E. coli are gram-negative, rod-shaped bacteria that belong to the Enterobacteriaceae family.
Although thousands of strains of E. coli exist, most of the strains are relatively harmless, existing as commensal organisms that inhabit the lower intestine of warm-blooded animals and humans alike.
However, there are some strains of E. coli that are pathogenic and can cause severe infections due to their virulent factors.
The virulent strains of E. coli can cause gastroenteritis, urinary tract infections (UTIs), and neonatal meningitis.
In some cases, these virulent strains are also responsible for peritonitis, hemolytic-uremic syndrome (HUS), mastitis, septicemia, and Gram-negative pneumonia.
Types of cultivation media
There are two types of cultivation media that are used in laboratories to isolate and identify microbes. They are selective media and differential media.
- Selective media: these contain ingredients that inhibit the growth of some organisms but allow others to grow. For example, mannitol salt agar contains a high concentration of sodium chloride that inhibits the growth of most organisms but permits staphylococci to grow.
- Differential media: these contain compounds that allow groups of microorganisms to be visually distinguished by the appearance of the colony or the surrounding media, usually on the basis of some biochemical difference between the two groups. Blood agar is one type of differential medium, allowing bacteria to be distinguished by the type of hemolysis produced.
Some differential media are also selective; for example, standard enteric agars such as MacConkey and EMB agars, which are selective for gram-negative coliforms and can differentiate lactose-fermenting and non-lactose-fermenting bacteria.
Examples of standard general-purpose media that will support the growth of a wide variety of bacteria include nutrient agar, tryptic soy agar, and brain heart infusion agar.
A medium may be enriched by the addition of blood or serum. Examples of such enriched media include sheep blood agar and chocolate (heated blood) agar.
Other media used for the cultivation of E. coli in the laboratory include: Columbia Horse Blood Agar medium; Violet Red Bile Agar medium; Cysteine Lactose Electrolyte Deficient Agar (CLED) medium; Eosin Methylene Blue Agar (EMB Agar) Medium (Selective medium);
Sorbitol MacConkey Agar medium (Selective medium); Modified Tryptic Soy Broth (Selective medium) and the liquid medium (Nutrient Broth medium, TSB medium, etc).
Characteristics of E. coli that make it a model organism for laboratory studies
- Shape – Escherichia coli is a straight, rod-shaped (bacillus) bacterium.
- Size – The size of Escherichia coli is about 1–3 µm × 0.4–0.7 µm (micrometer). Cell arrangement – Escherichia coli are arranged singly or in pairs.
- Motility – Escherichia coli are mainly motile, although some strains of E. coli are non-motile.
- Flagella – Escherichia coli have a flagellated peritrichous flagella arrangement. Spores – Escherichia coli is a non–sporing bacterium.
- Capsule – Capsules are present in some strains of E. coli which can easily be demonstrated using Indian ink preparation, and appear as a clear halo in a dark background.
- Gram Staining Reaction – Escherichia coli is a Gram -ve (Negative) bacterium.
- Temperature – Escherichia coli thrives if the temperature ranges from 10–40°C, but the optimum temperature for most E. coli strains is 37°C, being the temperature they are usually cultivated at in laboratories.
- pH – E. coli can survive at 4.5 – 9.5 pH but the maximum growth is observed at 7.0 i.e. neutral pH. Also, the pH requirements vary amongst the various strains of E. coli.
- Nutritional requirements – Escherichia coli have no complex nutritional requirements and readily grow in ordinary media like Blood agar, Nutrient Agar medium (NAM), and MacConkey Agar medium amongst others.
- Oxygen requirements – Escherichia coli is an aerobic bacterium i.e. it grows best in the presence of oxygen but it is also a facultative anaerobic organism i.e. it can grow in a low oxygen environment too.
Importance of culturing E. coli
E. coli is a relatively robust microorganism that can be grown on or in a variety of different solid or liquid media, respectively, over a wide range of temperatures. It is of great importance and
significance in the medical sciences and biotechnology because of its use in both teaching and research laboratories.
Typical laboratory strains are considered to be nonpathogenic microorganisms and grow rapidly in and on a wide range of liquid or solid media, especially in the presence of oxygen (usually doubling every 20 minutes), but can also grow under anaerobic conditions (as a facultative anaerobe).
The phenotypes and genotypes of common laboratory E. coli strains have been well characterized, and have led to their use as a model teaching and research organism since the early 1940s.
It is vitally important to culture E. coli because of their immense contribution to the fields of biotechnology, genetic engineering, and the medical sciences. E. coli are cultured
- To produce insulin by adopting the genetic engineering technique
- To produce certain vitamins in the intestine
- As a parameter to determine the fecal contamination of drinking water ∙ for plasmid study in the bacterial genetics
- To aid in the study of their morphology and identification
- To aid in diagnosing infectious diseases
Culture characteristics of E. coli grown on Blood Agar
Blood agar is one of the most commonly used cultural media. It is an enriched medium used to culture those bacteria or microbes that do not grow easily.
Such bacteria are called “fastidious” as they demand a special, enriched nutritional environment as compared to routine bacteria.
Blood agar medium is suitable for growing most common human and animal bacteria and allows for differentiating different types of hemolysis of the inoculated bacteria. Blood agar is made by adding 5–10% of blood (usually sheep or horse blood) to a nutrient medium.
E. coli grown on blood agar shows the following characteristics:
- Size – 1 to 3 mm
- Elevation – Convex
- Surface – Surface Smooth (fresh isolation); Rough (repeated subculture); mucoid (capsulated strains)
- Color – Greyish white
- Structure – Translucent –Opaque
- Hemolysis – β-Hemolysis (in some strains)
- Emulsifiability – Smooth forms are easily emulsifiable but rough forms are auto-agglutinable, hence do not emulsify easily.
In Blood Agar medium, some of the strains of Escherichia coli show beta hemolysis, especially those which are isolated from the pathologic conditions whereas those which are isolated from normal persons may or may not show hemolysis on blood agar.
Composition of Blood Agar
- 0.5% Peptone
- 0.3% beef extract/yeast extract
- 1.5% agar
- 0.5% NaCl
- Distilled water
- 5% Sheep Blood
- pH should be from 7.2 to 7.6 (7.4)
Note: Blood Agar is made from Nutrient Agar, the only difference being the addition of blood and the subsequent change in pH.
This enriched composition makes Blood Agar to be very useful as a general-purpose enriched medium often used to grow fastidious organisms and to differentiate bacteria based on their hemolytic properties (β-hemolysis, α-hemolysis and γ-hemolysis (or non-hemolytic)).
Choice of Blood for a Blood Agar Medium
Sheep blood is the first choice to prepare blood agar plates, followed by horse, rabbit, or goat blood. Human blood, particularly expired citrated donor blood, should not be used because it may contain some substances that are inhibitory to the growth of some pathogens.
Because of its richness, blood Agar is used to grow a wide range of pathogens including Escherichia coli and even fastidious organisms, such as streptococci, which do not grow well on ordinary growth media.
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Last Updated on July 12, 2023 by Our Editorial Team