Global gene expression patterns of Bacillus subtilis in response to subinhibitory concentrations of protein synthesis inhibitors (chloramphenicol, erythromycin, and gentamicin) were studied by DNA microarray analysis. B. subtilis cultures were treated with subinhibitory concentrations of protein synthesis inhibitors for 5, 15, 30, and 60 min, and transcriptional patterns were measured throughout the time course. Three major classes of genes were affected by the protein synthesis inhibitors: genes encoding transport/binding proteins, genes involved in protein synthesis, and genes involved in the metabolism of carbohydrates and related molecules. Similar expression patterns for a few classes of genes were observed due to treatment with chloramphenicol (0.4؋ MIC) or erythromycin (0.5؋ MIC), whereas expression patterns of gentamicin-treated cells were distinct. Expression of genes involved in metabolism of amino acids was altered by treatment with chloramphenicol and erythromycin but not by treatment with gentamicin. Heat shock genes were induced by gentamicin but repressed by chloramphenicol. Other genes induced by the protein synthesis inhibitors included the yheIH operon encoding ABC transporter-like proteins, with similarity to multidrug efflux proteins, and the ysbAB operon encoding homologs of LrgAB that function to inhibit cell wall cleavage (murein hydrolase activity) and convey penicillin tolerance in Staphylococcus aureus.
Natural isolates of Bacillus subtilis exhibit a robust multicellular behavior known as swarming. A form of motility, swarming is characterized by a rapid, coordinated progression of a bacterial population across a surface. As a collective bacterial process, swarming is often associated with biofilm formation and has been linked to virulence factor expression in pathogenic bacteria. While the swarming phenotype has been well documented for Bacillus species, an understanding of the molecular mechanisms responsible remains largely isolated to gram-negative bacteria. To better understand how swarming is controlled in members of the genus Bacillus, we investigated the effect of a series of gene deletions on swarm motility. Our analysis revealed that a strain deficient for the production of surfactin and extracellular proteolytic activity did not swarm or form biofilm. While it is known that surfactin, a lipoprotein surfactant, functions in swarming motility by reducing surface tension, this is the first report demonstrating that general extracellular protease activity also has an important function. These results not only help to define the factors involved in eliciting swarm migration but support the idea that swarming and biofilm formation may have overlapping control mechanisms.It has long been recognized that bacteria can facilitate their growth and survival by forming cooperative, multicellular communities that are most often associated with surfaces (38). Such organized populations of microorganisms have been found in both clinical and environmental settings, where they have positive and negative impacts on human health and environmental ecology (4, 24). For these reasons, bacterial multicellularity is being actively studied, and while biofilms are most commonly ascribed to this type of behavior, swarming is another primary example of a surface-associated collective bacterial process. A form of migration, swarming facilitates the rapid colonization of surfaces by a population of bacteria. Swarming motility has been linked with biofilm formation, antibiotic resistance, and virulence factor production (1, 21, 39). Many of the virulence factors reported to be associated with the occurrence of swarming are exoenzymes and, more specifically, extracellular proteases (12, 47). While a strong link between swarming, extracellular protease production, and pathogenesis has been noted (12,44,47), only a few studies have focused on the requirement of proteases for swarm motility. In a recent investigation, the extracellular protease Epr was found to be essential for swarming motility in the domesticated Bacillus subtilis laboratory strain 168 (6). Since laboratory strains of B. subtilis have been shown to exhibit less-robust biofilm formation and swarm motility (3, 20), we were interested in investigating whether Epr and other extracellular proteases were involved in swarming in an undomesticated B. subtilis strain.First described more than a century ago for the gram-negative organism Proteus, swarming motility has now been...
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