Bacteriology

Second-Line Drug Susceptibility Testing (SL-DST) for Tuberculosis:Introduction, Principle, Procedure, Result-Interpretation, and Keynotes

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Phenotypic drug susceptibility testing (DST) of Mycobacterium tuberculosis using the BACTEC MGIT 960 system

Introduction Second-line drug susceptibility testing (SL-DST) for tuberculosis is a crucial laboratory procedure designed to detect resistance to anti-TB drugs used when first-line treatment fails (i.e., multidrug-resistant TB – MDR-TB).  Principle SL-DST measures the susceptibility of Mycobacterium tuberculosis to specific agents by determining the Critical …

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Xpert MTB/RIF: Introduction, Principle, Procedure, Result-Interpretation, Uses, and Keynotes

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GeneXpert® Xpert® MTBRIF Test Result- MTB Detected (Very Low) and Rifampicin Resistance Not Detected

Introduction The Xpert MTB/RIF assay is an automated, cartridge-based molecular test used to detect Mycobacterium tuberculosis (MTB) and rifampicin (RIF) resistance, the most potent first-line anti-TB drug. Endorsed by the WHO in 2010, the Xpert MTB/RIF assay revolutionized TB diagnosis by providing results in less than two …

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Bacterial Load (Decoding the CFU): Introduction, Ranges, Application, and Keynotes

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Bacterial Count in a Colony Forming Unit (CFU)

Introduction of Bacterial Load (Decoding the CFU) Bacterial load, measured in Colony-Forming Units (CFU), is a fundamental concept in microbiology used to estimate the number of viable (living) microorganisms in a sample. Unlike total cell counts that include dead bacteria, the CFU method only counts …

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Xpert MTB/RIF Ultra: Introduction, Principle, Procedure, Result-Interpretation, Uses, and Keynotes

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GeneXpert Dx System software interface displaying a completed diagnostic result for an Xpert MTB-RIF Ultra assay

Introduction Xpert MTB/RIF Ultra is an automated diagnostic test designed for the rapid and simultaneous detection of Mycobacterium tuberculosis (MTB) and resistance to rifampin (RIF). It is a second-generation assay developed by Cepheid to provide significantly higher sensitivity than the original Xpert MTB/RIF, particularly for …

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Contamination in L-J Media: Introduction, Common Contaminant, Identification clues, Minimization tricks, and Keynotes 

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Advanced Fungal Contamination and Decolorization of L-J Medium: Fungal Overgrowth-The slants show dense, white, "cotton-like" mycelial growth. This morphology is a classic indicator of contamination by environmental molds, such as Aspergillus or Penicillium. Media Alteration- There is a total loss of the characteristic malachite green color. The medium has turned pale yellow or colorless, suggesting a significant shift in pH or the enzymatic degradation of the selective agent by the contaminants. Diagnostic Interpretation-The rapid growth of these organisms has completely masked any potential slow-growing Mycobacterium tuberculosis. These cultures are invalid and must be recorded as contaminated.

Introduction Lowenstein-Jensen (L-J) medium is the gold standard for the cultivation of Mycobacterium tuberculosis. However, because it is an egg-based, non-selective (or semi-selective) medium that requires long incubation periods, it is highly susceptible to contamination. L-J medium is rich in nutrients (eggs, glycerol, potato flour), …

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Xpert MTB-XDR: Introduction, Principle, Procedure, Result-Interpretation, Uses, and Keynotes

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The image shows a GeneXpert software interface displaying an Xpert MTB/XDR test result with M. tuberculosis (MTB) detected and a specific drug resistance profile.Key Findings:MTB DETECTED: Positive for Mycobacterium tuberculosis complex.Low FLQ Resistance DETECTED: Indicates low-level resistance to fluoroquinolones (e.g., levofloxacin or moxifloxacin).Resistance NOT DETECTED: No mutations were found for isoniazid (INH), amikacin (AMK), kanamycin (KAN), capreomycin (CAP), or ethionamide (ETH).This result is consistent with a pre-XDR-TB diagnosis, as there is resistance to fluoroquinolones but susceptibility to second-line injectables.

Introduction The Xpert MTB/XDR assay is an advanced molecular diagnostic tool designed by Cepheid to rapidly detect Mycobacterium tuberculosis complex (MTBC) and its resistance to multiple first- and second-line drugs. Launched in 2021, it is categorized as a “low complexity” automated nucleic acid amplification test …

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Middlebrook 7H9 Broth: Introduction, Composition, Principle, Procedure, Result-Interpretation, Uses, and Keynotes

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The BBL MGIT tube, showing Modified Middlebrook 7H9 Broth paired with an oxygen-sensitive fluorescent sensor for rapid diagnostic results

Introduction Middlebrook 7H9 broth is a liquid growth medium first described in 1958 by Middlebrook and Cohn for the specialized cultivation of Mycobacterium species. It is widely used in clinical and research settings for rapid growth, biochemical testing, and preparing inocula for drug susceptibility testing …

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BACTEC MGIT 960: Introduction, Principle, Procedure, Result-Interpretation, Application, and Keynotes

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BD BACTEC MGIT 960 instrument display showing the real-time status and distribution of mycobacterial culture tubes

Introduction The system consists of an incubator-reader that can hold up to 960 culture tubes (7 ml size), allowing it to process approximately 8,000 specimens annually. It is widely considered a “gold standard” for culture-based drug susceptibility testing (DST) due to its speed and high recovery rate …

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