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Appl. Environ. Microbiol. doi:10.1128/AEM.02737-07
Copyright (c) 2008, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The role of ureolytic activity in nitrogen metabolism and acid survival of Bacillus cereus

Top Institute Food and Nutrition, Wageningen, The Netherlands; Laboratory of Food Microbiology, Wageningen University, Wageningen, The Netherlands

^* To whom correspondence should be addressed. Email: tjakko.abee{at}wur.nl.

	Abstract

The presence and activity of urease genes was investigated in 49 clinical, food and environmental Bacillus cereus isolates. Ten strains showed to have urease genes, with eight of these strains showing growth on urea as sole nitrogen source. Two of the urease positive strains, including the sequenced strain ATCC 10987, could not use urea for growth, despite their capacity to produce active urease. These observations can be explained by the inability of the latter two strains to use ammonium as a nitrogen source. The impact of urea hydrolysis on acid stress resistance was subsequently assessed among the ureolytic B. cereus strains. However, none of these strains displayed an increased fitness under acidic conditions nor showed an enhanced acid shock survival in the presence of urea. Expression analysis of urease genes in B. cereus ATCC 10987 revealed a low level of expression of these genes and a lack of pH-, nitrogen-, urea-, oxygen- and growth phase-dependent modulation of mRNA transcription. This is in agreement with the low urease activity observed in strain ATCC 10987, and the nine other strains tested. Although a role of B. cereus ureolytic activity in acid survival cannot be excluded, its main role appears to be in nitrogen metabolism, where ammonium may be provided to the cells in nitrogen-limited, urea-containing environments.