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Swarm-Cell Differentiation in Salmonella enterica Serovar Typhimurium Results in Elevated Resistance to Multiple Antibiotics. Wook Kim, 2003.Although a wealth of knowledge exists about the molecular and biochemical mechanisms governing the swimming motility of Salmonella enterica serovar Typhimurium, its surface swarming behavior has not been extensively characterized . When inoculated onto a semisolid agar medium supplemented with appropriate nutrients, serovar Typhimurium undergoes a morphological differentiation whereby single cells hyperflagellate and elongate into nonseptate, multinucleate swarm cells . Swarm migration is a collective behavior of groups of cells . We have isolated a MudJ insertion mutant of serovar Typhimurium 14028 that failed to swarm under any conditions . The site of the MudJ insertion was determined to be in the pmrK locus within the pmrHFIJKLM operon, which was previously demonstrated to confer resistance to cationic antimicrobial peptides . ß-Galactosidase assays, using the pmrK::lacZ transcriptional fusion, showed increased expression of the pmr operon in swarm cells compared to that in vegetative cells . In concurrence with the expression data, swarm cells exhibited greater tolerance to polymyxin . To compare the profiles of vegetative and swarm-cell resistance to other antibiotics, E-test strips representing a wide range of antibiotic classes were used . Swarm cells exhibited elevated resistance to a variety of antibiotics, including those that target the cell envelope, protein translation, DNA replication, and transcription . These observations, in addition to the dramatic morphological changes associated with the swarming phenotype, provide an intriguing model for examining global differences between the physiological states of vegetative and swarm cells of serovar Typhimurium . Open Reading Frame all0601 from Anabaena sp . Strain PCC 7120 Represents a Novel Gene, cnaT, Required for Expression of the Nitrate Assimilation nir Operon. José E. Frías, 2003.Expression of the nitrate assimilation nir operon in the filamentous, heterocyst-forming cyanobacterium Anabaena sp . strain PCC 7120 requires the action of both the global nitrogen control transcription factor NtcA and the pathway-specific transcriptional regulator NtcB . In the genome of this cyanobacterium, the ntcB gene is found in a cluster of genes located in the complementary strand, upstream from the nir operon . Just downstream of ntcB, there is an open reading frame, all0601 (previously designated orf356 and now designated the cnaT gene), that putatively encodes a protein similar to proteins with glycosyl transferase activity and that is also present clustered together with ntcB homologues or nitrate assimilation structural genes in other cyanobacterial genomes . An insertional mutant of cnaT was generated and found to be unable to assimilate nitrate, although it could use ammonium or dinitrogen as a source of nitrogen for growth . In the mutant, under derepression conditions, nir operon mRNA (as determined by RNA-DNA hybridization and primer extension analysis) and enzymes of the nitrate reduction system (i.e., nitrate reductase and nitrite reductase) were expressed at low or undetectable levels . Inactivation of cnaT did not impair expression of ntcB, and expression of cnaT itself was constitutive and regulated by neither NtcA nor NtcB . Regulation of expression of the nir operon in Anabaena sp . strain PCC 7120 by CnaT and the previously described regulatory elements, NtcA and NtcB, is discussed .
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