Gram positive Bacilli Branching Non-Branching Actinomycetes Spore formers Non-spore formers Aerobic Anaerobic Acid-fast Non-acid fast Bacillus anthracis Bacillus cereus Clostridium perfringens Clostridium tetani Corynebacterium Listeria Lactobacillus Mycobacterium

The Genus Bacillus The Gram’s Positive Spore Former Aerobic Bacilli: The genus Bacillus: Microscopic characteristics: Gram-positive spore-forming, non-motile, large bacilli, 5-8 µm x 1.5 µm, arranged in chains. Bacillus anthracis species are capsulated in tissue and spore-former. in the culture in vitro.

The Genus Bacillus Cultural characteristics: Bacillus anthracis grows on nutrient agar or blood agar in presence of oxygen (obligate aerobic) produce large 2-5 mm grey white irregular colonies. On the blood agar, Bacillus anthracis colonies are non-haemolytic, While Beta-hemolytic activity is demonstrated by Bacillus cereus colonies. L: Bacillus cereus. R: Bacillus anthracis.

The Genus Clostridium The Gram Positive Spore-Forming Anaerobic Bacilli: The genus Clostridium: Microscopic characteristics: Gram-positive spore-forming, anaerobic large bacilli, 5 µm x 1 µm. Some members are saprophytic in soil and sewage; others are commensals in the intestine of man and animals. Clostridium tetani species forms terminal spores, while sub-terminal or central oval spores are produced by Clostridium perfringens. Microscopic Drum-stick Appearance of Clostridium tetani

The Genus Clostridium Cultural characteristics: All Clostridium species are obligatory anaerobic. On blood agar, Clostridium perfringens species produce smooth, large, regular, convex, slightly opaque colonies, and zone of complete hemolysis surrounded by wider zone of incomplete hemolysis. Clostridium tetani colonies show fine branching projections with raised central part and filamentous edge.

The Genus Clostridium Biochemical activities of Clostridium species: All Clostridium species are Catalase and oxidase negative. All Clostridium species are negative for lipases and proteases. Clostridium perfringens ferments lactose and produce Lecithinase. Other species can not ferment lactose and Lecithinase negative.

The Genus Mycobacterium The Non-Spore Former Acid-Fast Bacilli: The genus Mycobacterium: Microscopic characteristics: The Mycobacteria are acid-fast, non-motile, non-spore forming rods, measuring 1-4 µm x 0.2-0.6 µm. The microbe can not be stained well by Gram’s stain due to the presence of waxy capsule.

The Genus Mycobacterium Mycobacterium is best demonstrated by Ziehl -Neelsen staining technique; the microbes appear as thin pink rods arranged singly or in groups. Once stained, they resist decolorization with 2% H2SO4 and 95% alcohol or 3% HCl in 95% ethyl alcohol (Acid-fast, Alcohol fast).

The Genus Mycobacterium Cultural characteristics and colony morphology: Mycobacterium is strict aerobic, grows very slowly (2-4 weeks) on Lowenstein-Jensen medium; produce dry creamy colored colonies. Mycobacterium tuberculosis produces rough, and tough colonies on glycerol and pyruvate egg media. Culture characteristics of Tubercle bacilli on Lowenstein Jensen medium (raised dry cream colored colonies.

The Genus Corynebacterium The Non-Spore Non-Acid Fast Bacilli: The genus Corynebacterium: Gram positive pleomorphic long rods with swollen one end (club-shaped). They lie in small groups joined at angles to each other giving characteristic Chinese-letter arrangement Non-motile, non-spore formers, non capsulated, Catalase positive. Irregularly distributed within the rode are metachromatic volutin granules that stain dark with Albert stain or methylene blue stain.

The Genus Corynebacterium Cultural characteristics: Aerobe or facultative anaerobe; grows rapidly on Loeffler’s serum at 37ᵒ C giving opaque white colonies within 4-12 hours. On blood tellurite, the organism reduces tellurite and produce grey- black colonies measuring 0.5-2 mm in diameter after 24-48 hours. Two different strains are identified: 1- Toxigenic strain. 2-Non-Toxigenic strain. In vitro Elek’s test is used to differentiate between the two strains.