[Malithi Weerakkody]

Many bacterial species that are clinical pathogens or industrial contaminants are capable of forming biofilms. These biofilms provide a safe haven to bacteria, protecting them from the attack of antimicrobials, host defences and other microorganisms hence making it practically impossible to eradicate them with the available control strategies.

Biofilms may form on any surface inert or living. Biofilms on industrial pipelines, such as industrial or potable water system piping or pipelines of the oil and gas industry can lead to corrosion problems causing considerable economic losses. When formed on food contact surfaces and food-processing equipment, biofilms may pose a significant risk to the safety and microbiological quality of food products, resulting in food-borne diseases and food spoilage. Biofilms are a serious health threat when formed on catheters, orthopaedic devices and other clinical surfaces aw well as on living tissues such as teeth, heart valves, middle ear and lungs of cystic fibrosis patients etc.

These biofilms resist removal by physical abrasion and are not susceptible to chemical biocides such as antibiotics and disinfectants. For this reason, study of biofilm structure and the mechanisms by which they resist antimicrobials has become an area of immense importance in order to come up with successful methods of control.

Biofilm formation and Architecture

A biofilm is initiated when a planktonic (i.e. free-moving) bacterial cell reversibly attaches to a surface through van der Waals forces. This cell then irreversibly binds to the surface and begins multiplication forming a microcolony on the surface. Bacterial cells in the microcolony continue to multiply and produce a polymer matrix around the colony; a process which is known as the maturation of the biofilm. This biofilm continues to mature further, often trapping other microorganisms such as other bacteria, algae and protozoa in the sticky matrix during the process. This leads to the formation of a complex microbial community, each of these groups serving a specialized function. The biofilm provides a significant survival advantage to the microorganisms entrapped. A fully mature biofilm is often a mushroom-like or tower-like structure and exhibits maximum resistance. When the biofilm is matured, some of the biofilm cells may detach from the structure and initiate a new biofilm by attaching to a new surface, a stage known as biofilm dispersion.



See also:
Microbial Biofilms: Sticking together for success

A Protective Fortress

Biofilm bacteria are more resistant to antimicrobials and host defenses than their free-floating colleagues. The structure of the biofilm itself and specific genetic mechanisms of the cells contribute to this recalcitrance. This article summarises some of the main strategies that are important for the antimicrobial resistance (and tolerance) of biofilms.

A Drug-Proof Shield

The extracellular polymer matrix is the first line of defence of the biofilm inhabitants against the antibiotics. This matrix, composed mainly of polysaccharide along with other components such as adhesive proteins and DNA, acts as a physical barrier against the diffusion of antimicrobial compounds. The compounds in the external polymeric matrix either react with these chemical agents or bind to them, thus restricting the penetration of the drug molecules into the interior.

Slow Growth Rates

Biofilm bacteria grow at slower rates than planktonic bacteria due to the limited availability of nutrients. Since the antimicrobials usually target rapidly growing cells, this slower growth rate is a major factor contributing to the increased tolerance to the antibiotics of biofilms.

A Stratified Assembly

Due to the multi-layered structure of the biofilm, there exists a gradient of essential growth factors such as oxygen and nutrients. The gene expression depends on the environmental conditions and this gives rise to phenotypically distinct cells in different growth stages with different metabolic activities. The activity levels are higher at the surface of the biofilm and gradually reduce towards the centre. The cells at the core of the biofilm show very low or no activity. As a result, the susceptibility to the biocides of bacteria within the structure greatly varies, thus making it difficult to eradicate the biofilm by any given antibiotic.

Responding to Stress

Another proposed mechanism of antibiotic resistance is the expression of the stress-response genes by the slow growing cells in the biofilm community. It is suggested that RpoS-mediated stress responses- which induce physiological alterations that protect the cells from various environmental stresses- may also play a role in conferring antibiotic resistance to the biofilm bacteria.

Persisters - The Selfless Warriors

Increased number of persister cells - cells that cease proliferation and growth in the presence of antibiotics- than in a planktonic colony is another hypothesis that explains the elevated resilience of the biofilms.

These persisters, though genetically similar to the other bacteria in the biofilm, are resistant to antibiotics. However, they are distinctly different from the drug resistant mutants. Their recalcitrance is thought to occur due to their induction of dormancy and the over-expression of toxin–antitoxin systems which blocks the important cellular functions targeted by the drugs. These “toxins”, conflicting with their name, act to protect the cell form destruction by biocides. The persisters may resume growth once the antimicrobial is removed from the system.



You can learn more about persister cells here: and here


In Addition

Along with those mechanisms, general drug resistant strategies such as efflux pumps, mutations in target molecules and quorum-sensing also play a significant role in the persistence of biofilms. For instance, the increased incidence of mutation and horizontal gene transfer in a biofilm community account for the multidrug resistance of biofilm-associated bacteria.

Bringing Down the Fortress

Armed with this knowledge, scientists are trying to come up with new, more effective strategies to eradicate biofilms. Use of anti-biofilm enzymes, quorum-sensing inhibitors and bacteriophages as novel therapeutics to prevent formation of new biofilms and destroy the existing ones is being considered currently.

The following image sums up the above described mechanisms:



Sources:

1.Høiby, N., Bjarnsholt, T., Givskov, M., Molin, S., & Ciofu, O. (2010). Antibiotic resistance of bacterial biofilms. International journal of antimicrobial agents, 35(4), 322-332.

2.Lewis, K. (2001). Riddle of biofilm resistance. Antimicrobial agents and chemotherapy, 45(4), 999-1007.

3.Lewis, K. (2001). Riddle of biofilm resistance. Antimicrobial agents and chemotherapy, 45(4), 999-1007.

4.López, D., Vlamakis, H., & Kolter, R. (2010). Biofilms. Cold Spring Harbor perspectives in biology, 2(7).