Patient 3 - Posting 2
Alvin

Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative rod measuring 0.5 to 0.8 µm by 1.5 to 3.0 µm. Almost all strains are motile by means of a single polar flagellum.

The bacterium is ubiquitous in soil and water, and on surfaces in contact with soil or water. Its metabolism is respiratory and never fermentative, but it will grow in the absence of O2 if NO3 is available as a respiratory electron acceptor.

The typical Pseudomonas bacterium in nature might be found in a biofilm, attached to some surface or substrate, or in a planktonic form, as a unicellular organism, actively swimming by means of its flagellum. Pseudomonas is one of the most vigorous, fast-swimming bacteria seen in hay infusions and pond water samples.

In its natural habitat Pseudomonas aeruginosa is not particularly distinctive as a pseudomonad, but it does have a combination of physiological traits that are noteworthy and may relate to its pathogenesis.

· Pseudomonas aeruginosa has very simple nutritional requirements. It is often observed "growing in distilled water" which is evidence of its minimal nutritional needs. In the laboratory, the simplest medium for growth of Pseudomonas aeruginosa consists of acetate for carbon and ammonium sulfate for nitrogen.

· P. aeruginosa possesses the metabolic versatility for which pseudomonads are so renowned. Organic growth factors are not required, and it can use more than seventy-five organic compounds for growth.

· Its optimum temperature for growth is 37 degrees, and it is able to grow at temperatures as high as 42 degrees.

· It is tolerant to a wide variety of physical conditions, including temperature. It is resistant to high concentrations of salts and dyes, weak antiseptics, and many commonly used antibiotics.

· Pseudomonas aeruginosa has a predilection for growth in moist environments, which is probably a reflection of its natural existence in soil and water.
These natural properties of the bacterium undoubtedly contribute to its ecological success as an opportunistic pathogen. They also help explain the ubiquitous nature of the organism and its prominance as a nosocomial pathogen.

P. aeruginosa isolates may produce three colony types. Natural isolates from soil or water typically produce a small, rough colony. Clinical samples, in general, yield one or another of two smooth colony types. One type has a fried-egg appearance, which is large, smooth, with flat edges and has an elevated appearance. Another type, frequently obtained from respiratory and urinary tract secretions, has a mucoid appearance, which is attributed to the production of alginate slime. The smooth and mucoid colonies are presumed to play a role in colonization and virulence.

Pseudomonas aeruginosa on MacConkey agar

E.coli

E. coli is the head of the large bacterial family, Enterobacteriaceae, the enteric bacteria, which are faculatively anaerobic Gram-negative rods that live in the intestinal tracts of animals in health and disease. The Enterobacteriaceae are among the most important bacteria medically. A number of genera within the family are human intestinal pathogens (e.g. Salmonella, Shigella, Yersinia). Several others are normal colonists of the human gastrointestinal tract (e.g. Escherichia, Enterobacter, Klebsiella), but these bacteria, as well, may occasionally be associated with diseases of humans.

The Enterobacteriaceae are distinguished from the Pseudomonadaceae in a number of ways known reflexively to competent microbiologists. The pseudomonads are respiratory, never fermentative, oxidase-positive, and motile by means of polar flagella. The enterics ferment glucose producing acid and gas, are typically oxidase-negative, and when motile, produce peritrichous flagella.

Physiologically, E. coli is versatile and well-adapted to its characteristic habitats. It can grow in media with glucose as the sole organic constituent. Wild-type E. coli has no growth factor requirements, and metabolically it can transform glucose into all of the macromolecular components that make up the cell. The bacterium can grow in the presence or absence of O2. Under anaerobic conditions it will grow by means of fermentation, producing characteristic "mixed acids and gas" as end products. However, it can also grow by means of anaerobic respiration, since it is able to utilize NO3, NO2 or fumarate as final electron acceptors for respiratory electron transport processes. In part, this adapts E. coli to its intestinal (anaerobic) and its extraintestinal (aerobic or anaerobic) habitats.

E. coli can respond to environmental signals such as chemicals, pH, temperature, osmolarity, etc., in a number of very remarkable ways considering it is a single-celled organism. For example, it can sense the presence or absence of chemicals and gases in its environment and swim towards or away from them. Or it can stop swimming and grow fimbriae that will specifically attach it to a cell or surface receptor. In response to change in temperature and osmolarity, it can vary the pore diameter of its outer membrane porins to accommodate larger molecules (nutrients) or to exclude inhibitory substances. With its complex mechanisms for regulation of metabolism the bacterium can survey the chemical contents its environment in advance of synthesizing any enzymes necessary to use these compounds. It does not wastefully produce enzymes for degradation of carbon sources unless they are available, and it does not produce enzymes for synthesis of metabolites if they are available as nutrients in the environment.

E.coli on Blood Plate Agar

References

Textbookofbacteriology
Singapore General Hospital Protocol Document No. 70300-13W-411