Swarm-cell differentiation in Salmonella enterica serovar Typhimurium (S. typhimurium) results in a biosynthetic mode of growth, despite growing on a rich medium, and cells that have elevated antibiotic resistance. These phenotypes are not a prerequisite for swarm motility. By blocking the switch to anabolic growth using amino acid auxotrophs and screening for the presence of elevated antibiotic resistance in the swarm state, we found that cysteine biosynthesis is crucial for complete swarm-cell differentiation. Mutants were made in each cys biosynthetic operon and all had decreased antibiotic resistance in the swarm state, while swim-cell resistance remained the same as that of wild-type cells. This swarm-state-specific decreased resistance in Dcys strains could be restored to wild-type levels by the addition of cysteine to swarm medium. Two regulatory mutants, DcysB and DcysE, failed to swarm unless cysteine was supplemented to the medium. We show that all CysB-responsive operons involved in cysteine biosynthesis are upregulated in the swarm state, even though swarm cells are cultivated on a medium that represses cysteine biosynthesis in the swim state. While swarm medium has sufficient cysteine for growth of S. typhimurium, it does not contain enough for swarm-cell differentiation. We hypothesize that in these cells, the additional cysteine requirement is for use in pathways not directly related to cell growth.

No abstract

BackgroundThe Salmonella enterica serovar Typhimurium PhoPQ two component system (TCS) is activated by low Mg2+ levels, low pH and by antimicrobial peptides (AP). Under Mg2+ limitation, the PhoPQ system induces pmrD expression, which post-translationally activates the PmrAB TCS. PhoPQ and PmrAB control many genes required for intracellular survival and pathogenesis. These include the polymyxin resistance (pmr) operon, which is required for aminoarabinose modification of LPS and protecting the outer membrane from antimicrobial peptide disruption and killing. Extracellular DNA is a ubiquitous polymer in the matrix of biofilms and accumulates in some infection sites. Extracellular DNA chelates cations and thus activates the Pseudomonas aeruginosa PhoPQ/PmrAB systems, leading to expression of the orthologous arn (pmr) operon.ResultsHere we show that extracellular DNA induces expression of the S. Typhimurium pmr antimicrobial peptide resistance operon in a PhoPQ and PmrAB-dependent manner. Induction of the pmr genes by DNA was blocked when present with excess Mg2+. Exogenous DNA led to increased resistance of planktonic cultures to aminoglycosides, antimicrobial peptides (AP) and ciprofloxacin, but only AP resistance was PhoPQ/PmrAB-dependent. Extracellular DNA was shown to be a matrix component of S. Typhimurium biofilms cultivated in flow chambers and on glass surfaces. A pmrH-gfp fusion was highly expressed in flow chamber biofilms cultivated in medium with repressing levels of 10 mM Mg2+ and co-localized with eDNA. Expression of pmrH-lux was monitored in plastic peg biofilms and shown to require PhoPQ and PmrAB. Biofilms had higher levels of pmrH expression compared to planktonic cultures. We propose that DNA accumulation in biofilms contributes to the increased pmrH-lux expression in biofilms.ConclusionsThe Salmonella PhoPQ/PmrAB systems and antimicrobial peptide resistance are activated by the cation chelating properties of extracellular DNA. DNA-induced AP resistance may allow immune evasion and increased survival of S. Typhimurium biofilms formed during extracellular growth stages of an infection or outside the host.

No abstract

SUMMARYCallus and onion leaf tip material were exposed to ["C]cysteme, ["C]serine or ["Qvaline and the distribution of labelling in the amino acids and cysteine derivatives assessed after two to fourteen days. With ["C]cysteine and serine radioactivity vras found in the callus in a range of amino-acids and in the flavour precursor compounds, S-methyl-L-cysteine sulphoxide and 5-trans-prop-1-enyl-L-cysteine sulphoxide, particularly in the former precursor. ["C]valine was much more metabolically inert and little distribution of radioactivity occurred. In comparison the shoot tip showed a more active metabolism of ["CJserine and cysteine than callus but again much less activity with ['*C]valine, With all three compounds, however, the heaviest labelling in the shoot tip was in the cysteine derivatives, particularly the main flavour precursor of the intact onion, .S-trans-prop-1-enyl-L-cysteine sulphoxide. When unlabelled cysteine derivatives were fed to callus, S-trans-prop-t-enyl-L-cysteine sulphoxide was formed. The presence of this precursor compound in callus showed that the callus was capable of synthesizing 5-trans-prop-l-enyl-L-cysteine from early intermediates, such as the amino-acids, and by a pathway similar to that in the intact plant.