GROWTH, SURVIVAL AND ADAPTATION OF Shigella flexneri UNDER ACIDIC CONDITIONS RELEVANT TO THOSE APPLIED DURING DAIRY PROCESSING

Document Type : Original Article

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Dairy Dept., Fac. of Agric., Mansoura University,

Abstract

Shigella flexneri is an important fooborne pathogen that has been associated with dairy products. This study aimed to assess the growth, survival and adaptation of S. flexneri under acidic conditions relevant to those adopted in dairy processing. S. flexneri could grow at mildly acidic pH values of 6.0 and 5.0 adjusted by HCl, lactic acid, or acetic acid, but at slower growth rates, compared to unacidified conditions of pH 7.3. Lower pH values ranging from 3.0 to 4.5 caused declines in the initial numbers of S. flexneri, but the pathogen could survive exposure to pH 4.0 and 3.5 for variable times depending on the acidifying agent. The order of sensitivity of S. flexneri to acids at those lower pH values were acetic acid > lactic acid > HCl. The survival of S. flexneri under the acidic conditions of yoghurt prepared from full fat (6%) and half fat (3%) buffalo's milk was also studied. The viable numbers of S. flexneri were generally higher in full fat yoghurt than half fat yoghurt on milk coagulation and during the cold storage of yoghurt. S. flexneri could survive for at least 14 days in full fat yoghurt, compared with 7 days in half fat yoghurt. This could be attributed to a potential protective effect of fat. S. flexneri exhibited an ability to adapt to acidic conditions, since its previous exposure to pH 5.5 increased its tolerance to pH 3.5. The pattern of acid tolerance in pH 3.5 by acid-adapted cells of S. flexneri depended upon the adaptation time that ranged from 30 min to 120 min.

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