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Competence without a Competence Pheromone in a Natural Isolate of Streptococcus infantis. Ola Ween, 2002.Many streptococcal species belonging to the mitis and anginosus phylogenetic groups are known to be naturally competent for genetic transformation . Induction of the competent state in these bacteria is regulated by a quorum-sensing mechanism consisting of a secreted peptide pheromone encoded by comC and a two-component regulatory system encoded by comDE . Here we report that a natural isolate of a mitis group streptococcus (Atu-4) is competent for genetic transformation even though it has lost the gene encoding the competence pheromone . In contrast to other strains, induction of competence in Atu-4 is not regulated by cell density, since highly diluted cultures of this strain are still competent . Interestingly, competence in the Atu-4 strain is lost if the gene encoding the response regulator ComE is disrupted, demonstrating that this component of the quorum-sensing apparatus is still needed for competence development . These results indicate that mutations in ComD or ComE have resulted in a gain-of-function phenotype that allows competence without a competence pheromone . A highly similar strain lacking comC was isolated independently from another individual, suggesting that strains with this phenotype are able to survive in nature in competition with wild-type strains . The SecB Chaperone Is Bifunctional in Serratia marcescens: SecB Is Involved in the Sec Pathway and Required for HasA Secretion by the ABC Transporter. Guillaume Sapriel, 2003.HasA is the secreted hemophore of the heme acquisition system (Has) of Serratia marcescens . It is secreted by a specific ABC transporter apparatus composed of three proteins: HasD, an inner membrane ABC protein; HasE, another inner membrane protein; and HasF, a TolC homolog . Except for HasF, the structural genes of the Has system are encoded by an iron-regulated operon . In previous studies, this secretion system has been reconstituted in Escherichia coli, where it requires the presence of the SecB chaperone, the Sec pathway-dedicated chaperone . We cloned and inactivated the secB gene from S . marcescens . We show that S . marcescens SecB is 93% identical to E . coli SecB and complements the secretion defects of a secB mutant of E . coli for both the Sec and ABC pathways of HasA secretion . In S . marcescens, SecB inactivation affects translocation by the Sec pathway and abolishes HasA secretion . This demonstrates that S . marcescens SecB is the genuine chaperone for HasA secretion in S . marcescens . These results also demonstrate that S . marcescens SecB is bifunctional, as it is involved in two separate secretion pathways . We investigated the effects of secB point mutations in the reconstituted HasA secretion pathway by comparing the translocation of a Sec substrate in various mutants . Two different patterns of SecB residue effects were observed, suggesting that SecB functions may differ for the Sec and ABC pathways . Role of Listeria monocytogenes  B in Survival of Lethal Acidic Conditions and in the Acquired Acid Tolerance Response.Adriana Ferreira, 2003.The food-borne pathogen Listeria monocytogenes can acquire enhanced resistance to lethal acid conditions through multiple mechanisms . We investigated contributions of the stress-responsive alternative sigma factor, B, which is encoded by sigB, to growth phase-dependent acid resistance (AR) and to the adaptive acid tolerance response in L . monocytogenes . At various points throughout growth, we compared the relative survival of L . monocytogenes wild-type and
 sigB strains that had been exposed to either brain heart infusion (pH 2.5) or synthetic gastric fluid (pH 2.5) with and without prior acid adaptation . Under these conditions, survival of the
sigB strain was consistently lower than that of the wild-type strain throughout all phases of growth, ranging from 4 orders of magnitude less in mid-log phase to 2 orders of magnitude less in stationary phase . Survival of both
sigB and wild-type L . monocytogenes strains increased by 6 orders of magnitude upon entry into stationary phase, demonstrating that the L . monocytogenes growth phase-dependent AR mechanism is
B independent .
B-mediated contributions to acquired acid tolerance appear to be greatest in early logarithmic growth . Loss of a functional
B reduced the survival of L . monocytogenes at pH 2.5 to a greater extent in the presence of organic acid (100 mM acetic acid) than in the presence of inorganic acid alone (HCl), suggesting that L . monocytogenes protection against organic and inorganic acid may be mediated through different mechanisms .
B does not appear to contribute to pHi homeostasis through regulation of net proton movement across the cell membrane or by regulation of pHi buffering by the GAD system under the conditions examined in this study . In summary, a functional
B protein is necessary for full resistance of L . monocytogenes to lethal acid treatments .
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