation according to Garner et al.⁷, involving bromination, with pyridinum hydrobromide perbromide (PBPB) (Sigma P- 3179). The detection limit was 0.001 mg/kg. Standard Aflatoxins B₁, B₂, G₁ and G₂ were purchased from Sigma (A-6636).

Correlation fungal biomass/aflatoxin production: Fungal biomass of each culture flasks (blank and interactive cultures) in MCD, at 8^th and 12^th days, were removed to a Petri disk, dehydrated at 120ºC for 15 min and then weighted, to correlate with production (productivity mg of AFs/g biomass).

RESULTS

Concerning the five wild strains of A. flavus previously tested and proved to be non-toxigenic, it was verified, in all cases, that they were synergic with productive strain (Tab. 1, 2, and ⁴). Synergism was also observed in mycelial development, since biomasses of mixed colonies were higher than the reference culture. It was evident that there was a relative constant average concerning Aflatoxins production per unity of fungal biomass (Tab. 3).

Table 1

Production of Aflatoxins (AFs) and comparative deviation ratio, at 8 days of incubation in MCB (mg/kg). Lisboa, 1998/1999.

Table 2

Production of Aflatoxins (AFs) and comparative deviation ratio at 12 days of incubation in MCB (mg/kg). Lisboa, 1998/1999.

Table 3

Productivity ratio of Aflatoxins (AFs) in MCB at 8 and 12 days of incubation in function of fungal biomasses. Lisboa, 1998/1999.

Legend – (*) – mg AFs/g fungal biomass; T8 – Time of incubation – 8 days (25ºC); T12 – Time of incubation – 12 days (25ºC).

Aflatoxins global production, in both substrates, was higher in mixed cultures than in the control one (A. parasiticus ATCC 15517), cultured individually. The wild A. flavus 2 had a remarkable synergic activity, with the competent strain, in MCD, presenting an increase of production that was 106.5% higher than the average of the control culture, at the 12^th day (Tab. 2).

In a general appreciation, the synergism observed on those mixed cultures increases between 5.6% and 106.5% over the average control values (Tab. 1, 2, ⁴ and ⁵); with an exception registered with A. flavus 1, 2 and 3 strains cultured on cracked corn at 12^th day, which the production decreased considerably (-7.6% and -3.6%) (^{Tab. 5}).

DISCUSSION AND CONCLUSIONS

A. flavus are natural contaminants of many feeds and raw materials for human and animal consumption. On a survey conducted during four years, Martins¹² found A. flavus in about 70.0% of feeds (1,103 samples), and in 42.6% of raw materials. In Australia, Bryden et al.⁴ found prevalence over 80% of A. flavus on those materials. Martins¹⁸ in a study about in vitro aflatoxin production by Aspergillus flavus (114 strains) isolated from raw materials and mixed feed found 60.5% toxigenic strains and the range of the aflatoxin B₁ was 0.004 to 391.9 mg/kg.

The present study shows that indigenous strains of A. flavus previously tested and classified as non-toxigenic can be synergetic with toxigenic strains. This synergism may be due to the capacity of the non-toxigenic strains to produce ethylene and to the fact that this metabolite, being a precursor of acetate, is useful to the biogenesis chain of aflatoxins²³. Badii et al.¹ related that precursor metabolites of aflatoxin biosynthesis may justify the synergism between interactive cultures. Another possibility concerns to the fact that the non toxigenic strains can also achieve the production of sterigmatocystin and 0-metil-sterigmatocystin, chemical precursors of the aflatoxins Pro et al.²¹. The same authors showed that the addition of these precursors to a culture medium, where the development of a toxigenic strain takes place, increases the production levels of 3 to 25 times. Nevertheless, Brown et al.³ verified that in natural conditions the addition of telluric non-toxigenic A. flavus to corn, during the harvest, prevented the posterior colonisation by toxigenic strains, and even when those strains develop on the maize, during the storage, they are capable of biodegrading the previously formed aflatoxins. The assimilation by mycelia, in the vegetative phase of the development of non-toxigenic strains, seems to justify detoxification process, of the toxin in the subtracts²⁰.

Martins et al.¹⁹ studied the role of Fusarium moniliforme on aflatoxins production, and verified that the global aflatoxins productivity decreased about 30% on MCD, and 26% on cracked corn. In another interactive studies with A. terreus, A. niger and with Mucorales the in vitro aflatoxin production also decreased^5,14,17. According to Jarvis¹⁰, Rhizopus oryzae strains have biological capacity to degrade the aflatoxins.

Other interactions performed with Saccharomyces cerevisae ATCC 97 631¹³ and with Penicillium spp¹⁶ showed increase of the aflatoxin productivity.

The present study confirms synergic interaction of mould development and a potential increase of aflatoxin productivity. In the evaluation of the toxigenic capacity of the different strains of A. flavus, the interaction with the other mycoflora components of the substrate should always be taken in consideration. Otherwise the intervenue on the aflatoxinogenesis mechanism can be under or over evaluated.

RESUMO

A produção de aflatoxinas é afetada por parâmetros abióticos, bióticos e genéticos. As condições eugenésicas e disgenésicas da aflatoxinogênese estão relativamente bem estudadas, tal como as influências ecológicas (temperatura, pH, Aw, tensão de oxigênio, pressão osmótica, nutrientes e substâncias fungistáticas). Contudo, é muito escasso o conhecimento relativo aos mecanismos reguladores endógenos, à cinética do anabolismo e à interação dos fungos produtores com a restante microflora presente em substratos eutrofizantes. O principal objetivo deste estudo é avaliar a interação de cinco estirpes indígenas de A. flavus, comprovadamente atoxígenas, com uma geneticamente apta (A. parasiticus ATCC 15517), cultivadas simultaneamente em dois substratos: um natural (milho triturado) e outro sintético (Caldo de Czapeck-Dox Modificado). A quantificação das aflatoxinas foi efetuada no 8º e 12º dias de incubação, por Cromatografia Líquida de Alta Resolução (HPLC). Os resultados demonstraram que todas as estirpes foram sinérgicas, aumentando o rendimento da produção entre 5,6 e 106,5%, quando comparados com os valores das testemunhas.

UNITERMOS: Interação de plantas; Aflatoxinas; Produção vegetal; Aspergillus flavus; Aspergillus parasiticus.

Received: 23/11/1999

Accepted: 10/01/2001

• ¹
  - BADII, F.; MOSS, O.; WILSON, K. The effect of sodium biselenite on the growth and aflatoxin production of A. parasiticus and the growth of other Aspergilli. Letters Applied Microbiology, v.2, p.61-2, 1986.
• ²
  - BETINA, V. Mycotoxins: chemical, biological and environmental aspects. New York : Elsevier, 1989. p.42-145.
• ³
  - BROWN, R.L.; COLTY, P.J.; CLEVELAND, T.E. Reduction in aflatoxin content of maize by atoxigenic strains of Asp. Flavus Journal Food Protection, v.54, n.8, p.626-33,1991.
• ⁴
  - BRYDEN, W.L.; RAJION, M.A.; LLOYD, A.B.; CUMMING, R.B. Surveys of Australian feedstuffs for toxigenic strains of Aspergillus flavus and for Aflatoxin. Australian Veterinary Journal, v.51, p.491-3, 1975.
• ⁵
  - DIENER, M.L.; COLE, T.J.; SANDERS, T.H.; PAYNE, P.A.; LEE, L.S.; KLICH, M.A. Epidemiology of Aflatoxin formation by Aspergillus flavus Annais Review Phytopathology, n.25, p.249-70, 1987.
• ⁶
  - DOMSCH, K.H.; GAMS, W.; ANDERSON, T.H. Compendium of soil fungi New York : Academic Press, 1980. 2v. p.1156.
• ⁷
  - GARNER, C.; WHATTAM, M.; TAYLOR, L.; STOW, W. Analysis of U.K. purchased spices for aflatoxins using an immunoaffinity column clean-up procedure followed by high-performance liquid chromatographic analysis and post-column derivatisation with pyridinium bromide perbromide. Journal of Chromatography, n.648, p.485-90, 1993.
• ⁸
  - HARA, S.; FENNELL, D.J.; HESSELTINE, C.W. Aflatoxin producing strains of Aspergillus flavus detected by fluorescence agar medium under ultra violet light. Applied Microbiology, v.27, n.6, p.1118-23, 1974.
• ⁹
  - HSIEH, D.P.H. Mycotoxins in food: mode of action of mycotoxins. New York : Academic Press, 1987. p.149-76.
• ¹⁰
  - JARVIS, B. Factors affecting the production of mycotoxins. Journal of Applied Bacteriology, v.34, p.199-213, 1971.
• ¹¹
  - MAGGON, K.; GUPTA, S.; VENKITASUBRANIAN, T. Biosynthesis of aflatoxins. Bacteriology Review, v.41, n.4, p.822-55, 1977.
• ¹²
  - MARTINS, H.M. Relatório de actividades Lisboa : LNIV, 1987. p.202.
• ¹³
  - MARTINS, H.M.; BERNARDO, F.; MARTINS, M.L. Effect of Saccharomyces cerevisae ATCC 97631 on aflatoxin production. In: WORLD CONGRESS OF FOODBORNE INFECTIONS AND INTOXICATIONS, 4., Germany, 1998. Abstracts Book Germany : s.c.p., 1998. p.288.
• ¹⁴
  - MARTINS, H.M.; BERNARDO, F.M.; MARTINS, M.L. Interferência dos Aspergillus niger e A. terreus na aflatoxigenese. In: CONGRESSO DE ZOOTECNIA, 6., Évora, 1996. Proceedings Évora : s.c.p. 1996. p.90.
• ¹⁵
  - MARTINS, H.M.; BERNARDO, F.M.; MARTINS, M.L. Produção de Aflatoxinas "in vitro". Revista Portuguesa de Ciências Veterinárias, v.94, n.532, p.177-81, 1999.
• ¹⁶
  - MARTINS, H.M.; BERNARDO, F.M.; MARTINS, M.L. Synergic action of some Penicillia on aflatoxinogenesis. In: NORDIC VETERINARY CONGRESS, 18., Helsinki, 1998. Proceedings Helsinki : s.c.p., 1998. p.95.
• ¹⁷
  - MARTINS, H.M.; MARTINS, L.M.; BERNARDO, F.M. Interaction of Mucorales with A. parasiticus of aflatoxin production. In: WORLD CONGRESS ON FOOD HYGIENE. The Hague, 1997. Proceedings The Hague : World Association of Veterinary Food Hygienists, 1997. p.255.
• ¹⁸
  - MARTINS, M.L. Capacidade de produção de aflatoxinas por Aspergillus flavus em substratos naturais. Repositório de Trabalhos LNIV, v.21, p.123-32, 1989.
• ¹⁹
  - MARTINS, M.L.; MARTINS, H.M.; BERNARDO, F. Role of Fusarium moniliforme on aflatoxins production in interactive cultures with A. flavus In: INTERNATIONAL SEMINAR ON FUSARIUM, MYCOTOXINS, TAXONOMY AND PATHOGENICITY. Italy, 1995. Book of Abstracts Italy : s.c.p., 1995. p.50.
• ²⁰
  - MASIMANGO, N.; REMACLE, J.; RANAUT, J. Rôle du mycelium dans l’elimination de l’aflatoxine B₁, des milieux contaminés. Annais Nutritionnal Alimmentar, v.33, p.149-62, 1979.
• ²¹
  - PRO, M.L.; MORENO, M.A.; SUAREZ, G. Transformation of sterigmotocystin and o - metal sterigmatocystin by aflatoxigenic and nonaflatoxigenic field isolates of the Asp flavus Mycopathologia, v.116, p.71-5, 1991.
• ²²
  - RAPER, K.B.; FENNELL, D.I. The genus Aspergillus Baltimore : Williams & Wilkins, 1965. p.704.
• ²³
  - SHARMA, A.; DESSAI, S.R.P.; NAD KARNI, G.B. Possible implications of reciprocity between ethylene and Aflatoxin Biogenesis in A. flavus and A. parasiticus Applied Environmental Microbiology, v.49, n.1, p.79-82, 1985.
• ²⁴
  - VAN EGMOND, H.P.; SIZOO, E.A.; PAULSCH, W.E. Evaluation of new methods of analysis for the determination of Aflatoxin B₁ in Feeding - stuffs. R.I.V.M. The Netherlands : Bilthoven, 1985. p.212.
• ²⁵
  - WOGAN, G.N.; PAGLIALUNGA, S.; NEWBERNE, M. Carcinogenicity effects of low dietary levels of aflatoxin B₁ in rats. Food Cosmetic Toxicology, v.12, p.681-5, 1974.

Table 4

Production of Aflatoxins (AFs) in cracked corn at 8th day of incubation (mg/kg). Lisboa, 1998/1999.

Table 5

Production of Aflatoxins (AFs) in cracked corn in five interactive cultures at 12^th day of incubation (mg/ kg). Lisboa, 1998/1999.

¹

Laboratório Nacional de Investigação Veterinária – Lisboa, Portugal

²

Faculdade de Medicina Veterinária, Polo Universitário da Ajuda – Lisboa, Portugal

Publication Dates

• Publication in this collection
  22 Aug 2001
• Date of issue
  Dec 2000

History

• Accepted
  10 Jan 2001
• Received
  23 Nov 1999