The Staphylococcus aureus alternative sigma factor sigmaB controls the environmental stress response but not starvation survival or pathogenicity in a mouse abscess model

Abstract

The role of sigmaB, an alternative sigma factor of Staphylococcus aureus, has been characterized in response to environmental stress, starvation-survival and recovery, and pathogenicity. sigmaB was mainly expressed during the stationary phase of growth and was repressed by 1 M sodium chloride. A sigB insertionally inactivated mutant was created. In stress resistance studies, sigmaB was shown to be involved in recovery from heat shock at 54 degreesC and in acid and hydrogen peroxide resistance but not in resistance to ethanol or osmotic shock. Interestingly, S. aureus acquired increased acid resistance when preincubated at a sublethal pH 4 prior to exposure to a lethal pH 2. This acid-adaptive response resulting in tolerance was mediated via sigB. However, sigmaB was not vital for the starvation-survival or recovery mechanisms. sigmaB does not have a major role in the expression of the global regulator of virulence determinant biosynthesis, staphylococcal accessory regulator (sarA), the production of a number of representative virulence factors, and pathogenicity in a mouse subcutaneous abscess model. However, SarA upregulates sigB expression in a growth-phase-dependent manner. Thus, sigmaB expression is linked to the processes controlling virulence determinant production. The role of sigmaB as a major regulator of the stress response, but not of starvation-survival, is discussed.

FIG. 1

Expression of sigB::lacZ during growth. S. aureus MC100 (sigB::lacZ) was grown in BHI (circles) or BHI supplemented with 1 M NaCl (squares) at 37°C. S. aureus MC102 (sar, sigB::lacZ) was grown in BHI (diamonds) at 37°C. Bacterial growth was measured as the A₆₀₀ (open symbols), and sigB::lacZ expression was determined by β-galactosidase activity (solid symbols) as described in Materials and Methods.

FIG. 2

Role of sigB in virulence. Bald mice were inoculated subcutaneously with ≈10⁸ cells of S. aureus strains 8325-4, PC400 (sigB), or PC6911 (agr), as represented by the solid bars. Mice were sacrificed after 7 days, and the number of bacteria recovered from skin lesions was counted (open bars).

FIG. 3

Role of sigB in the response to different environmental stresses. S. aureus 8325-4 (circles) and PC400 (sigB) (squares) were grown to different growth phases at 37°C and then subjected to various environmental stresses as described in Materials and Methods. (A) Effect of heat shock (54°C for 10 min, T = 0). Growth was measured as the A₆₀₀. The arrow denotes the initiation of heat shock. (B) Effect of H₂O₂ (7.5 mM) on stationary-phase cells. (C) Effect of ethanol (6% [vol/vol], solid symbols) or no addition (open symbols) on exponential-phase cells. The arrow indicates when the ethanol was added. (D) Effect of 2 M NaCl (solid symbols) or no addition (open symbols) on exponential-phase cells. The arrow indicates when the NaCl was added.

FIG. 4

Role of sigB in the acid stress response. S. aureus strains were grown and treated with acid as described in Materials and Methods. Cells were either resuspended at pH 4 at 37°C for 1 h (solid symbols) or left untreated (open symbols) prior to the pH 2 treatment. Circles, S. aureus 8325-4; squares, PC400 (sigB).