Production of the Bioactive Compounds Violacein and Indolmycin Is Conditional in a maeA Mutant of Pseudoalteromonas luteoviolacea S4054 Lacking the Malic Enzyme

It has previously been reported that some strains of the marine bacterium Pseudoalteromonas luteoviolacea produce the purple bioactive pigment violacein as well as the antibiotic compound indolmycin, hitherto only found in Streptomyces. The purpose of the present study was to determine the relative role of each of these two compounds as antibacterial compounds in P. luteoviolacea S4054. Using Tn10 transposon mutagenesis, a mutant strain that was significantly reduced in violacein production in mannose-containing substrates was created. Full genome analyses revealed that the vio-biosynthetic gene cluster was not interrupted by the transposon; instead the insertion was located to the maeA gene encoding the malic enzyme. Supernatant of the mutant strain inhibited Vibrio anguillarum and Staphylococcus aureus in well diffusion assays and in MIC assays at the same level as the wild type strain. The mutant strain killed V. anguillarum in co-culture experiments as efficiently as the wild type. Using UHPLC-UV/Vis analyses, we quantified violacein and indolmycin, and the mutant strain only produced 7-10% the amount of violacein compared to the wild type strain. In contrast, the amount of indolmycin produced by the mutant strain was about 300% that of the wild type. Since inhibition of V. anguillarum and S. aureus by the mutant strain was similar to that of the wild type, it is concluded that violacein is not the major antibacterial compound in P. luteoviolacea. We furthermore propose that production of violacein and indolmycin may be metabolically linked and that yet unidentified antibacterial compound(s) may be play a role in the antibacterial activity of P. luteoviolacea.

Surface modification of an organic hessian substrate leads to shifts in bacterial biofilm community composition and abundance

Antifouling strategies to limit biofilms on submerged surfaces in the marine environment are of particular interest due to the economic and environmental impacts in industries such as shipping and aquaculture. Here, we investigate the influence of chemically modified hessian bag surfaces on the bacterial abundance and community composition of biofilm formation using flow cytometry and 16S rRNA pyrosequencing. Hessian bags were coated with 5% and 10% Propyl(trimethoxy)silane (PTMS) and half of the bags had their lignin and hemicellulose removed via NaOH mercerisation. Significantly lower bacterial abundance was observed on mercerised bags treated with 5% PTMS (p<0.01). Significant shifts in bacterial taxa were also observed (p=0.0004), whereby unmercerised bags exhibited higher relative abundances of the anaerobic family Desulfovibrionaceae (4.5±1.7%), while mercerised bags displayed higher relative abundances of the aerobic family Phyllobacteriaceae (3.6±1.7%). This suggests that the mercerisation process may lower colonization rates and subsequently produce a thinner biofilm. This hypothesis is strengthened by the lower abundance of bacteria on mercerised bags, particularly on the 5% PTMS coating. Our results show that modifying a hessian surface via non-toxic coating and mercerisation reduces biofilm formation and also shifts the dominant taxa, increasing our understanding of antifouling strategies in the marine environment.

Comparison of microcolony formation between Vibrio sp. strain S141 and a flagellum-negative mutant developing on agar and glass substrata

A flagellum-negative mutant, M8.2, of the marine bacterium Vibrio sp. S141 was produced by transposon mutagenesis. Time-lapse video imaging of surface colonisation behaviour and microcolony formation of S141 compared to M8.2 cells was carried out to investigate the role of the flagellum of Vibrio sp. S141 in microcolony formation on agar and glass substrata. On an agar surface, S141 cells formed a tetrad pattern after the first two cell divisions, during initial surface colonisation. Developed microcolonies consisted of tight circular arrangements of cells with infrequent branching of cells from the main body. In contrast, M8.2 cells did not form tetrad patterns and micro-colonies generally showed enhanced branching and did not develop circular arrangements of cells. On a glass surface under flow conditions, S141 cells displayed several types of movement behaviours at the surface which may have assisted microcolony formation. M8.2 cells appeared unable to develop micro-colonies, but rather displayed a behaviour which enabled them to spread out across the substratum. Laser scanning confocal microscopy revealed S141 mature biofilms consisted of characteristic towers of bacterial growth with scattered troughs. The flagellum-negative M8.2 biofilm did not form such architecture, displaying a homogeneous distribution of cells throughout the biofilm and across the entire substratum. Although not required for attachment to the glass substratum, the flagellum was required for alignment as well as specific movement behaviours by S141 cells.