Farm animals are known reservoirs of Salmonella infection in humans. Thousands of cases of human infection are reported worldwide each year resulting in hundreds of deaths. Salmonella have adapted to survive the harsh acidic conditions in the digestive systems of animals by employing a mechanism known as acid tolerance response (ATR). ATR is triggered by mild acidic conditions; it allows colonisation of the gut resulting in infected faeces, the main source of food contamination.

At the molecular level, ATR may confer cross-resistance to antibiotics, i.e. the same mild acidic conditions that trigger ATR may also activate resistance to a range of antibiotics. A better understanding of Salmonella ATR is key to controlling both animal and human infection.

Manipulating the diet of animals may help control the level of Salmonella in animal digestive systems. Diets that result in high levels of volatile fatty acids (VFA) in the rumen and acidic feed additives for pigs could be used to regulate the gut conditions, thereby inhibiting or reducing Salmonella numbers.

Scientists from Teagasc Food Research Centre, Ashtown and University College Dublin investigated the Salmonella ATR to determine if diet could be used to limit the amount of Salmonella excreted by cattle and pigs. They tested a number of strains of Salmonella as well as Salmonella that had been preexposed to acid conditions (acid-adapted) and those that had not been exposed to acidic conditions (non acidadapted).

Salmonella Acid Tolerance Response (ATR)

Some of the major genes that are important to acid adaptation and the acid tolerance response of different Salmonella species were identified in this study. The results suggest that there is not a unified regulation system that controls the Salmonella ATR. When analysed, key ATR-genes in different Salmonella isolates displayed different gene expression profiles. Also acid-adaptation caused different genes to become active within different species.

Link to Antibiotic Resistance

Results from gene expression profiles of acid adapted and non-adapted Salmonella isolates showed a relationship between acid resistance and antibiotic resistance with respect to global gene regulators (those genes involved in switching on and off gene expression). However, the efflux pump based resistance to antibiotics does not appear to support acid resistance and there was no relationship between acid adaptation and increased efflux activity.

Manipulating the Diet of Cattle

Cattle were fed one of five diets for a two-month period ranging from typical grass and silage based regimes to high grain diets (Diet 5 in Table 1). The survival of acid adapted and non-acid adapted Salmonella in the faeces and rumen fluid recovered from cattle on the different diets was analysed.

The results indicated that survival of non-acid and acid adapted Salmonella in bovine rumen fluid and faeces were not reduced by any of the diets. The study demonstrates that dietary regimes alone will not control the shedding of Salmonella in cattle, indicating that other factors, such as, feeding interval, water intake and rumen microbial population need to be taken into account.

Benefit of Additives in Controlling Salmonella in Pigs

Formi and Bactacid are feed acid treatments used in the pig industry and were included in a pig feeding trial conducted in Teagasc, Moorepark. Six diets were fed to pigs as outlined in Table 2.

Samples of the contents of the stomach, intestine and faeces from these pigs were collected at slaughter and then inoculated with non-acid-adapted and acid-adapted Salmonella.

The findings indicate that the type of feed (meal versus pellet) and the addition of acid additives may have an effect on survival under certain circumstances. Both acid treatments reduced the survival of Salmonella in faeces when it was stored at 22°C but not when stored at 4°C. Fewer Salmonella survived in the faeces of meal fed animals, which was finely ground, compared to the pelleted diets.

These results have practical implications for the pig industry as they suggest that acid supplementation may reduce survival of Salmonella in the environment of pigs indoors where temperatures in the house are kept about 20-25oC. However, Salmonella in faeces in passages between houses and other areas, where the temperature is cooler may not be affected.

Pig Trial 2

The results from earlier pig feeding trials, along with the knowledge gained on ATR, were used in a further pig feeding trial. This trial showed no significant effect of acid supplement on the control of Salmonella on a farm with high Salmonella status. While not definitive, the results do suggest that farms with a heavy burden of Salmonella may not be able to improve their Salmonella status through addition of dietary acids alone.

Farmers cannot regard acid supplementation as a ‘silver bullet’ for all Salmonella problems on the farm, other control measures such as improved hygiene, must also be undertaken. This is an important message to convey to industry as new EU regulations for control of Salmonella will be introduced in 2011.