Unveiling the Expression Characteristics of IspC, a Cell Wall-Associated Peptidoglycan Hydrolase in Listeria monocytogenes, during Growth under Stress Conditions

Listeria monocytogenes serotype 4b is a food-borne pathogen of public health concern, since it accounts for approximately 40% of human listeriosis cases. We have recently identified IspC, a surface-localized peptidoglycan hydrolase, as the antigen recognized by a number of monoclonal antibodies (MAbs) produced against a serotype 4b strain for diagnostic applications. To determine whether IspC, which is well conserved among various serotype 4b strains, is a useful diagnostic marker in antibody-based methods, we assessed the expression of IspC inL. monocytogenes cultured under normal and stress conditions. A functional promoter directing the transcription of theispC gene was identified upstream of theispC open reading frame by constructing a promoterlesslacZ gene fusion with the putativeispC promoter region and by 5′ rapid amplification of cDNA ends analysis. Using both thelacZ reporter gene system and immunofluorescent staining with an IspC-specific MAb, we provide evidence that IspC is expressed on the cell surface in all growth conditions tested (temperature, osmotic stress, pH, ethanol, oxidative stress, anaerobic conditions, carbon source, and type of growth media) that allow for cellular division, although the level ofispC gene expression varies. These results demonstrated the usefulness of IspC as an excellent diagnostic marker for the serotype 4b strains and imply that IspC, in conjunction with specific MAbs, can be targeted for detection and isolation ofL. monocytogenes serotype 4b strains directly from food, environmental, and clinical samples with minimal or no need for culture enrichment.