So, what initiates biofilm formation in the first place? Quorum sensing, Bacterial Density, Autoinducers?
How do 10, 20 or 1000+ individual bacteria all minding their own business at a specific threshold, 1001 for example, initiate quorum sensing?
Has anyone studied the mechanism behind population density? Like why 100 bacteria can peaceful bacteria can cohabitate, but at 101 it becomes too much to handle and the whole environment changes, and not for the better.?
This was posed to Dr. Gregory Anderson, biofilm researcher, in an email exchange. The more data I have the better I’m able to come up with an effective and comprehensive, natural biofilm busting protocol. I truly appreciate the time and effort Dr. Anderson put into the information he shared with me.
The numbers really don’t matter. What matters is the population density. You could have 10 bacteria in a really small space, and they would be experiencing all sorts of quorum sensing behavior. On the other hand, you could have 10^9 bacterial cells in a huge volume and no quorum sense at all. This is because, at high cell density, you also have a high density of the autoinducer molecule (the molecule produced by the bacteria that they then sense to initiate quorum sensing).
def. Quorum sensing is a process of cell-cell communication that allows bacteria to share information about cell density and adjust gene expression accordingly. This process enables bacteria to express energetically expensive processes as a collective only when the impact of those processes on the environment or on a host will be maximized. Among the many traits controlled by quorum sensing is the expression of virulence factors by pathogenic bacteria.
Bassler BL, et. al., Bacterial quorum sensing: its role in virulence and possibilities for its control. Cold Spring Harb Perspect Med. 2012 Nov 1;2(11). pii: a012427.
In fact, in the early days of quorum sensing research, they took cell-free supernatants from dense cultures and added them to low-density cultures, and they saw quorum sensing regulated gene expression. This was because the culture supernatant contained high concentrations of the autoinducer molecules.
def. Autoinduction (AI), the response to self-produced chemical signals, is widespread in the bacterial world. This process controls vastly different target functions, such as luminescence, nutrient acquisition, and biofilm formation, in different ways and integrates additional environmental and physiological cues.
Burkhard A. Hense, et. al., Core Principles of Bacterial Autoinducer Systems. Microbiology and molecular biology reviews
In a growing culture, though, you are right; at low numbers (or, more accurately, low cell density), there is no quorum sensing. But once that threshold density is achieved, then that pathway is initiated. What’s happening is that the autoinducers, which is steadily pumped out of each cell, finally accumulate to a sufficient level to be simultaneously recognized by each cell.
Imagine 2 people standing on opposite sides of a tennis court, tossing out tennis balls at random. The probability is low that any tennis balls will hit either person. If there are 300 people on the tennis court throwing balls, the probability is high that each person will be hit constantly. So as you increase from low density (2 people) to high density (300), the probability steadily increases that each person will be hit (4 people, probably not; 8 people probably not; 16 people, maybe; 32 people, likely). Once they reach that threshold density, then each person will be hit constantly.
Similarly in quorum sensing, as bacteria increase in number in a confined space, they will eventually reach a threshold density at which they each constantly receive the quorum sensing signal (autoinducers). Is it possible that they get hit before then? Yes. But since they are free-floating, as they diffuse away it is less likely that they will get continual autoinducer signal, until the threshold density is achieved. Thus, they will only experience transient quorum sensing activated gene transcription.
In a biofilm, however, bacteria are stuck to each other. Thus, in a growing biofilm microcolony, there is a local high density of bacteria, which is ideal for constant quorum sensing activation. So once more than 1 bacterium starts to bind to a surface (or one binds and then divides), quorum sensing will play a large role in that biofilm development. It’s all based on probability and statistics.
I’m not sure I have the best article to describe quorum sensing, but the attached review is from one of the greats in the field.
Dr. Gregory Anderson, PhD
Assistant Professor Biology Department at Indiana University–Purdue University Indianapolis
My laboratory studies the interactions between bacterial pathogens and the host epithelium. Specifically, we are interested in understanding how Pseudomonas aeruginosa exploits underlying lung dysfunction in individuals with cystic fibrosis (CF) to establish and maintain chronic lung infection. After CF lung colonization, P. aeruginosa undergoes genetic regulatory changes leading to the formation of antibiotic-resistant biofilms, which persist in the lung for the life of the patient despite aggressive antimicrobial therapy. We have developed a novel system for the development of P. aeruginosa biofilms on human CF-derived airway epithelial cells in vitro. Using this model, we are identifying factors that impact biofilm antibiotic resistance as well as bacterial virulence in the context of CF lung infection.
We are also interested in understanding the mechanisms of biofilm formation in chronic wound infections. While a number of different pathogens have been identified in chronic wounds, P. aeruginosa is found in the most severe. We are investigating how this microbe colonizes these sites and maintains long-term infection.
The overall goal of our research is to better understand the nature of chronic infections so that new and better therapies can be developed. Toward that end, we are testing novel compounds for antimicrobial and antibiofilm activity.
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