No abstract

Summary Mesophilic Aeromonas strains express a single polar flagellum in all culture conditions and produce lateral flagella on solid media. Such hyperflagellated cells demonstrate increased adherence. Nine lateral flagella genes, lafA–U for Aeromonas hydrophila, and four Aeromonas caviae genes, lafA1, lafA2, lafB and fliU, were isolated. Mutant characterization, nucleotide and N‐terminal sequencing demonstrated that the A. hydrophila and A. caviae lateral flagellins were almost identical, but were distinct from their polar flagellum counterparts. The aeromonad lateral flagellins exhibited higher molecular masses on SDS–PAGE, and this aberrant migration was thought to result from post‐translational modification through glycosylation. Mutation of the Aeromonas lafB, lafS or both A. caviae lateral flagellins caused the loss of lateral flagella and a reduction in adherence and biofilm formation. Mutations in lafA1, lafA2, fliU or lafT resulted in strains that expressed lateral flagella, but had reduced adherence levels. Mutation of the lateral flagella loci did not affect polar flagellum synthesis, but the polarity of the transposon insertions on the A. hydrophila lafT/U genes resulted in non‐motility. However, mutations that abolished polar flagellum production also inhibited lateral flagella expression. We conclude that Aeromonas lateral flagella: (i) play a role in adherence and biofilm formation; (ii) are distinct from the polar flagellum; (iii) synthesis is dependent upon the presence of a polar flagellum filament; and (iv) that the motor proteins of the polar and lateral flagella systems appear to be shared.

Group A streptococci (Streptococcus pyogenes or GAS) freshly isolated from individuals with streptococcal sore throat or invasive (''flesh-eating'') infection often grow as mucoid colonies on primary culture but lose this colony appearance after laboratory passage. The mucoid phenotype is due to abundant production of the hyaluronic acid capsular polysaccharide, a key virulence determinant associated with severe GAS infections. These observations suggest that signal(s) from the human host trigger increased production of capsule and perhaps other virulence factors during infection. Here we show that subinhibitory concentrations of the human antimicrobial cathelicidin peptide LL-37 stimulate expression of the GAS capsule synthesis operon (hasABC). Up-regulation is mediated by the CsrRS 2-component regulatory system: it requires a functional CsrS sensor protein and can be antagonized by increased extracellular Mg 2؉ , the other identified environmental signal for CsrS. Up-regulation was also evident for other CsrRS-regulated virulence genes, including the IL-8 protease PrtS/ScpC and the integrin-like/IgG protease Mac/IdeS, findings that suggest a coordinated GAS virulence response elicited by this antimicrobial immune effector peptide. LL-37 signaling through CsrRS led to a marked increase in GAS resistance to opsonophagocytic killing by human leukocytes, an in vitro measure of enhanced GAS virulence, consistent with increased expression of the antiphagocytic capsular polysaccharide and Mac/IdeS. We propose that the human cathelicidin LL-37 has the paradoxical effect of stimulating CsrRS-regulated virulence gene expression, thereby enhancing GAS pathogenicity during infection. The ability of GAS to sense and respond to LL-37 may explain, at least in part, the unique susceptibility of the human species to streptococcal infection.