One of Bacteria’s Deadliest Weapon
The Role of LPS and “Lipid A” Endotoxin in disease
How exactly do bacteria make you sick?
[note: The following is an informational article, not meant to be used for medical purposes. Consult a doctor or physician for medical advice]
It’s a complicated question, and the answer is multifaceted, but there are two broad categories of disease mechanisms: Exotoxins and Endotoxins. Exotoxins are toxic elements that bacteria and other dangerous micro-organisms create within themselves and excrete through their membranes to poison a host. Endotoxins, the subject of today’s article, are agents that are embedded within the bacteria’s cell wall before excretion, and are released upon bacterial death, or excised from the cell wall. Today we’re going to specifically talk about the well known endotoxin: LPS or lipopolysaccharide.
Note: I’m thankful to live in the modern day and age when all this information is freely available and composable, thanks to the efforts of countless others who came before me. All relevant links to primary and secondary sources have been posted on the term, and for any that sources I have missed citing, I will work to update this to include those references.
Disclaimer: None of the literature contained in this article is medical advice or to be taken as medical advice. This article is for informational purposes and potentially subject to errors which I’m constantly working to correct. Consult a physician if you have a medical issue.
A Recap on Bacterial Defense
Last time I talked about bacteria that form impenetrable shells called Endospores. Endospores are bacteria’s defense against harsh extracellular conditions, which these little critters are exposed to quite often. A bacteria will enter the Endospore state for years, not metabolizing, not reproducing and not living at all until external conditions become favorable again. After this, they germinate again back into a fully functional bacteria.
Unfavorable conditions that induce sporulation include super cold environments:
- low oxygen environments for aerobic bacteria
- high oxygen environments for anaerobic bacteria
- low Alanine and other essential nutrient environments
- enzymatic environments introduced by predators like amoebas and even immune system’s macrophages
- and a whole host of other deadly conditions.
The bacteria that are able to form endospores can be seen as good at defending themselves against attack. They usually Gram Positive bacteria (for the type of staining which can identify them). There are also Gram Negative bacteria that are deadly to humans and other species in their own right. These bacteria use the aforementioned LPS endotoxin as mechanism of their attack.
Note: Gram positive bacteria can also have LTA (lipotacoic acid which performs a similar function to LPS in Gram negative bacteria causing septic conditions in the body, but via a different mechanism)
A Tour of The Cell Wall
Before we get to Lipopolysaccharides (LPS), let’s talk a little bit about the bacterial cell wall. The cell wall of a bacteria, a prokaryote, shares some similarity with the cell wall of a plant cell, at least in function. Both types of cell wall help to maintain both the shape and osmotic pressure balance of the cell. Bacteria, like plant cells, are subject to hydraulic stresses and ion gradients of varying degrees. These phenomena, unless protected against, will either destroy the bacteria mechanically through sterolysis, or through an electrochemical imbalance, unless a mitigating structure is in place, such as you guessed it: The Cell Wall.
All cells, including bacteria, also have phospholipid bilayers, the primary cellular membrane, which is something I believe we learn about in grammar school now. These double layered membranes, with the assistance of membrane embedded proteins and channels, are usually sufficient to control the flow of substances inside and outside the cell. Standard functions of the cell membrane including bringing macro-nutrients inside, secreting hormones, other signalling molecules and toxins outside, and maintaining the electrical charge balance of the cell by exchanging positive and negative ions.
Normal eukaryotic cells of all varieties are usually in extracellular environments that don’t necessitate more physical endurance than that provided by the standard cell membrane to survive. This is not so for lone prokaryotes like walled bacteria, which are individual organisms, not just a part of a larger host. Individual organism need to survive on their own to pass their DNA along. Before going into the endospore state, which is considered an extreme measure, in their living vegetative motile state, bacteria need an extra barrier.
Note: There are eukaryotic organisms such as ameoba and paramecia that don’t have cell walls persay, but they have other supporting structures and forms that enable their survivability.
The bacterial cell wall varies in thickness, but for Gram positive bacteria, the cell wall can be anywhere from 20nm-to-80nm thick, which is essentially a bacterial fortress compared to the 2–4 nm thin cell walls of the gram negative bacteria. Gram positive bacteria form endospores though, and as we indicated before, seem to be more focused on defense against perforation in general, but Gram negative bacterial walls contain LPS, which we will get to.
The bacterial cell wall is composed of a layer of a substance called peptidoglycan, which is beyond the scope of this article to discuss very in-depth, but suffice to say its a somewhat complex cross linked multi-chain of amino acids and sugars, with another lipid bilayer on top, and the so called periplasmic space between. I’ll devote an entire large article to it, when I swing back to bacterial defensive structures.
LPS
The lipopolysaccharide is a multicomponent structure which runs from the outside of the bacterial cell wall, all the way down to being embedded in in either the outer lipid layer of the cell wall, or down through the peptidoglycan layer, and embedded amongst the inner membrane’s phospholipids.
They look like the “little hairs” sticking out of the bacterial cell wall as pictured below:
Chemically zoomed in, they look like the below structure:
The LPS has 3 principal molecular domains (or parts). The top most domain is exposed to the extra-bacterial space. It’s known as the O-Antigen domain consists of long chains of sugar molecules that as the name implies, are features of recognition used by white blood cells, particularly macrophages, to identify, engulf and kill the bacteria. This O-Antigen site can be largely variant amongst different species of bacteria, but even worse, amongst different strains of bacteria, making it more challenging to for the immune system to mount effective responses rapidly against these bacteria.
The sugar chains of the O-Antigen region are bound to what’s known as a core sugar, which structurally similar across all bacterial species. The core sugar functions as an anchor mechanism of the O-Antigen and lower sections of the LPS to the cell wall. It is further sectioned into an inner core and an outer core, and both cores contains various kinds of heptose and pentose sugar chains and rings that are embedded and modified with amino acids.
The lowest structure, and the part that makes the LPS dangerous is known as Lipid A. Lipid A is a group of fatty acid molecules that is attached to the core sugar via another interfacing sugar. The fatty acid molecule group consists of one change of hydromyristic acid, and several other amino modified fatty acids.
Lipid A — The Atom Bomb of the Immune System
So how does the bacteria use the LPS to spread disease per say? Also, since the Lipid A portion of LPS is embedded in the cell wall, how is it exposed to even cause any damaged to begin with? The answer would be in the immune response of white blood cells to the presence of LPS.
Let’s first discuss the mechanism of disease expansion. All of this will be done at a very high, non-chemical level, because I’m just introducing the concept here. More in-depth resources are available on the internet and I’ll likely do deeper dives in the future.
As a bacteria travels through the body of the host organism and does its “bacteriological thing”, it starts to shed pieces of the LPS, and sometimes into the bloodstream to be carried off to distant parts of the body. LPS fragments are considered primordial antigens for macrophages and also the so called complement immune system.
Remember, macrophages are first line of defense white blood cells, and their function is to track down and consume threats to the body by digesting them through a process called phagocytosis (“Cell Eating”). Macrophages also travel to the lymph nodes and signal other parts of the immune system to activate more specialized immune cells when necessary.
When the Lipid A portion of these fragments are picked up by the macrophage, the macrophages go into overdrive and start producing signalling cytokines en masse. IL-1, IL-2, TNF-alpha, Neutrophil Chemotaxis factor…
Way too many to discuss in depth in this article!
This cytokine activation is initiated when LPS binds to the so-called Toll-like receptors (TLR-4) of the macrophage and other cells to initiate this process. Without diverting to much into the details of immunology, it deserves mention that the 2011 Nobel Prize in Medicine was awarded to Bruce Beutler in discovering this interaction.
These signalling molecules, travel all over the body — to the hypothalmus where they induce a pyrogenic reaction causing a fever — to the blood vessels causing vasodilation, which is the expansion of the blood vessels to allow more immune cells to travel to the location of infections, and many other standard components of immune responses. Some of these are even designed to help recruit and ramp up the immune system’s cells to fight the bacterial infection, further causing the inflammatory cascade runs amok.
So what is the problem? This is what’s supposed to happen right?
The prime issue is the overwhelming number of bacteria associated with a typical infection, which reproduce exponentially, and product an overwhelming number of LPS fragments, which are picked up by more and more macrophages. On top of that, when a macrophage consumes the offending bacteria, the cell wall is digested, and releases all the remaining LPS. This induces a thunderous and more direct version of this immune response.
For those familiar with immunology: While immune responses are good and necessary, they can often be deadly if they are too extreme. The presence of excess LPS will elevate the immune response to dangerous levels. This is why, often times for bacterial infections, antibiotics are needed, to prevent endotoxic bacterial responses from causing septic shock and ultimately death in a person.
I won’t go too much into this because I’m not a medical practitioner
I hope this was a good introduction to the world of bacterial weaponry! Next time, I’ll talk more about the structure of bacterial cell wall and shape of bacteria.
