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Open Access Highly Accessed Research article

Staphylococcus epidermidis strains isolated from breast milk of women suffering infectious mastitis: potential virulence traits and resistance to antibiotics

Susana Delgado1, Rebeca Arroyo1, Esther Jiménez1, Maria L Marín1, Rosa del Campo2, Leonides Fernández1 and Juan M Rodríguez1*

Author Affiliations

1 Dpt. Nutrición, Bromatología y Tecnología de los Alimentos. Universidad Complutense de Madrid, 28040 Madrid, Spain

2 Servicio de Microbiología, Hospital Ramón y Cajal, 28034 Madrid, Spain

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BMC Microbiology 2009, 9:82  doi:10.1186/1471-2180-9-82

Published: 7 May 2009

Abstract

Background

Although Staphylococcus aureus is considered the main etiological agent of infectious mastitis, recent studies have suggested that coagulase-negative staphylococci (CNS) may also play an important role in such infections. The aims of this work were to isolate staphylococci from milk of women with lactational mastitis, to select and characterize the CNS isolates, and to compare such properties with those displayed by CNS strains isolated from milk of healthy women.

Results

The milk of 30 women was collected and bacterial growth was noted in 27 of them, of which Staphylococcus epidermidis was isolated from 26 patients and S. aureus from 8. Among the 270 staphylococcal isolates recovered from milk of women with mastitis, 200 were identified as Staphylococcus epidermidis by phenotypic assays, species-specific PCR and PCR sequencing. They were typified by pulsed field gel electrophoresis (PFGE) genotyping. The PFGE profiles of the S. epidermidis strains were compared with those of 105 isolates from milk of healthy women. A representative of the 76 different PFGE profiles was selected to study the incidence of virulence factors and antibiotic resistance. The number of strains that contained the biofilm-related icaD gene and that showed resistance to oxacillin, erythromycin, clindamycin and mupirocin was significantly higher among the strains isolated from mastitic milk.

Conclusion

S. epidermidis may be a frequent but largely underrated cause of infectious mastitis in lactating women. The resistance to diverse antibiotics and a higher ability to form biofilms found among the strains isolated from milk of women suffering mastitis may explain the chronic and/or recurrent nature of this infectious condition.