Direct fluorescent antibody DFA stain of a patient respiratory tract specimen. Delisle and Lewis Tomalty. Queens University, Kingston, Ontario, Canada. In spite of its natural occurrence in water cooling towers and air conditioners, Legionella is a fastidious bacterium grown in the laboratory, which led to the long lag in identification of the first outbreak of Legionnaire's disease in Philadelphia in Had fluorescent antibody to the bacterium been available at that time, diagnosis could have been made as quickly as the time to prepare and view this slide.
Most pathogenic bacteria of animals, which have adapted themselves to growth in animal tissues, require complex media for their growth. Blood, serum and tissue extracts are frequently added to culture media for the cultivation of pathogens. Even so, for a few fastidious pathogens such as Treponema pallidum , the agent of syphilis, and Mycobacterium leprae , the cause of leprosy, artificial culture media and conditions have not been established.
This fact thwarts the the ability to do basic research on these pathogens and the diseases that they cause. Other concepts employed in the construction of culture media are the principles of selection and enrichment. One can also adjust the physical conditions of a culture medium, such as pH and temperature, to render it selective for organisms that are able to grow under these certain conditions.
A culture medium may also be a differential medium if allows the investigator to distinguish between different types of bacteria based on some observable trait in their pattern of growth on the medium. Thus a selective, differential medium for the isolation of Staphylococcus aureus, the most common bacterial pathogen of humans, contains a very high concentration of salt which the staph will tolerate that inhibits most other bacteria, mannitol as a source of fermentable sugar, and a pH indicator dye.
From clinical specimens, only staph will grow. Mannitol-fermenting colonies S. Types of Culture Media Culture media may be classified into several categories depending on their composition or use. An enrichment medium employs a slightly different twist.
This will help us improve our understanding and ability to fight resistance including through antimicrobial stewardship, infection control policies and prevention measures. The media enable growth of pathogenic organisms while guaranteeing minimum interference from the constituents of the formula in the antimicrobial susceptibility test results.
Mueller Hinton E agar enables the growth of non-fastidious bacteria enterobacteria, non-fermenting Gram-negative bacilli, staphylococci and enterococci found in human pathology. These cost-effective, easy-to-use reagent strips are well-recognized for their performance in determining on-scale Minimum Inhibitory Concentration MIC of antimicrobials. Thanks to this extended performance, they enable:.
A validated Certificate of Compatibility , available in the Resource Center , clearly shows the product compatibility and makes it easy to ensure your accreditation requirements. Mueller Hinton Cloxa agar. Mueller Hinton E agar mm x mm. Mueller Hinton E agar 90 mm. Brucella Blood agar. Conveniently bundled yet flexible solutions for all your Envir An example of a selective medium is MacConkey agar. It contains bile salts and crystal violet, which interfere with the growth of many gram-positive bacteria and favor the growth of gram-negative bacteria , particularly the Enterobacteriaceae.
These species are commonly named enterics, reside in the intestine, and are adapted to the presence of bile salts. The enrichment cultures foster the preferential growth of a desired microorganism that represents a fraction of the organisms present in an inoculum. For example, if we want to isolate bacteria that break down crude oil, hydrocarbonoclastic bacteria , sequential subculturing in a medium that supplies carbon only in the form of crude oil will enrich the cultures with oil-eating bacteria.
The differential media make it easy to distinguish colonies of different bacteria by a change in the color of the colonies or the color of the medium. Color changes are the result of end products created by interaction of bacterial enzymes with differential substrates in the medium or, in the case of hemolytic reactions, the lysis of red blood cells in the medium. In Figure 1, the differential fermentation of lactose can be observed on MacConkey agar. The lactose fermenters produce acid, which turns the medium and the colonies of strong fermenters hot pink.
The medium is supplemented with the pH indicator neutral red, which turns to hot pink at low pH. Selective and differential media can be combined and play an important role in the identification of bacteria by biochemical methods. The microbiology department is celebrating the end of the school year in May by holding its traditional picnic on the green. The speeches drag on for a couple of hours, but finally all the faculty and students can dig into the food: chicken salad, tomatoes, onions, salad, and custard pie.
By evening, the whole department, except for two vegetarian students who did not eat the chicken salad, is stricken with nausea, vomiting, retching, and abdominal cramping. Several individuals complain of diarrhea. One patient shows signs of shock low blood pressure. Blood and stool samples are collected from patients, and an analysis of all foods served at the meal is conducted.
Bacteria can cause gastroenteritis inflammation of the stomach and intestinal tract either by colonizing and replicating in the host, which is considered an infection, or by secreting toxins, which is considered intoxication. Signs and symptoms of infections are typically delayed, whereas intoxication manifests within hours, as happened after the picnic.
Blood samples from the patients showed no signs of bacterial infection, which further suggests that this was a case of intoxication. Since intoxication is due to secreted toxins, bacteria are not usually detected in blood or stool samples. MacConkey agar and sorbitol-MacConkey agar plates and xylose-lysine-deoxycholate XLD plates were inoculated with stool samples and did not reveal any unusually colored colonies, and no black colonies or white colonies were observed on XLD.
All lactose fermenters on MacConkey agar also ferment sorbitol. These results ruled out common agents of food-borne illnesses: E. Figure 2. Gram-positive cocci in clusters. Analysis of the chicken salad revealed an abnormal number of gram-positive cocci arranged in clusters Figure 2.
A culture of the gram-positive cocci releases bubbles when mixed with hydrogen peroxide.
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