Breath in… Breath out…
And another deep breath in… A trembling in the chest… “Now, go!” – I screamed.
My older siblings left, and I returned to my grandparents to deliver the news. A bitter battle currently rules deep in the lungs that once called home. Antibiotics are threatening our existence. We are trying our best to survive yet suffered great losses. Some of us, including all my older siblings, already left to find a new host. When I saw my grandfather, he asked nervously, “Was someone able to escape?” I replied, “All of them. I wonder how far they’ll get.” With a sigh of relief, he said, “Well, it is a crowded place here and I believe they will soon find a healthy host. Just as I did.” Exhaustedly I added, “Really, I cannot wait until it is our turn. How did you even make it through all the attacks?” My response triggered his memories. “It was not always like this…”, and so he started to recall his story.
A story from a silent observer to an active contributor
I persisted here for almost a decade. It’s hot and humid at the same time. A perfect place to call home. For a long time, I remained passive and in a dormant state. Silently, I observed my host’s behavior; waiting for my chance to grow. My host was a healthy, middle-aged man living with a large family inside a tiny house. He picked me up at a nursing home while taking care of his sick mother who was home to our active ancestors and my first host (1). After some time, he got quite busy with work and always felt exhausted at the end of the day. It was foreseeable that one day his immune system would become weaker, and when it did, I finally woke up. Switching from a passive to an active state enabled me to multiply and add new members to our family. Even if we became detectable for various immune cells, they did not keep up with our growth. We were slowly gaining the upper hand. From this point on, our life would change. We no longer had to fear or stay silent about our existence.
Time passed and sooner or later our host had to realize that he could not deal with us all by himself. That was when he started to take the antibiotics which we now fear. Isoniazid is one of them – a cunning one. It took advantage of our catalase-peroxidase enzyme encoded by the katG gene which we usually use to remove the toxic byproduct hydrogen peroxide from our system. By interacting with the enzyme, isoniazid gets converted into an active state where it inhibits another enzyme encoded by the inhA gene which we need to synthesize mycolic acid – an essential building block of our cell wall. Our fundament became wobbly, still, there were other building blocks that held us together. A characteristic one was arabinogalactan which needs support from the active enzyme arabinosyltransferase to secure our cell wall. This enzyme however was targeted by another antibiotic called ethambutol. We gradually lost the ability to maintain a stable cell. As though playing Jenga, we prayed that the next block taken away from us wouldn’t cause a collapse of our cell wall. We were already weakened, still, our host was determined to completely get rid of us. So, he added another antibiotic called rifampicin. Do you remember, what we needed RNA polymerase for? Exactly, it always supported us in synthesizing messenger RNA from a DNA template which was our blueprint. We needed this indispensable step to translate our inherited blueprint into essential proteins that secured our survival. Rifampicin was designed in a way that it fits perfectly to the ß-subunit of our RNA polymerase encoded by the rpoB gene. By docking, the RNA polymerase was no longer able to bind the DNA. Losing essential proteins meant that the mechanisms to ensure our survival slowly shut down. As though that was not already enough, the host continued to add one more antibiotic called pyrazinamide which similar to isoniazid also took advantage of our enzymes to gain an active state. Pyrazinamide creates an environment that is acidic enough to inhibit essential enzymes that we use for energy production. Not even those of us who have returned to a passive state of persistence were able to survive in this acidic state. (2) We are now close to giving in if our host continues his treatment.
“It’s true,” I responded right after Grandfather finished his story, “I predict that in a few months, none of us will be left. Now it is almost time for the next round of attacks. We should better get prepared.” Yet, the attackers never came …
Changing roles at a critical time
Days passed without a sign of any antibiotics. It seems like our host stopped his treatment earlier than expected. By now, we have greatly reduced in number. Yet without hesitation, we took the chance with the remaining survivors to protect ourselves against the attackers. Now we had the knowledge of what those antibiotics target. And we used this knowledge to introduce changes in our genome to alter the target site. Becoming resistant against Rifampicin was an easy task, we just used mutations in the rpoB gene by which the ß-subunit of our RNA polymerase slightly changes while still fulfilling its task of binding to the DNA and taking forward protein synthesis. Isoniazid resistance was trickier to obtain as we needed several mutations for example at the inhA and katG gene which, as I recall from my grandfather’s story, encodes for enzymes involved in the activation and action of isoniazid. Even if isoniazid makes its way into our cells, the antibiotic will never be activated or reach its target. Though we use mutations in the inhA gene less commonly, those mutations have the potential to evoke cross-resistance to another drug called ethionamide which our host also used against us, and which was structurally similar to isoniazid (2). Once our essential proteins and cell walls were secured and stabilized, we returned on stage and took over the leading role. Again, we grew and multiplied stronger than before. The moment our host recognized the comeback, he again took the drugs that are now no longer effective against us. At this point, we have acquired multidrug resistance (3).
To celebrate our revival, some of us decided to go on a journey including me and my grandfather. Our host was living together with a large household in a crowded and poorly ventilated place. When he coughed, a healthy host was within immediate range (1). At departure time, we traveled on droplets to our new host. She inhales and through her airways, we travel down to the lungs. Arrival Time.
An uncertain home
As we got off, we started again to grow and multiply while equipping all our fellows with the previously obtained resistance mechanisms. When the same antibiotics attacked, we were all perfectly prepared and had no struggles to survive. Just like that the weeks passed without any great losses. Then suddenly, a drug we have not yet dealt with attacked. I heard them call it bedaquiline. My grandfather never told me anything about this drug, it was the first time we got exposed to it. None of our resistance mechanisms seem to work and the drug again took away our fellows one by one. Reports indicated that this new drug would target our ATP synthase which is an enzyme embedded in our membrane. This enzyme synthesizes Adenosine triphosphate (ATP) – our source of energy. Yet, there was no time to figure out a new mechanism to prevent bedaquiline from reaching its target and carrying out his actions on such short notice. Slowly, the drug approaches the c-ring of my own ATP synthase which the enzyme needs to rotate and set free ATP for me to function. Slowly, I lost my last energy reserves. I knew that this was it for me. Now it’s on my fellows and the behavior of the new host. Will she make the same mistake as our previous host and stop the treatment earlier? Or, have both learned from the consequences? Will my fellows find a way to develop new resistance mechanisms and a way to survive? Or will there be a new effective drug with the potential to completely eradicate our existence? It is not on me to predict the future, yet what I know for the moment is that during my lifetime I was a resistant Mycobacterium tuberculosis and one that the humans stand in awe of.
References
1. Centers for Disease Control and Prevention. Tuberculosis (TB)- How TB Spreads. 2021 [cited 2021 14 December]. Available from: https://www.cdc.gov/tb/topic/basics/howtbspreads.htm.
2. Palomino JC, Martin A. Drug Resistance Mechanisms in Mycobacterium tuberculosis. Antibiotics (Basel). 2014;3(3):317-40.
3. World Health Organization. Tuberculosis (TB). 2021. Available from: https://www.who.int/news-room/fact-sheets/detail/tuberculosis.W