Effect of bismuth subnitrate on in vitro growth of major mastitis pathogens.

Because of concerns regarding increasing antimicrobial resistance and the need for responsible use of antibiotics, there is a need to investigate the feasibility of selective dry cow therapy and non-antimicrobial alternatives. The most common alternative to antibiotics at the time of dry-off is internal teat sealants that contain bismuth sub-nitrate, a heavy metal compound, in a paraffin-based substance. They have proven to be effective at preventing new dry-period infections. However, the mechanism of action of bismuth-based products remains uncertain, although it is thought that creating a physical barrier is the main method of protection. Internal teat sealants are also commonly used in combination with dry cow therapy (DCT), to prevent and treat intra-udder infection (IUI) during the dry period, but the use of the combination is not always more effective. The proposed mechanisms through which teat sealants exert their function include assisting the teat canal in forming a better keratin plug and creating a physical barrier against bacteria in the teat canal, but this has not been scientifically proven. Other studies testing different physical barriers, such as wax plugs or intra-udder polystyrene devices, were unsuccessful in the long-term protection of cows against IUI and mastitis. Thus, Dr S. Notcovich and colleagues hypothesized that creating a physical barrier against bacteria traversing the teat canal is not the sole mode of action of bismuth-based formulations. They are of the opinion that bismuth sub-nitrate also inhibits the growth of bacteria associated with the colonization of the udder during the dry period and the development of new IUI. The objective of their study was therefore to assess the effect of bismuth sub-nitrate on the growth of mastitis-causing bacterial strains in vitro. The results of the study were published in the Journal of Dairy Science, Volume 103 of 2020, page 7249 to 7259, the title being: Effect of bismuth subnitrate on in vitro growth of major mastitis pathogens. 

They tested a strain of Streptococcus uberis (SR115), two strains of Staphylococcus aureus (SA3971/59 and SA1) and a strain of Escherichia coli (P17.14291) in vitro for their ability to grow in the presence or absence of bismuth sub-nitrate. Disk diffusion testing, impedance measurement and evaluation of bacterial growth in shaking conditions were the methods they used to test the hypothesis.

The results showed that bismuth sub-nitrate slowed bacterial growth in all four strains, but to different degrees. This suggests that the efficacy of bismuth sub-nitrate based teat sealants in preventing clinical mastitis may at least in part be due to a reduction in growth of the major mastitis-causing agents, particularly Strep. uberis, in the teat canal during the early stages of IUI in the dry period. In addition to the barrier effect described in other work, the presence of an inhibitory effect in bismuth sub-nitrate formulations might be crucial for their documented high efficacy. It may also indicate that new intra-udder methods to prevent mastitis over the dry period will require inhibitory substances in addition to barrier components.