China blue lactose agar

from Wikipedia, the free encyclopedia
Colonies of Lactobacillus delbrueckii subsp. bulgaricus on China blue lactose agar, the blue color indicates lactose- positive bacteria.

China blue lactose agar (also abbreviated as CLA ) is a solid nutrient medium for the cultivation and differentiation of various lactose- positive and lactose-negative bacteria . Its use for the determination of the germ count in milk and milk products is recommended in Germany by the food, consumer goods and feed code.

Mode of action

China blue lactose agar shows the utilization of lactose through acid formation. Many different types of bacteria can grow on the inhibitor-free nutrient medium.

Acid formation

The only carbohydrate in the nutrient medium is lactose (milk sugar). This disaccharide can only be used by bacteria that have the enzyme β-galactosidase and can split the disaccharide into the two monosaccharides glucose and galactose . They are said to be lactose positive. The breakdown of monosaccharides is linked to the formation of acid. In lactic acid bacteria is the lactic acid (or lactate as the anion of the acid) is a typical product, the fermentation is therefore lactic acid fermentation called. In the case of bacteria of the Enterobacteriaceae family , to which many intestinal bacteria belong, the subdivision into lactose-positive and -negative species is a criterion for their differentiation. With them, the breakdown of the monosaccharides usually takes place in the mixed acid fermentation , typical products here are lactic acid (or lactate), acetic acid (or acetate), succinic acid (or succinate) and, in smaller quantities, formic acid (or formate). In addition, carbon dioxide (CO 2 ), hydrogen (H 2 ) and ethanol are formed. The amount of acids formed is lower in the Enterobacteriaceae than in the lactic acid bacteria because of the other products. China blue lactose agar contains the color China blue , which serves as a pH indicator . It changes color from colorless to deep blue in the presence of acids. This leads to a blue coloration of the colonies , and the culture medium around the colonies also changes color . The colonies of the lactose-positive Enterobacteriaceae often appear light blue with a darker center.

Other components

The addition of sodium chloride (table salt) serves to maintain the osmotic pressure of the cells in the nutrient medium. Peptones and meat extract serve as a source of carbon, nitrogen and provide other supplines . Lactose-negative bacteria can utilize these components and thus also grow on the CLA. Since no acid is formed in this case, their colonies do not appear blue. The specialty of China Blue Lactose Agar is that it does not contain any inhibitors . In the culture media used for the differentiation of the gram-negative Enterobacteriaceae into lactose-positive and -negative species (e.g. MacConkey agar ), the dyes and bile salts used act as an inhibitor against gram-positive bacteria. Lactic acid bacteria, but also staphylococci, are gram-positive and cannot be cultivated with these media.

use

Colonies of Escherichia coli on China blue lactose agar, the bacterial colonies and the surrounding nutrient medium are colored blue as a result of the lactic acid formation; the light blue color of the uninoculated CLA can still be seen at the bottom left.

China blue lactose agar is used for the microbiological analysis of milk and milk products . Since it does not contain any inhibitors, it enables both lactic acid bacteria and representatives of the Enterobacteriaceae to grow. The latter can get into the milk in the course of the processing process (e.g. during milking ) from the cow , from the environment or from non-hygienic working materials and represent an undesirable contamination . CLA is suitable for the differentiation of lactose-positive and -negative bacteria and is therefore counted among the differential culture media . It is also used to determine the germ content in the products as part of the colony count (germ count), in which the bacteria present are quantitatively recorded as so-called " colony-forming units ".

The use of China Blue Lactose Agar is recommended by the method book published by the Association of German Agricultural Testing and Research Institutes . The medium also complies with the recommendations of the German Food and Feed Code § 64 LFGB on the official collection of analysis method (formerly § 35 LMBG) and is, for example, in the method of acid-forming determination and non-acid forming microorganisms in infants and young children food based on milk used . In addition to its use in laboratories for quality assurance of dairy products, China Blue Lactose Agar is also used in research and teaching (including schools), also outside of Germany and, in addition to food, in samples from the clinical sector.

Test results

The uninoculated nutrient medium in Petri dishes is light blue and transparent to opalescent and has a pH of 7.0 ± 0.2. After inoculation , the nutrient medium is incubated at 35-37 ° C. for 18-24 hours . For some types of bacteria, the incubation time must be increased to 48 hours. The bacteria that have grown can then be differentiated as lactose-positive (blue) or lactose-negative (colorless). Since China Blue Lactose Agar does not contain any inhibitors, both gram-positive (e.g. streptococci ) and gram-negative bacteria (e.g. coliform bacteria ) can grow. These can be further identified with the help of the Gram stain , the catalase and oxidase test and the tests in a colored series .

Growth of various bacteria on CLA and results of lactose utilization
Microorganism Color of colonies, remarks Lactose utilization Grief behavior
Enterococcus durans blue + gram positive
Enterococcus faecalis ATCC 29212
and E. faecalis ATCC 11700		 light blue
weak blue		 +
+
Lactobacillus delbrueckii subsp. bulgaricus blue, culture medium turned blue +
Lactococcus lactis subsp. lactis blue, culture medium turned blue +
Staphylococcus aureus ATCC 6538p blue, culture medium turned blue +
Staphylococcus epidermidis ATCC 12228 blue +
Streptococcus agalactiae ATCC 13813 blue +
Escherichia coli ATCC 25922 blue + gram negative
Escherichia coli ATCC 8739 light blue with a darker center,
culture medium discolored blue		 +
Bacillus cereus ATCC 11778 colorless - gram positive
Enterococcus faecium 147f colorless -
Staphylococcus aureus ATCC 25923 colorless -
Proteus mirabilis ATCC 29906 colorless - gram negative
Proteus vulgaris ATCC 13315 colorless -
Pseudomonas aeruginosa ATCC 27853 colorless -
Salmonella Typhi ATCC 6539 colorless -
Salmonella Typhimurium ATCC 13311 colorless -
Serratia spp. colorless -
Shigella flexneri ATCC 12022 colorless -

Typical composition

The original composition from 1963 has been slightly modified over time. The nutrient medium consists roughly of (data in grams per liter ):

See also

Individual evidence

  1. ↑ a b c d e Eckhard Bast: Microbiological methods: An introduction to basic work techniques . 2nd Edition. Spectrum Academic Publishing House, Heidelberg / Berlin 2001, ISBN 978-3-8274-1072-6 , p. 48-50 .
  2. ↑ Michael T. Madigan, John M. Martinko, Jack Parker: Brock Microbiology. German translation edited by Werner Goebel, 1st edition. Spektrum Akademischer Verlag, Heidelberg / Berlin 2000, ISBN 3-8274-0566-1 , pp. 532-535, 559-563, 693.
  3. ↑ a b c d e f China Blue Lactose Agar. Oxoid Microbiology Products, accessed July 14, 2014 .
  4. ↑ a b c d e f g h i j China blue lactose agar. heipha Dr. Müller GmbH, accessed on July 14, 2014 . - China blue lactose (PDF; 48 kB)
  5. ↑ a b c d e f g h i j China Blue Lactose Agar. HiMedia Laboratories, accessed July 15, 2014 . - M580 (PDF); 36 kB
  6. ↑ a b E. Brandl, E. Sobeck-Skal: On the method of determining the bacterial count in milk with China blue-lactose agar. In: Milchwiss. Ber. Volume 13, 1963, pp. 1-9.
  7. ^ Association of German Agricultural Testing and Research Institutes [VDLUFA] (Ed.): Method Book Volume VI . 4th edition. VDLUFA-Verlag, Darmstadt 1985, ISBN 3-922712-78-9 (with 5th supplement from 2000).
  8. ↑ L 48.01-7: Determination of acid-forming and non-acid-forming microorganisms in milk-based food for babies and small children (1988). In: Official collection of investigation procedures according to § 64 LFGB. 2014, ISBN 978-3-410-11220-4 , available from Beuth-Verlag .
  9. ↑ Determination of the living bacteria count of milk with a dilution series. State education server Baden-Württemberg , accessed on July 15, 2014 .
  10. ↑ a b c Microorganisms in class . In: Horst Bayrhuber, Eckhard R. Lucius (ed.): Handbook of practical microbiology and biotechnology . 1st edition. tape  3 . Metzler-Schulbuchverlag, Hannover 1992, ISBN 3-8156-3351-6 , p. 55, 58-59 .
  11. ↑ BC Viljoen, JFR Lues: The microbial populations associated with post-fermented dough and compressed baker's yeast. In: Food Microbiology. Volume 10, No. 5, October 1993, pp. 379-386, doi: 10.1006 / fmic.1993.1044 .
  12. ↑ M. Krasowska, A. Reps u. a .: Effect of high pressure on mesophilic starters for cheesemaking. In: High Pressure Research. Volume 23, No. 1-2, March 2003, pp. 93-96, doi: 10.1080 / 0895795031000147830 .
  13. ^ E. Pfeil, B. Wiedemann: Pronounced postantibiotic effect of quinupristin-dalfopristin in static cultures of Staphylococcus aureus: an effect not seen with other antibiotics. In: Antimicrobial Agents and Chemotherapy . Volume 44, No. 4, April 2000, pp. 1059-1061, PMID 10722512 , PMC 89813 (free full text).
  14. ↑ J. Yang, Y. Chen et al. a .: Dissemination and characterization of NDM-1-producing Acinetobacter pittii in an intensive care unit in China. In: Clinical Microbiology and Infection . Volume 18, No. 12, December 2012, pp. E506-E513, doi: 10.1111 / 1469-0691.12035 , PMID 23036089 .
  15. ↑ a b c d e f g Data sheet China blue lactose agar (PDF) from Merck , accessed on July 15, 2014.
  16. ↑ a b China blue lactose agar. (PDF; 64 kB) sifin diagnostics GmbH, archived from the original on July 27, 2014 ; accessed on July 15, 2014 .