Microbial Growth in Biofilms
Typical growth curves of microbial cultures are readily observed, when the bacteria propagate in artificial liquid nutrient media. However, in real conditions most of the bacteria form the complex microbial communities that exist in closest contact with surrounding non-living or living matter. The bacteria demonstrate outstanding capacity of adherence – they easily attach one to another, or settle and bind to any underlying surface (inorganic grounds, natural or artificial polymers, or body tissues). Futhermore, the nascent microbial clusters and microcolonies produce a vast number of substances that become an integral part of growing culture. This multicomponent united microbial complex, demonstrating the common behavior and the evident tendency to spatial expansion, is termed as biofilm.
The International Union of Pure and Applied Chemistry (IUPAC) gives the next definitions and essential characteristics for microbial biofilms:
biofilm is: “Aggregate of microorganisms in which cells that are frequently embedded within a self-produced matrix of extracellular polymeric substance (EPS) adhere to each other and/or to a surface”;
and gives further clarifications:
“Note 1: A biofilm is a system that can be adapted internally to environmental conditions by its inhabitants.
Note 2: The self-produced matrix of extracellular polymeric substance, which is also referred to as slime, is a polymeric conglomeration generally composed of extracellular biopolymers in various structural forms.”
An alternative to biofilm is the existence of bacteria in their free, or planktonic forms, where the cells are not directly connected to each other.
Now it is generally ascertained that the largest majority of infections (not less than 60-80%) are caused by bacteria, growing as biofilm. Among them are the most deleterious and resistant microbial pathogens, such as S. aureus, streptococci, P. aeruginosa, numerous enterobacteria. Likewise, motile microorganisms are more active producers of the biofilm.
Every biofilm community evolves through certain common steps: a) initial and irreversible attachment; b) biofilm maturation; c) biofilm dispersion.
Biofilm formation can be triggered not only by microbial contact with some external surface, but by any stress affecting microbial population (e.g., heat exposure, antibiotic treatment, etc.).
Biofilm maturation is followed by intensive synthesis of various kinds of extracellular biopolymers as the essential components of biofilm matrix. The main components of matrix are usually DNA and polysaccharides.
After primary biofilm establishment it begins to demonstrate quasi-multicellular behavior reacting as “a single whole” against external or internal challenges. This ensues from the activity of a system of intercellular microbial signalling and coordination known as quorum sensing. Various environmental challenges, affecting microbial population within biofilm, stimulate active synthesis of signal quorum sensing molecules by most of bacteria (e.g., N-acyl-homoserine lactone or regulatory oligopeptides). These inducer molecules spread throughout the biofilm and trigger coordinated gene expression by microbial cells. This, in turn, leads to the concordant changes of individual bacterial reactions within the biofilm that stimulates biofilm progression. In addition, quorum sensing activates production of virulence factors and enhances antimicrobial resistance of bacterial communities.
As the result, microbial biofilm is characterized by active release of depolymerizing enzymes that provide bacterial feeding and invasion; it also develops intensive synthesis of extracellular polymeric matrix and elevated production of toxic substances. Enhanced lateral gene transfer between cooperating microbial cells ensures their rapid adaptation to worsened environmental conditions.
After maturation, microbial biofilm becomes poorly permeable for most of the biocides. By fact, it usually demonstrates the increase of antibiotic resistance more than 10-100 times over the resistance of respective planktonic forms of bacteria. The dispersal of biofilm accelerate microbial contamination of various prosthetic devices and artificial appliances in clinics (e.g., catheters, drains or implants) that is responsible for about 60-70% of hospital-acquired infections.
In sum, the problem of bacterial biofilm formation still remains unsolved and poses serious difficulties for existing health care services.