MacConkey lactose Agar. MacConkey Agar - uninoculated. Purpose: Contains bile salts and crystal violet which selects for gram-negative enterics, differentiates lactose-fermenters from non-fermenters. Can include sugars other than lactose for further differentiation for example, to differentiate enterohemorrhagic E. Interpretation: Selects for non-fastidious gram-negatives; red colonies indicate fermentation of lactose, white indicates no fermentation of lactose. MacConkey Agar - Escherichia coli Note: Red colonies and red precipitate due to acid production as a result of lactose fermentation.
Colorless colonies, medium is slightly yellow due to the increased pH resulting from bacterial digestion of peptone in the medium. Eosin Methylene Blue Agar - uninoculated. Type: Differential lactose and selective dye inhibition and precipitation at acid pH. Purpose: Differentiates lactose fermenters E. Eosin Methylene Blue Agar - Salmonella enteritidis Note: pink colonies indicative of non-lactose fermentation.
Eosin Methylene Blue Agar - Escherichia coli Note: Green metallic sheen indicative of dye precipitation due to lactose fermentation. Eosin Methylene Blue Agar - Klebsiella pneumoniae Note: Mucoid colonies with dark centers due to capsule production and lactose fermentation respectively. Hektoen Agar. Hektoen - uninoculated. Purpose: Detects lactose fermentation, H 2 S production, inhibits non-enterics. Interpretation: Lactose fermenters yellow or salmon, non-fermenters colorless; H 2 S production produces black precipitate.
Hektoen - Escherichia coli Note: Orange color indicates acid production as a result of lactose fermentation. Mannitol Salt Agar. Mannitol Salt Agar - uninoculated. Purpose: Selects for Staphylococci, which grow at high salt concentrations; differentiates Staphylococcus aureus from other Staphylococci.
Interpretation: Staphylococcus aureus is yellow ferments mannitol , other staphylococci are white. Nutrient Agar: Nutrient agar is a solid medium. Nutrient Broth: Nutrient broth is a liquid medium. Nutrient Agar: Nutrient agar is used to grow non-fastidious organisms. Nutrient Broth: Nutrient broth is used to grow fastidious organisms. Nutrient Agar: Nutrient agar is poured into Petri dishes. Nutrient Broth: Nutrient broth is poured into test tubes and culture bottles.
Nutrient Agar: Nutrient agar is used for colony formation of microorganisms. Nutrient Broth: Nutrient broth is used to maintain stocks of microorganisms.
Nutrient agar and nutrient broth are two types of media used to grow microorganisms. It is used to obtain colonies of microorganisms. Nutrient broth lacks agar, and it is a liquid medium. It is used to maintain stocks of microorganisms. Therefore, the main difference between nutrient agar and nutrient broth is the texture and purpose of use.
These media can also be used to select for or against the growth of specific microbes. Usually a fair amount of information must be known about the microbe to determine its minimal media requirements.
Selective media — Used for the growth of only selected microorganisms. For example, if a microorganism is resistant to a certain antibiotic, such as ampicillin or tetracycline, then that antibiotic can be added to the medium in order to prevent other cells, which do not possess the resistance, from growing.
Differential media — Also known as indicator media, are used to distinguish one microorganism type from another growing on the same media. This type of media uses the biochemical characteristics of a microorganism growing in the presence of specific nutrients or indicators such as neutral red, phenol red, eosin y, or methylene blue added to the medium to visibly indicate the defining characteristics of a microorganism.
This type of media is used for the detection and identification of microorganisms. These few examples of general media types provide some indication only; there are a myriad of different types of media that can be used to grow and control microbes. In defined media all the chemical compounds are known, while undefined media has partially unknown chemical constituents. There are many types of culture media, which is food that microbes can live on. Two major sub types of media are complex and synthetic medias, known as undefined and defined media.
Undefined Media : Luria Broth as shown here is made with yeast extract, as yeast extract is not completely chemically defined Luria Broth is therefore an undefined media. An undefined medium has some complex ingredients, such as yeast extract or casein hydrolysate, which consist of a mixture of many, many chemical species in unknown proportions.
A defined medium also known as chemically defined medium or synthetic medium is a medium in which all the chemicals used are known, no yeast, animal, or plant tissue is present.
A chemically defined medium is a growth medium suitable for the culture of microbes or animal cells including human of which all of the chemical components are known.
A chemically defined medium is entirely free of animal-derived components including microbial derived components such as yeast extract and represents the purest and most consistent cell culture environment.
By definition chemically defined media cannot contain either fetal bovine serum, bovine serum albumin, or human serum albumin as these products are derived from bovine or human sources and contain complex mixes of albumins and lipids.
Animal serum or albumin is routinely added to culture media as a source of nutrients and other ill-defined factors, despite technical disadvantages to its inclusion and its high cost. Technical disadvantages to using serum include the undefined nature of serum, batch-to-batch variability in composition, and the risk of contamination. There are increasing concerns about animal suffering inflicted during serum collection that add an ethical imperative to move away from the use of serum wherever possible.
Chemically defined media differ from serum-free media in that bovine serum albumin or human serum albumin with either a chemically defined recombinant version which lacks the albumin associated lipids or synthetic chemical such as the polymer polyvinyl alcohol which can reproduce some of the functions of serums.
Selective media allows for the growth of specific organisms, while differential media is used to distinguish one organism from another. There are many types of media used in the studies of microbes. Two types of media with similar implying names but very different functions, referred to as selective and differential media, are defined as follows. Selective media are used for the growth of only selected microorganisms. Media lacking an amino acid such as proline in conjunction with E. Selective growth media are also used in cell culture to ensure the survival or proliferation of cells with certain properties, such as antibiotic resistance or the ability to synthesize a certain metabolite.
Normally, the presence of a specific gene or an allele of a gene confers upon the cell the ability to grow in the selective medium. In such cases, the gene is termed a marker. Selective growth media for eukaryotic cells commonly contain neomycin to select cells that have been successfully transfected with a plasmid carrying the neomycin resistance gene as a marker.
Gancyclovir is an exception to the rule as it is used to specifically kill cells that carry its respective marker, the Herpes simplex virus thymidine kinase HSV TK. Some examples of selective media include:. Non-selective versus selective media. While the plate on the right selectively only allows the bacteria Neisseria gonorrhoeae, to grow white dots. Differential media or indicator media distinguish one microorganism type from another growing on the same media.
This type of media is used for the detection of microorganisms and by molecular biologists to detect recombinant strains of bacteria. Examples of differential media include:. Microbiologists rely on aseptic technique, dilution, colony streaking and spread plates for day-to-day experiments.
Microbiologists have many tools, but four relatively simple techniques are used by microbiologists daily, these are outlined here. Aseptic technique or sterile technique is used to avoid contamination of sterile media and equipment during cell culture. This technique involves using flame to kill contaminating organisms, and a general mode of operation that minimizes exposure of sterile media and equipment to contaminants.
Serial Dilution : Example of Serial dilution of bacteria in five steps. The diluted bacteria were then spread plated. When working with cultures of living organisms, it is extremely important to maintain the environments in which cells are cultured and manipulated as free of other organisms as possible. This means passing rims and lids through the flame produced by a Bunsen burner in order to kill microorganisms coming in contact with those surfaces. Sterile technique, in general, is a learned state-of-being, or mantra, where every utilization of any sterile material comes with the caveat of taking every precaution to ensure it remains as free of contaminants as possible for as long as possible.
A serial dilution is the step-wise dilution of a substance in solution. Usually the dilution factor at each step is constant, resulting in a geometric progression of the concentration in a logarithmic fashion.
A ten-fold serial dilution could be 1 M, 0. A culture of microbes can be diluted in the same fashion. For a ten-fold dilution on a 1 mL scale, vials are filled with microliters of water or media, and microliters of the stock microbial solution are serially transferred, with thorough mixing after every dilution step. The dilution of microbes is very important to get to microbes diluted enough to count on a spread plate described later.
Streak plate : Four streak plates. Successful streaks lead to individual colonies of microbes. In microbiology, streaking is a technique used to isolate a pure strain from a single species of microorganism, often bacteria. Samples can then be taken from the resulting colonies and a microbiological culture can be grown on a new plate so that the organism can be identified, studied, or tested. The streaking is done using a sterile tool, such as a cotton swab or commonly an inoculation loop.
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