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Thursday, October 27, 2011

Multi-Drug Resistance and Detection (MDR TB & XDR TB)


Multi-Drug Resistance and Detection (MDR TB & XDR TB)
Antibiotic resistance is a type of drug resistance where a microorganism is able to survive exposure to an antibiotic. While a spontaneous or induced genetic mutation in bacteria may confer resistance to antimicrobial drugs, genes that confer resistance can be transferred between bacteria by conjugation, transduction, or transformation. Evolutionary stress such as exposure to antibiotics then selects for the antibiotic resistant trait. Many antibiotic resistance genes reside on plasmids, facilitating their transfer. If a bacterium carries several resistance genes, it is called multidrug resistant (MDR).Organisms that display multidrug resistance can be pathologic cells, including bacterial and cancerous cells.
Bacteria that resist treatment with more than one antibiotic are called multidrug-resistant organisms (MDROs for short). Multidrug-resistant organisms are found mainly in hospitals and long-term care facilities.
Widespread use of antibiotics plus the natural growth of bacteria over time has lead to a number of MDROs. It is not a new problem Penicillin resistance in Staphylococcus aureus, a common type of bacteria, was first found in the 1940s.

Multidrug Resistant Organisms Risks
Multidrug-resistant organisms develop when antibiotics are taken longer than necessary or when they are not needed.people who are healthy are at low risk of becoming infected by multi drug resistant organisms,
The risk of infection is increased if you have:
  • An existing severe illness
  • An underlying disease or condition such as diabetes, chronic kidney disease, or skin lesions
  • Previous use of antibiotics
  • Invasive procedures, such as dialysis, and the use of medical devices that enter the body such as tubes used to drain urine (urinary catheters) or tubes in blood vessels used to give fluids, medicines, or nutrients (vascular catheters)
  • Repeated contact with the healthcare system such as numerous admissions to the hospital or regular dialysis visits
  • A long stay in the hospital
  • Previous colonization with a MDRO
  • If you are elderly or are on immune-suppressing medicine
Many different bacteria like staphylococci, enterococci, gonococci, streptococci, salmonella, mycobacterium tuberculosis,etc exhibit multidrug resistance.

Use of right antibiotics instead of wide range antibiotics, using antibiotics only for bacterial infections (not for viral) can reduce the development of multidrug resistance.

MultiDrug Resistance TB (MDR TB) & its Detection
Tuberculosis (TB) continues to be a leading cause of morbidity and mortality in developing countries. Global efforts for TB control are being challenged by the steady increase in drug-resistant TB, particularly multidrug resistant tuberculosis (MDR TB), defined as resistance to at least rifampicin (RIF) and isoniazid (INH). The World Health Organization (WHO) estimates that 500,000 new cases of MDR TB occur globally every year and MDR TB has been reported in 2.9% and 15.3% among the new and previously treated cases, respectively .
MDR TB requires 18–24 months of treatment with expensive second line drugs some of which are injectable agents. The cure rate is much lower than for drug susceptible TB, only around 60%. Therefore, it is crucial that MDR TB should be detected as soon as possible, and measures implemented to effectively control its further spread.
Conventional methods for detection of MDR TB involve primary culture of specimens and isolation of Mycobacterium tuberculosis (MTB), followed by drug susceptibility testing (DST). This process, referred to as indirect susceptibility testing has a long turn around time (TAT) of around 2 months. The TAT is longest in the TB high burden low-income countries where primary isolation and indirect DST are almost exclusively performed on solid medium. Use of liquid systems such as the BACTEC MGIT 960 system (Becton Dickinson, Sparks, Maryland, USA) has improved TAT to about 25–45 days, but liquid culture systems are in most cases not available where the need is greatest .
Even though liquid-based indirect susceptibility tests have improved the TAT, they are still not rapid enough to allow timely decisions on patient management in case of MDR TB. More rapid TB susceptibility tests are needed, particularly in TB high burden countries. Recently, the focus has shifted to rapid direct tests in which decontaminated respiratory samples are directly inoculated in drug-free and drug-containing medium or amplified for detection of MDR TB. Some of the direct tests being studied with prospects for applicability in developing countries include the Nitrate Reductase Assay (NRA); Microscopic Observation Drug Susceptibility (MODS) assay, and more recently molecular assays such as the Line Probe Assay -  Genotype® MTBDR (Hain Life sciences, Nehren, Germany), and its newer version – the Genotype® MTBDRplus.

The Nitrate Reductase Assay (NRA) test, initially introduced as an indirect assay is performed on solid medium as for the proportion method, though liquid-based assays have recently been studied. The medium is supplemented with potassium or sodium nitrate at a concentration of 1000 mg/L to act as a growth indicator. Live M. tuberculosis organisms possess the nitro-reductase enzyme and will reduce nitrate to nitrite, which is then detected as a pink-purple colour when a detection reagent (Griess reagent) is added to the tube. A colour change in a drug-containing tube indicates resistance. The MODS assay is a low-technology liquid culture system performed in OADC-supplemented 7H9 broth on an ordinary tissue culture plate . A cock-tail of antibiotics – polymyxin B, amphotericin B, Nalidixic acid, trimethoprim and azlocillin (PANTA) is added to prevent growth of contaminating bacteria and fungi. Incorporation of isoniazid and rifampicin in the wells followed by inoculation of processed samples in the drug-free and drug containing wells allows direct detection of MDR TB. When M. tuberculosis grows in the broth, characteristic cord-like structures can be seen under an inverted microscope, permitting early detection of resistance . The MODS assay has been studied on both smear positive and smear negative sputum samples with good results, which is not the case with any other tests. The GenoType®MTBDR assay is a molecular test that detects the common mutations in the rpoB and katG genes responsible for resistance to rifampicin and isoniazid, respectively . The test involves DNA extraction, multiplex polymerase chain reaction (PCR), solid phase reverse hybridization and detection of the resistance mutations. The Genotype® MTBDRplus assay detects additional mutations in the rpoB gene and also in the inhA gene promoter region, giving a higher sensitivity in resistance detection.
(This part is from the paper publish in bio-med central "Direct susceptibility testing for multi drug resistant tuberculosis: A meta-analysis")

XDR-TB(Extensively Drug resistant Tuberculosis) is the TB that has developed resistance to at least rifampicin and isoniazid (resistance to these first line anti-TB drugs defines Multi-drug-resistant tuberculosis, or MDR-TB), as well as to any member of the quinolone family and at least one of the following second-line anti-TB injectable drugs: kanamycin, capreomycin, or amikacin.


Nucleic acid Detection Methods & System

Cephid's GeneXpert - Xpert ® MTB/RIF
Molbio Diagnostics - Truelab 


References:
http://www.biomedcentral.com/1471-2334/9/67 (Direct susceptibility testing for multi drug resistant tuberculosis: A meta-analysis)