This is a multi-part series on antibiotic resistance in bacteria.
Eventually, we'll reach the ways in which bacteria develop antibiotic resistance, but before we get there, we'll spend a little more time on antibiotics themselves.
What have we learned so far?
1. Antibiotics are natural products, made by bacteria and some fungi. We have also learned about the difference between antibiotics and synthetic drugs. There isn't always a clear distinction since chemical groups can be added to antibiotics, making them partly synthetic and partly natural.
2. Antibiotics are a chemically diverse group of compounds. Antibiotics are not DNA. Neither are they proteins, although some antibiotics contain amino acids, which are the building blocks of proteins. In a way, antibiotics are kind of luxury molecules, since they aren't essential for life.
Bacteria don't even make them until the population reaches a certain density and phase of growth. Even though we can describe all antibiotics with a single word, there is no single description that does justice to these fascinating compounds.
We can group antibiotics into classes, either by chemical similarity - peptide antibiotics all contain amino acids held together by peptide bonds, ß-lactams all have a ß lactam ring - by the range of organisms they can kill - broad spectrum antibiotics kill a wide variety of bacteria, where narrow spectrum antibiotics have more specific targets; or by the metabolic pathway that they target.
Perhaps the easiest way to categorize antibiotics is by the organisms that produce them. Even there, antibiotics defy easy groupings. The majority are made by denizens of the dirt, but both fungi and bacteria get into the act.
But what do antibiotics do?
How do they fulfill their role as agents of warfare? They do kill bacteria - but how? In a bacterial population, do all members get killed? Unfortunately, no, many antibiotics work by preventing bacterial growth. This means that most antibiotics only kill growing bacteria.
They keep bacteria from getting bigger?
No. When we talk about animals, plants, or people growing, we're really describing individual organisms getting larger. But, when we talk about bacterial growth, we're referring to the size of a bacterial population and not just the size of a cell.
In order for bacteria to grow, then, they need to make all the parts necessary for building new bacterial cells. DNA must be copied. New RNA, ribosomes, and proteins must be made. Cell walls must be built. Membranes have to be synthesized. And, then, of course the cells must divide. Many, if not most, antibiotics act by inhibiting the events necessary for bacterial growth. Some inhibit DNA replication, some, transcription, some antibiotics prevent bacteria from making proteins, some prevent the synthesis of cell walls, and so on. In general, antibiotics keep bacteria from building the parts that are needed for growth.
There are some antibiotics that act by attacking plasma membranes. Most antibiotics, though, work by holding bacterial populations in check until the immune system can take over. This also brings us, to our first mechanism of antibiotic resistance.
Persistance is resistance.
If bacteria need to grow in order to be killed by antibiotics, then bacteria, can escape from antibiotics, by NOT growing or by growing very slowly. This phenomenon has been observed with biofilms (colonies of bacteria living on a surface) (1), E. coli in urinary tract infections (2), and most notably in the slow growing bacteria, that cause tuberculosis, Mycobacterium tuberculosis, and leprosy, Mycobacterium leprae (3).
It seems funny to think that not growing can be a mechanism for survival. But if you're a bacteria, and you can hang around long enough in an inactive, non-growing state, enventually your human host will stop taking antibiotics, they will disappear from your environment and you can go back to growing.
1. P.S. Stewart 2002. "Mechanisms of antibiotic resistance in bacterial biofilms." Int J Med Microbiol. 292(2):107-13.
2. Trülzsch K, Hoffmann H, Keller C, Schubert S, Bader L, Heesemann J, Roggenkamp A. 2003. "Highly Resistant Metabolically Deficient Dwarf Mutant of Escherichia coli Is the Cause of a Chronic Urinary Tract Infection." J Clin Microbiol. 41(12): 5689-5694.
3. Gomez JE, McKinney JD. 2004. Tuberculosis (Edinb). 84(1-2):29-44.
Other articles in this series:
1. A primer on antibiotic resistance: an introduction to the question of antibiotic resistance.
2. Natural vs. synthetic drugs: what is the difference between an antibiotics and synthetic drugs?
3. How do antibiotics kill bacteria? a general discussion of the pathways where antibiotics can act and one characteristic that helps some bacteria survive.
4. Are antibiotics really only made by bacteria and fungi? It depends on what you'd like to call them.
5. The Five paths to antibiotic resistance: a quick summary