LOSS OF VIABILITY BY STAPHYLOCOCCUS AUREUS IN ACIDIFIED MEDIA

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Abstract
Determinations were made on the extent to which Staphylococcus aureus was inactivated [inability to grow on Trypticase Soy agar (TSA)] or injured [inability to grow on TSA + 7% added NaCl (TSAS)] in acidic artificial media and how inactivation was affected when some stresses common to food processing operations were inflicted on the organism. When cells were held in TS broth at >PH 6 (24 hr, 37 C), presence of 7% salt added to the medium did not affect inactivation of staphylococci; but, at ≤pH 5, salt enhanced inactivation. Cells that survived this treatment were not sensitive to salt present in the plating medium, whereas staphylococci from the stationary growth phase (TS broth, pH 7) were sensitive to salt. Cells exposed to low pH were only slightly more sensitive to salt when plated on TSAS than were cells held at pH 7. Exposure of organisms to salt did not render them appreciably more sensitive to subsequent acid treatment than were untreated organisms. Freezing (−30 C, 24 hr) did not substantially enhance inactivation of S. aureus at low pH but injury was greatly increased. A sublethal heat treatment (60 C, 15 min) failed to markedly enhance injury or death of cells at low pH values. Exposure to high and low temperatures failed to increase susceptibility of staphylococci to acid and acid-treated cells were no more sensitive to effects of these temperatures than were untreated cells. Staphylococci exposed to a low pH exhibited an extended lag phase when transferred to neutral nutrient media.

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