Culture media are substances used to grow and maintain microorganisms in the laboratory. They typically consist of a mixture of nutrients and other substances that support the growth and metabolism of microorganisms. Culture media can be liquid, semisolid, or solid, and they can be tailored to the specific requirements of different types of microorganisms.
Culture media are an important tool in microbiology, as they allow scientists to isolate, identify, and study microorganisms in the laboratory. They are used in a wide range of applications, including the identification of pathogens, the production of biotechnology products, and the study of the biology and behavior of microorganisms.
There are many different types of culture media, including:
Culture media are an essential tool in microbiology and are used in a wide range of research and applied settings. They allow scientists to study the biology, behavior, and ecology of microorganisms and to identify and characterize new species. They also play a crucial role in the production of biotechnology products and in the diagnosis and treatment of diseases caused by microorganisms.
Hektoen Enteric Agar, a selective and differential medium, is used to distinguish Salmonella and Shigella species from other Enterobacteriaceae. Sylvia King, William I. Metzger introduced the medium in 1968. They developed HE Agar medium during their time at the Hektoen Institute, Chicago, in order to improve the recovery of Salmonella and Shigella from clinical specimens.
Schiemann first described Yersinia selective agar as an alternative to MacConkey agar and other media commonly used for isolating Yersinia Enterocolitica, a causative organism of gastroenteritis. Yersinia Enterocolitica, a major food- or waterborne enteric pathogen, has been reported to cause epizootic outbreaks in animals such as diarrhea, lymphadenopathy and pneumonia. Yersinia Selective Agar, a selective and differentiated medium that supports the growth of Y. Enterocolitica and other Yersinia spp.
Cary-Blair Transport Medium is simple, semi-solid, and non-nutritive medium that is used to collect and storage of samples of microbiological organisms. The low levels of nutrients in the medium aid in the life of the organisms, without multiplication. The semisolid consistency facilitates the ability to transport easily and the medium is able to be stored for up to a year after its preparation at temperatures of room temperature. Cary-Blair Transport Medium is a modification to Stuart’s Medium which is comprised of a more effective buffering system that replaces sodium glycerophosphate by organic phosphates. This new formulation helps prevent the growth of Enterobacteriaceae and aids in the long-term conservation for Salmonella as well as Shigella for extended durations. It is employed for the transport of clinical specimens believed to have enteric pathogenssuch as Shigella, Salmonella, Vibrio Cholerae, and Escherichia Coli O157 H7.
Deoxycholate Citrate Agar is an alteration of Leifson formula that is suggested for the identification of Salmonella as well as Shigella spp. It is comparable to deoxycholate agar however it is slightly more selective for enteric pathogens due to higher levels of both deoxycholate and citrate salts. The sodium deoxycholate pH range of 7.3 to 7.5 inhibits gram-positive bacteria. Citrate salts in the amount contained into the composition, act as inhibitors to gram-positive bacteria, as well as other intestinal organisms that are normal. This makes it an effective and selective media, commonly used to isolate intestinal pathogens.
Cystine Glucose blood agar is also known as Cystine Heart Agar. Francis developed blood-dextrose-cystine agar after determining F. tularensis would only grow on an artificial medium supplemented with sulfhydryl compounds (i.e., cystine). F. Tularensis is extremely meticulous organism that requires cystine to grow at its best. To honor the achievements of all time of Francis in the field of understanding the disease it was named Francisella Tularensis. The hemoglobin-rich medium is suggested to cultivate Francisella Tularensis. Without enrichment supplies, it facilitates the growth of cocci gram-negative and other pathogenic organisms.
The most meticulous organisms that have a high nutritional requirements can be developed using infusion media. Meat infusions were among the first media utilized to cultivate bacteria. Huntoon created a medium with fresh beef heart and peptone that was later named the Heart Infusion Aggar. He showed that it can be utilized to help support the growth of fastidious nutritional microorganisms, without the need for enrichment like blood from animals. Heart Infusion Agar can be used as a general-purpose growth medium. It is recommended for cultivation of fastidious and nutritious microorganisms and as a medium for basal growth that can be used for a wide range of purposes.
PCA, also known as plate count (PCA) is an bacteriological medium that is used to determine the total amount of aerobic live bacteria present in a sample. This is not considered to be a specific medium. The quantity of bacteria is expressed in units of colony-forming units per Gram (CFU/g) in solid samples, and per milliliter (CFU/ml) for liquid samples. The preferred method is to use the pour plate method. The samples are dilute and the appropriate dilutions added to Petri plates. Sterile molten Agar is added to the plates. The plates are rotated with care to ensure an even mixing of the sample with the agar. The plates are then incubated at 20 or 30degC over three days. After incubation, the amount of colonies counted is recorded on the plate using 25-250 colonies, which is believed to provide the most precise results. When calculating the amount of bacteria present within the specimen, the dilution factors is to be considered.
Czapek medium (also known as Czapek’s agar or Czapek Dox medium) is a medium used to grow fungi and other organisms. This medium is suitable for qualitative cultivation of soil bacteria and saprophytic fungi. Czapek originally created the medium in 1902 to cultivate saprophytic mushrooms. Czapek-Dox Agar, a modified formula of the Czapek (1902-1903) and Dox (1910) formulas, is prepared according to Thom & Church. Medium contains sucrose as the only source of carbon, and nitrate is the only inorganic nitrogen source.
Anaerobic blood agar is a solid media that can be used in qualitative methods for the isolation and cultivation anaerobic organisms. V.R. Dowell and T.M. Hawkins, Centers for Disease Control and Prevention Atlanta, Georgia. Anaerobic blood agar supports the growth of anaerobes with typical pigmentation, both fastidious and slow-growing, and other anaerobes that are of clinical significance.
Thiosulfate-citrate-bile salts-sucrose (TCBS) Agar, is a type of selective agar that is used in microbiology laboratories to isolate Vibrio species. TCBS Agar can be used to cultivate Vibrio cholerae from clinical specimens or other materials. Kobayashi et. al. developed TCBS Agar, which modified the Nakanishi selective medium. This medium was originally intended to isolate V. cholerae from V. parahaemolyticus. However, Vibrios can grow healthy colonies with many different colonial morphologies.
Cystine-Lactose-Electrolyte-Deficient (CLED) medium, first described by Sandys and later modified by Mackey and Sandys, is generally used for diagnostic routine urinary bacteriology as a non-selective medium capable of supporting the growth of most urinary pathogens. CLED Agar, a differential medium for the isolation and counting of bacteria from urine, is used. It supports the growth all potential urinary pathogens. The medium also provides distinct colony morphology. It is suitable for the growth of all urinary pathogens, contaminants, and provides good colonial differentiation. However, it does not allow for the spread of Proteus species because of its low electrolytes.
In 1979, Dr. A. Rambach invented and patented the first chromogenic medium for E.coli detection. This technology uses a color-based differentiation technique. It uses soluble colorless molecules, also known as chromogens. They are composed of a substrate that targets a specific enzyme activity and a chromophore. The chromophore can be released when the enzyme of the target organism cleaves the colorless, chromogenic conjugate.
Rosenow developed a medium that could be used to cultivate streptococci using a dextrose broth and brain tissue in 1919. Hayden modified Rosenow’s formula and found that crushed marble promoted the growth of dental pathogens. The current formulation uses infusions from calf brain instead of brain tissue, and disodiumphosphate has been replaced by calcium carbonate.
Campylobacter blood agar (CVA), is a selective medium that allows for the primary isolation from stool specimens of Campylobacter Jejuni. Dekeyser et al. Dekeyser et al. reported that Campylobacter jejuni was isolated from patients suffering from diarrhea and acute gastroenteritis using a filtration method and a selective media with antimicrobials. Skirrow, however, reported that a select medium containing three antimicrobics was used for isolation. Blaser and colleagues reported that they were able to isolate C. jejuni from feces using a selective medium that contained four antimicrobials: amphotericin (vancomycin), polymyxin B and trimethoprim. Reller et al. in 1983 also introduced a better selective medium containing cefoperazone and Vancomycin. This combination of antimicrobials allowed for better suppression of normal fecal bacteria, thereby allowing for better isolation of C.jejuni from the fecal specimen.
Loeffler Medium, a modified version of the 1887 Loeffler formula, is now called Loeffler Medium. Loeffler medium is a modified formula that Loeffler developed in 1887. It enhances primary and secondary isolation, and cultivates fastidious pathogenic microorganisms. After prolonged subculturing or prolonged incubation, Loeffler medium restores virulence as well as other identifying properties (microscopic/colonial). High serum levels are useful in determining organisms’ proteolytic activity. It can also be used to demonstrate pigmentation and ascospores.
A selective medium called phenylethyl alcohol (PEA), is used to cultivate Gram positive organisms, especially cocci, in a sample containing pathogens. Phenylethyl alcohol is the active ingredient that inhibits or significantly reduces growth of Gram-negative organisms.
To ensure that clinical specimens are properly cultured, two types of media are required. The non-selective medium (e.g. Brain Heart Infusion Agar) should be used. Other media should be specific and tailored to isolate pathogenic fungi.
Raymond Sabouraud created Sabouraud Dextrose Aga or SDA in 1892. Sabouraud Dextrose Aggar is useful in the cultivation of fungi (yeasts and moulds), especially for skin infections.
Cystine tryptic agar (CTA) which is also referred to as cystine trypticase agar was created by Vera as an semi-solid growth medium to identify and maintain the health of microorganisms for long time period of time when stored at suitable temperatures. It’s used for determination of the motility of bacteria and, when combined with carbohydrate, is involved in the fermentation process and the differentiation of microorganisms that are fastidious, such as Neisseria and streptococci, as well as pneumococci and non-spore-forming anaerobes.
Peptone Yeast Extract Broth Based Media are enriched nonselective media which include hemin and vitamin K to aid in the recovery of aggressive organisms like Prevotella Spp., Porphyromonas species, as well as the Bacteroides fragileis group.
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