Modulation of the microbiota and reduction of S. Suis virulence. What mechanism could be behind it?

Streptococcus suis has become an important cause of antimicrobial prescription in pig production today. With a prevalence of infected farms close to 100%, it is one of the most important bacterial pathogens in swine. Besides posing a zoonotic risk, it carries a significant economic cost and is an animal welfare problem. This challenge is compounded by the increasing control of veterinary prescriptions and the use of antimicrobials, including the restrictions applied to metaphylaxis, which until now was used to control the appearance of symptoms, but not the presence of S. suis.

There are at least 33 serotypes of Strep suis. Serotypes 2 and 9 are the most commonly implicated in disease and outbreaks. Their virulence is especially related to mrp, epf, and sly proteins, and others, such as luxS, which are involved in biofilm formation, increasing their resistance to treatment and immune response. Within each serotype, there is great variability in the bacterial capsule and virulence factors, which complicates the design and efficacy of vaccines. Corsault et al., 2021 concluded that vaccination of sows increased maternal immunity transmitted to piglets, maximizing at 7 days of life, but was drastically reduced by day 18 of life and therefore unable to protect piglets after weaning. We must therefore explore other control tools.

The role of the microbiota in Streptococcus suis infection

As early as 1996, reference was made to the early contact of piglets with S. suis during farrowing when they come into contact with vaginal secretions. There are studies on how the transmission of an unfavorable microbiota from the mother to the piglets as well as an insufficient consumption/quality of colostrum affect the development of the disease.

A critical function of the microbiota is to prevent the overgrowth of potential pathogens under homeostatic conditions by mechanisms of colonization resistance: competition for nutrients, production of antimicrobial compounds, or indirectly by stimulating host innate and adaptive immunity.

The modification of the sow's fecal microbiota has been correlated with that of the vaginal microbiota and intervention at the level of the sow's intestinal microbiota led to an improvement of the vaginal microbiota with a reduction of Streptococcus at the genital level. The presence of a greater diversity of microbiota enhances the presence of commensal bacterial populations that compete for space and nutrients with S. suis reducing the infection pressure and facilitating disease control.

The use of nutritional solutions that improve the intestinal microbiota of the sow, positively influencing the vertical transmission of the microbiota to the piglet, would lead to a better response capacity of the animal to the disease.

Given these arguments, a field study was carried out on a 3000-sow commercial farm with a history of severe outbreaks of S. suis that required continuous antibiotic treatment for symptom control. The objective was to obtain a better understanding of the mechanism by which a specific combination of medium-chain fatty acids (MCFAs) and short-chain fatty acids (SCFAs) administered to the sow and piglet feed leads to an improvement of the intestinal microbiota allowing a reduction in the use of antibiotics against S. suis, as we were observing.

To assess the evolution of the microbiota, sow fecal samples were taken directly from the rectum, and tonsil swabs were taken from a group of piglets, which were followed from farrowing throughout the post-weaning period, before the beginning of the nutritional strategy and for four months after its implementation.

The following analyses were done:

• Feces (320 samples): ratio analysis of certain bacterial populations in sows (Masterlab laboratories).
• Tonsils (176 samples): analysis of serotypes, virulence factors, and commensal microbiota, through the analysis of other Streptococcus spp (S. hyovaginalis, S. oralis, S. mitis (In collaboration with the BACRESP research group, University of León).

How did the incorporation of SCFAs and MCFAs affect the microbiota?

At the beginning of the study, the bacterial ratios in the feces of the sows showed great variability in all the production phases (beginning, middle, and end of gestation and lactation). Successive samples taken every four weeks from the same animals over four months showed an evolution towards stability, with similar values detected in all of them at the end.

As a consequence of an influenza outbreak in the middle to end of the study, treatment with amoxicillin was necessary starting the second week after weaning, but even with the complications derived from the influenza outbreak, it was possible to withdraw the antibiotic treatment in the piglets until 10 days post-weaning, without a significant increase in mortality during those weeks.

The results obtained at the tonsillar level in piglets showed clear differences compared to the initial ones in the presence of continuous antibiotic treatments.

In the case of S. suis:

• Before nutritional treatment: its detection increased progressively over time and the presence of gapdh and luxS virulence factors could be detected.
• After nutritional treatment: the number of samples with the presence of S. suis isolated by culture was reduced and remained constant over time during sampling. In cases where we were able to characterize virulence factors, they were initially positive for gapdh and luxS genes, but then no longer had the gapdh gene. One isolate was positive for epf.

In the case of commensal/environmental bacteria:

• Before the incorporation of SCFAs and MCFAs: more bacteria are detected at the end of the sampling period.
• After the incorporation of SCFAs and MCFAs: the opposite happens, as they are detected at the beginning of the sampling and are not found in the last week of sampling, probably due to the action of the antibiotic treatment.