Pseudomonas
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 P. aeruginosa colonies on an agar plate.
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P. aeruginosa |
The bacterial genus Pseudomonas includes plant pathogenic bacteria such as P. syringae (various pathovars), the opportunistic human pathogen P. aeruginosa, the ubiquitous soil bacterium P. putida, and some species that are known to cause spoilage of unpasteurised milk and other dairy products. The Pseudomonads are metabolically diverse, can consequently colonise a wide range of niches, and are generally perceived to be agents of spoilage and degradation. Since the mid 1980s, members of the Pseudomonas have been applied to cereal seeds or applied directly to soils as a way of preventing the growth or establishment of crop pathogens. This practice is generically referred to as biocontrol.
Members of Pseudomonas are Gram-negative, aerobic (able to consume oxygen) rods. Most are flagellated and are thus described as motile (able to move about). A significant number can produce exopolysacchardies that are known as slime layers. Secretion of exopolysaccharide makes it difficult for Pseudomonads to be phagocytosed by mammalian white blood cells. Slime production also contributes to surface-colonising biofilms which are difficult to remove from food preparation surfaces. P. fluorescens is one of the many Pseudomonads that secretes large amounts of a fluorescent, yellow-green siderophore under iron-limited conditions. Many Pseudomonadaceae are capable of producing these fluorescent pigments called pyoverdines and pyocyanins. Growth of Pseudomonads on spoiling foods can generate a "fruity" odor.
Pseudomonas have the ability to metabolise a variety of diverse nutrients. Combined with the ability to form biofilms, they are thus able to survive in a variety of unexpected places. For example, they have been found in areas where pharmaceuticals are prepared. A simple carbon source, such as soap residue or cap liner-adhesives is a suitable place for the Pseudomonads to thrive. Other unlikely places where they have been found include antiseptics such as quaternary ammonium compounds and bottled mineral water. Being gram-negative bacteria, most Pseudomonas spp. are naturally resistant to penicillin and the majority of related beta-lactam antibiotics, but a number are sensitive to piperacillin, imipenem, tobramycin, or ciprofloxacin.
This ability to thrive in harsh conditions is a result of their hardy cell wall that contains porins. Their resistance to most antibiotics is attributed to efflux pumps called ABC transporters, which pump out some antibiotics before they are able to act.
The Pseudomonads can also grow in the human ear canal causing pain and a discharge. It can be treated with Ciprofloxacin and Gentisone HC ear drops though this treatment is not recommended for children.
The key identifying characteristics of genus Pseudomonas are: absence of gas formation from glucose, glucose is oxidised in oxidation/fermentation test using Hugh and Leifson O/F test, positive oxidase test, most produce water-soluble blue pigment called Pyocyanin, haemolytic (on blood agar), catalase test is always positive, indole negative, methyl red negative, Voges Proskauer test negative
Genus Commamonas form the acidovorans sub-group. They are non-pigmented and do not use carbohydrates. C. acidovorans uses muconic (organic) acid as sole source of carbon and electron donor.
