13. Characterize mycobacteria and nokardia forms. What features of their morphology and structure of cells. The habitat and representatives.

Mycobacteria are nonflagellated gram-positive microorganisms. Previously thought to be nonmotile, we show here that Mycobacterium smegmatis can spread on the surface of growth medium by a sliding mechanism. M. smegmatis spreads as a monolayer of cells which are arranged in pseudofilaments by close cell-to-cell contacts, predominantly along their longitudinal axis. The monolayer moves away from the inoculation point as a unit with only minor rearrangements. No extracellular structures such as pili or fimbriae appear to be involved in this process. The ability to translocate over the surface correlates with the presence of glycopeptidolipids, a mycobacterium-specific class of amphiphilic molecules located in the outermost layer of the cell envelope. Although most mycobacteria are free-living saprophytic organisms, much of the research on this genus has focused on those species that are pathogenic to humans. These include obligate pathogens such as the leprosy bacillus, M. leprae, and the tubercule bacillus, M. tuberculosis, which kills more than 3 million people per year and infects one-third of the world population. Others are opportunistic pathogens which occur naturally in the environment but can occasionally cause disease, especially in immunocompromised individuals. The most important of the opportunistic pathogens are the members of the M. avium-M. intracellulare complex, which are a leading cause of bacteremia in AIDS patients One of the most striking characteristics of mycobacteria is the enormous complexity of their cell envelope. Extensive chemical analyses have shown that the cell wall of mycobacteria consists of three components. The outside layer is composed of mycolic acids, a complex mixture of long-chain α-branched β-hydroxy fatty acids which are arranged as a densely packed monolayer. The mycolic acids are covalently linked to arabinogalactan, which is in turn attached to the peptidoglycan layer. This complex cell wall is surrounded by a capsule of noncovalently bound polysaccharides, proteins, and a small amount of lipids, which include the species- and type-specific glycopeptidolipids (GPLs) and phenolic glycolipids. This unusual envelope provides mycobacteria with remarkable impermeability to external substances, a critical virulence determinant for these organisms.

Nocardia is a genus of weakly staining Gram-positive, catalase-positive, rod-shaped bacteria. It forms partially acid-fast beaded branching filaments (acting as fungi, but being truly bacteria). It has a total of 85 species. Some species are non-pathogenic while others are responsible for nocardiosis.^[1] Nocardia are found worldwide in soil that is rich with organic matter. In addition, Nocardia are oral microflora found in healthy gingiva as well as periodontal pockets. Most Nocardia infections are acquired by inhalation of the bacteria or through traumatic introduction.Nocardia colonies have a variable appearance, but most species appear to have aerial hyphae when viewed with a dissecting microscope, particularly when they have been grown on nutritionally limiting media. Nocardia grow slowly on non-selective culture media, and are strict aerobes with the ability to grow in a wide temperature range. Some species are partially acid fast (meaning that a less concentrated solution of sulfuric or hydrochloric acid should be used during the staining procedure) due to the presence of intermediate-length mycolic acids in their cell wall. Majority of strains possess the cord factor (trehalose 6-6' dimycolate) an important virulence factor.The various species of Nocardia are pathogenic bacteria with low virulence; therefore clinically significant disease most frequently occurs as an opportunistic infection in those with a weak immune system, such as small children, the elderly, and the immunocompromised (most typically, HIV).Nocardia asteroides is the species of Nocardia most frequently infecting humans, and most cases occur as an opportunistic infection in immunocompromised patients. Other species of medical interest are N. brasiliensis and N. caviae. Because it is acid-fast to some degree, it stains only weakly gram positive.The most common form of human nocardial disease is a slowly progressive pneumonia, whose common symptoms include cough, dyspnea (shortness of breath), and fever.