Most antibiotics are microbicins – substances with which one microorganism inhibits another. The famous observation made by Sir Alexander Fleming was of Penicillium notatum – a mold inhibiting the growth of Staphylococcus aureus – a bacterium. This led to the discovery of penicillin and ushered in the antibiotic age.
Two things about this momentous event are worth pondering. It occurred in 1928, a very short time ago in ecologic terms. It is likely that Staphylococcus aureus and Penicillium notatum had known each other for several hundreds of millions of years by this time. Secondly, penicillin the substance that was inhibiting Staphylococcus aureus in minute concentrations is now useless against this important human pathogen because of development of resistance in S. aureus found the world over. Amazing things have happened in 80 short years.
Mechanisms of action
How do Antibiotics work?
Put simply, antibiotics kill microorganisms while doing little or no harm to human cells. All take advantage of differences between human cell metabolism and microorganism cell metabolism. Though strictly speaking there are antibiotics that kill fungi (antifungals), protozoa (antiprotozoals), viruses (antivirals) and worms (antihelminths) the term antibiotic is usually reserved for substances that inhibit bacteria. I’ll limit the rest of this discussion to antibacterial agents though many of the concepts are similar.
There are several mechanisms by which antibiotics kill bacteria. These are four predominant ones:
Inhibition of cell wall synthesis
The Beta-lactam ring is the nucleus of penicillins and cephalosporins. These agents inhibit cell wall synthesis by binding to bacterial cell wall construction enzymes referred to as penicillin binding proteins.
Glycopeptides – e.g. vancomycin also inhibit cell wall synthesis but by another mechanism.
Inhibition of protein synthesis
- Macrolides – e.g. erythromycin, clarithromycin, azithromycin
- Aminoglycosides – e.g. gentamicin, tobramycin, amikacin
- Lincosamides – e.g. clindamycin
- Tetracyclines – e.g. tetracycline, doxycycline, minocycline
These agents all bind to specific components of bacterial ribosomes inhibiting translation. Prokaryotic ribosomes are fundamentally different from eukaryotic ribosomes allowing for these agents to be relatively non-toxic to human cells.
Inhibition of folate synthesis
Trimethroprim and sulphonamides – e.g. sulfamethoxazole
These two types of compounds are usually used in combination with one another. They inhibit two different steps in bacterial synthesis of tetrahydrofolate which ultimately stops purine synthesis and DNA construction.
Binding DNA gyrase
The fluoroquinolones – e.g. ciprofloxacin, ofloxacin, norfloxacin, levofloxacin, moxifloxacin are relatively new agents that stop DNA replication by inhibiting DNA gyrase, a bacterial enzyme that is involved in the quaternary folding of double stranded DNA.
Perhaps the most important public health problem of today is the rapid development and proliferation of antibiotic resistant bacteria. The 60 or so short years of widespread use of antibiotics has witnessed enormous changes in the bacteria that commonly infect human beings. Indiscriminate use of these relatively easily produced compounds has led to crises in many areas. In many countries, antibiotics are widely available over the counter with “black market” production and distribution common. Use in animal husbandry as “growth promoters” has reached massive levels as consumers demand inexpensive meat.
In North America, antibiotics are very freely prescribed and consumed. In contrast, the Scandinavian countries have long been watchful of their antibiotic prescribing habits and can now boast some of the lowest levels of resistance in the world.
There are many mechanisms by which bacteria are able to evade the effects of antibiotics. These are the predominant ones:
Inactivate the antibiotic
The best examples are beta-lactamases produced by many kinds of bacteria. These enzymes open the beta-lactam ring of penicillins and cephalosporins.
Many different types of beta-lactamases have been described with different affinities for different antibiotic compounds. This is a very important mechanism of resistance with very large clinical implications. Other examples of substances which inactivate antibiotics include aminoglycoside modifying enzymes and chloramphenicol acetyltransferase.
Alteration of target sites
- Ribosomal alterations can lead to resistance to any of the agents that bind ribosomes most notably tetracyclines and macrolides.
- Changes in Penicillin Binding Proteins is an important means by which some organisms evade the effects of penicillins and cephalosporins. Methicillin resistant Staphylococcus aureus (MRSA) is a serious problem in hospitals and is the primary example.
- Modifications of folate synthesis enzymes confers resistance to trimethoprim and sulfa drugs.
- Altered DNA Gyrases don’t bind fluoroquinolones conferring resistance.
Alterations in Membranes
Gram negative outer membranes contain porins – channels that facilitate transmembrane passage of substances in and out of the cell. Most antibiotics gain access to the inside of bacteria in this way. Alterations in porin structure and number can decrease activity of a wide variety of antibiotics.
Several types of bacteria can pump antibiotics out of themselves using energy dependant pumps. This relatively recently described mechanism is being more and more reported for more and more types of antibiotics and is gaining in clinical importance.
Spread of Resistance
Bacteria are very promiscuous!! They share genetic elements freely and willingly.
Plasmids are self-replicating, extrachromosomal circular pieces of DNA that facilitate exchange of resistance determinants. Many plasmids carry several different resistance genes making the acquisition of multiple resistances possible with one genetic event.
Bacteriophages are viruses that infect bacteria. They spread genetic material by a process known as transduction.
Transposons are DNA segments that have the ability to translocate between different areas of the chromosome or between plasmids, bacteriophage and chromosomes allowing for ready dissemination. Recently, there have been transposons described that have the ability to translocate from one bacterium to another without a plasmid or bacteriophage intermediate. These so-called “conjugative” transposons have been referred to as “jumping genes”.