BACKGROUND

The 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.

RESULTS

Here 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.

CONCLUSIONS

The 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.