Clostridium

Gram positive anaerobic, spore forming bacilli. Spores are refractile, oval or spherical and usually wider than the parent cell. They may be terminal, subterminal or central within the cell. Most clostridia possess flagella and motile but C. perfringens not. In 48 hour cultures, many of the bacilli may be Gram negative.

Clostridia lack the cytochromes required for electron transport to oxygen. They contain flavoprotein enzymes that reduce oxygen to H₂O₂ and to superoxide, and they lack the catalase, peroxidases and superoxide dismutase that destroy these toxic products. Therefore, they can grow only under anaerobic conditions. Some are found in animal, human intestines. Many species are pathogenic, but most saprophytes found in soil and water decomposing plant and animal matter. Some are of industrial importance for the production of chemical such as acetone and butanol. They are highly pleomorphic, rod shaped, motile. They are obligate anaerobes. The optimum temperature is 37⁰C, pH 7-7.4, growth relatively slow. The characteristic media is Robertson’s cooked meat broth- contains unsaturated fatty acids which take up oxygen. Clostridia can produce disease only when the conditions are appropriate.

ENTEROBACTERIACEAE

             The predominant aerobic bacterial flora of the large intestine of human beings and animals is composed of non-sporing, non-acid fast, Gram negative bacilli. Members of this group are included in a complex family, Enterobacteriaceae. Some are capsulated, some not, some are motile and some non-motile. They are oxidase negative and catalase positive and reduce nitrates to nitrites, aerobes and facultative anaerobes, grow readily on ordinary media, MacConkey agar and ferment glucose in peptone water with the production of either acid or acid and gas. The property of lactose fermentation is of great importance for the isolation and identification of enterobacteria from clinical specimens.

Vibrios

            Gram negative, rigid, curved rods, actively motile by polar flagellum, nonsporing, noncapsulated, present  in  marine  environments  and  surface  waters. The most  important  member  of  the  genus  is    V. cholerae, the causative agent of cholera / Asiatic cholera. It was first isolated by Robert Koch.

Rickketsia

General characters

·         They are small Gram negative bacilli adapted to obligate intracellular parasitism, transmitted by arthropod vectors. They are primary parasites of arthropods such as lice, fleas, ticks, in which they are found in the alimentary canal. In vertebrates they infect vascular endothelium, reticuloendothelial cells.

·         This pleomorphic coccobacilli are nonmotile, noncapsulated. They stain bluish purple with Giemsa stain. It has a 3 layered cell wal  , a trilaminar plasma membrane and an outer slime layer.

·         Growth occurs in the cytoplasm of infected cells, nucleus etc. the optimum temperature is 32-35⁰C. they are readily cultivated in the yolksac of chick embryos, HeLa cells etc., in arthropods.

·         They multiply by binary fission.

·         They are inactivated by physical and chemical agents. They are rapidly destroyed at room temperature when seperated from host components, susceptible to tetracycline, chloramphenicol, ciprofloxacin, lysozyme. Sulfonamides enhance the growth and worsen the condition of patients.

·         They possess both RNA and DNA.

·         They are large enough to be seen under the light microscope and are held back by bacterial filters.

Pseudomonas

 They are large group of aerobic,non-spore forming, Gram negative, motile. They are ubiquitous, mostly saprophytic, found in water, soil etc. Some are pathogens(oppurtunistic).

P.aeruginosa- slender, Gram negative bacillus 1.5-3µm X 0.5µm,actively motile by polar flagella. Pathogens are piliated. Many strains possess slime layer. It is an obligate aerobe,growth at 6-42⁰C, grow well on ordinary media, produce large opaque irregular with a distinctive musty or earthy smell. Patches with metallic sheen on nutrient agar. It grows on Mac Conkey forming nonlactose fermenting colonies, hemolytic on blood agar. It produces a number of pigments- pyocyanin(bluish green) and fluorescein(greenish yellow),pyorubrin(bright red),pyomelanin(brown to black). Glucose is utilised forming acid, nitrates reduced to nitrites and to gaseous nitrogen,oxidase,catalase positive.

It is an important cause of hospital infection. It is killed at 55⁰ C in 1 hour, resistant to chemical agents,antiseptics and disinfectants, but sensitive to acids and phenolic disinfectants. They show considerable degree of natural resistance to antibiotics, cephalosporins are effective.

Neisseria

                The genus Neisseria consists of Gram negative, aerobic, non-sporulating, nonmotile, oxidase positive cocci typically arranged in pairs(Diplococci).They occur in CSF, urinogenital tract, commensals of mouth or the upper respiratory tract. They are saccharolytic, catalase positive, G+C content 47-52mol %. Pathogenic Neisseria species are fastidious organisms, requiring enriched media for optimal recovery.

Bordetella

            They are small Gram negative coccobacilli that do not ferment carbohydrates. They are strict aerobes. They are parsitic in the respiratory tract of human beings, animals or birds. They produce toxins.The most significant type in humans is B.pertussis which causes pertussis or whooping cough or violent cough. It is a small coccobacilli, in cultures they are uniform in size and shape but later become longer and thread-like. It is nonmotile and nonsporing. It is capsulated and some possess fimbriae. They are aerobic, grow best at 35-36⁰C, commonly used media is Bordet-Gengou Blood agar, slow growth ,after incubation small, dome shaped, smooth , opaque, greyish white, glistening ,resembling ‘mercury drops’

It is biochemically inactive,does not ferment sugars,oxidase and catalse positive. It is a delicate organism being killed readily by 55⁰C, drying, disinfectants. Outside the body it survives in dried droplets for 5 days. It secretes pertussis toxin and excretes outside, also produces other toxins, LPS also contributes antigenicity.