Salmonella protects itself through a smart infection style
Salmonella have developed a clever way to protect itself. Upon reaching the intestine, they attach to epithelial cells and form small molecular “syringes” to inject various substances into intestinal cells (type 3 injection system). These signaling substances cause the intestinal cells to swell their membranes and enclose the bacteria. Finally, the manipulated intestinal cell absorbs the Salmonella, the host “allows” the bacteria to enter, and the bacteria can proliferate in the intestinal cells.
When an antibiotic attacks the bacteria, the Salmonella stops its cell division. Since many antibiotics are only effective during cell growth and division, Salmonella survives the attack by remaining dormant until treatment is stopped.
Knowing the optimal growth conditions for Salmonella facilitates its control
Salmonellae are gram-negative, rod-shaped (size: approx. 2 µm), glucose-fermenting, facultative anaerobes and motile by flagella.
Its optimal growth conditions are temperatures between 5 and 46°C (the optimum is 38°C), a water activity of 0.94-0.99 and a pH of 3.8-9.5. Since Salmonella do not form spores, they can be easily destroyed by heating at 60°C for 15-20 minutes, especially in food/feed with higher water content. Also, for food storage, most Salmonella serotypes do not grow at temperatures below 7°C and at pH below 4.5.
The fat in the food increases the probability of Salmonella infection: it protects the bacteria during their passage through the stomach. Doses of 10 to 100 Salmonella cells can already pose a serious risk.
Prevention is the key
Considering that several Salmonella serotypes are critical for animals and humans, and that more than 91,000 cases of salmonellosis are reported in Europe each year (EFSA, 2022), its spread must be prevented by all means. The basis for effective control of Salmonella infections is good biosecurity management and a high level of hygiene at all levels of farming and production. Due to the increased risk of vertical transfer, it is essential to keep breeder flocks and hatcheries free of Salmonella.
The transfer of natural intestinal flora from adult birds to chicks decreases the susceptibility of the latter to Salmonella infections. Against the relevant serotypes for birds, veterinary medicine offers vaccines. Other possibilities are acidification of drinking water, probiotics, and prebiotics to reduce intestinal colonization.
Are phytomolecules natural alternatives to antibiotics in Salmonella control? – a first trial gives promising results
One more tool to contain Salmonella can be the products based on phytomolecules offered by the additive industry. Phytomolecules are known for their immune system boosting and gut health enhancing properties. Additionally, some phytomolecules have antimicrobial effects. In general, they act through different modes of action, from the attack on the cell wall (terpenoids and phenols) to the influence on the genetic material of pathogenic cells or the change of the entire cell morphology.
Phytomolecules are hydrophobic, and the cell wall of gram-negative bacteria, such as Salmonella, only allow the passage of small hydrophilic solutes, so the bacteria may be less vulnerable to their action. However, mixing the phytomolecules with an emulsifier facilitates cell invasion. Its effectiveness depends on its chemical composition. It is also important whether individual substances or mixtures are used (possible positive or negative synergies).
The most studied mode of action is that of thymol and carvacrol, the main components of thyme and oregano oils. They can enter the bacterial membrane and disrupt its integrity. The permeability of the cell membrane for ions and other small molecules such as ATP increases, decreasing the electrochemical gradient with the consequent loss of energy equivalents.
An in vitro test demonstrates the effectiveness of Activo Liquid against Salmonella
To evaluate the potential of Activo Liquid, a mixture of phytomolecules and an organic acid with synergistic action, as a tool for the reduction of antibiotics, a trial was carried out to test its antimicrobial properties against Salmonella serotypes relevant to humans. Active Liquid is designed for application in drinking water.
The laboratory test was carried out at the Department of Veterinary Diagnostics at Kasetsart University in Thailand. Standardized suspensions [1×104 CFU/ml] of three Salmonella strains relevant to humans occurring in poultry were incubated in LB medium, with or without Activo Liquid. The tests were carried out with concentrations of 0.05%; 0.1%; 0.2% and 0.4%. Following incubation at 37°C for 6-7 hours, serial dilutions of cell suspensions were transferred to LB agar plates and incubated for 18-22 hours at 37°C. Subsequently, the colonies (CFU/ml) were determined.
Results: Activo Liquid inhibited the growth of all Salmonella strains tested from a concentration of 0.1%. At 0.2%, Activo Liquid already showed bactericidal efficacy against Salmonella strains, which was confirmed at a concentration of 0.4%.