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Pesquisa Veterinária Brasileira

versão impressa ISSN 0100-736Xversão On-line ISSN 1678-5150

Pesq. Vet. Bras. vol.28 no.12 Rio de Janeiro dez. 2008 

Staphylococcal toxin genes in strains isolated from cows with subclinical mastitis


Detecção de genes de toxinas em linhagens de estafilococos isolados de vacas com mastite subclínica



Manuela F.L. de FreitasI, II; Isabelle da S. LuzII; Vladimir da M. Silveira-FilhoI, II; José W.P. JúniorIII; Tânia L.M. StamfordI; Rinaldo A. MotaIII; Maria J. de SenaIII; Alzira M.P. de AlmeidaII; Valdir de Q. BalbinoI; Tereza C. Leal-BalbinoII, *

IUniversidade Federal de Pernambuco (UFPE), Cidade Universitária, Recife, PE 50670-901, Brazil
IIDepartamento de Microbiologia, Centro de Pesquisas Aggeu Magalhães (CPqAM), Fiocruz, Av. Prof. Moraes Rego s/n, Campus da Cidade Universitária, Recife, PE 50670-420
IIIDepartamento de Medicina Veterinária, UFRPE, Dois Irmãos, Recife, PE 52171-900




The present study was carried out in 11 dairy herds in four municipal districts of the rural area of the State of Pernambuco, Brazil. Out of 984 quarter milk (246 cows), 10 (1.0%) were positive for clinical mastitis, 562 (57.1%) for subclinical mastitis and 412 (41.9%) were negative. A total of 81 Staphylococcus spp. isolates were obtained from milk samples from the cows diagnosed with subclinical mastitis. From these, 53 (65.0%) were S. aureus, 16 (20.0%) coagulase-positive staphylococci (CPS) and 12 (15.0%) coagulase-negative staphylococci (CNS). The isolates were further investigated for the presence of toxin genes by multiplex and uniplex PCR. The main gene observed was seg followed by seh, sei and sej. The distribution of these observed genes among the isolates obtained from different areas showed a regional pattern for the SEs. The presence of toxin genes in the strains isolated from bovine milk demonstrates a potential problem for public health.

Index terms: Staphylococcal toxins, toxin genes, milk, dairy cows.


O presente estudo foi realizado em 11 rebanhos leiteiros de quatro municípios da área rural do estado de Pernambuco, Brasil. Dos 984 quartos mamários examinados (246 vacas), 10 (1,0%) foram positivos para a mastite clínica, 562 (57,1%) para a mastite subclínica e 412 (41,9%) foram negativos para mastite. Foram isoladas 81 linhagens de Staphylococcus spp. do leite de vacas com mastite subclínica. Destes, 53 (65,0%) foram S. aureus, 16 (20,0%) estafilococos coagulase-positivo (SCP) e 12 (15,0%) estafilococos coagulase-negativo (SCN). O principal gene observado nos estafilococos foi o seg seguido pelo seh, sei e sej. Foi constatada distribuição regional dos genes dos estafilococos isolados dos animais nos municípios estudados. A presença dos genes das toxinas nas linhagens isoladas do leite de vacas representa risco potencial para a Saúde Pública.

Termos de indexação: Toxinas estafilocócicas, genes das toxinas, leite, vacas leiteiras.




Staphylococci cause several human and animal diseases. These pathogenic groups of microorganisms play an important role in the etiology of infectious bovine mastitis. Classically, staphylococci are related predominantly in subclinical form of bovine mastitis. Subclinical mastitis is very important because of its high prevalence among herds, decreases milk production, difficult therapy and diagnosis (Bradley et al. 2002).

Some staphylococci produce staphylococcal enterotoxins (SEs) involved in staphylococcal food poisoning syndrome in humans, especially in the toxic shock syndrome toxin 1 (TSST-1), in humans patients and the exfoliate toxins (ETA and ETB) that cause staphylococcal scalded skin syndrome in children and newborns. Recently, 19 serologically distinct SEs have been identified. SEA, B, C, D and E are the classical five major types. However, other new enterotoxins have been described (Arbuthnott et al. 1990, Loncarevic et al. 2005, Thomas et al. 2007).

Several studies reported the production of SEs or the presence of toxin genes in Staphylococcus aureus from milk and derivates associated with mastitic cows in different countries (Ercolini et al. 2004, Loncarevic et al. 2005, Normanno et al. 2005, Rosec et al. 1997, Zschöck et al. 2004). However, in Brazil little attention has been dispended in order to evaluate the prevalence of staphylococcal toxins in strains isolated from bovine mastitis.

The purpose of our study was to determine the presence and distribution of the se, tst, eta and etb toxin genes in Staphylococcus spp. from milk obtained from cows diagnosed with subclinical mastitis, in the rural area of the State of Pernambuco, Brazil.



Source of the staphyloccocal strains

The study was carried out in 11 dairy herds in four municipal districts (Angelim, São Bento do Una, Caetés and Correntes) located in the rural area of the State of Pernambuco, Brazil. Cows were screened for clinical mastitis by the Tamis Test (Radostitis 2007) and for subclinical mastitis by the California Mastitis Test - CMT (Schalm & Noorlander 1957). Milk samples were collected for the analysis from the quarter milk of cows that tested positive for clinical or subclinical mastitis.

Phenotypic diagnosis of Staphyloccocal strains were performed by the classical microbiological test, including hemolysis and pigmentation production in sheep blood agar, Gram staining and the biochemical tests of coagulase and acetoin production, thermonuclease, catalase, glucose (anaerobic) and mannitol (anaerobic and aerobic) fermentation.

Genomic DNA extraction

Staphylococcal DNA was extracted from 1mL of culture grown in brain heart infusion (BHI) broth, centrifuged at 14,000rpm at 4ºC. The pellet was homogenized in 500µL of TE buffer along with the addition of 10µL of lysozyme (10mg/mL) and 10µL of proteinase K (5mg/mL). The suspension was incubated at 60ºC for 20 min followed by the addition of 100µL of STE buffer (2.5% SDS, 10mM Tris-HCl, pH 8, 0.25 M EDTA) and incubation for 15min at 60ºC, 5min at room temperature and 5min in an ice bath. The reaction was neutralized with 130µL of 7.5 M ammonium acetate, kept in an ice bath for 15min and then centrifuged for 5min. Approximately 700µL of the supernatant was transferred to another tube and mixed with the same volume of phenol-chloroform-isoamyl alcohol (25:24:1), followed by centrifugation for 5min. The supernatant was transferred to a new tube, and DNA was precipitated with approximately 420µL of isopropanol at either -70ºC for 30min or -20ºC for 24 h. After centrifugation, the supernatant was discarded, and the precipitate was resuspended in 10µL of sterile deionized water and kept at -20ºC. The DNA product was quantified using the 1D Image Analysis Software program, version 3.5 from Kodak Digital Science, DC 120 zoom Digital Camera, after electrophoresis in 1% agarose gel using ë HindIII DNA as standard.

Detection of toxin genes by PCR

Multiplex-PCR. Two multiplex-PCR essays were developed: one to detect the sea, seb, sec, sed, see genes and the other for eta, etb and tst. Reactions were prepared for a 25µL final volume, with 20 pmol of each primer, 10mM Tris-HCl, pH 9.0, 50mM KCl, 160µM of each dNTP, 3mM MgCl2, 20çg of genomic DNA and 1.2 U of Taq DNA polymerase (Invitrogen, Brazil). Amplifications were carried out in a thermocycler (Biometra) programmed for 30 cycles, each consisting of 95º C for 1 min (denaturation), 55º C for 1 min (annealing), and 72º C for 2 min (extension). Primer sequences and the size of the expected products are shown in Table 1. The following FRI (Food Research Institute, Madison, Wisconsin, USA) S. aureus strains were used as positive controls: FRI 361 harboring sec, sed, seg, sei and sej genes; FRI MN8, tst gene; FRI 722, sea gene; FRI S6, seb gene; and FRI 1151, sed gene. The amplified products were separated by electrophoresis in 1.5% (w/v) agarose gel, stained with ethidium bromide (10mg/mL) for 15min, visualized under a UV transilluminator and photographed.



Uniplex-PCR. Detection of seg, seh, sei and sej was carried out separately in uniplex-PCR reactions. Reaction mix consisted of a mixture of 20pmol of each primer, 160µM of each dNTP, 1.5mM of MgCl2, 10mM Tris-HCl pH 9.0, 50mM KCl, 20çg of genomic DNA and 1U of Taq DNA polymerase (Invitrogen, Brazil) for a final volume of 25µL. Amplifications were performed in a thermocycler (Biometra) programmed for 30 cycles, each one consisting of 94º C for 3min, 94º C for 30 sec, 60º C for 30 sec and 72º C for 30 sec to amplify seg, seh and sej. For sei amplification, the cycle was modified to 94º C for 30 sec, 60º C for 30 sec and 72º C for 60 sec. Primer's sequences and the size of the expected products are shown in Table 1. S. aureus strain FRI 361 was used as positive control. Amplification products were analyzed as previously described.

Restriction analysis and sequencing

To confirm their identity, the PCR-generated fragments for seh, seg, sei and sej were sequenced, and their restriction profiles were analyzed. The selection of the enzymes, based on the restriction sites, was determined by the Generunner DNA Sequence Analyses software, version 3.05, available for free on the internet. The amplified fragment of seh was digested with DraI, and seg, sei and sej fragments were digested with RsaI. The size of the fragments obtained after digestion was determined by electrophoresis in 1.8% agarose gels.

The amplified fragments were purified using the PureLink PCR Purification commercial kit (Invitrogen, Brazil) and analyzed in an automatic sequencer ABI 3100 (Applied Biosystem, USA). The sequences obtained were examined using the program S. aureus sequencing assembly-forward and reverse (Huang & Madan 1999) and compared to the GenBank sequences and by the Blast search program version 2.2.12 (Altschul et al. 1990).



Out of 984 quarter milk samples from 246 cows investigated, 10 (1.0%) were positive for clinical mastitis, 562 (57.1%) for subclinical mastitis and 412 (41.9%) were negative for mastitis. A total of 81 Staphylococcus spp. isolates were obtained from milk samples from the cows diagnosed with subclinical mastitis. Among them, 53 (65.0%) were S. aureus, 16 (20.0%) were coagulase-positive staphylococci (CPS) and 12 (15.0%) were coagulase-negative staphylococci (CNS). Table 2 shows the differences on distribution of the isolates based on the municipal district.



None of the isolates analyzed amplified of the classical sea-see, tst, eta and etb toxin genes. Sixty-five (80.2%) isolates amplified the seg, seh, sei and sej genes, whereas 16 isolates (19.8%) amplified no toxin gene (Table 2). The seg, seh and sei genes were found alone or in combinations of two or three genes (Table 2). Accordingly the expected segments were amplified by the reference strains used as positive controls.

DraI restriction fragments of seh and RsaI restriction fragments of seg, sei and sej generated the expected segments confirming the identity of the PCR amplified fragments (Fig.1).



The nucleotide sequences obtained for seg, seh, sei and sej were deposited in the GenBank with the accession numbers: DQ916163, DQ917579, DQ917580 and DQ917581, respectively. Comparison of these sequences using the Blast software revealed homology of 98.0%, 99.0%, 99.0% and 95.0% respectively with the sequences available in GenBank.



Staphylococcal toxin genes have a broad distribution worldwide. Differences in the geographical distribution of these genes and toxin gene combinations have been reported (Omoe et al. 2002, Cabral et al. 2004, Lim et al. 2004, Salasia et al. 2004, Katsuda et al. 2005, Silva et al. 2005).In our study, the prevalence of genes encoding the most recently described enterotoxins SEG, SEH, SEI and SEJ was very high (80.2%) in the isolates of Staphylococcus spp. obtained from milk samples from cows diagnosed with subclinical mastitis. Interestingly, none of the genes encoding the classical toxins, TSST-1, ETA and ETB, were found.

The gene seg alone was predominant. It was found in 35.0% (23/65) of the isolates, but it was also found in combination with genes sej, sei and seh. The prevalence of SEG and combination of the seg and sei has reported elsewhere (Abe et al. 2000, Jarraud et al. 2001, Omoe et al. 2002, Cabral et al. 2004, Katsuda et al. 2005, Zschöck et al. 2005). The association seg and sei is attributed to their localization in tandem orientation in the entero-toxigenic gene cluster (egc). Nonetheless, a low incidence of the seg and sei combination has also been described (Jorgensen et al. 2005), indicating probably that they are not always associated with the same strain.

The seh gene was detected in 32.0% (21/65) of the isolates, either alone or associated. This gene is rarely reported. However, similar study found seh genes in isolates from municipal districts of Caetés and Correntes, Brazil (Jorgensen et al. 2005). The occurrence of seh in these two areas can be explained by their geographical proximity and similar management of the dairy farms studied.

The gene sej has been found only in Staphylococcus aureus associated with seg and sei. The occurrence of multiple toxin genes in S. aureus is considered rare (Jorgensen et al. 2005). However, the prevalence of seg in S. aureus has been noted (Abe et al. 2000), indicating the potential importance of these gene in pathogenicity of Staphylococcus spp. strains in occurrence of subclinical bovine mastitis.

Usually CNS is not taken into account in most of the investigations and its toxigenic ability is seldom analyzed (Su & Wong 1996). Interestingly, in the present study all CNS samples analyzed harbored toxin genes. Each strain harbored either seg, seh or combinations of genes. These findings suggest a toxigenic involvement of CNS in subclinical bovine mastitis.

In conclusion, our results show evidence of a regional distribution of enterotoxin genes in staphylococci strains isolated from milk from cows with subclinical mastitis in the rural area of the State of Pernambuco, Brazil, reinforcing the geographical distribution of toxigenic isolates. The studies of the gene pattern of the staphylococcal strains isolated from bovine mastitis highlighted in present study can contribute in the knowledge of pathogenicity of microorganism, and consequent measures indicated for control of contagious mastitis in dairy herds. Furthermore, the presence of toxigenic staphylococci in milk samples isolated from cows, especially in subclinical mastitis, represent a public health threat.

Acknowledgements.- To Banco do Nordeste do Brasil (BNB) and PAPESIV/CNPq for supporting the Project and to Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the fellowship granted to the first and second authors. To the Laboratório de Enterotoxinas Estafilocócicas da Fundação Ezequiel Dias (FUNED-MG) and to Dr. Luiz Simeão do Carmo for kindly providing the standard (FRI) strains.



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Received on November 1, 2007.
Accepted for publication on July 18, 2008.



* Corresponding author:

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