Question

The disinfectants which are resistant to staphylococcus from milk samples

Answer 1

Explanation:

We demonstrate here a widespread distribution of genes mediating efflux-based resistance to quaternary ammonium compounds (QACs) in staphylococci from unpasteurized milk from 127 dairy cattle herds and 70 dairy goat herds. QAC resistance genes were identified in 21% of the cattle herds (qacA/B, smr, qacG, and qacJ) and in 10% of the goat herds (qacA/B and smr). Further examination of 42 QAC-resistant bovine and caprine isolates revealed the following genes: qacA/B (12 isolates) was present in four different species of coagulase-negative staphylococci (CoNS), smr (27 isolates) was detected in eight different CoNS species and in Staphylococcus aureus on a previously reported plasmid (pNVH99), qacG (two isolates) was detected on two plasmids (pST94-like) in Staphylococcus cohnii and Staphylococcus warneri, and qacJ (two isolates) was found in Staphylococcus hominis and Staphylococcus delphini on a plasmid (pNVH01) previously found in equine staphylococci. Isolation of indistinguishable pulsed-field gel electrophoresis (PFGE) CoNS types from tank milk and mammary quarter milk samples in a dairy cattle herd suggested that these QAC-resistant staphylococci were of intramammary origin. Indistinguishable or closely related PFGE types of bovine QAC-resistant CoNS were observed in different herds. One particular bovine S. warneri PFGE type was isolated repeatedly from samples collected during a 30-month period in a herd, showing long-term persistence. In conclusion, it seems that the widespread distribution of staphylococci carrying QAC resistance genes in Norwegian dairy cattle and goat herds is the result of both the intra- and interspecies spread of QAC resistance plasmids and the clonal spread of QAC-resistant strains.

Staphylococcus aureus and coagulase-negative staphylococci (CoNS) are common causes of bovine and caprine intramammary infections. S. aureus infections, which can be clinical or subclinical, frequently persist for a long time, and infected mammary glands thus serve as reservoirs from which the organism may spread to other cows within a herd and occasionally to other herds (4, 8, 30). CoNS are mainly causing subclinical mastitis, particularly in prepartum heifers and primiparous cows (12, 15, 16, 31). CoNS are part of the normal skin microflora of dairy ruminants and occur in their environment. Together with organisms of extramammary origin, bacteria causing subclinical intramammary infections are present in tank milk shipped from dairy farms. As part of quality assessment of raw milk, bacteriological examination of tank milk is performed regularly. Human pathogens are occasionally found in raw milk, giving rise to some concern regarding the consumption of products from unpasteurized milk (28). In addition, attention should be paid to the possible presence in raw milk of bacteria carrying antimicrobial resistance determinants (27). Both antibiotic resistance genes and disinfectant resistance genes should be considered.

Disinfectants based on quaternary ammonium compounds (QACs) have various applications in veterinary medicine and play an important role in the control of animal disease. The QACs benzalkonium chloride (BC) and cetyltrimethylammonium bromide (CTAB) are active components in various teat preparations commonly used for mastitis prevention in dairy herds in Norway and other countries (5, 6). Efflux-mediated resistance to QACs has been observed in staphylococci of various origins, and several staphylococcal QAC resistance genes have been identified (2, 5, 6, 7, 9, 10, 11). These genes are in general plasmid-borne (21), encoding efflux proteins capable to expel hydrophobic drugs including QACs, the intercalating dye ethidium bromide (EtBr), and some other cationic biocides. The nearly identical genes qacA and qacB (denoted qacA/B gene here) are normally harbored by large plasmids of >20 kb, whereas the remaining QAC resistance genes have most frequently been found within their gene cassettes on small plasmids of less than 3 kb, where they encode efflux proteins belonging to the small multidrug resistance family (1, 14, 17, 18