Minimum inhibitory concentrations of chlorhexidine- and lactic acid-based teat disinfectants: An intervention trial assessing bacterial selection and susceptibility.

Teat disinfection is a recommended preventive tool to improve udder health and to prevent new intramammary infections. However, side effects are discussed, such as bacterial selection of less-susceptible bacteria with the application of certain teat disinfectants. The objective of this study was to assess the species composition and bacterial in vitro susceptibility by means of an interventive trial. For this purpose, 3 different postmilking teat treatments (disinfection with 0.215% chlorhexidine or 3.5% lactic acid, or control group with no dipping) were applied to 28 cows in a 6-d intervention approach using a split-udder design. Milk samples were taken before and after intervention. Bacteria were cultured and differentiated to species or genus level by MALDI-TOF mass spectrometry. Minimum inhibitory concentrations (MIC) were determined, and MIC changes over time were recorded. Susceptibilities to chlorhexidine and lactic acid were compared between species of the genera Staphylococcus, Streptococcus, Corynebacterium, and others. Species composition changed during the intervention. Under the treatment of chlorhexidine and lactic acid, the proportion of coagulase-negative staphylococci (CNS) decreased. An increased proportion of species belonging to the genus Corynebacterium was observed especially under the application of lactic acid. Although both teat disinfectants were basically effective, isolates differed in their susceptibility to both teat disinfectants. Populations of CNS, Staphylococcus aureus, and Corynebacterium spp. showed significantly lower absolute MIC values for chlorhexidine. Compared with other species, Corynebacterium spp. showed the lowest susceptibility for chlorhexidine as well as for lactic acid. A significant increase in MIC values after 6 d of intervention was observed with the lactic acid treatment in all isolates, as well as in CNS. This increase can be interpreted as either adaptation of isolates or displacement of more-susceptible species by less-susceptible species. Further studies using long-term intervention might reveal more pronounced effects on MIC values and species composition.