Fighting fire with fire and bacteria with bacteria

Most bacteria found in natural, medical and industrial settings persist in complex microbial communities attached to surfaces or associated with interfaces known as biofilms. Recently, there is a renewed interest in the use of biological control agents against these surface-associated bacteria. Research conducted with “predatory bacteria” has shown that these microorganisms have the ability to attack and destroy existing biofilms as well as the potential to be used in the future in the fight against harmful bacteria.

For many years microbiologists have studied bacteria as free-floating organisms in liquid suspension. However, it became quite evident that in nature bacteria are typically not free floating but grow upon submerged surfaces. Biofilms form when bacteria adhere to surfaces and begin to excrete a glue-like substance that protects them and anchors them to material such as metals, plastics, soil, medical implants and tissue. Biofilms cost the nation billions of dollars yearly in equipment damage, product contamination, energy loss and medical infections. In industry, biofilms can cause corrosion of pipes, reduce heat transfer in cooling systems and plug water filters. Biofilms can cause contamination of drinking water and food processing surfaces. In our daily life, harmful bacteria can form biofilms on surfaces such as kitchen sponges and cutting boards and lead to contamination and cross contamination of foods and produce. In addition, biofilms have a significant impact on human health. They are believed to be a major cause of chronic infections and form on various medical and surgical implants.


One of the major difficulties in controlling surface-attached bacteria is their enhanced resistance to antimicrobial agents—biofilms can be up to 1,000 times more resistant to antimicrobial agents than their planktonic counterparts. Thus, the huge doses of antimicrobials required to rid systems of biofilm bacteria can be environmentally undesirable and medically impractical. The problem of enhanced biofilm resistance has led researchers to examine other methods of biofilm control. Among these alternative techniques is the use of biological control agents; these include the use of invertebrates and protozoa to reduce biofilms by means of grazing and the use of bacteriophages.

In my lab we are studying the unique biology of predatory prokaryotes from the genera Bdellovibrio and Micavibrio and evaluating their ability to destroy pathogenic biofilms. Bdellovibrio and Micavibrio are both gram-negative bacteria. However, unlike most bacteria, these organisms are obligatory parasites that can survive only by feeding on other gram-negative bacteria. Bdellovibrio was first isolated from soil in the early 1960s by Heinz Stolp as he was trying to isolate bacteriophages. In order to find its host, Bdellovibrio swims rapidly in its environment and is considered to be one of the fastest moving organisms in nature. After colliding with its prey, Bdellovibrio penetrates the prey’s periplasm and starts multiplying in the periplasmic space, feeding on its host from within. Subsequently, it bursts the cell envelope and starts a new cycle of attack. In a previous study, we demonstrated that Bdellovibrio bacteriovorus has the ability to reduce an existing Escherichia coli biofilm by more than 99% in just a few hours. Since Bdellovibrio multiplies rapidly within its host, even low numbers of Bdellovibrio (1-10 cells/ml) are sufficient to initiate an attack. To mimic the development of more naturaly occurring biofilms, such as those that develop in pipes and in the urinary tract, we have grown E. coli and Pseudomonas fluorescens biofilms under high flow conditions. B. bacteriovorus was able to successfully attack the thicker biofilms grown in flow cell systems, suggesting that the action of this predator is not restricted to the surface of the biofilm, as is frequently observed with invertebrates and protozoan biofilm grazing experiments and with studies using bacteriophage. Furthermore, our data suggest that the predator not only can survive in the biofilm but also could feed, proliferate, and escape in order to start a new cycle of predation.

Another predator we are currently studying is Micavibrio, which was first isolated from sewage water over 25 years ago in the former Soviet Union. However, virtually nothing is known of its unique biology. Unlike Bdellovibrio, which feeds on its prey as an endoparasite, Micavibrio exhibits a “vampire” like lifestyle, leeching to its host as it feeds. We have found that, like Bdellovibrio, Micavibrio has an ability to attack and destroy existing biofilms of major human pathogens such as Pseudomonas aeruginosa, Burkholderia cepacia and Klebsiella pneumoniae, including numerous clinical isolates from these species.

With the increasing interest in developing improved methods for controlling biofilms, there are potential advantages to using predatory prokaryotes as biological control agents. For example, they are highly specific for infecting bacteria and thus are harmless to nonbacterial organisms. The initial dose of the predator can be low since this organism multiplies rapidly as it feeds, and as we have observed, the predator’s population is maintained in the biofilm even though a majority of the host bacteria have been destroyed. Bdellovibrio and Micavibrio are also effective against bacteria that have multiple resistances to antibiotics, which is the situation in many biofilm settings. Finally, a key difficulty encountered in the use of biological control agents in reducing biofilm population is the inability of the agent to access the cells within the biofilm. Our data suggest that both Bdellovibrio and Micavibrio have the capability to access extremely thick biofilms and are not restricted to the surface of the biofilm.

Currently, we are focusing our efforts on developing genetic approaches that will allow us to learn more about the biology of these unique organisms and the biological mechanisms important for predator-prey interactions. We have developed new methods to isolate predatory bacteria-derived proteins which have antibacterial properties for use as therapeutic agents, as well as investigating the role of these proteins in predator-host-biofilm interactions. We are continuing to evaluate the ability of the predators to reduce existing biofilms of oral pathogens that cause periodontal disease, a chronic biofilm-based disease that occurs in more than 35% of the adult population in the U.S. The long-term goal of the research is to harness the therapeutic potential of microbial predators and develop novel systems for controlling biofilms by using microbial predators, as well as the integration of these systems with classical biofilm control strategies.

http://www.umdnj.edu/research/publications/spring07/3.htm