Safety in numbers: how TB infects

For a tuberculosis infection to progress, around 50 bacterial cells need to infect a single immune cell.
15 June 2017

Interview with 

Alex Sigal, Africa Health Research Institute


For a tuberculosis infection to progress, around 50 bacterial cells need to infect a single immune cell.


It was in 1882 that Robert Koch announced to the world that he’d discovered the cause of the disease tuberculosis; he did it, in part, by looking down a microscope. But although Koch linked the TB bacterium to the disease, despite more than a century of study since, we still don’t really know how the infection unfolds in the average person. Now, speaking with Chris Smith, Africa Health Research Institute scientist Alex Sigal explains how he has also been looking down the microscope and has got a new piece to add to the TB puzzle…

Alex - TB exposure is very common. One-third of individuals throughout the world are exposed mostly in developing countries, 10 million active infections, and about 2 million deaths a year. It’s quite a healthcare burden and it actually is a burden in countries which can least afford to deal with it.

Chris - The causative organism, it’s a bacterium, a mycobacterium.

Alex - Yeah. So it’s Mycobacterium tuberculosis and it’s been studied for 100 years or more.

Chris - What's our present understanding of how it causes the diseases that it does and how it spreads?

Alex - It’s mostly a disease of the lung. It spreads by aerosol – so people cough and somebody actually breathes in the droplets, inside the droplets are the microorganisms.

Chris - Once they land in your lungs, what do they do?

Alex - They're taken up by a cell called a macrophage. At some point, that cell may attract additional cells and this whole conglomeration of cells becomes a structure called a granuloma where you have some infected macrophages at the core and outer cuff of fibres and other immune cells.

Chris - The mycobacterium is actually inside the infected macrophages – not killed. It’s in there and viable.

Alex - It’s in there. In most cases, it’s quiescent and the person’s immune system is able to control it.

Chris - What about that model were you uncomfortable with which provoked you to start looking in the study that you're presently publishing?

Alex - The transition to really, the active disease is not that clear. I mean, it’s known that the immune system controls it. But let’s say now the immune system stops controlling it, what are the steps that need to happen for the infection to actually grow?

Chris - In other words, in the vast majority of people, the infection gets into the lung but then the immune system holds it back and doesn’t go anywhere, but in a number of people and for various reasons, it suddenly becomes productive and it starts to grow in the infected cells, in the infected person, but we don’t really understand what the triggers are or what the immune processes are that allow that escape.

Alex - Yeah, that allow not only the escape but also the continuous growth of the bacteria.

Chris - So, how did you manage to study it, given that people have been looking at TB for a century and they haven't yet been able to find this?

Alex - The tools that we use are fairly recent, so it’s time-lapse microscopy. We figured that the best way to understand it is just to see it. We just filmed the infection. So we used a very simplified model, so we just infected macrophages and we looked at what happens – how the infection actually grows or does not grow.

Chris - What did you find?

Alex - We’re actually surprised. First, that these cells are tough as nails. The person’s cells which contain these mycobacteria, they can contain the infection when they're in fact with low numbers. Up to about 50 bacteria, they're okay, but above that, they start to die. But what surprised us more is that the bacteria don’t die with the host cell and they grow very nicely in the dead cell where before, in the live cell, they were actually controlled even at high numbers.

Chris - So basically, by killing the cell, they’ve now released a whole bunch of nutrients and food stuffs that those microbes can then assimilate and used to fuel the further amplification of numbers.

Alex - Yes, so it seems. So, the cell before was contained then in a compartment called the phagosome. Once the cell is dead, it’s dead. It’s like a sack of nutrients – whatever is left of it.

Chris - And then what happens?

Alex - Well, macrophages are phagocytes so they pick up all the junk and bacteria, and so on that surround the lung. But it will now then go for another live cell but what they will do is engulf a dead cell even if it is infected with a lot of bacteria. So, you have the first cell which died because of a high bacteria load. It’s going to be engulfed by another cell. Now, that cell is faced with an equal or greater problem because now, the bacteria had some chance to grow. Now, once that cell dies, the bacteria have extra nutrients and the next cell is going to come along, and pick that cell up. So basically, cells, at that stage of infection have two functions – one is they provide fuel for the infection to grow and second, they're bait for other cells to come in and engulf them and die.

Chris - So in a nutshell, we have a 3-phase lifecycle going on where you have an initial infection. If the number of bacteria in the initial infected cell increases to beyond a threshold of about 50, the cell is compromised. In the course of dying, it then becomes bait to pull in more potential victims and so, you get this sort of positive feedback going on where your amplifying the bacteria, feeding them in the process, and attracting more food as they grow.

Alex - Right. The only thing you need for it to start this process is you need enough bacteria in one place. So the bacteria need to be concentrated in a clump; this is in fact what a TB naturally do.


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