Microbiology Lecture Notes: The Genus Mycobacterium
Learning Outcomes for Mycobacterial Diseases
Describe Three Characteristics of Mycobacteria: Identify and detail the primary identifying traits of bacteria within the Mycobacteria genus.
Clinically Relevant Diseases and Transmission: Describe the mycobacterial diseases significant to clinical practice, identifying their hosts, how they infect individuals, and the mechanisms of transmission.
Pathogenesis of Different Mycobacteria: Detail the host-pathogen interactions and the specific disease processes involved in various mycobacterial infections.
Diagnosis and Prevention: Describe the methodology for diagnosing various mycobacterial species and the strategies used for their prevention and control.
Significance of Combined Tuberculosis and Johne's Disease: Explain why these two specific diseases are considered important within the context of microbiology and veterinary medicine.
Revision of Bacterial Taxonomy and Classification
Binomial Nomenclature: Bacterial taxonomy utilizes two names for identification: the Genus (e.g., Staphylococcus) and the Species (e.g., aureus).
Phenotypic Taxonomy: This classification is based on observable pathology and physical characteristics. * Morphology: Observations of shape, such as cocci (spheres) or bacilli (rods). * Staining Characteristics: * Gram-Positive: Bacteria with a very large, thick layer of peptidoglycan (cube-like structure) and a single inner cell membrane. They do not have an outer membrane. These stain purple/blue with Crystal Violet because they absorb and retain the primary stain. * Gram-Negative: Bacteria with a very thin peptidoglycan layer and two membranes: an inner cell membrane and an outer membrane. They are decolorized by acetone and take up the counterstain, Safranin, appearing red or pink. * Examples: Gram-positive examples include Staphylococcus species, Bacillus species, and fecal species. Gram-negative examples include E. coli, Salmonella, and Campylobacter. * Oxygen Requirements: * Aerobic: Bacteria that cannot survive without oxygen. * Anaerobic: Bacteria that may be killed by the presence of oxygen. * Microaerophilic: Bacteria that require only a very small amount of oxygen but can sometimes survive without it. * Obligate Anaerobic: Strict requirement for the absence of oxygen. * Culture Characteristics: Identification based on colony shape, color, elevation, and texture. * Biochemical Reactions: Testing for the presence of specific enzymes or sugar utilization patterns : * Fermentation: The way bacteria utilize sugar. * Enzyme Tests: Tests such as Catalase and Oxidase (practical application scheduled for March 4).
Genotypic Taxonomy: Classification based on genetic characteristics. * Old Methods: Focused on genomic content such as DNA hybridization and Guanine-Cytosine () content. * Modern Methods: Utilization of Ribosomal RNA (rRNA) sequencing, Multi-Locus Sequence Typing (MLST), and Whole Genome Sequencing (WGS) paired with phylogenetic analysis. * 16S rRNA Gene: This gene is the primary target for genetic taxonomy. It contains highly conserved regions of nucleotide sequences, with specific sequences being unique to different bacteria, allowing for accurate differentiation between species.
Taxonomy of Mycobacterium Species
Historical Context: Tuberculosis (TB) is an ancient, historically fatal disease. In old films, it was depicted as causing patients to become emaciated and cough up blood from ruptured lung tissue. While previously untreatable, it can now be managed.
Major Groupings: Mycobacteria are divided into two primary clinically significant groups: * Mycobacterium tuberculosis Complex (MTBC): A group of phylogenetically close species that cause tuberculosis in humans and animals. This includes bovine and human tuberculosis. * Mycobacterium avium Complex (MAC): A group containing members closely related to each other that cause avian tuberculosis and Johne’s disease.
Significant Species and Host Associations: * M. tuberculosis: Part of the MTBC; primary host is man and captive primates; causes Tuberculosis (TB). * M. bovis: Part of the MTBC; primary host is cattle; causes Tuberculosis. * M. africanum: Part of the MTBC; primary host is man; causes Tuberculosis specifically in Africa. * M. avium subspecies avium: Part of the MAC; primary host is all avian species; causes Tuberculosis. * M. microti: Host is voles; causes Tuberculosis. * M. marinum: Host is fish; causes Tuberculosis in fish. * M. leprae: Host is man; causes Leprosy (a skin disease). * M. lepraemurium: Hosts include rats, mice, and cats; causes rat leprosy and feline leprosy. * M. avium subspecies paratuberculosis (MAP): Affects cattle, sheep, goats, and deer; causes Johne’s Disease (also called paratuberculosis).
Characteristics and Staining of Mycobacteria
General Phenotype: Mycobacteria are aerobic, non-spore forming, non-motile, and rod-shaped bacilli.
Growth Rate: They are exceptionally slow-growing. While most pathogens grow in to hours, Mycobacteria require a minimum of (three weeks) to show colony growth on agar. They require complex, egg-enriched media for the cultivation of pathogenic species.
Acid-Fast Properties: * Mycobacteria possess Mycolic Acid in their thick cell wall. * They are referred to as Acid-Fast Bacteria (AFB) because they resist decolorization by acids or alkalis after staining. * They stain poorly with Gram stain; they do not absorb the dye well and often appear as "ghost-like" unstained rods.
Ziehl-Neelsen (ZN) Staining Method: * This is the definitive method for identifying Mycobacteria. * It uses a specific staining process that takes approximately . * Visual result: Mycobacteria appear as bright red rods (red bacilli) against a blue background. * It is the most useful diagnostic tool, allowing clinicians to quickly check sputum, saliva, or feces (e.g., feces from cows suspected of Johne's disease).
Structure and Pathogenesis of Mycobacteria
Cell Wall Composition: The cell wall is thick and contains mycolic acid, glycolipids, and a high lipid/wax content. This composition provides several survival advantages: * Resists penetration by standard dyes. * Provides a "shield" or resistance against acids and alkalis. * Protects the bacteria from being digested by macrophages. * Allows environmental persistence; bacteria can survive in soil for up to one year.
Intracellular Survival Mechanism: * Mycobacteria utilize a secretion system (noted as distinct from the T3SS in Salmonella, though the transcript mentions T7SS briefly). * Upon entry into the host (via aerosol/inhalation for lungs or ingestion for enteric disease), they are engulfed by alveolar macrophages. * They are contained within a phagosome. * Typically, a lysosome would fuse with the phagosome to digest the bacteria. However, Mycobacteria prevent the contact between the lysosome and the phagosome. * This allows them to survive and multiply inside the host macrophage rather than being killed.
Granuloma and Tubercle Formation: * The infection causes an inflammatory response characterized by the accumulation of macrophages and other immune cells. * Components of a Granuloma: Epithelioid cells, Langhans giant cells, lymphocytes, and the bacilli themselves. * Caseous Necrosis: The center of these lesions often contains "caseous material." The word "caseous" comes from the Latin for "cheese." It is described as having the texture of soft "spreading cheese" (thick and white, not watery like regular pus). * Tubercle: When the granulomatous lesion is enclosed in a connective tissue capsule, it is called a tubercle, from which the name "Tuberculosis" is derived.
Mycobacterium bovis (Bovine Tuberculosis)
Clinical Significance: A major worldwide disease with zoonotic implications, meaning humans can contract it from diseased animals.
Transmission: * Inhalation of aerosols from infected cattle. * Ingestion of contaminated, raw milk (particularly in calves). * In the UK, the badger serves as a wildlife reservoir, meaning it carries the pathogen without necessarily exhibiting the disease, spreading it to livestock.
Hosts and Spillover: * Reservoir Hosts: Badgers and possums (maintain the bacteria in their bodies and spread it). * Spillover Hosts: Hedgehogs and wild boars (may carry the bacteria on their hair or mucosa and shed it everywhere, but do not necessarily develop the clinical infection).
Prevention in Humans: The introduction of pasteurization in milk production has greatly reduced human exposure and the incidence of fatal human infection from cattle.
Diagnostic Test: The Tuberculin Test: * Utilizes a protein extract from the bacteria (Tuberculin). * The protein is injected intradermally (inside the skin). * It triggers a delayed hypersensitivity reaction. * Clinicians check the site after three days. A positive result is indicated by an elevation (swelling) of the skin.
Control Measures: * Treatment with antibiotics in cattle is inappropriate as high doses would contribute to antimicrobial resistance (AMR). * Culling: The primary method of control is the identification of "reactors" (positive animals) via the Tuberculin test followed by isolation and slaughter. * Economic Impact: Significant losses in the dairy industry due to trading restrictions and the cost of culling. * Vaccination: The BCG (Bacillus Calmette-Gurin) vaccine is currently the most suitable but requires more research to improve its effectiveness in cattle.
Mycobacterium avium Complex (MAC) and Johne's Disease
Mycobacterium avium subspecies avium (Avian TB): * Affects both wild and domestic birds. * Characterized by chronic wasting, weakness, and decreased egg production. * It is an enteric disease; post-mortem examination of the intestine reveals nodules filled with caseous material. * There is no vaccine available for poultry; control involves moving the flock to clean ground and disinfecting soil.
Mycobacterium avium subspecies paratuberculosis (MAP) / Johne’s Disease: * Hosts: Common in sheep, goats, and cattle (also seen in UAE camels). * Clinical Signs: Chronic, intermittent, and eventually profuse diarrhea; emaciation (the animal eats well but loses weight); no fever. * Incubation Period: Very long, lasting from to . First symptoms may not appear until the animal is an adult. * Pathogenesis in the Intestine: 1. Ingestion of contaminated feces or milk. 2. Bacteria enter the intestinal epithelium and are engulfed by macrophages. 3. Immune-mediated granulomatous reaction causes significant thickening of the intestinal mucosa. 4. Result: Loss of mucosa function leads to malabsorption of nutrients and water, even though the animal is digesting food. * Economic Impact: Leads to premature culling and loss of productivity in livestock.
Other Mycobacterial Species
Mycobacterium tuberculosis (Human TB): The same group as M. bovis. Human treatment involves specific antibiotics like Isoniazid.
Mycobacterium lepraemurium (Feline Leprosy): * A skin disease affecting rodents and cats. * Transmitted to cats through bites and scratches from infected rodents. * Causes subcutaneous lesions and nodules, typically on the head and limbs. * Treatment: Primarily through surgical excision of the lesions. * Zoonotic Status: Not considered a zoonotic disease; humans do not get it from cats.
Questions & Discussion
Question (Lecturer): What are the differences between phenotypic and genotypic taxonomy?
Participant Answer: Phenotypic is pathology/physical; genotypic is genetic characteristics.
Question (Lecturer): What does "acid-fast" mean?
Participant Answer: It means there is mycolic acid in the cell wall.
Question (Lecturer): What stain is used for Mycobacteria?
Participant Answer: The Ziehl-Neelsen stain (ZN).
Question (Lecturer): What is a chronic disease?
Participant Answer (Lecturer Refinement): A disease that lasts a long time, often years (e.g., TB), as opposed to acute diseases which have severe, quick onset symptoms within to .