Growth of Staphylococcus Aureus under stress conditions

Abstract

Staphylococcus aureus is considered as one of the most important food-borne, halotolerant pathogenic bacteria. An investigation was thus performed to determine parameters that could limit its growth in "Shelf Stable Meat Products". Altogether 35 coagulase positive strains were isolated from fresh minced meat and "Shelf Stable Meat Products". Of the isolates 46% were found to be typable using the international basic human set of phages, indicating human origin. Strains relatively sensitive and resistant to low aw and pH were found. The minimal aw for bacterial growth of the aw sensitive and resistant strains were 0,866 and 0,846 respectively. However the type of humectant used to lower aw greatly influenced bacterial growth. When investigating the effect of humectants, its strength for binding water, its bactericidal effects and the concentration of the humectant required to produce the desired aw must be considered. The inorganic acid HCl had the least effect on staphylococcal growth when compared to the organic acids. The minimal pH enabling bacterial growth of the pH sensitive and resistant strains was 5, 1 and 5, 0 respectively when an organic acid was used to adjust the pH, and 4, 43 and 4, 08 respectively when an inorganic acid was used. Acetic acid was the most effective organic acid for growth inhibition. Lowering the aw to 0,925 produced a much greater bacteriostatic effect than lowering the pH to 5, 5. The greatest bacteriostatic effect was obtained at aw 0,925 adjusted with sodium chloride and pH 5, 5 adjusted with gluconic acid. As the aw was lowered, it was found that the amount of cardiolipin in the cell membrane increased, while the amount of phosphatidylglycerol decreased, irrespective of the humectant used to lower the aw. The average diameters of the cocci were found to increase from 0, 87 μm to 1, 25 μm when grown in BHI adjusted to aw 0,925 with glycerol, propylene glycol, sucrose or sodium chloride and to +, 65 μm when the aw was adjusted using sodium lactate.