Colonization and local host response following intramammary Staphylococcus chromogenes challenge in dry cows

Abstract

Although extensive research has been performed on bovine non-aureus staphylococci (NAS), several aspects such as bacteria-host interaction remain largely unstudied. Moreover, only a few mastitis pathogen challenge studies in cows have been conducted in the dry period, an important period that allows intramammary infection (IMI) to cure and new IMI to occur. We challenged 16 quarters of 4 Holstein Friesian cows at dry off with 100; 100 000 or 10 000 000 CFU of the udder-adapted S. chromogenes IM strain. Four quarters from one cow served as negative controls. Internally sealed quarters remained untouched, whereas non-sealed quarters were sampled 3 times during the dry period. After parturition, colostrum and daily milk samples were taken during the first week of lactation of all quarters. In total, 8 quarters appeared to be colonized, since S. chromogenes IM was recovered at least once during the experiment, as substantiated using Multilocus Sequence Typing. S. chromogenes IM shedding was highest in dry quarters inoculated with 10 000 000 CFU. Colonized quarters had the highest quarter somatic cell count (qSCC) in early lactation. Inoculated quarters (both colonized and non-colonized) had lower IL-6 and IL-10 concentrations in the dry period, whilst IFN-γ levels tended to be higher in colonized quarters compared to non-inoculated quarters. Also, IgG2 levels were higher in inoculated compared to non-inoculated quarters and the IgG2/IgG1 ratio was on average above 1. To conclude, we showed that dry quarters can be colonized with S. chromogenes IM, resulting in a shift towards a Th1 response in late gestation and early lactation characterised by an increased IgG2 concentration. However, further research is needed to confirm our findings.

Keywords: Dry period; IFN-γ; IL-10; IL-6; IgG1; IgG2; Staphylococcus chromogenes; intramammary challenge; non-aureus staphylococci.

Figure 1

Overview of the experimental set-up. Milk samples were taken to check the quarters for the presence of bacteria and cytokine and antibody levels were determined before inoculation, and throughout the dry period and the first week of lactation. The quarter milk somatic cell count was determined before inoculation, in the colostrum sample and during the first 6 days in lactation. Sealed quarters remained untouched during the entire dry period, thus no dry cow secretion samples were taken from these quarters.

Figure 2

Bacterial load of recovered S. chromogenes IM as a function of challenge dose. Three challenge doses (i.e., 100, 100 000 and 10 000 000 CFU) were compared with the unchallenged controls. Values shown represent the least square means and the error bars represent the standard error of the mean (+SEM).

Figure 3

Quarter somatic cell count as a function of different quarter strata. The quarter somatic cell count (qSCC) was measured on the day before dry off and during the first week of lactation. Values shown represent the least square means and the error bars represent the standard error of the mean (+ SEM).

Figure 4

Cytokine levels as a function of different quarter strata. IL-6 (A), IFN-γ (B) and IL-10 (C) concentrations. Values shown represent the least square means and the error bars represent the standard error of the mean (+SEM).

Figure 5

Antibody levels as a function of different quarter strata. IgG1 (A) and IgG2 (B) concentrations. Values shown represent the least square means and the error bars represent the standard error of the mean (+SEM).

Figure 6

Antibody ratio as a function of different quarter strata. The IgG2/IgG1 ratio, as main immune response of the quarter. Values shown represent the least square means and the error bars represent the standard error of the mean (+SEM).