Patterns of inhibitory activity of Streptomyces from Myrick Marsh sediments

Background/Question/Methods

Given the extensive benefits freshwater marshes provide for their inhabitants, as well as for surrounding human populations, it is critical to understand the relative health of these ecosystems.  To comprehend the health of freshwater marshes, like Myrick Marsh in La Crosse, WI, sediment dwelling bacterial communities can be employed as bioindicators because they are unable to escape pollution and have short generation times and are, therefore, highly responsive to conditional variability. In this study, fifty strains of Streptomyces isolated from sediment samples at an undisturbed control site and locations that are directly inundated with storm water runoff from several land use areas (commercial/industrial, commercial/residential, transportation, and residential) were examined for their inhibitory capabilities of four non-indigenous reference Streptomycesisolates (previously obtained from agricultural fields).  Additionally, randomly selected isolates from each of the five locations were examined for their inhibitory capabilities against other indigenous isolates to search for any local adaptation.  Variations in isolate inhibitory activities were determined by performing inhibition assays in which spore suspensions of isolates were dotted on starch casein agar and subsequently overlayed with water agar and the given test isolate. From these assays, zones of inhibition were identified and measured for each isolate. 

Results/Conclusions

Differences in mean inhibition zone size against reference isolates were found among locations with the largest observed for the control site and the smallest within the transportation site.  Conversely, cross-site comparisons revealed the lowest mean zones of inhibition were within the commercial/residential location and the highest were observed among isolates from the transportation site.  Additionally, cross-site comparisons revealed trends consistent with local adaptation in three of the five locations as inhibition was greater against isolates from the same location than against isolates from other locations (particularly at the control site).

Given that inhibition of other microbes through production of antibiotics is a primary competitive tactic used by Streptomyces, any lessened inhibitory capability could indicate reduced health of the system.  Furthermore, as each drainage site was characterized by a different drainage regime these assays can potentially yield data on the varying hazard levels each land usage poses.  Comparisons with reference isolates indicated that isolates from the transportation site were potentially more severely impacted than isolates from other sites in their broad inhibitory abilities.  However, the strong indication of local adaptation of isolates from this site suggests that different inputs could be leading to differentiation among Streptomyces communities in Myrick Marsh.