Drug susceptibility testing – the MGIT system

The MGIT system

MGIT stands for Mycobacteria Growth Indicator Tube. It is a system that determines whether or not TB bacteria will grow in the presence of TB drugs. If the bacteria grow, then they are resistant to the drugs, because the drugs could not stop their growth. If the bacteria don't grow, the TB drugs work. In this case, researchers and health care workers say the bacteria are 'susceptible' to the drugs.

The MGIT is a so-called liquid culture system. It has been developed by a company called Becton Dickinson (BD) in the United States. It consists of a small tube with a solution in which TB bacteria can grow – a so-called 'medium'. The medium is a special solution, which contains oxygen. It is then mixed with a TB drug. Then TB bacterium is added (for example from a sputum sample), and the tube with the mix is left for a few days. Researchers call this period the 'incubation period', which means the time given to the bacterium to grow. At the bottom of the tube is a special substance which starts to glow (researchers call it fluorescence) when the oxygen in the tube is reduced. When TB bacteria grow, they 'breathe' and use up the oxygen. The tube is monitored under a special light to detect whether or not the oxygen is used up and the bottom of the tube shows fluorescence. If the bacterium is susceptible to the TB drug in the tube, it will not grow, because the drug can do its work. The tube will not show fluorescence because there is no bacterium to use up the oxygen. If this is the case, the patient can be treated with this drug. If the TB bacterium is resistant to the TB drug in the tube, then it will grow – because the drug does not work. The tube will start to show fluorescence after a few days, because the growing bacteria use up the oxygen. In this case, the patient should not be treated with this drug, because it will not have any effect. A tube is monitored for up to 42 days to see whether or not it contains growing TB bacteria and starts to show fluorescence.

Liquid culture systems are currently the standard for drug susceptibility testing in industrialised countries. Experts at the World Health Organisation have recommended liquid culture systems for low and middle income countries as well. There are advantages and disadvantages to liquid culture systems compared to solid culture systems. The conventional TB drug testing cultures are solid culture systems, where TB bacteria are grown on a solid surface in a laboratory (in a container called a petri dish) over a long period of time, usually more than 30 days. The advantage of liquid culture systems is that the bacteria grow much faster. So health care workers can determine much faster whether or not a TB bacterium grows in the culture, meaning whether or not that TB bacterium is susceptible to the TB drugs that are being tested. This means that patients can receive the correct drugs faster, and be treated in an efficient and timely way. A big disadvantage of liquid culture systems is that they are more prone to contamination. This means that other bacteria can enter the system more easily and jade results.

A laboratory that wishes to start MGIT-based TB drug susceptibility testing will need to buy the 'BD BACTECMGIT 320 System'. This system can incubate the tubes and contains all TB drugs to test for. It currently costs $19.500 (about R165.000). Additionally the laboratory will need a 'reader' to monitor for fluorescence. This currently costs $850 (about R7.200). The software for the system costs $12.500 (about R105.000). BD charges $5.500 (about R47.000) for servicing the system in the first 3 years.

The MGIT system is a good system to test for TB drug susceptibility in laboratories. It cannot be used in clinics or remote health facilities. It is the current standard in industrialised countries and should be used in laboratories that have the capacity to use it.