The Bacillus anthracis SleL (YaaH) Protein Is an N -Acetylglucosaminidase Involved in Spore Cortex Depolymerization

Abstract

Bacillus anthracis spores, the infectious agents of anthrax, are notoriously difficult to remove from contaminated areas because they are resistant to many eradication methods. These resistance properties are due to the spore's dehydration and dormancy and to the multiple protective layers surrounding the spore core, one of which is the cortex. In order for B. anthracis spores to germinate and resume growth, the cortex peptidoglycan must be depolymerized. This study reports on analyses of sleL (yaaH), which encodes a cortex-lytic enzyme. The inactivation of sleL does not affect vegetative growth, spore viability, or the initial stages of germination, including dipicolinic acid release. However, mutant spores exhibit a slight delay in the loss of optical density compared to that of wild-type spores. Mutants also retain more diaminopimelic acid and N-acetylmuramic acid during germination than wild-type spores, suggesting that the cortex peptidoglycan is not being hydrolyzed as rapidly. This finding is supported by high-pressure liquid chromatography analysis of the peptidoglycan structure used to confirm that SleL acts as an N-acetylglucosaminidase. When sleL is inactivated, the cortex peptidoglycan is not depolymerized into small muropeptides but instead is retained within the spore as large fragments. In the absence of the sleL-encoded N-acetylglucosaminidase, other cortex-lytic enzymes break down the cortex peptidoglycan sufficiently to allow rapid germination and outgrowth.