E.coli Metabolism Products on Different Culture Mediums

E. coli typically produces positive tests for indole, lysine decarboxylase, and mannitol fermentation and produces gas from glucose. an isolate from urine can be quickly identified as E. coli by its hemolysis on blood agar typical colonial morphology with an iridescent "sheen" on different media such as EMB agar, and a positive spot indole test.


Lactose fermented rapidly E. coli: Metallic sheen on different media: motile: flat: nonvicous growth


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Source: Brooks, G., Butch, J., & Ornston, N. (1991). Jawetz,Melnick and Adel Berg's Medical Microbiology (19th ed.). United States of America: Appleton and Lange.

Selective and differential agar

EMB (Eosin Methylene Blue) agar inhibits gram positive bacteria, so it is instrumental in isolating gram negative pathogens of the lower digestive tract. the normal flora bacterium E.coli is readily detected on EMB agar by the green sheen that the colonies develop. most pathogens don't develop such as sheen, so microbiologists has a much easier job of isolating the organism responsible for the disease. EMB therefore both a selective and differential medium.

Source: Kandel, J. and Larry McKane. (1996). Microbilogy: Essentials and Application (2nd Ed). United States of America: McGrawHill,Inc.

Mac Conkey Agar

Escherichia coli: Mac conkey agar

peptone base with lactose. Gram positive organism inhibited by crystal violet and bile salts. neutral ed as indicator

(HE) Hektoen Enteric agar

Peptone base agar with bile salts, lactose, sucrose, salicin, and ferric ammonium citrate.indicators include bromthymol blue and acid fuchsin.

(XLD) agar=yeast extract agar with lysine, xylose, lactose, sucrose, and ferric ammonium citrate, sodium desoxychocolate inhibits gram positive organism ; phenol red as indicator

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LF: flat,dry,pink colonies with a surrounding darker pink area of precipitated bile salts

yellow

yellow

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Source:Forbes, B., Shan, D., & Weissfeld, A. Baileys and Scott's

Diagnostic Microbiology (12th ed). Philippines: Elsevier.

A Bacterial Growth Curve Illustrates the Dynamics of Growth in Culture Medium

In growing bacteria, a standard growth curve can be observed; it illustrates the events that occur over time within the population of bacteria. There are four distinct phases that take place: the lag phase, the logarithmic phase, the stationary phase and the decline phase.

In the lag phase cover the first few hours of the curve, no cell divisions occur, because bacteria are adapting to the new environment. In cases of infection in the respiratory tract, white blood cells may swallow up the bacteria, in culture medium; they may die from the inability to adapt to the new environment. The actual length of the lag phase depends on the metabolic activity of the remaining bacteria which were able to adapt. Then the preparation for binary fission begins, they grow in size, store nutrients, and synthesize enzyme. Then the population now enters the active stage called the logarithmic phase or log phase. This is the exponential growth for E.coli. During the log phase, all cells undergo binary fission; the length of the lag phase depends on the species and the environmental conditions. As each generation time passes, the number of bacteria doubles and the graph rises in a straight line on a logarithmic scale.

In human infection, symptoms develop during the lag phase. Bacteria and their metabolic wastes cause tissue damage. Coughing or fever may occur, and fluid may enter the lungs if air sacs are damaged. However, this is also the stage where bacteria are very susceptible antibiotics. In liquid medium preparations, bacterial growth is implies bacterial growth. If prepared in solid medium, growth seen through the emergence of colonies, a single colony may consist of millions of organisms.

After some time: days in cases of infection and hours in culture preparations, the dynamism of the population changes, as the reproductive and death rate equalizes, the population enters a plateau. This is called the stationary phase. In an infection, antibodies, and possibly antibiotics too are now attacking the bacteria. In an in vitro environment, available nutrients become scarce and waste products accumulate.  If the bacterium is a species that secretes antibiotics, they are produced during the stationary phase, if the organism is a species of Bacillus or Clostridium, all vegetative cells will produce endospores.

If such conditions persist, the environment reaches its limit and the population declines thus entering the exponential death phase or decline phase. At this point, the number of dying cells exceeds the number of new cells formed. A bacterial glycocalyx may hinder death by acting as buffer to the environment. For many species, though, the history of the population ends with death of the last cell.

source: please ask joymyrn

Growth Requirements of E. coli

In growing a bacteria there must be consideration of factors that helps the cell grow, or synthesize. Growth factors that include all the nutrients required by the organism to be cultivated, and such factors as pH, temperature and aeration. Agar,is a good medium where microbes are capable to grow in them, this is an extract form marine alga.

First factor to consider is its nutrients,A very good medium includes the exact and accurate qualities and amount of concentrations in every component f the medium, to make the bacteria adapt to its new environment. Otherwise, it is much cheaper and simpler to use natural materials such as yeast extract, protein digest, or similar substances. A factor to consider is the Hydrogen Ion Concentration (pH)A, where it is very important to determine the optimum range they can grow in. Most organisms grow best at a pH of 6.0-8.0, although some forms have optima as low as pH 3.0 and others have optima as high as pH 10.5. Another factor is temperature, microorganisms vary on the range of the temperature they are capable to grow in with such as psychrophilic forms grows best at low temperature( 15-20 C) mesophilic forms grow best at 30-37 C; and most thermophilic forms grow best at 50-60C. Proper aeration & ionic Strength & Osmotic Pressure are also factors affecting the growth of bacteria.

Escherichia coli occurs in enormous numbers in normal feces and is widely distributed in the intestinal canal of animals and humans. Ordinarily it does no harm.

Extensive studies of bacteria in the feces of infants with diarrhea indicate that many cases of infant diarrhea are due to certain particular kinds of Escherichia coli. These pathogenic E. coli can be distinguished from less harmful varieties by immunologic studies of their antigenic nature. 14 enteropathogenic E. coli serotypes are generally recognized. In a study by Rantz it is pointed out that in patients with urinary infections, about 45% of these infections were acquired in the hospital as the result of catheterization.

Since Escherichia coli is always present in feces, and since other species of Escherichiae tribe frequently accompany it and closely resemble it, this tribe is frequently referred to as "the coliform group." They are easily cultivated and recognized and usually remain alive in foods and water for considerable periods of time. The coliform group is therefore commonly sought after in bacteriologic examinations of water, milk, and food as evidence of fecal pollution, rather than Salmonella or Shigella.

Enterotoxic E. coli

Some strains of E. coli produce two kinds of soluble endotoxin, one heat-labile and antigenic, that acts somewhat like cholera toxin, causing severe diarrhea in humans. These enterotoxic strains should be considered as a distinct group of enteropathogenic E. coli; they differ serologically from the common H and O varieties that do not produce any soluble exotoxin.

Since the coliforms rapidly ferment lactose with the production of acid and gas, measured portions of the suspected water are put into broth containing lactose. If gas is present after 24 to 48 hours, it is not necessarily due to coliform species. Several other organisms also produce gas from lactose. Plates or tubes of selective medium are therefore inoculated from the lactose-broth tubes showing gas. After incubation, colonies resembling those of the cloriform group are selected for pure-culture study and are subjected to simple biochemical tests that easily identify them.

source:  Revised by Robert Fuerst, PhD. (1978). Frobisher and Fuerst's Microbiology in health and disease. (14th ed.). West Washington Square, Philadelphia: W.D. Saunders Company.

source :Brock,Thomas D. Biology of microorganisms.6th ed. Englewood Cliffs, N.J.