AIMS S. aureus is the most important causative agent of subclinical mastitis in cattle. In the affected farms the clinical outcome and the consequent economic losses can vary in relation to the strains involved in the mammary infection. Methicillin-resistant strains (MRSA) have zoonotic importance, especially for farm workers and were also isolated from bovine mastitis in Italy and other countries[1,2]. The aim of our study was to evaluate the prevalence of S. aureus and MRSA in dairy herds and to identify the main circulating genotypes. MATERIALS AND METHODS We examined 509 samples of bulk tank milk from eight provinces of Lombardy region. Samples were plated directly on Blood agar and the S. aureus count was determined on Baird Parker Agar + RPF. For each sample, at least 5 colonies referable to S. aureus were tested for susceptibility to oxacillin by disk diffusion test. The same milk samples were examined by subsequent enrichment in MH broth + 7.5% NaCl and TSB + 5mg/l of oxacillin and plating on Brilliance MRSA agar (Oxoid). The presence of the mecA gene was confirmed by PCR [3]. Genotyping was performed by RS-PCR (16S-23S intergenic spacer PCR) [4] and Multilocus Sequence Typing (MLST) [5]. RESULTS A total of 211 out of 509 samples were positive for S. aureus (41.4%), with a prevalence in the different provinces from 9.1% to 56.8%. The counts of S. aureus showed values ranging between 10 and > 30000 cfu/ml with a median value of 100 cfu/ml. MRSA were isolated from 26 samples (5.1%). Five of these were demonstrated by both direct plating and plating after enrichment, 10 and 11 by enrichment or direct plating only, respectively. The RS-PCR identified several different profiles, including the so-called genotype B (GTB) [4] that was detected in 45 samples (21.3%). The characterization by MLST of 18 of the 26 samples positive for MRSA identified ten ST398, four ST1, three ST97 and one ST5. DISCUSSION The high prevalence of positive bulk milk samples confirms the importance of the S. aureus infection in Lombardy dairy farms. The analysis of circulating genotypes showed considerable variability, but it should be noted that a fairly significant rate is represented by GTB genotype which is associated with high infectivity and pathogenicity. MRSA were demonstrated in more than 10% of S. aureus positive samples, confirming the data obtained previously in the province of Lodi (personal data). The genotyping of MRSA strains showed a prevalence of typical LA-MRSA (ST398 and ST97) and ST1, the latter associated with community-acquired human infection. In the case of MRSA the application of more stringent control plans to reduce the risk of transmission to humans is recommended. REFERENCES 1) Benedetti V et.al (2010) Large Animal Review, 16:67-70; 2) Holmes MA and Zadocks RN (2011) J. Mammary Gland Biol. Neoplasia 16, 373-382; 3) McClure JA., et al. (2006) J. Clin. Microbiol., 44:1141-44; 4) Graber HU et al. (2009). J. Dairy Sci., 92(4) 1442-1451; 5) Enright MC et al (2000) J. Clin. Microbiol., 38, 1008-1015
Prevalence of Staphylococcus aureus and MRSA in bulk tank milk of lombardy dairy herds
Cremonesi P;
2013
Abstract
AIMS S. aureus is the most important causative agent of subclinical mastitis in cattle. In the affected farms the clinical outcome and the consequent economic losses can vary in relation to the strains involved in the mammary infection. Methicillin-resistant strains (MRSA) have zoonotic importance, especially for farm workers and were also isolated from bovine mastitis in Italy and other countries[1,2]. The aim of our study was to evaluate the prevalence of S. aureus and MRSA in dairy herds and to identify the main circulating genotypes. MATERIALS AND METHODS We examined 509 samples of bulk tank milk from eight provinces of Lombardy region. Samples were plated directly on Blood agar and the S. aureus count was determined on Baird Parker Agar + RPF. For each sample, at least 5 colonies referable to S. aureus were tested for susceptibility to oxacillin by disk diffusion test. The same milk samples were examined by subsequent enrichment in MH broth + 7.5% NaCl and TSB + 5mg/l of oxacillin and plating on Brilliance MRSA agar (Oxoid). The presence of the mecA gene was confirmed by PCR [3]. Genotyping was performed by RS-PCR (16S-23S intergenic spacer PCR) [4] and Multilocus Sequence Typing (MLST) [5]. RESULTS A total of 211 out of 509 samples were positive for S. aureus (41.4%), with a prevalence in the different provinces from 9.1% to 56.8%. The counts of S. aureus showed values ranging between 10 and > 30000 cfu/ml with a median value of 100 cfu/ml. MRSA were isolated from 26 samples (5.1%). Five of these were demonstrated by both direct plating and plating after enrichment, 10 and 11 by enrichment or direct plating only, respectively. The RS-PCR identified several different profiles, including the so-called genotype B (GTB) [4] that was detected in 45 samples (21.3%). The characterization by MLST of 18 of the 26 samples positive for MRSA identified ten ST398, four ST1, three ST97 and one ST5. DISCUSSION The high prevalence of positive bulk milk samples confirms the importance of the S. aureus infection in Lombardy dairy farms. The analysis of circulating genotypes showed considerable variability, but it should be noted that a fairly significant rate is represented by GTB genotype which is associated with high infectivity and pathogenicity. MRSA were demonstrated in more than 10% of S. aureus positive samples, confirming the data obtained previously in the province of Lodi (personal data). The genotyping of MRSA strains showed a prevalence of typical LA-MRSA (ST398 and ST97) and ST1, the latter associated with community-acquired human infection. In the case of MRSA the application of more stringent control plans to reduce the risk of transmission to humans is recommended. REFERENCES 1) Benedetti V et.al (2010) Large Animal Review, 16:67-70; 2) Holmes MA and Zadocks RN (2011) J. Mammary Gland Biol. Neoplasia 16, 373-382; 3) McClure JA., et al. (2006) J. Clin. Microbiol., 44:1141-44; 4) Graber HU et al. (2009). J. Dairy Sci., 92(4) 1442-1451; 5) Enright MC et al (2000) J. Clin. Microbiol., 38, 1008-1015I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
