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Biofilm research aims at fighting hospital germs

Pseudomonas aeruginosa is a common bacterium that can be found in many different places – soil, water, washing basins, toilets and washing machines, to name but a few examples. Due to its resistance to antibiotic treatment, P. aeruginosa is mainly known as the cause of hospital-acquired infections. David Schleheck, a biologist from Konstanz University, deals specifically with the bacterium’s presence in biofilms. His research could open up new possibilities for combating P. aeruginosa.

Pseudomonas aeruginosa, which has a diameter of between two to four micrometres, thrives in a broad range of different environments. It is even able to reproduce under nutrient-deficient conditions due to its ability to use a broad range of different substances for growth. Although P. aeruginosa does not normally occur in the human body, the bacterium is nevertheless also able to adapt to the conditions in the human body and colonise the urinary tracts, the sinuses, the ear canal and the lungs. “Under normal circumstances, P. aeruginosa does not represent a danger for humans, but can lead to severe inflammation in immunocompromised people,” said Schleheck.

From single cells to biofilms

Schleheck’s research is specifically focussed on P. aeruginosa biofilms. P. aeruginosa bacteria seem to thrive particularly in biofilms where they are embedded in a self-generated extracellular polymeric matrix which protects them against macrophages, antibiotics and elimination from the body. Biofilms make the bacteria resistant to treatment. “Research into bacterial physiology, biochemistry and molecular biology has long been focussed on dispersed single free-swimming cells,” said Schleheck. However, research also needs to focus on bacteria in biofilms because more than sixty percent of all chronic bacterial infections are due to bacterial biofilms. 

Bacterial cells (green) in a biofilm on a solid surface (photo on the left, top view; photo below, lateral view) and in a liquid culture biofilm floc, i.e. a suspended aggregate of bacteria (photo on the right). Dead cells and extracellular DNA are stained red. © Dr. Schleheck, modified from PLoS ONE 7(8): e42874 (left photo) and PLoS ONE 4(5): e5513 (right-hand photo)

Biofilms in aqueous solutions – a previously unknown phenomenon

Schleheck closely examined the formation and dispersal of cellular aggregates formed by P. aeruginosa in liquid cultures and compared this with biofilm formation on a solid surface. P. aeruginosa bacteria have long been known to form biofilms on solid surfaces. In addition, prior to Schleheck’s studies most researchers were of the opinion that P. aeruginosa cells were only present as dispersed single cells in liquid cultures. However, Schleheck has been able to show that P. aeruginosa bacteria also form cellular aggregates and biofilm flocs when grown in liquid cultures. He was thus able to show P. aeruginosa research in a new light: previously, bacterial cultures grown on surfaces were compared with liquid bacterial cultures in the hope of being able to elucidate the specific properties of biofilm bacteria. “However, what the researchers actually did was to compare different biofilms with each other,” said Schleheck referring to his observation that P. aeruginosa also formed biofilms in liquid cultures. It therefore comes as no surprise that the studies did not achieve any clear results.

Biofilm dispersal puts old measurement methods in a new perspective

Schleheck also found that the process of biofilm formation consists of several phases: initially, the bacteria grow in unstructured biofilms as they divide and synthesise extracellular polymeric matrix constituents. Later, under conditions of carbon starvation and oxygen limitation, the bacteria switch back to their dispersed single-cell lifestyle and are able to ‘swim’ to areas with better nutrient conditions and divide again. This phenomenon used to cause misunderstandings related to the measurement of P. aeruginosa growth in aqueous solutions. Bacterial growth is usually determined from optical density (OD), which involves the use of spectrophotometry for measuring culture turbidity. As a larger number of individual cells leads to greater turbidity, the method seemed to be well suited for determining the growth of P. aeruginosa in liquid cultures. Schleheck’s findings about the formation and dispersal of biofilms, however, show that photometric measurements are prone to produce false results. Higher optical densities are usually measured as the bacteria are depleted of limited growth substrate. However, these higher values are not due to continued growth, but rather to the dissolution of the aggregates and dispersal of single free-swimming cells.

Growth curve of a Pseudomonas aeruginosa liquid culture and increase of the optical density (OD) after the onset of starvation (panel A). The additional increase of OD is not due to continued growth, but to the dispersal of suspended aggregates into dispersed single cells during starvation. Panel A also shows a schematic of the apparatus used for automated OD monitoring. Panel B shows the growth curve of a Pseudomonas aeruginosa biofilm when grown on glucose compared to the curve resulting from the dispersal of the biofilm after the onset of starvation (no glucose). The onset of starvation is indicated by dashed vertical lines. © Dr. Schleheck, panel A modified from PLoS ONE 4(5): e5513. doi:10.1371/journal.pone.0005513

The goal: improving the control of P. aeruginosa bacteria

The occurrence of P. aeruginosa is not only a big problem in hospitals; the bacteria can also cause major harm in technical areas such as water management and distribution systems as well as in the food industry. Schleheck hopes that his basic research leads to a better understanding of P. aeruginosa biofilm formation and dispersal and enables the more effective destruction of the bacteria. He is particularly focussed on finding ways to control the dissociation of biofilms and the elimination of the bacteria. He is interested in working with companies, especially in the development and testing of biofilm control methods.


 

Further information:
Dr. David Schleheck
University of Konstanz
Tel.: +49 (0)7531/ 88-3270
E-mail: david.schleheck(at)uni-konstanz.de

Website address: https://www.gesundheitsindustrie-bw.de/en/article/news/biofilm-research-aims-at-fighting-hospital-germs