Results from a thesis from the Sahlgrenska Academy at the University of Gothenburg indicate that S. aureus-induced mortality is associated with dietary fat consisting of saturated and monounsaturated fatty acids, but not polyunsaturated fatty acids.
Doctoral student Louise Strandberg, in her thesis, also investigated different variants of three genes that are important for the immune system and noted that several of the gene variants that strengthen immunity also result in less obesity.
Strandberg said that S. aureus-induced mortality was investigated in mice fed a lard-based high fat diet (HFD), a diet rich in saturated and monounsaturated fatty acids (HFD/S) or a low fat diet (LFD).
She explained that after eight weeks on these diets, the mice were intravenously inoculated with S. aureus.
She said that a fourth group was added that included mice fed a HFD rich in polyunsaturated fatty acids (HFD/P) from fish.
According to Strandberg her results showed that the obese HFD/S-fed mice had increased S. aureus induced mortality compared with the lean LFD-fed mice. The HFD/S-fed mice showed signs of immune suppression as evident by increased bacterial load and decreased capacity to phagocytose bacteria.
Furthermore, she found that the HFD/P-fed mice displayed a degree of obesity and glucose intolerance that was milder than in the HFD/S-fed mice, but higher than in LFD mice.
But, she added, the S. aureus induced mortality and the bacterial load of HFD/P-fed mice were comparable with that of LFD-fed mice, and markedly lower than that of mice fed HFD/S.
"Obesity is usually associated with inflammation that does not result from an infection, which simply means that the immune defences are activated unnecessarily," said Strandberg. "Ironically, the mice on the high-fat diet (HFD/S) seem to have a less active immune system when they really need it."
The doctoral student concluded that the white blood cells of the mice on the HFD/S diet got worse at dealing with bacteria in the blood, which, she claims, could have contributed to many of them dying of sepsis.