Global gene expression profile for swarming Bacillus cereus bacteria

Abstract

Bacillus cereus can use swarming to move over and colonize solid surfaces in different environments. This kind of motility is a collective behavior accompanied by the production of long and hyperflagellate swarm cells. In this study, the genome-wide transcriptional response of B. cereus ATCC 14579 during swarming was analyzed. Swarming was shown to trigger the differential expression (>2-fold change) of 118 genes. Downregulated genes included those required for basic cellular metabolism. In accordance with the hyperflagellate phenotype of the swarm cell, genes encoding flagellin were overexpressed. Some genes associated with K(+) transport, phBC6A51 phage genes, and the binding component of the enterotoxin hemolysin BL (HBL) were also induced. Quantitative reverse transcription-PCR (qRT-PCR) experiments indicated an almost 2-fold upregulation of the entire hbl operon during swarming. Finally, BC1435 and BC1436, orthologs of liaI-liaH that are known to be involved in the resistance of Bacillus subtilis to daptomycin, were upregulated under swarming conditions. Accordingly, phenotypic assays showed reduced susceptibility of swarming B. cereus cells to daptomycin, and P(spac)-induced hyper-expression of these genes in liquid medium highlighted the role of BC1435 and BC1436 in the response of B. cereus to daptomycin.

Fig. 1.

Growth of B. cereus ATCC 14579 on swarm and nonswarm plates. (A) Growth of swarmer (sw+) and nonswarmer (sw−) populations was monitored over time by measuring colony diameters (▪, ⧫) and CFU numbers (□, ⋄). (B) Swarming (left) and nonswarming (right) colonies produced at 8 h after the inoculum. (C) Examples of swarm (left) and vegetative (right) cells collected from the swarming halo and the nonswarming colony, respectively. Samples were treated with flagellar staining and photographs taken by phase-contrast microscopy (magnification, ×1,000). Insets show higher-magnification views of swarm and nonswarm cells.

Fig. 2.

Number of differentially expressed genes (P < 0.05) during swarming motility reported on the basis of COG classification. Some genes belong to more than one category. Genes that have not been assigned to a COG category (listed in Table S2 in the supplemental material) are not included in the figure.