Interim Laboratory Biosafety Guidance for Extensively Drug-Resistant (XDR) Mycobacterium Tuberculosis strains

The recent emergence of a new Mycobacterium tuberculosis strain that causes extensively drug-resistant tuberculosis (XDR TB) has prompted the issuance of these interim guidelines for clinical and research laboratories handling XDR TB strains.

XDR TB stems from poor general TB control and the consequent development of multidrug-resistant TB (MDR TB). The current definition of XDR TB is “XDR TB is TB showing resistance to at least rifampicin and isoniazid, which is the definition of MDR TB, in addition to any fluoroquinolone, and to at least 1 of the 3 following injectable drugs used in anti-TB treatment: capreomycin, kanamycin and amikacin.” 1

Currently, no differences in modes of infection, pathogenesis, transmissibility, or other risk assessment factors have been demonstrated for XDR-TB strains other than its increased resistance to antibiotic treatment. At the time of this writing, the emergence of XDR TB is a recent phenomenon; evidence indicating that XDR TB may be a much higher risk organism from a laboratory safety perspective may yet emerge. In that event, laboratory facilities must reevaluate their own site-specific risk assessments in order to determine if additional safety measures, beyond those described in this document, are required.

Occupational Infections

The risk of occupational infections from XDR TB strains has not been shown to differ from that of non-resistant M. tuberculosis strains. As described in the 5th edition of “Biosafety in Microbiological and Biomedical Laboratories” (BMBL), M. tuberculosis infections are a proven hazard to laboratory personnel as well as others who may be exposed to infectious aerosols in the laboratory, autopsy rooms, and other healthcare facilities.2 The incidence of tuberculosis in laboratory personnel working with M. tuberculosis has been reported to be three times higher than that of those not working with the agent. Naturally or experimentally infected non-human primates are a proven source of human infection. Experimentally infected guinea pigs or mice do not pose the same hazard because droplet nuclei are not produced by coughing in these species; however, litter from infected animal cages may become contaminated and serve as a source of infectious aerosols.

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Laboratory Safety

Like other strains of M. tuberculosis, XDR tubercle bacilli may be present in sputum, gastric lavage fluids, cerebrospinal fluid, urine, and in a variety of tissues.2 Exposure to laboratory-generated aerosols is the most important hazard encountered. Tubercle bacilli may survive in heat-fixed smears and may be aerosolized in the preparation of frozen sections and during manipulation of liquid cultures. Because of the low infective dose of M. tuberculosis (i.e., ID50 <10 bacilli), sputa and other clinical specimens from suspected or known cases of tuberculosis must be considered potentially infectious and handled with appropriate precautions. Accidental needle-sticks are also a recognized hazard.

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Containment Recommendations

Clinical specimens

Most laboratories handling clinical specimens of suspected tuberculosis will not know if an XDR TB strain is present until after testing is completed on the materials. Initially, BSL-2 practices and procedures, containment equipment, and facilities are required for non-aerosol-producing manipulations of clinical specimens such as preparation of acid-fast smears.2 All aerosol-generating activities must be conducted in a biological safety cabinet (BSC). Use of a slide-warming tray, rather than a flame, is recommended for fixation of slides. Liquefaction and concentration of sputa for acid-fast staining may be conducted safely on the open bench by first treating the specimen in a BSC with an equal volume of 5% sodium hypochlorite solution (undiluted household bleach) and waiting 15 minutes before processing.

If samples are being received from a known or highly suspected source of XDR TB, BSL-2 with full BSL-3 practices are highly recommended for manipulations of the clinical specimens, including additional personal protective equipment (PPE) and autoclaving of waste before leaving the laboratory (see 5th edition BMBL for full description of BSL-3 practices).

Laboratory activities with known XDR TB strains

Research activities on XDR TB strains, especially protocols involving aerosolization of infectious materials, should only be undertaken if absolutely necessary. Researchers and institutions should review protocols to determine if less resistant strains of M. tuberculosis can be used instead of XDR TB strains.

BSL-3 practices, containment equipment, and facilities with enhancements are required for laboratory activities in the propagation and manipulation of cultures of XDR TB. BSL-3 enhancements must include the use of respiratory protection, the implementation of specific procedures and use of specialized equipment to prevent and contain aerosols, and the autoclaving of laboratory waste before removal from the laboratory. The use of Powered Air-Purifying Respirators (PAPRs) or higher level of protection is highly recommended. Loose fitting PAPRs can be used; however a site and procedure specific risk assessment should be conducted to determine the exact type of respiratory protection used. An OSHA respiratory protection program will be required for individuals required to wear respirators.

Disinfectants proven to be tuberculocidal should be used. (See Appendix B in the 5th edition BMBL for additional information.)

Vertebrate Biological Safety Level (ABSL) 3 is required for animal studies using non-human primates experimentally or naturally infected with XDR-TB. Animal studies using guinea pigs or mice can be conducted at ABSL-2.

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  1. Extensively drug-resistant tuberculosis (XDR-TB): recommendations for prevention and control. Wkly Epidemiol Rec. 2006 Nov 10;81(45):430-2.
  2. CDC/NIH Biosafety in Microbiological and Biomedical Laboratories, 5th Edition. (2007) Retrieved March 1, 2007.