How your body beats disease-causing pathogenic bacteria.
It was about 130 years ago – in the late 1880s – that the term pathogen was first used to describe a microbe that can cause disease.
Perhaps it was no coincidence that, around the same time, Russian zoologist Ily Ilyich Mechnikov began his groundbreaking work investigating the mechanisms through which white blood cells can help to fight off microbial infection, for which he would ultimately share a joint Nobel Prize.
Not surprisingly, we talk a great deal about microorganisms in our weekly newsletters, but we figured that it was high time for us to produce a short primer on the basics of bacterial infection.
It’s definitely not intended as a deep scientific drive, nor as some kind of definitive clinical handbook. It’s just a reminder of the underlying microbiology.
Although both bacteria and viruses can be responsible for disease, right now we’ll solely focus on the former.
It’s important to recognize that pathogenic bacteria are not the disease themselves: they cause disease, generally by releasing toxins, which are poisonous substances.
Bacteria, of course, are microscopic organisms. Most are no larger than 10 micrometers long. That’s 10 millionths of a meter.
When the conditions favor it, bacteria can multiply rapidly, meaning that the wrong kinds of microbes have the potential to produce substantial levels of toxins in a short space of time.
Of course, in order to affect a human, these microbes need to enter the body, and they can do this in many ways.
It might be through a cut or wound.
You can breathe in bacteria, or transfer them to yourself by touching your mouth, nose, or even eyes, after your hands have been in contact with a dirty surface.
Bacteria can also get into your body if you eat or drink foods or liquids containing pathogens.
Some of the diseases that can be caused by bacterial infection include food poisoning, cholera, typhoid, whooping cough, and gonorrhea.
However, if this is all starting to sound a bit disturbing, it’s probably reassuring to know that your body has some remarkable natural defense systems that enable it to fight off pathogenic microbes without external assistance.
These systems are mainly centered around your white blood cells, invaluable components produced in your bone marrow, which work hard for you in two main ways.
First, they’re able to ingest (or absorb) and destroy pathogens.
Secondly they have an extraordinary ability to produce antibodies that can destroy pathogens, and also generate antitoxins that neutralize toxins.
While it’s tempting to imagine that white blood cells somehow eat pathogens, this is not the case – when a human cell ingests a bacterial cell, it takes place as a kind of absorption, almost like a paper towel mopping up a kitchen spill.
White blood cells come in two main varieties: phagocytes and lymphocytes.
Phagocytes were first recognized by the aforementioned Russian Ily Ilyich Mechnikov in 1882, when he noticed that specialized cells seemed to be involved in defending against microbial infections.
Somewhat improbably, he discovered this while poking a tangerine thorn into the larvae of a starfish.
While this may seem an unusual thing to have done, as we said earlier, it did lead to him being jointly awarded (with Paul Ehrlich) the Nobel Prize for Medicine in 1908, recognising his work on phagocytes and “phagocytosis,” which describes the useful bug-busting work carried out by phagocytes.
The process builds on the ability of phagocytes to easily pass through the walls of blood vessels into surrounding tissue, where they can then move towards pathogens and toxins.
When good guy meets bad, a phagocyte is able to either ingest and absorb a pathogen or toxin, or release an enzyme that zaps the unwanted guest.
When phagocytes have absorbed a pathogen, they may then broadcast a chemical message that helps the other main type of white blood cell – lymphocytes – to identify the flavor of antibody that’s required to neutralize this particular pathogen.
Lymphocyte circulation was discovered in 1959. Lymphocytes contain chemicals that are foreign to the body, known as antigens.
A lymphocyte carries a specific type of antibody, antibodies being proteins that have a chemical “fit” to a certain antigen.
Things happen quickly when a lymphocyte with the appropriate antibody comes across its matching antigen.
The lymphocyte reproduces rapidly, manufacturing many copies of the antibody, which in turn neutralize the pathogen.
Antibodies are able to neutralize pathogens in several ways.
They may bind themselves to the attacking bacterium, damaging or destroying it.
Alternatively, they can coat pathogens, causing them to clump together, allowing phagocytes to more easily ingest them.
Another helpful mechanism is the ability of antibodies to bind to pathogens, releasing chemical signals that attract more phagocytes.
Fortunately it’s relatively rare for humans to be seriously affected by pathogenic bacteria, but it’s illuminating to be reminded that the microorganisms making up the human microbiome don’t just sit there in silence.
There’s an ongoing, highly complex, set of interactions that takes place between different microorganisms, and between microbes and the human host.