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Pesquisa Veterinária Brasileira

Print version ISSN 0100-736XOn-line version ISSN 1678-5150

Pesq. Vet. Bras. vol.38 no.7 Rio de Janeiro July 2018

http://dx.doi.org/10.1590/1678-5150-pvb-55141 

LIVESTOCK DISEASES

Detection of Staphylococcus aureus, Streptococcus agalactiae and Escherichia coli in Brazilian mastitic milk goats by multiplex-PCR

Detecção de Staphylococcus aureus, Streptococcus agalactiae e Escherichia coli em leite caprino mastítico no Brasil, por multiplex-PCR

Gustavo P. Machado2 

Rodrigo C. Silva3 

Felipe F. Guimarães4 

Anelise Salina4 

Hélio Langoni4  * 

2 Médico Veterinário, Rua Rodrigues Alves 227, Centro, Itápolis, SP 14900-000, Brazil.

3 Centro de Ciências Agrárias, Universidade Estadual do Norte do Paraná, Campus Luiz Meneghel, sala 88, Rodovia BR-369 Km 54, Vila Maria, Bandeirantes, PR 86360-000, Brazil.

4 Departamento de Higiene Veterinária e Saúde Pública, Universidade Estadual Paulista (Unesp), Rua Prof. Dr. Walter Maurício Correa s/n, Botucatu, SP 18618-681, Brazil.


ABSTRACT:

This study evalueted the prevalence of Staphylococcus aureus, Streptococcus agalactiae and Escherichia coli in milk samples from 257 goats (513 half-udders) and ten bulk tanks, from ten dairy goat farms of São Paulo State, Brazil, by multiplex-PCR. The samples were screened by microbiological culture (gold-standard), and tested by different multiplex-PCR protocols for the detection of each bacterium. A total of 178 half-udders resulted positive by microbiological culture, with coagulase-negative staphylococci (70%), S. aureus (13.5%), S. intermedius (7.9%), and Enterobacteriaceae (4%) the prevalent pathogens. In other way, multiplex-PCR detected 173 pathogens in 151/523 (28.9%; CI95% 25.2-32.9%) milk samples 144/513 (28.1%) half-udders and 7/10 (70%) bulk tanks, with E. coli (86/162, 51.9%) and S. aureus (50/162, 30.9%) the prevalent ones in half-udders, and S. aureus (6/10, 60%) and E. coli (4/5, 36.4%) in bulk tanks. Multiplex-PCR showed a high performance for the detection of three bacteria at a time in mastitic goat milk direct from half-udders or bulk tanks. Thus, this multiplex-PCR protocol proved to be an adequate tool for the identification of the most common mastitis pathogens, independent of their phenotypic characteristics in the diagnosis of clinical mastitis in goats, allowing a continuous and better vigilance and monitoring the herd, being included in quality programs.

INDEX TERMS: Staphylococcus aureus; Streptococcus agalactiae; Escherichia coli; mastitis; milk; goats; multiplex-PCR; bulk tanks; microbiological culture

RESUMO:

Este estudo avaliou por multiplex-PCR a prevalência de Staphylococcus aureus, Streptococcus agalactiae e Escherichia coli em amostras de leite de 257 caprinos (513 tetos) e dez tanques de expansão, em dez fazendas leiteiras do estado de São Paulo, Brasil. As amostras foram triadas por cultura microbiológica (padrão-uro) e testadas por diferentes protocolos multiplex-PCR para a detecção de cada bactéria. Um total de 178 amostras de leite foram positivos na cultura microbiológica, com estafilococos coagulase-negativos (70%), S. aureus (13,5%), S. intermedius (7,9%) e Enterobacteriaceae (4%) como patógenos prevalentes. Por outro lado, a PCR multiplex detectou 173 patógenos em 151/523 (28,9%, IC95% 25,2-32,9%) amostras de leite, 144/513 (28,1%) amostras de tetos e 7/10 (70%) em tanques de expansão, E. coli (86/162, 51,9%) e S. aureus (50/162, 30,9%) foram identificados nas amostras de tetos e S. aureus (6/10, 60%) e E. coli (4/5, 36,4%) em tanques expansão. Multiplex-PCR mostrou um alto desempenho para a detecção das três bactérias em leite de cabra com mastite ou em tanques de expansão. Dessa forma, este protocolo multiplex-PCR provou ser uma ferramenta adequada para a identificação dos patógenos mais comuns da mastite, independentemente de suas características fenotípicas no diagnóstico de mastite clínica em caprinos, permitindo uma vigilância contínua e melhor acompanhamento do rebanho, sendo incluído em programas de qualidade.

TERMOS DE INDEXAÇÃO: Staphylococcus aureus; Streptococcus agalactiae; Escherichia coli; leite; caprinos; mastítis; multiplex-Pcr; tanques de expansão; cultura microbiológica

Introduction

Mastitis is an inflammation of the mammary gland, caused by over 150 different contagious or environmental micro-organisms (Kuang et al. 2009). Although a wide spectrum of bacterial species have been identified as causative agents of mastitis, only few species of staphylococci, streptococci and coliforms present economic and epidemiological importance. In many countries, Staphylococcus aureus is the most significant pathogen causing intra-mammary infection in dairy animals (Graber et al. 2007, Kuang et al. 2009). In addition, coagulase-negative staphylococci (CNS), a group of Staphylococcus spp., have become the predominant pathogens causing subclinical or clinical mastitis in several countries (Pyorala & Taponen 2009, Schukken et al. 2009).

Streptococcus agalactiae is an important pathogen in caprine mastitis, causing fibrosis and reduced milk production (Bergonier et al. 2003). Escherichia coli is one of the main pathogens associated with environmental mastitis, causing to a wide range of systemic disease, from acute to severe (Gunther et al. 2011). Raw milk and dairy products made from nonpasteurized milk have been responsible for S. aureus and E. coli outbreaks and could represent a potential hazard for consumers (Little et al. 2008). Accurate species identification is essential to enable rapid intervention and the use of the appropriate treatment for the control of caprine mastitis (Ghebremedhin et al. 2008).

As the number of causative agents of mastitis is large and many of these are genetically closely related, the differentiation of the different species within this large group is complex (Koskinen et al. 2009). Moreover, identification based on phenotypic differences is unreliable because there is lack of unique biochemical markers for species identification (Capurro et al. 2009). Although DNA sequence-based molecular methods have proved to be proficient approach for species identification, reliable, feasible, fast and accurate identification methods still remain a necessity (Cremonesi et al. 2009, Zadoks & Watts 2009).

In this way, the present study was aimed to determine the prevalence of S. aureus, S. agalactiae and E. coli in milk samples from goats and bulk tanks by the use of a sensitive and specific multiplex-PCR assay for the simultaneous detection of these causative agents of caprine mastitis.

Materials and Methods

Sampling. A total of 508 half-udder milking samples (254 goats) and five bulk tanks were sampled in an unique timepoint for each one of 10 flocks during the lactation period. The first few streams of milk were discarded, and duplicate half-udder milk sample were aseptically collected into sterile tubes after cleaning and disinfection of each teat end. One sample (10mL) was used for bacteriological culture and molecular analysis. The milk sample was stored at 4°C.

Microbiological methods. Each half-udder milk sample was homogenized and 10μL were inoculated onto 5% Sheep Blood Agar (SBA) and MacConkey Agar (MA) plates. The plates were incubated aerobically at 37°C and examined after 24, 48 and 72 hours. Bacterial colonies were isolated on brain heart infusion agar (BHI) (Oxoid, UK), and identified using routinely microbiological procedures such as colony morphology, microscopic characteristics and Gram staining, hemolysis pattern on SBA, catalase and oxidase reactions. Additional classification was carried out by specific biochemical sets for each bacteria Staphylococcus aureus, Streptococcus agalactiae (Quinn et al. 2005) and E. coli (Trabulsi et al. 2005).

Molecular methods. The milk samples were stored and frozen in 1.5-mL DNase and RNase free centrifuge tubes at -80°C. The extraction of DNA from milk samples of 10 dairy goats farms were performed using illustra genomicPrep blood Mini Spin Kit (GE Healthcare, USA) according to the instructions of the manufacturer with some modifications. All milk samples of half-udders, bulk tanks, and ATCC standards were submitted to the DNA extraction.

The analytical sensitivity for each analyzed bacterium, each bacterium was inoculated into BHI broth and incubated for 24 hours at 37°C. After, 900μL sterile ultrapure water were aliquoted into sterile microtubes free DNase and RNase, and other microtubules 900μL goat milk long life commercialized in Botucatu, SP. An aliquot of 100μL of the 109 bacterias/mL-1 suspension, obtained according to the McFarland scale 4(1.2 x 109 bacteria/mL-1) was diluted in 900μL of ultrapure water in order to obtain a serial dilution, in water, of 108, 107, 106, 105, 104, 103, 102, 101 and 100 bacterias/mL-1. Likewise, the same suspension 108 bacterias/mL-1 was diluted in 900μL goat milk sample in order to obtain a serial dilution of 108, 107, 106, 105, 104, 103, 102, 101, and 100 bacteria/mL-1.

All samples were assayed to PCR detection of S. aureus, S. agalactiae and E. coli by single PCR protocols firstly and, then, multiplex-PCR by the use of the primers described on Table 1. The primers targeted the species-specific regions of the DNA coding for 16S and 23S rRNA (16S or 23S rRNA), according to Straub et al. (1999), Riffon et al. (2001) and Chotár et al. (2006), and .the encoding gene sip (surface immunogenic protein), designated as Sequence Specific Sip (SSS) for GBS (Lancefield Group B Streptococcus), according to Chotár et al. (2006). The concentrations of reagents used in the multiplex-PCR was used to compare the Straub et al. (1999) and Chotár et al. (2006) studies. Each 0.2-mL microtube received 25μL of mix containing 10x magnesium-free PCR buffer (50mM KCl, 10mM Tris-HCl pH8.0), 1.5mM MgCl2, 0.2mM desoxynucleotide solution, 0.5U (Straub et al. 1999) Platinum Taq DNA polymerase (Life Technologies, USA), 10μM (Straub et al. 1999) or 25μM (Chotár et al. 2006) each oligonucleotide (Integrated DNA Technologies, USA) 10ng sample, and ultrapure water q.s.p (Life Technologies, USA). All reactions were run in a Mastercycler EP gradient thermocycler (Eppendorf, USA).

Table 1 Primers used for amplification of Staphylococcus aureus, Streptococcus agalactiae and Escherichia coli DNA in goat milk samples by multiplex-PCR 

Agent Target Primersa Primersb Sequences (5’-3’) Size (pb) Reference
S. aureus 23S rRNA Sau 327 SAU1 GGACGACATTAGACGAATCA 1300 Riffon et al. (2001)

Chotár et al. (2006)
Sau 1645 SAU2 CGGGCACCTATTTTCTATCT
23S rRNA STAUR4 - ACGGAGTTACAAAGGACGAC 1250 Straub et al. (1999)
STAUR6 - AGCTCAGCCTTAACGAGTAC
S. agalactiae 16S rRNA Sag 432 SAGA1 CGTTGGTAGGAGTGGAAAAT 590 Riffon et al. (2001)

Chotár et al. (2006)
Sag 1018 SAGA2 CTGCTCCGAAGAGAAAGCCT
sip* SIP3 SIP3 TGAAAATGCAGGGCTCCAACCTCA 293 Chotár et al. (2006)
SIP4 SIP4 GATCTGGCATTGCATTCCAAGTAT
E. coli 23S rRNA Eco 2083 Ecoli1 GCTTGACACTGAACATTGAG 660 Riffon et al. (2001)

Chotár et al. (2006)
Eco 2745 Ecoli2 GCACTTATCTCTTCCGCATT

sip* = Surface immunogenic protein (protein immunogenic surface), designated as Sequence Specific Sip (SSS), only for GBS (Lancefield Group B Streptococcus); a Names of primers of according to the original reference (Riffon et al. 2001), b Names of primers for multiplex-PCR according to Chotár et al. (2006).

Two cycling protocols were tested: the first, described by Chotár et al. (2006), using the primers SAU1 and SAU2, SIP3 and SIP4 or SAGA1 and SAGA2, Ecoli1 and Ecoli2, consisted of an initial incubation of 5 minutes at 96°C, followed by 30 cycles of 1 minute at 96°C, 1 minute at 55°C and 2 minutes 72°C, with a final incubation of 8 minutes at 72°C. The second, described by Straub et al. (1999), used the primers STAUR4 and STAUR6, SIP3 and SIP4, Ecoli1 and Ecoli2, and consisted of an initial incubation of 5 minutes at 96°C, followed by 37 cycles of 1 minute at 96°C, 1 min (gradient annealing test: 56°C, 56.2°C, 56.6°C, 57.3°C, 58.2°C, 59.3°C, 60.4°C, 61.5°C, 62.5°C, 63.4°C, 63.8°C, 64°C) and 2 minutes at 72°C with final incubation of 8 minutes at 72°C.

Aliquots of 8μL of the amplicons were mixed to 2μL bromophenol blue solution and subjected to horizontal electrophoresis in 1.5% agarose gel containing 0.3 ethidium bromide μL/mL-1 diluted in 1X tris-borate-EDTA (TBE). The amplification products were visualized and photographed in a digital photo documentation system, Gel-Doc System it (UVP, USA), VisionWorks LS software.

Statistical analysis. Descriptive statistics PROC FREQ (SAS Institute 2009) were produced to characterize the studied population, describe and estimate the prevalence of the etiological causative agents of mastitis. Chi-square or Fisher’s exact test (PROC FREQ, SAS Institute 2009) was used to test the hypothesis that the prevalence of mastitis was heterogenous among the farms. The concordance between the results of microbiological culture and multiplex-PCR, for half-udders and bulk tank, was analyzed by the Cohen’s kappa index. The statistics performance of the multiplex-PCR was analyzed by the use of Mackinnon (2000) spreadsheet. Statistical significance was defined as P<0.05.

Results

Microbiological culture

By microbiological culture, 178/513 (34.7%; CI95% 30.6-38.8%) half-udders presented positive results, with CNS (n=123; 70%), Staphylococcus aureus (n=24; 13.5%), Streptococcus intermedius (n=14; 8.0%), and Enterobacteriaceae (n=7; 4%) the prevalent pathogens observed. Additionally, Corynebacterium spp. (n=3; 1.7%), S. hyicus (n=3; 1.7%), Streptococcus spp. (n=1; 0.6%), Micrococcus spp. (n=1; 0.6%), Staphylococcus schleiferi subsp. coagulans (n=1; 0.6%) and Staphylococcus lutrae (n=1; 0.6%) were identified. Considering the frequency in all 513 studied half-udders the respective percentage was 24% CNS, 4.7% S. aureus, 2.73% S. intermedius, 1.4% Enterobacteriaceae and 0.2% Streptococcus spp. (Table 2).

Table 2. Frequency of Staphylococcus aureus, Streptococcus spp. and Enterobacteriaceae isolates by microbiological culture and multiplex-PCR for S. aureus, Streptococcus agalactiae and Escherichia coli in half-udders (hu) and status in bulk tanks (bt) of 10 dairy goat farms 

Farms Microbiological culture Multiplex-PCR
S. aureus Streptococcus spp. Enterobacteriaceae S. aureus S. agalactiae E. coli
hu(bt) % hu(bt) % hu(bt) % hu(bt) % hu(bt) % hu(bt) %
1 7(-) 11.5 0(-) 0.0 1(+) 1.6 9(-) 14.5 2(-) 3.2 18(-) 29.0
2 9(+) 12.9 0(+) 0.0 0(+) 0.0 17(+) 24.3 7(+) 10.0 10(+) 14.3
3 1(+) 2.3 0(-) 0.0 1(-) 2.3 2(-) 4.6 1(-) 2.3 13(-) 29.6
4 2(+) 4.0 0(-) 0.0 1(-) 2.0 2(+) 4.0 1(-) 2.0 0(-) 0.0
5 1(-) 2.2 0(-) 0.0 0(-) 0.0 3(-) 6.5 1(-) 2.2 7(-) 15.2
6 1(+) 1.3 0(-) 0.0 2(+) 2.6 6(+) 7.9 2(-) 2.6 19(+) 25.0
7 3(+) 7.5 0(-) 0.0 0(-) 0.0 5(+) 12.5 4(-) 10.0 5(-) 12.5
8 0(+) 0.0 1(-) 2.2 0(+) 0.0 4(+) 8.7 8(-) 17.4 2(+) 4.4
9 0(+) 0.0 0(-) 0.0 0(-) 0.0 2(+) 5.0 1(-) 2.5 2(-) 5.0
10 0(-) 0.0 0(-) 0.0 2(+) 5.0 0(-) 0.0 1(-) 2.5 8(+) 20.0
TOTAL 24(7) 4.7(70.0) 1(1) 0.2(10.0) 7(5) 1.4(50.0) 50(6) 9.7(60.0) 28(1) 5.5(10.0) 84(4) 16.4(40.0)

hu = half-udders, bt = bulk tanks; + positive results for the pathogen research in bulk tank of each farm, - negative result for the pathogen research in bulk tank of each farm.

Additionally, the frequency of CNS, S. intermedius, S. aureus, Corynebacterium spp., Enterobacteriaceae, S. hyicus, and Streptococcus spp. ranged from 0 to 50%, 0 to 13.1%, 0 to 12.9%, 0 to 5%, 0 to 3.4%, 0 to 2.6%, and 0 to 2.2% respectively, among the dairy goat farms. The distribution of isolated pathogens from the milk samples were not homogenous among the farms (P<0.01).

Multiplex-PCR

Specific PCR was performed for each agents using the protocol and primers described by Chotár et al. (2006), which uses an annealing temperature of 55°C. When examining the efficiency of the primer in the detection of the agent in water samples and goat milk samples contaminated, was observed that the sets of primers SAU1 and SAU2, SAGA1 and SAGA2, and Ecol1 and Ecol2 detected until 100 bacteria/mL-1 in water, but when analyzing milk samples contaminated, the same primers detected a minimum of 102 bacteria.mL-1 (Staphylococcus aureus), 104 bacteria.mL-1 (Streptococcus agalactiae), and 100 bacteria.mL-1 (E. coli). The low analytical sensitivity observed for S. aureus and S. agalactiae may be suggested by the low frequency of copies of regions flanked by primers SAU1 and SAU2, and mainly SAGA1 and SAGA2, used in the work described by Chotár et al. (2006).

Based on these results, we tested the set of primers STAUR4 and STAUR6, described by Straub et al. (1999) for detection of S. aureus, and SIP3 and SIP4 described by Chotár et al. (2006) for detection of S. agalactiae. Chotár et al. (2006) standardized the multiplex-PCR protocol used previously. We reused the same protocol modifying primers specific for S. aureus and S. agalactiae, modifying the temperature gradient annealing, since the annealing temperature used by Straub et al. (1999), STAUR6 and STAUR4, was 64°C, it is considered very high when compared to the temperature used for Chotár et al. (2006), 55°C. It was observed that separately by testing concentrations of 106 bacterias/mL-1, the same amplification was obtained in all tested temperatures.

However, when analyzing the gradient temperature with all three pairs of primers together (STAUR4 and STAUR6, SIP3 and SIP4, and Ecol1 and Ecol2) and the three agents present at concentration of 107 bacteria.mL-1, it was observed that the annealing temperature with better results in simultaneous detection of three agents was 64°C, as described by Straub et al. (1999) for detecting by STAUR4 and STAUR6.

Using primers STAUR4 and STAUR6, SIP3 and SIP4, or Ecol1 and Ecol2 on individually PCR assays, and comparing the concentrations of primers, was observed that the use of primers in lower concentrations (10μM), as shown by Straub et al. (1999), showed better results (Fig.1), obtaining the respective analytical sensitivity for S. aureus: 100 (10μM) and 102 (25μM) bacteria.mL-1 goat milk samples. For S. agalactiae, the analytical sensitivity was 100 (10μM) and 101 (25μM) bacteria.mL-1 in milk samples, whereas for it was 100 (10 um) and 102 (25μM) bacteria.mL-1 for E. coli. Another advantage observed for the detection of S. agalactiae was compared to the size of the base pairs observed on amplification with primers SAGA1 and SAGA2. These primers amplify a region of 660bp, closed size to that amplicon produced by Ecol1 and Ecol2 (590bp), which one can cause confusion on the analysis. As the set of primers SIP3 and SIP4 produces an amplicon 293bp, the confusion on the analysis can be avoided (Fig.1). The 10μMprimer concentration showed better results in comparison with the 25μM concentration, and was selected to be used in this study. When tested together on a multiplex-PCR, the set of primers SATUR4:STAUR6 (S. aureus), S1P3:SIP4 (S. agalactiae) and Ecol1:Ecol2 (E. coli) showed again better results than the others, by the use of 10μM each primer, and were selected to run multiplex-PCR on all 513 samples (Fig.2).

Fig.1. Detection threshold (107 to 100) separately using primers STAUR4 and STAUR6, SIP3 and SIP4, and Ecol1 and Ecol2 in different concentrations: 10μM according with Straub et al. (1999), 25μM according Chotár et al. (2006) in goat milk sample, using the PCR protocol described by Straub et al. (1999). M = molecular weight standard (100 base pairs). 

Fig.2. Multiplex-PCR with primers STAUR4 and STAUR6, SIP3 and SIP4, and Ecol1 and Ecol2 isolated and together, using various concentrations of primers: 10μM according with Straub et al. (1999), 25μM according Chotár et al. (2006), in sample of goat milk. M = molecular weight standard (100 base pairs). 

Multiplex-PCR detected 173 pathogens in 151/523 (28.9%; CI95% 25.2-32.9%) milk samples [144/513 (28.1%) hald-udders and 7/10 (70%) bulk tanks], with the following frequency: E. coli (86/162, 51.9%), S. aureus (50/162, 30.9%) and S. agalactiae (28/162, 17.3%) in half-udders; and S. aureus (6/10, 60%) and E. coli (4/5, 36.4%) in bulk tanks. S. agalactiae presented negative results for all bulk tank samples. In this way, the prevalence of S. aureus ranged from 0 to 24.3%, S. agalactiae from 2 to 17.4%, and E. coli from 0 to 29.6% among the dairy goat farms (Table 2).

Analyzing just the bulk-tanks, the prevalence of S. aureus was 60% (6/10), S. agalactiae 10% (1/10) and E. coli 40% (4/10) by multiplex-PCR, similar results to those observed by microbiological culture, with 70% (7/10) S. aureus, 50% (5/10) Enterobacteriaceae and 10% (1/10) Streptococcus spp., presenting an almost perfect agreement (κ = 0.8618), when considering just the half-udders results, and multiplex-PCR presented a sensitivity of 84.6%, specificity of 100%, and positive and negative predictive values of 100% and 89.5%, respectively. A low positive (30.9%) and high negative (85.4%) agreements were still observed between both tests. In other way, regarding to the half-udders a fair concordance was observed (κ = 0.2312), and multiplex-PCR presented a sensitivity of 87.5%, specificity of 75.1%, and positive and negative predictive values of 18.8% and 98.9%, respectively. High positive (91.7%) and negative (94.4%) agreements were still observed between both tests.

Animals presenting co-infection were observed in seven farms by multiplex-PCR. Seventeen pathogens were detected causing co-infection. One animal from farm #7 presented all three studied pathogens. The most common co-infection was between S. aureus and E. coli (10/17, 58.8%) in six farms, followed by S. agalactiae and E. coli (4/17, 23.5%) in four farms, and S. aureus and S. agalactiae (2/17, 11.8%) in two farms.

Discussion

A technique that has been used in an increasing order for bacterial identification is the multiplex-PCR, which uses more than one pair of primers in the same reaction, amplifying simultaneously various DNA sequences. This technique allows the identification of more than one bacterial species in the same PCR, promoting a more rapid and wide of pathogenic bacteria in food (Wilson et al. 1991). Thus, the objective was to research directly in the milk of 513 half-udders and 10 bulk tanks pathogens: Staphylococcus aureus, Streptococcus agalactiae and Escherichia coli.

Specific PCR was performed for each of the agents using the protocol and primers described by Chotár et al. (2006). By low analytical sensitivity observed for S. aureus and S. agalactiae, which may be suggested by low frequency copies of regions flanked by the primers SAU1 and SAU2, and SAGA1 and SAGA2, and was tested primers STAUR6 and STAUR4 described by Straub et al. (1999) for detection of S. aureus, and SIP3 and SIP4 described by Chotár et al. (2006) for detection of S. agalactiae.

Chotár et al. (2006) standardized multiplex-PCR protocol used previously, and attempt to reuse the same protocol modifying primers specific for S. aureus and S. agalactiae, we tested the temperature gradient annealing, since the annealing temperature used by Straub et al. (1999) for STAUR6 and STAUR4 to 64°C, was considered very high in relation to the temperature used for Chotár et al. (2006), 55°C, however, the temperature described by Straub et al. (1999) was the best result presented in the simultaneous detection of three agents.

Utilizing primers STAUR4 and STAUR6, SIP3 and SIP4, and Ecol1 and Ecol2, and comparing the concentrations of primers, was observed that the use of primers in lower concentrations (10μM), as shown by Straub et al. (1999) showed better results (Table 1, Fig.2), obtaining analytical sensitivity in detecting S. aureus 100 (10μM) and 102 (25μM) bacterias/mL-1 goat milk samples. For the detection of S. agalactiae, sensitivity to 100 (10μM) and 101 (25μM) bacterias/mL-1 was obtained in samples of milk, while for Escherichia coli 100 (10μM) and 102 (25μM) bacterias/mL-1.

The primers SIP3 and SIP4 produce an amplicon 293pb, different of the 660pb produced by SAGA1 and SAGA2, and 590pb produced by Ecol1 and Ecol2, no problem in the agarose gel analysis (Table 1).

The prevalent pathogens by multiplex-PCR in half-udders were E. coli (16.4%), S. aureus (9.7%) and S. agalactiae (5.5%) and, in bulk tanks, S. aureus (60%), E. coli (40%) and S. agalactiae (10%).

Of the 10 dairy goat farms that had the half-udders evaluated by microbiological culture, seven were isolated S. aureus, five were Enterobacteriaceae family and in only one was isolated Streptococcus spp., being correctly diagnosed by multiplex-PCR six bulk tanks with S. aureus, one with S. agalactiae and four E. coli. This shows the high sensitivity of the technique for the diagnosis of these pathogens and their applicability to monitoring the health of livestock and quality of milk bulk tanks. The prevalent pathogens by multiplex-PCR in half-udders were E. coli (16.4%), S. aureus (9.7%) and S. agalactiae (5.5%) and, in bulk tanks, S. aureus (60%), E. coli (40%) and S. agalactiae (10%).

The bulk tank of farm #2 was positive for all three agents by the multiplex-PCR, however, the bulk tanks of the farms #1, #3 and #5 were negative for all three bacteria. The absence of positive results for S. aureus, S. agalactiae and E. coli agreed to the results of the microbiological culture and reinforces the point that the sensitivity and specificity of molecular methods for the detection of bacterial pathogens in milk samples are closely related to both the characteristics of microorganisms, such as complexity of the cell wall, which may hinder lysis during the extraction of DNA, as well as the types of primers used (Cremonesi et al. 2006).

Microbiological culture of half-udders detected 24 positive samples for S. aureus, against 50 by multiplex-PCR. S. aureus is the most pathogenic infectious agent to the mammary gland of goats, both subclinical and clinical disease, its importance is evident in public health by the production of toxins, serving as an indicator of sanitary-hygiene in the food industry (Murray et al. 1992, Anderson et al. 2004).

Streptococcus spp. was diagnosed only by microbiological culture in just one half-udders milk sample, against 28S. agalactiaepositive results by multiplex-PCR . Sporadically, S. agalactiae causes mastitis in goats. The infection with this pathogen may result in fibrosis and decrease in milk production (Bergonier et al. 2003, Anderson et al. 2004).

In the same way, E. coli detection presented higher sensitivity by multiplex-PCR (84 positive half-udders milk samples detected), when compared to the detection of Enterobacteriaceae family by microbiological culture (seven positive samples). E. coli can be found in organic matter (feces and soil), water, earth, air and animal bedding, and hardly will not be present in the milk of animals after milking (Smith & Sherman 2009, Fuquay et al. 2011). The prevalence of infection with E. coli in goats is low, occurring mainly in the periods between milking, representing public health risk (Radostitis et al. 2007, Fuquay et al. 2011).

The multiplex-PCR technique standardized in this study showed an almost perfect agreement for the detection of S. aureus, S. agalactiae and E. coli in bulk tanks when compared to microbiological culture, but a fair one in goat half-udders milk samples. In other way, multiplex-PCR presented a high quality perfomance on the detection of real positive and negative results in goat milk samples obtained direct from the half-udders or bulk tanks, presenting at least high negative agreement for both type of samples, and a high positive agreement for bulk tank samples, when compared to the microbiological culture. Thus, the combined use of this technique with microbiological culture allows maximizing the sensitivity and specificity of technique, increasing the efficiency of the mastitis diagnosis on goats.

Conclusions

The standardized multiplex-PCR showed good accuracy for the detection of Staphylococcus aureus, Streptococcus agalactiae and Escherichia coli in mastitic goat milk directly of the mammary gland and bulk tanks.

As correct species identification is important for mastitis treatment, prevention, control and in epidemiological investigations, as well as to understand of the significance of infections caused by different bacterial species, this assay proved to be an adequate tool for the identification of the most common mastitis pathogens, independent of their phenotypic characteristics in the diagnosis of clinical mastitis, allowing a continuous and better vigilance and monitoring of the herd, which one could be included in quality programs.

Acknowledgments

We would like to thank the São Paulo Research Foundation (FAPESP) for PhD grant and financial support, Processes (# 2010/17254-7) and (# 2011/07107-0).

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Received: June 29, 2017; Accepted: January 08, 2018

*Corresponding author: hlangoni@fmvz.unesp.br

Conflict of interest.- The authors declare no conflict of interest.

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