Vaccines
In many non-US countries, TB vaccination is quite common. The vaccine is called BCG (Bacillus of Calmette and Guerin, named after its inventors), and uses an attenuated strain of cow infecting bacterium called Mycobacterium bovis35. However, in the US the vaccine is not routinely used for several reasons35. For one, the vaccine cannot prevent disease from being reactivated in patients already infected with the latent form of TB35. Thus, unless administered at birth, it has limited effectiveness. A related concern is that the vaccine does not prevent infection, thus the entire population would have to be vaccinated for the treatment to be uniformly effective35. Finally, the vaccine would complicate the previously described diagnostic tests, making it harder to read the skin test because the attenuated Mycobacterium would show up as a false positive35.
The Rise of Antibiotics
With the knowledge that bacteria were disease vectors came a consequent desire by the scientific world to find ways to combat these harmful microorganisms. The first isolation of an antibiotic (e.g. antibacterial) substance was accomplished in 1889 by E. de Freudenreich, a German scientist who discovered a bacterial pigment capable of arresting the division of other bacteria44. Though effective, the pigment proved toxic to humans so could not be used in drugs44. An important moment in the fight against bacterial infections occurred in 1928, when British scientist Alexander Fleming discovered a mold contaminant growing in his bacterial cultures44. Amazingly, the mold seemed to lyse the bacteria, an effect Fleming had previously observed when human tears contacted certain bacterial cells44. Since the antibacterial substance had been produced by the Penicillium mold, the resulting drug was named penicillin. However, Fleming was unable to purify much of this compound himself, leaving most work on penicillin to be done by his scientific descendents44. Ultimately, it became one of the most widely used agents to treat allied soldiers during WWII, with the US partnering with the economically deprived Britain to generate supplies of penicillin45,46. The public first became aware of the drug after it was used to treat burn victims of a Boston club fire at Massachusetts General Hospital, and by 1946 it was widely available45.
Penicillin, like other antibiotics, works by blocking key processes in the bacterial cell – in this case the synthesis of the bacterial cell wall46. By preventing the cross-linking of peptidoglycans in the bacterial wall, penicillin prevents new bacterial cells from assembling a cell wall rigid enough to resist osmotic pressure. Normally, the connections between the components of the cell wall form a kind of “safety-net” for the plasma membrane. Even if water pushes on membrane from the inside, expanding it, the cell wall will keep the bilayer from actually bursting. However, with the cell wall weakened by penicillin, bacteria may well burst when placed in a solution of pure water46. However, it turns out that this same cell wall was the major determinant of penicillin’s effectiveness, as it can only treat Gram positive bacterial infections. M. tuberculosis however is Gram negative30. Thus, a cure was still missing, though researchers had a promising lead.
Until the 1940s, there was no cure for TB. However, a breakthrough came in 1943, when Selman Waksman of the University of California identified a potent antibiotic called streptomycin that showed effectiveness against M. tuberculosis47. So how does streptomycin work? By binding to the ribosome – a complex of protein and RNA responsible for synthesizing proteins from mRNA instructions – streptomycin shuts down the production of proteins in bacterial cells by ensuring that RNA messages are either terminated at too short a length or formed with the wrong amino acid components48. Without this translation, the cell cannot make enzymes, cofactors, structural proteins, or any of the other components it requires to function. Many other antibiotics work on similar principles, shutting down biochemical pathways essential for life, be it protein synthesis, DNA replication, or the generation of cell wall components49. Alternately, the antibiotic might change the bacterial membrane’s permeability, making it impossible for it to maintain the carefully balanced chemical conditions normally present in the cytoplasm49.
Patients are not usually treated with just one antibiotic at one time. Instead, they are given a “cocktail” of different antibiotics which lowers the risk of encouraging drug resistance in the bacteria, an issue addressed in depth below35. By using multi-drug therapy, the bacteria is less likely to develop resistance35. Thus, even if the bacteria do acquire resistance to one antibiotic, another compound in the mixture will destroy it and prevent the resistance mutation from passing to the next generation of bacteria.
The Fall of Antibiotics: The Looming Threat of Drug Resistance
Alexander Fleming would be shocked if he knew that Staphylococcus aureus, the first bacterium to be effectively treated with penicillin, is once again a major health threat. In fact, the medical community now routinely deals with antibiotic-resistant bacteria, particularly in an inconvenient setting: the hospital! It happens like this: a patient checks into a hospital with a normal illness. During their stay, they become infected with bacteria lingering in the hospital, and, when the staff attempts to treat them, they find the infection is resistant to standard antibiotics. The consequences can be dire; these nosocomial infections can be more life-threatening than the surgery or treatment for which a patient is admitted.
How could this happen? Like any organism, bacteria evolve or change over time. Given their quick reproduction time (mere hours in some species), any advantageous mutations they acquire can be speedily propagated to millions of individuals. As we will see below, bacteria also communicate with each other, swapping DNA (intentionally or unintentionally) that can contain genes for drug resistance.
The Evolutionary Waltz and the Plasmid Shuffle: How Resistance Dances Between Bacteria
During a bacterium’s rapid replication of its DNA – sometimes every few hours – mistakes can be made. An incorrect base pair may be inserted within a codon, sometimes resulting in a different amino acid to be incorporated during protein synthesis. This change may make the protein nonfunctional or have incidentally have no effect at all. Every so often though – about once in every billion new bacteria – a beneficial change will occur, perhaps changing the bacterium’s metabolic processes for cell wall synthesis, protein generation, or other essential pathways so that these biochemical pathways are no longer inhibited by an antibiotic.
When this effect is isolated to a single bacterium, the effect is small. However, if this bacterium is in a population exposed to an antibiotic, the rest of the population will die, leaving only the mutated bacterium as a survivor. Once it replicates, the new population of bacteria will all be resistant, as the antibiotic has become a force of natural selection. The fit survive, and the offspring are similarly hardy: they are officially termed antibiotic-resistant. This scenario – where bacteria acquire resistant genes from their progenitors – is known as Vertical Gene Transfer (VGT)51. The reason it is called vertical is clear if bacterial generations are thought of as a ladder, with each generation on a different rung. Passing a resistant gene to an offspring, antibiotic resistance moves vertically down the ladder of the bacterial lineage. That this is possible would be frightening enough if it were the only way for bacteria to acquire drug resistance: but it’s not.
The action on the ladder of bacterial generations does not just happen vertically. When that activity involves the exchange of genetic information between bacteria, it is called Horizontal Gene Transfer (HGT) because genes are being passed back and forth among bacteria in the same generational rung. There are several ways that this exchange can take place:
1: Bacteria, like humans, are preyed upon by organisms. In fact, bacteria have a class of infectious predators all their own: small viruses called bacteriophages. In some cases, bacteriophages are harmful to the bacteria (just as viral infections harm human cells). However, bacteriophages can repackage bacterial DNA and shuttle it to other bacterial cells51. Consequently, a bacteriophage can transmit drug resistance genes between individuals through this process known as transduction41.
2: As mentioned earlier, bacteria can have independently replicating plasmids in addition to their chromosome. Just like the chromosome, the plasmid can be mutated and acquire drug-resistance genes51. Unlike the chromosome however, the plasmid can jump between bacterial cells with ease. In some cases, this occurs through a process called conjugation, in which two bacteria exchange a plasmid through direct cellular contact. This process, in fact, is thought to be the main culprit behind drug resistance. A second possibility is that a plasmid escapes the cell membrane of its original bacterium (this can happen, for example, when the first bacterium dies) and is taken up by another cell in a process called transformation51. The plasmid begins replicating in its new home, and the host cell acquires any benefits of its augmented genetic information, including drug resistance.
Terminating the Antibiotic
How exactly does a mutation make a bacterium resistant to an antibiotic? There are many ways to turn off an antibiotic. A common mutation alters a bacterium’s existing enzymes so that it now turns the formerly lethal antibiotic into a harmless molecule52. Another, related strategy is to interfere with the antibiotic’s target: whatever site in an essential enzyme was originally blocked by the antibiotic compound is altered so that the drug no longer binds, preventing its activity52. Lastly, the bacteria can simply expel the drug before it has a chance to work through an advantageous mutation in a pump bridging the cell membrane52.
How did this Resistance Epidemic Happen?
The drug development process itself is to blame on at least two counts. During the 1980s, too little attention was given to the development of new antibiotics. Indeed, developing new drugs has become an exceedingly high-risk and high-cost endeavor, with new products on average requiring 14 or more years of testing before the FDA will allow them to be commercially distributed53. Secondly, even though there is a diversity of antibiotics on the market, many of them are very similar in function. This is in part because developing new drugs is expensive, leading pharmaceutical companies to create variations on an old compound instead of pursuing the expensive venture of creating a new one from scratch. However, because these antibiotics work through similar mechanisms, a bacterium can develop resistance to a whole class of antibiotics with a single mutation51.
Social conditions have also contributed to the rise of antibiotic resistance. Increased infections mean more antibiotic prescriptions, so situations that facilitate the transmission of bacteria contribute to the problem. For example, studies have correlated the rise of ear infections in preschoolers over the last 30 years with the rising prevalence of daycare. Children are crowded into public spaces more often in this system, giving greater opportunity to pass infections between each other. Another example is homelessness whereby the presence of more unsheltered individuals on U.S. streets (usually without adequate access to health care) leads to more urban congestion in general, and specifically a population who, when in poor health, can serve as a ready means for an infection to spread through a city54.
Another major cause of the resistance dilemma is the overuse of antibiotics. For example, in order to keep slaughter animals healthy in unsanitary conditions, antibiotics are used in massive amounts in livestock feed, effectively cultivating antibiotic-resistant bacterial strains. Antibiotics are also often overused in clinical settings as a treatment for viral infections (for which they are useless). Ironically, improvements in medical treatment may be to blame as well. The antibiotic overprescription is, at least in part, due to the increasing number of patients that can be kept alive in the hospital. The elderly and the immune-compromised now stand a much better chance of survival but as a result require antibiotics to stave off infections in their weakened immune systems. Consequently, more antibiotics are being used than ever before, and patients tend to expect them immediately as a treatment for bacterial illnesses54.
Comprehension Questions:
1. What are two ways in which bacteria may acquire drug resistance?
2. How does the drug streptomycin work? How is this different from how penicillin works?
3. Why is an antibiotic “cocktail” usually more effective than any single antibiotic compound?
4. What is in a TB vaccine, and how does it prevent infection?
