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Acid Tolerance
Stationary phase E.coli K-12 and S.flexneri can survive for several hours at low pH (2-3) (Small and others 1994). This may contribute to the low infective dose associated with shigellosis. S. flexneri cultures grown initially at an external pH range of 5-8 were reported to be 100% acid resistant, surviving at pH 2.5 for 2 h. Exponential phase cultures grown at pH 5 were also acid resistant; however, survival decreased 10-100-fold as the pH of the growth medium was raised to pH 8. Lin and others (1995) reported the minimum growth pH for S. flexneri to be pH 4.8, which was greater than that required for both E. coli (pH 4.4) and S. typbimurium (pH 3.0). S. flexneri and E.coli, however, both survived exposure to low pH levels better than S. typbimurium. Small and others (1994) reported a 1.3 kb fragment cloned from S. flexneri was found to confer acid resistance on E. coli HB101 and S. flexneri Tn10. This fragment was identified as homolog of rpoS, the growth-phase-dependent sigma factor 83 (Small and others 1994). RpoS is a component of the RNA polymerase that directs the polymerase to promoters that are poorly recognized by the housekeeping sigma factor 70(Waterman and Small 1996). RpoS is a major regulator of late-log and stationary phase growth. Waterman and Small (1996) used transposon (TnlacZ and TnphoA) mutagenesis to create acid-sensitive S.flexneri mutants. Mutations were observed in the bdeA gene in two mutants and in an open reading frame (ORF) downstream of the gadB gene. This ORF encodes a protein that has homology to several inner membrane amino acid antiporters. The fusions, positively regulated by RpoS, were induced upon entry into late-exponential phase growth.
Zaika (2001) examined the ability of S.flexneri 5348 to survive in brain heart infusion broth adjusted to pH 2?5 with HCl at various temperatures. Survival increased as pH increased at all temperatures. At pH 2 and 19C S. flexneri was undetectable after 30 h. S. flexneri was present at 2 log CFU/mL after 8 days at pH 3 and after 23 days at pH 4. At 12C a 5-order-of-magnitude decrease in CFU/mL was observed at pH 3 after 13 days. A 1.1-log decrease in the S. flexneri population was observed after 58 days at pH 5. At pH 5 growth occurred at 19, 28, and 37C. Fehlhaber (1981) examined the ability of strains of S. flexneri and S sonnei to survive in nutrient broth at pH 3-4.5 at room temperature. Fifteen strains of S.flexneri survived for 30 min at pH 3. All strains survived for 4 h at pH 4.5, two strains survived for 2 days. This author used mid-exponential phase cells, which have been reported to be less acid tolerant than cells in the stationary phase (Fehlhaber 1981).
Bagamboula and others (2002) examined the behavior of multiple strains of S. sonnei and S.flexneri in media at different pH from 3.25 to 5 and in different fruit products. At pH 5 all Sbigella strains tested grew to maximal levels within 6 h. 'The minimum pH for growth of S. flexneri was 4.75.S. sonnet was able to grow at low pH 4.5. At pH 4.25 a great deal of variability in the ability of strains to survive was observed. At pH 4.0 and 3.75 all strains were recovered after 24 h, but populations were greatly reduced. The authors reported that laboratory strains survived better in acidic conditions than clinical isolates. This is contrary to what was expected as clinical isolates have been exposed to the acidic conditions of the stomach. Sbigella decreased rapidly at 22C in apple juice (pH 3.3-3.4) and tomato juice (pH 3.9-4.1). Some strains were detected for as long as 14 days in tomato juice and 8 days in apple juice. The survival of low numbers of Sbigella on strawberries and fresh fruit salad was examined. S.flexneri was not detected on strawberries stored at 4C for 4 or 48 h. It was, however, detected in all samples of fresh fruit salad 48 h (Bagamboula and others 2002).
Tetteh and Beuchat (2001) examined the effects of organic acids on the survival and growth of unadapted, acid-adapted, and acid-shocked S.flexneri. At the same pH, propionic acid was the most inhibitory, followed by acetic and lactic acids. Acid-adapted cells were found to be more resistant to acid environments than were acid-shocked or unadapted cells.

[ Last edited by telomerase on 2009-1-9 at 12:27 ]

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