The Effect of Protozoan grazing of Freshwater Biofilms, a Bacteria's Story.

Background

Ciliates are considered the most evolved and complex of protozoans, with various feeding mechanisms for devouring prey bacteria. Hundreds of in situ experiments have been conducted on the role of ciliates within the food web and their effect on microbial communities within a pelagic environment, however the impact of ciliates on a microbial biofilm is often overlooked. Along with studying the impact of ciliates feeding on microbial biofilms, the defence mechanisms are much less understood than those for the pelagial.

The increase in published papers focusing on biofilms over the last decade is largely due to the improvements of methods that provide unbiased estimates of phytoplankton growth and micro-zooplankton grazing rates. These developments in molecular techniques have opened up various opportunities to study the biofilm in greater depth. This has sparked a large increase of publications based on the interactions between ciliates and bacteria.

What we are looking at

The reviewed paper tackles how environmental nutrient conditions and the presence of Tetrahymena pyriformis, a predatory ciliates affect the bacteria’s biofilm structure and its resistance to grazing. Scherwass et al., 2016 conducted the experiment with the bacterium, Acinetobacter sp. Strain C6, A variant of the genus Acinetobacter, which is known for forming into microcolonies during biofilm growth.

Sodium benzoate and citrate were used as the mineral mediums , with benzoate providing the more optimal carbon source for Acinetobacter’s development. Scherwass et al., 2016 found that the presence of a grazing pressure led to different shapes of Acinetobacter biofilms depending on varying nutrients levels and grazing pressures. Throughout the controls the biofilms developed into round colonies, regularly distributed over the surface of the medium. However when a grazer pressure was introduced, the microcolonies showed an irregular, elongated shape. This change in structure occurred across all treatments. When the bacteria was cultured under optimal carbon supply the size and biovolume increased indicated that the presence of the ciliates stimulates bacterial growth, however when grown with a low/medium sodium benzoate supply they decreased in microcolony size and biovolume. These changes were even more pronounced in the biofilms matured under high medium citrate medium with, more irregular structures and even smaller in size and biovolume.

Results

Numerous key papers have been published based on protozoan grazing of biofilms and how the biofilm phenotype provides protection against predation. However this current study dispels the popular opinion that biofilms are protected as it is clear that T. pyriformis is capable of grazing on the biofilm communities as well as influencing their structure and size.

Along with establishing a connection between nutrient sources and a biofilms capability to defend itself against predation, the study also identified various additional factors (Growth Rate of biofilm, aeration through ciliate movement and the ciliates feeding mechanisms) that would influence a biofilm’s  defence. All these additional factors imply that the interaction between a biofilm community and its predators may be far more complex that currently thought.

The role of protozoans on microbial communities has received a lot of attention in recent years with the study on the biofilm and filament formation being developed as well. Scherwass et al., 2016 was able to build upon this emerging field has revealed that contrary to our pre-existing knowledge there is still far more to study and find out.

The paper mentions that the surface properties of bacteria have also been shown to influence the rate of their ingestion by protozoa, listing two studies, that show gram‐positive bacteria being grazed to a lower extant than gram‐negative bacteria, however the experiment they conducted only analysed Acinetobacter a Gram-negative bacteria, the study may be easier to link to natural communities if they carried out the study on both groups of bacteria.

References: 

Reviewed Paper:

Anja Scherwass, Martina Erken and Hartmut Arndt (2016). Grazing Effects of Ciliates on Microcolony Formation in Bacterial Biofilms, Microbial Biofilms - Importance and Applications, Dr. Dharumadurai Dhanasekaran (Ed.), InTech, DOI: 10.5772/63516. Available from: http://www.intechopen.com/books/microbial-biofilms-importance-and-applications/grazing-effects-of-ciliates-on-microcolony-formation-in-bacterial-biofilms

Further Reading:

Matz C, Kjelleberg, S. Off the hook – how bacteria survice protozoan grazing. Trends Microb 2005; 13:302–307. DOI: 10.1016/j.tim.2005.05.009

Matz C, Jürgens K. High motility reduces grazing mortality of planktonic bacteria. Appl Environ Microb 2005; 71:921–929. DOI: 10.1128/AEM.71.2.921–929.2005

Dopheide GL, Stott R, Lewis G. Preferential Feeding by the ciliates Chilodonella and Tetrahymena spp. and effects of these protozoa on bacterial biofilm structure and composition. Appl Environ Microb 2011; 77:4564–4572.