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Arquivo Brasileiro de Medicina Veterinária e Zootecnia

Print version ISSN 0102-0935

Arq. Bras. Med. Vet. Zootec. vol.66 no.3 Belo Horizonte June 2014

https://doi.org/10.1590/1678-41625796 

Animal Science And Technology And Inspection Of Animal Products

Isolation, enumeration, molecular identification and probiotic potential evaluation of lactic acid bacteria isolated from sheep milk

L.B. Acurcio 1  

M.R. Souza 1  

A.C. Nunes 2  

D.L.S. Oliveira 1  

S.H.C. Sandes 2  

L.B. Alvim 2  

1Escola de Veterinária - Universidade Federal de Minas Gerais - EV-UFMG - Belo Horizonte, MG

2Instituto de Ciências Biológicas - Universidade Federal de Minas Gerais - ICB-UFMG- Belo Horizonte, MG


ABSTRACT

Lactic acid bacteria species were molecularly identified in milk from Lacaune, Santa Inês and crossbred sheep breeds and their in vitro probiotic potential was evaluated. The species identified were Enterococcus faecium (56.25%), E. durans (31.25%) and E. casseliflavus (12.5%). No other lactic acid bacteria species, such as lactobacilli, was identified. Most of the isolated enterococci were resistant to gastric pH (2.0) and to 0.3% oxgall. All tested enterococci were resistant to ceftazidime, oxacillin and streptomycin and sensible to clindamycin, erythromycin and penicillin. The resistance to ciprofloxacin, gentamicin, tetracycline and vancomycin varied among tested species. All tested enterococci strongly inhibited (P<0.05) Escherichia coli and Listeria monocytogenes, moderately inhibited E. faecalis and Staphylococcus aureus and did not inhibit Pseudomonas aeruginosa, Salmonella enterica var. Typhimurium and also one E. durans sample isolated from sheep milk. Four samples of E. faecium, one of E. durans and one of E. casseliflavus presented the best probiotic potential.

Key words: sheep milk; enterococci; probiotic potential

RESUMO

Espécies de bactérias ácido-lácticas foram identificadas em nível molecular em leite das raças ovinas Lacaune, Santa Inês e suas mestiças, e o seu potencial probiótico in vitro foi avaliado. As espécies identificadas foram Enterococcus faecium (56,25%), E. durans(31,25%) e E. casseliflavus (12,5%). Nenhuma outra espécie de bactéria ácido-láctica, como Lactobacillus sp., foi identificada. A maioria dos enterococos isolados foi resistente ao pH gástrico (2.0) e a 0,3% de oxgall. Todos os enterococos testados foram resistentes à ceftazidima, oxacilina e estreptomicina e sensíveis à clindamicina, eritromicina e penicilina. A resistência à ciprofloxacina, gentamicina, tetraciclina e vancomicina variou entre as amostras. Todos os enterococos testados inibiram fortemente (P<0,05) Escherichia coli e Listeria monocytogenes, inibiram moderadamente E. faecalis e Staphylococcus aureus e não inibiram Pseudomonas aeruginosa, Salmonella enterica var. Typhimurium e uma amostra de E. duransisolada de leite de ovelha. Quatro amostras de E. faecium, uma de E. durans e uma de E. casseliflavus apresentaram o melhor potencial probiótico.

Palavras-Chave: leite de ovelha; enterococos; potencial probiótico

INTRODUCTION

Sheep milk represents only 1.3% of total global milk production, but is mostly used for cheese making, such as Roquefort, Pecorino and Serra da Estrela, with a better yield and higher value-added since its solids ratio is higher than other ruminant milks (Tsakalidou and Odos, 2012).

It is important to determine the sheep milk microbiota in order to understand the transformations that occur in cheese through its maturation. Studies from Abeijón et al. (2006) and Madrau et al. (2006) found predominance of the Enterococcus genus in sheep milk and determined their role in proteolysis during cheese maturation. Despite of that, there is not a focus on the probiotic potential of these microorganisms, such as their ability to resist gastric acid and biliary salts in order to reach their final destination - small or large intestine - in viable counts, and their antagonistic potential against important pathogenic microorganisms in the gastrointestinal tract (Silva et al., 2013). It is also important to evaluate antimicrobial susceptibility regarding probiotic properties because of the possible transference of antimicrobial resistance to pathogenic microorganisms from probiotic microorganisms (Lund and Edlund, 2001). Probiotic products are mainly represented by dairy products such as yogurts and fermented milks (Maragkoudakis et al., 2006).

Thus, the objective of this study was to determine the probiotic potential of microorganisms isolated from sheep milk for possible future use as probiotics in several dairies improving flavor and safety.

MATERIAL AND METHODS

Sheep milk was obtained at a small farm situated near Jaboticatubas city, Minas Gerais state, Brazil. Twenty samples were obtained from Lacaune, Santa Inês (a Brazilian breed) and their crossbreds. Dilutions were made up to 10-5 and were spread into Petri dishes containing MRS (Man Rogosa and Sharpe, Difco, USA) agar - incubated for 96h at 370C - and M17 (Difco) agar - incubated for 96h at 320C. Enumeration was made in Petri dishes containing from 20 to 200 CFU. Morphologically distinct colonies were submitted to Gram test, and the ones that had rod or round shape and were Gram positive were selected for further identification (adapted from IDF, 1983).

DNA from selected cultures was first obtained through treatment with LiCl (1M) for pellets obtained from each activated culture.Then they were incubated at 370C with constant mixing for 1h. New pellets were obtained and suspended in 1mL of protoplast buffer (50mM Tris HCl pH 8.0; 10mM EDTA and 10mg de lysozyme mL-1)with incubation at 370C with constant mixing for 1h. Total DNA was finally obtained with Wizard SV Genomic and DNA Purification System (Promega, USA), following the manufacturer indication.

All DNA samples were submitted to PCR reaction according to Tisala-Timisjarvi and Alatossava (1997). Primer 27F (5' AGAGTTTGATCCTGGCTCAG 3') was used as forward and 1492R (5' GGTTACCTTGTTACGACTT 3') as reverse to amplify the 16S rDNA gene of each sample. The program used was: 950C for the first 150sec, 35 cycles of 940C for 30sec, 550C for 60sec, 720C for 60sec and finally 720C for 10min, according to Moreira et al.(2005). Each sample was purified by Wizard SV Gel and PCR Clean-up System (Promega), according the manufacturer's instructions.

Each 16S rDNA sample was sequenced through Sanger's method with MegaBace 1000 (GE HealthCare, UK) according to Reysenbach et al. (2000). Results were submitted to the BLAST algorithm from GenBank located on the NCBI website.

Selected microorganisms were activated twice - 24h at 37oC - in 5mL of broth similar to the original solid medium that they grew in. Each sample was incubated at 37oC in the presence of gastric juice (0.85% NaCl, pH 2.0) for 3h and distributed (2% v/v) in three wells in a 96-well ELISA plate containing 0.2mL of pure MRS (Difco) broth each. The control of each sample was done with incubation at 370C for 3h in the presence of saline (0.85% NaCl) pH 7.0. They were alsodistributed (2% v/v) in three other wells containing 0.2mL of pure MRS broth each. Each plate containing 15 samples was incubated at 37oC for 18h. Absorbance at 620nm was read every 30min on a Spectramax 340 spectrophotometer (Molecular Devices, USA). Using Origin 8.5 (OriginLab, USA), differences in growth curve areas for each sample - control and in the presence of gastric juice - were calculated and the percentage of in vitro inhibition by gastric juice was obtained. This procedure was adapted from Walker and Gilliland (1993). Two repetitions were made.

Selected microorganisms were activated two times (24h at 370C) in 5mL of the broth similar to the original solid medium that they grew in. Each sample was distributed (2% v/v) in three wells in a 96-well ELISA plate containing 0.2mL of pure MRS (Difco) broth each, and in three wells containing 0.2mLof MRS (Difco) broth with 0.3% oxgall (Difco) each. Each plate containing 15 samples was incubated at 37oC for 18h. Absorbance at 620nm was read every 30min on a Spectramax 340 spectrophotometer (Molecular Devices). Using Origin 8.5 (OriginLab), differences in growth curve areas for each sample (control and in the presence of 0.3% oxgall - Difco) were calculated and the percentage of in vitro inhibition by biliary salts was obtained (adapted from Walker and Gilliland, 1993). Two repetitions were made.

Selected microorganisms were activated once - 24h at 370C - in 5mL of brothand then in agar similar to the original solid medium that they grew in. Then, each microorganism was transferred to 3.5mL of saline (0.85% NaCl) until turbidity equal to 0.5 on McFarland scale was obtained. Using a swab, each microorganism was transferred to a Petri dish containing MRS (Difco) agar. Antimicrobial discs (Oxoid, UK) were equally distributed on the surface of the agar. The antimicrobials used were: penicillin (PEN, 10U), oxacillin (OX, 1μg), vancomycin (VAN, 30μg), ceftazidime (CAZ, 30μg), streptomycin (S, 30μg), clindamycin (DA, 2μg), erythromycin (E, 5μg), ciprofloxacin (CIP, 5μg), gentamicin (CN, 10μg), and tetracycline (TE, 30μg). Each dish was incubated at 370C for 48h. Quality control was done using Escherichia coli ATCC 25922. The inhibition halos were measured with a Mitutoyo digital paquimeter (Mitutoyo, Brazil). This procedure was executed in duplicate with three repetitions and was adapted from Charteris (1998).

Selected microorganisms were activated twice - 24h at 370C - in 5mL of broth similar to the agar that they originally grew in. A 5μL spot was made from each activated microorganism in the center of a Petri dish containing MRS (Difco) agar. Each Petri dish was incubated for 48h at 370C. Then, 1mL of chloroform was added to the cover of each Petri dish and left to rest for 30min under UV. Another 30min with the cover open were needed to evaporate the chloroform. The revealing microorganisms were then added onto the surface of the former dishes through 7.5μL of their recent culture in 3.5mL of semi-solid BHI (Difco) for Enterococcus faecalis ATCC 19433, Escherichia coli ATCC 25922, Listeria monocytogenesATCC 15313, Pseudomonas aeruginosa ATCC 28853, Salmonella enterica var. Typhimurium ATTCC 14028 and Staphylococcus aureus ATCC 29213; or semi-solid MRS (Difco) agar for Enterococcus durans sample 23 isolated from sheep milk in the present study. Each dish was incubated for 48h at 37oC. The inhibition halos of each microorganism against the revealing microorganisms were measured with a Mitutoyo digital paquimeter (Mitutoyo). This procedure was executed in duplicate with three repetitions and was adapted from Tagg et al. (1976). The data obtained was analyzed using the Kruskal-Wallis test, since results from this kind of test usually present an abnormal behavior, and the level of significance was set at P<0.05.

RESULTS AND DISCUSSION

Using Sanger's method of sequencing the 16srDNA gene from microorganisms isolated from sheep milk, the following species were identified: nine (56.25%) out of 16 identified species were Enterococcus faecium, five (31.25%) were Enterococcus durans and two (12.5%) were Enterococcus casseliflavus (Table 1).

Medina et al. (2001), when identifying microorganisms isolated from sheep milk, also found a high percentage (48%) of bacteria from the Enterococcus genus. In a similar study, a lower (33%), but also significant percentage of bacteria from the Enterococcus genus was found in sheep milk (Oksuztepe et al., 2005).

According to Gilliland et al. (1984), when a 0.3 absorbance is achieved after at least 2h of incubation at 37oC in presence of gastric pH between 1.5 and 4.0, a microorganism can be considered tolerant or resistant to gastric pH. Considering this, 13 (81.25%) out of 16 samples tested can be considered tolerant to gastric pH (Table 2).

Table 1 Enterococcus spp. isolated from sheep milk identified by the sequencing of 16S rDNA gene using Sanger's method 

Sheep (Breed) Sample Species
1433 (half-blood) 3.1 E. faecium
L04 (Lacaune) 4.2 E. faecium
1516 (half-blood) 5 E. durans
L09 (Lacaune) 6.2 E. faecium
L09 (Lacaune) 6.4 E. faecium
194 (half-blood) 8.2 E. durans
1448 (half-blood) 10.3 E. casseliflavus
81 (Santa Inês) 12.1 E. faecium
81 (Santa Inês) 12.2 E. faecium
102 (half-blood) 14.1 E. faecium
102 (half-blood) 14.3 E. casseliflavus
102 (half-blood) 14.4 E. faecium
1469 (half-blood) 16 E. durans
300 (half-blood) 20.4 E. durans
300 (half-blood) 20.5 E. faecium
1508 (half-blood) 23 E. durans

Table 2 Maximum absorbance achieved by enterococci samples isolated from sheep milk after 3h incubation at 370C in the presence of gastric pH (2.0) and subsequent 18h incubation at 370C in plain MRS broth 

Sheep (Breed) Sample (Species) Maximum Absorbance
81 (Santa Inês) 12.1 (E. faecium) 0.987
L09 (Lacaune) 6.2 (E. faecium) 0.986
102 (half-blood) 14.1 (E.faecium) 0.966
300 (half-blood) 20.5 (E.faecium) 0.945
81 (Santa Inês) 12.2 (E.faecium) 0.944
300 (half-blood) 20.4 (E. durans) 0.933
1469 (half-blood) 16 (E. durans) 0.891
1516 (half-blood) 5 (E. durans) 0.699
102 (half-blood) 14.3 (E. casseliflavus) 0.493
L09 (Lacaune) 6.4 (E. faecium) 0.366
1433 (half-blood) 3.1 (E. faecium) 0.938
102 (half-blood) 14.4 (E. faecium) 0.324
L04 (Lacaune) 4.2 (E. faecium) 0.320
194 (half-blood) 8.2 (E. durans) 0.261
1448 (half-blood) 10.3 (E. casseliflavus) 0.223
1508 (half-blood) 23 (E. durans) 0.166

Morandi et al. (2005) found tolerance to acid pH from E. faecium samples such as what was found in the present work.

Tolerance to gastric juice was also considered for samples that achieved less than 40% of inhibition, therefore, 12 (75%) out of 16 samples were considered tolerant (Tab. 3). Sample 10.3 was not considered tolerant because it did not achieve a 0.3 absorbance (Table 2).

In a similar study, a 38.5% inhibition by gastric juice from E. duransand 56.8% from E. faecium samples were observed, confirming the resistance to gastric pH by enterococci samples observed in the present study (Cueto-Vigil et al., 2010).

Table 3 Inhibition percentage by gastric pH (2.0) of 16 enterococcisamples isolated from sheep milk 

Sheep (Breed) Sample (Species) Inhibition (%)
81 (Santa Inês) 12.1 (E. faecium) 0.00
102 (half-blood) 14.1 (E. faecium) 0.00
300 (half-blood) 20.4 (E. durans) 0.73
300 (half-blood) 20.5 (E. faecium) 2.08
81 (Santa Inês) 12.2 (E. faecium) 5.66
1469 (half-blood) 16 (E. durans) 7.26
L09 (Lacaune) 6.4 (E.faecium) 16.01
L04 (Lacaune) 4.2 (E. faecium) 19.74
1433 (half-blood) 3.1 (E. faecium) 26.29
1448 (half-blood) 10.3 (E. casseliflavus) 26.75
L09 (Lacaune) 6.2 (E. faecium) 31.97
1516 (half-blood) 5 (E. durans) 36.00
102 (half-blood) 14.3 (E. casseliflavus) 39.60
194 (half-blood) 8.2 (E. durans) 42.42
102 (half-blood) 14.4 (E. faecium) 57.62
1508 (half-blood) 23 (E. durans) 75.28

According to Gilliland et al. (1984), when 0.3 of absorbance is achieved after at least 8h of incubation at 370C in presence of 0.3% oxgall, a microorganism can be considered tolerant or resistant to biliary salts. Considering this, 11 (68.75%) out of 16 samples tested can be considered tolerant to biliary salts (Table 4).

Table 4 Maximum absorbance achieved by enterococci samples isolated from sheep milk after 18h incubation at 370C in presence of 0.3% of oxgall 

Sheep (Breed) Sample (Species) Maximum Absorbance
1433 (half-blood) 3.1 (E. faecium) 0.938
1516 (half-blood) 5 (E. durans) 0.883
194 (half-blood) 8.2 (E. durans) 0.843
L09 (Lacaune) 6.4 (E. faecium) 0.841
1469 (half-blood) 16 (E. durans) 0.841
L04 (Lacaune) 4.2 (E. faecium) 0.834
L09 (Lacaune) 6.2 (E. faecium) 0.827
102 (half-blood) 14.4 (E. faecium) 0.824
1508 (half-blood) 23 (E. durans) 0.799
102 (half-blood) 14.3 (E. casseliflavus) 0.768
300 (half-blood) 20.5 (E.faecium) 0.576
102 (half-blood) 14.1 (E.faecium) 0.245
81 (Santa Inês) 12.2 (E.faecium) 0.225
81 (Santa Inês) 12.1 (E. faecium) 0.213
300 (half-blood) 20.4 (E. durans) 0.132
1448 (half-blood) 10.3 (E. casseliflavus) 0.039

Tolerance to 1% oxgall from E. durans and E. faeciumsamples isolated from Feta cheese was observed in another study (Ambadoyiannis et al., 2005). Pereira and Gibson (2002) found tolerance to 0.4% oxgall from an E. durans sample.

Tolerance to biliary salt was also considered for samples that achieved less than 40% of inhibition, therefore, nine (56.25%) out of 16 samples were considered tolerant to biliary salts (Table 5). Sample 14.1 was not considered tolerant because it did not achieve a 0.3 absorbance (Table 4).

Table 5 Inhibition percentage by biliary salts (oxgall 0.3%) of 16 enterococci samples isolated from sheep milk 

Sheep (Breed) Sample (Species) Inhibition (%)
1433 (half-blood) 3.1 (E. faecium) 23.22
102 (half-blood) 14.3 (E. casseliflavus) 26.68
1516 (half-blood) 5 (E. durans) 26.69
194 (half-blood) 8.2 (E. durans) 26.89
102 (half-blood) 14.4 (E. faecium) 27.51
L04 (Lacaune) 4.2 (E. faecium) 30.14
1508 (half-blood) 23 (E. durans) 31.76
L09 (Lacaune) 6.2 (E. faecium) 32.55
L09 (Lacaune) 6.4 (E.faecium) 32.75
102 (half-blood) 14.1 (E. faecium) 34.14
1469 (half-blood) 16 (E. durans) 41.64
300 (half-blood) 20.5 (E. faecium) 44.27
81 (Santa Inês) 12.2 (E. faecium) 58.76
81 (Santa Inês) 12.1 (E. faecium) 58.76
300 (half-blood) 20.4 (E. durans) 63.01
1448 (half-blood) 10.3 (E. casseliflavus) 85.88

All enterococci samples were resistant to ceftazidime, oxacillin and streptomycin; and sensitive to clindamycin (sample 12.1 was only moderately sensitive), erythromycin, penicillin and tetracycline (sample 20.4 was only moderately sensitive). Samples 12.1 and 14.1 were the only ones moderately sensible to ciprofloxacin and sensitive to vancomycin, and the other samples were resistant. These two samples were also the only ones resistant to tetracycline, and the others were sensitive. Samples 14.4 and 20.5 were the only ones sensitive to gentamicin, and the other samples were resistant (Table 6).

Table 6 Enterococci antimicrobial susceptibilitya 

Sample Antimicrobial
CAZ CIP DA E GN OX P S TE VA
3.1 R R S S R R S R S R
4.2 R R S S R R S R S R
5 R R S S R R S R S R
6.2 R R S S R R S R S R
6.4 R R S S R R S R S R
8.2 R R S S R R S R S R
10.3 R R S S R R S R S R
12.1 R MS MS S R R S R R S
12.2 R R S S R R S R S R
14.1 R MS S S R R S R R S
14.3 R R S S R R S R S R
14.4 R R S S S R S R S R
16 R R S S R R S R S R
20.4 R R S S R R S R MS R
20.5 R R S S S R S R S R
23 R R S S R R S R S R

aCAZ: ceftazidime (30 μg), CIP: ciprofloxacin (5 μg), DA: clindamycin (2 μg), E: erythromycin (5 μg), GN: gentamicin (10 μg), OX: oxacillin (1 μg), PEN: penicillin (10 U), S: streptomycin (30 μg), tetracycline (30 μg), and vancomycin (VAN, 30 μg). R: resistant, MS: moderately sensible, S: sensible

Resistance to ciprofloxacin and vancomycin was found by most of the enterococci samples tested in a similar study (Coque et al., 1996), corroborating the findings of the present study. Cueto-Vigil et al. (2010) found sensitivity to clindamycin, erythromycin, penicillin and tetracycline by most of enterococci samples isolated from cheese, such as the enterococci isolated from sheep milk in this work.

Mannu et al. (2003), when comparing susceptibility to antimicrobials of enterococci from different origins, found that enterococci from raw sheep milk were sensitive to penicillin, tetracycline and vancomycin, and the ones isolated from sheep feces were sensitive to vancomycin, moderately sensitive to tetracycline and resistant to penicillin. This work leads to the supposition that the enterococci isolated from sheep milk in this study are from milk and not from feces - or any other contamination.

The inhibition of tested enterococci were considered significant (P<0.05) against E. coli and L. monocytogenes when compared to the other revealing microorganisms tested. The inhibition was less intense, but still significant (P<0.05), against E. faecalis and S. aureus when compared to the poor inhibition against S. Typhimurium, and the non-existent inhibition against P. aerugionosa and E. durans - sample 23 isolated from sheep milk (Table 7). It is interesting to notice here that an E. faecalis pathogenic sample was significantly (P<0.05) inhibited while an E. durans sample from sheep milk was not. None of the enterococci samples showed statistical difference in their behavior against all pathogens, indicating similar inhibition profiles according to Kruskal-Wallis test (P>0.05).

A diversity of bacteriocins produced by enterococci is known as good inhibitors to L. monocytogenes and S. aureus growth, according to Giraffa (1995). In Ennahar et al. (2001) and Sarantinopoulos et al. (2002) studies in different cheeses, different E. faecium samples inhibited reference L. monocytogenes and S. aureus samples. These results confirm the results observed in the present study.

A study developed with enterococci from goat cheese against reference pathogens found, as in this work, inhibition against E. coli, L. monocytogenes and S. aureus (Psoni et al., 2006).

In Strompfová et al. (2006) and Taras et al. (2006) works, in vivo inhibition of different E. faeciumprobiotic samples against E. coli in piglets were proven. These results confirm what was demonstrated in vitro in the present work.

Table 7 Means (mm) of inhibition halos of enterococci samples against reference microorganismsa 

Reference Microorganism Mean (mm) halo Inhibition
Escherichia coli 65.60a
Listeria monocytogenes 57.87a
Enterococcus faecalis 24.11b
Staphylococcus aureus 19.84b
Salmonella Typhimurium 3.85c
Pseudomonas aeruginosa 0.00d
Enterococcus durans(sample 23) 0.00d

aMeans followed by distinct letters are different by Kruskal-Wallis test (p<0.05)

CONCLUSION

E. durans, E. faecium and E. casseliflavus samples isolated from sheep milk (from Lacaune, Santa Inês and their crossbreeds) can present in vitro probiotic properties such as resistance to gastric juice, biliary salts and antagonism against reference pathogens such as Escherichia coli, Listeria monocytogenes and Staphylococcus aureus. Therefore, their use as probiotics in dairy products is promising, although more in vitro and in vivo studies are needed to prove their full probiotic potential and their inability to transfer antimicrobial resistance genes.

ACKNOWLEDGMENTS

We thank FAPEMIG (Fundação de Amparo à Pesquisa em Minas Gerais) for the funding and LGMPP at ICB UFMG and DTIPOA at EV UFMG laboratories for the equipment and help provided.

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Received: June 05, 2012; Accepted: November 07, 2013

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