Compact Dry™ LS

Cat. no. LS100 Compact Dry™ LS, 60x75mm Tray with 10x60mm Well 100 trays/box


Hardy Diagnostics Compact Dry™ LS is a ready-to-use test method recommended for the isolation and enumeration of Listeria spp . in raw materials, finished products, or on environmental surfaces pertaining to food and related industries.

This product is not intended to be used for the diagnosis of human disease.


The importance of food cleanliness cannot be understated. The Centers for Disease Control and Prevention (CDC) estimates that 48 million Americans get sick and 3,000 die each year from foodbourne disease. (1) Listeria monocytogenes , the causative agent of listeriosis, is the third leading cause of food related deaths. Listeriosis affects primarily pregnant, elderly, or immunocompromised individuals, and has a mortality rate of 20-30%. (2) L. monocytogenes and other Listeria spp . are gram-positive, catalase-positive bacilli that grow well in cold temperatures and haline environments, both of which are commonly used to limit microbial propagation. (3) While L. monocytogenes is the only common human pathogen in this genus, the presence of other Listeria contaminants indicates that suitable conditions exist for L. monocytogenes to grow, and that action should be taken to eliminate these conditions and potential sources of contamination.

Compact Dry™ LS is a ready-to-use chromogenic medium for performing Listeria spp . counts that contains dehydrated culture media and a cold water-soluble gelling agent in a non-woven cloth matrix. The medium is instantly hydrated when inoculated with a sample, and capillary action diffuses the sample evenly over the matrix to form a gel within seconds. Compact Dry™ LS contains a chromogenic substrate that yields a colored reaction when utilized and permits the differentiation of Listeria spp . from other types of bacteria.

Compact Dry™ LS performs comparably to the FDA MPN method for enumeration of Listeria spp. (6) Compact Dry™ LS utilizes ready-to-use trays that save space and greatly reduce the time needed to perform microbiological testing. Compared to other commonly used culture systems, Compact Dry ™ has a longer shelf life, can be stored at room temperature, does not require manual sample spreading, is rigid, stackable and easy to label, and allows for direct colony picking for further subculture.


Compact Dry™ LS contains dehydrated culture media, a gelling agent, and a chromogenic substrate to assist in differentiating Listeria spp . from other bacteria that may be present.

Final pH 7.2 +/- 0.2 at 25°C.




Method of Use:

1. It is recommended that samples are incubated at 20°C for 1 hour in buffered peptone water before inoculating to Compact Dry LS.

2. Remove the set of four trays from the foil pouch and separate each individual tray by gently bending along the connecting edge until each tray snaps free. Alternatively, if setting up a dilution series of the same sample, trays can be left connected to facilitate reading similar samples. Trays that are not used immediately should be resealed in the foil pouch. Refer to the "Storage and Shelf Life" section for proper storage of unused trays.

3. Remove the lid of the tray using two fingers to hold down one end of the lid and the thumb to lift the opposite end. Lids are easier to remove using a “peel back” method as opposed to a “pull off” method.

4. Inoculate by pipetting 1ml of sample directly to the center of a dry tray well, being careful not to touch the surface of the matrix with the pipet tip. Once dispensed, the sample will automatically diffuse across the surface by capillary action to form a gel; manual spreading of the inoculum is discouraged. Remember to account for the sample inoculum when calculating the dilution series.

5. Replace the lid and label the tray with appropriate information, including the sample dilution factor.

6. Invert the tray and incubate, upside down with the medium on top, at 35-37°C for 24-48 hours.

7. Count colonies illuminated from the backside of the tray to calculate CFU/ml using the Mini Light Box II (Cat. no. 378642000) or comparable back lighting. If the colony count is high, use the 1cm x 1cm molded grid on the back of the tray to assist in colony counting. Use a sheet of white paper with gridded lines to diffuse the light if the molded grids in the tray are difficult to visualize with a light box.


After incubation, read trays against a white background such as the Mini Light Box II(Cat. no. 378642000) or comparable back lighting.

Blue-colored colonies are indicative of Listeria spp . Count the total number of blue colonies to obtain a Listeria spp . count. The growth area is 20cm2 . If the colony count is high, the total count can be obtained by multiplying the average number of colonies observed in one 1cm x 1cm square grid by 20.

Bacteria other than Listeria spp . may grow, such as Proteus mirabilis, but will produce colorless or brown colonies and should not be included in the total Listeria count. Some other bacteria, such as some Enterococcus species, will not grow but may turn the media a light purple color. These should not be included in the total Listeria count.


During inoculation, do not touch the surface of medium and be careful to avoid any contamination by airborne microorganisms.

During incubation, keep cap tight on plates to avoid any possible dehydration.

A dilution may be needed when the sample has a dark color.

When the sample is viscous (thick), pipetting the sample on several points on a plate or an additional dilution may be needed for an even suspension.

When the sample contains an enzyme, it may react with the enzyme substrate in the dry sheet and affect the color.

If the nature of sample does affect the reaction of the medium, inoculate only after the factor is eliminated by means of dilution and other techniques. (e.g. samples with high viscosity, colored, reactive with chromogenic substrate, and with a high or low pH).

It is recommended to use a stomacher and filter homogenized sample afterwards to eliminate carry over of tiny particles of foodstuff onto the surface of the medium.

Counting colonies may be difficult against a dark background. For best results, count colonies with the tray held against a white background such as with the Mini Light Box II (Cat. no. 378642000) .

If using a light box, the molded grid lines or colonies may be difficult to view due to excessive brightness. Diffuse the light using a sheet of white, gridded (1cm x 1cm) paper underneath the tray to facilitate colony counting.

Colonies are not distinguishable on trays if concentrations are above 100 CFU/ml, as high colony counts will result in the whole surface becoming colored or no color development. The sample should be diluted to a concentration of less than 100 CFU/ml for best use.


Standard microbiological supplies and equipment such as loops, swabs such as EnviroTrans™ , applicator sticks, scoops, dilution buffers such as Dilu-Lok™ II , other culture media, a light box(Cat. no. 378642000), incinerators, and incubators, etc., as well as serological and biochemical reagents, are not provided.


Test Organisms Inoculation Method* Incubation Results
Time Temperature Atmosphere
Listeria monocytogenes
ATCC ® 7644
J 24-48hr 35-37°C Aerobic Growth; blue colonies
Listeria ivanovii
ATCC ® 33090
J 24-48hr 35-37°C Aerobic Growth; light blue-blue colonies
Escherichia coli
ATCC ® 25922
J 24-48hr 35-37°C Aerobic Inhibited


Physical Appearance

Compact Dry™ LS should appear dry, free of particles, and light yellow in color.

Compact Dry LS

Listeria monocytogenes (ATCC ® 19115) grown aerobically on Compact Dry LS for 24 hours at 35 degrees C.

Compact Dry LS

Listeria monocytogenes (ATCC ® 19115) grown aerobically on Compact Dry LS for 24 hours at 35 degrees C. The organisms were plated at a concentration of 10 6 CFU/ml, demonstrating confluent growth.


1. Estimating Foodbourne Illness: An Overview . 2014 . Centers for Disease Control and Prevention.

2. Samaswamy, Vidhya, et al. 2007. Listeria - Review of Epidemiology Pathogenesis. Jour. of Microbiolog, Immunology & Infection . 40: 4-13.

3. Ryser, Elliot T., and Elmer H. Marth, eds. 2007. Listeria, listeriosis, and food safety . CRC Press.

4. Association of Official Analytical Communities. Official Methods of Analysis . AOAC, Washington, D.C.

5. American Public Health Association. Standard Methods for the Examination of Dairy Products. APHA, Washington, D.C.

6. APHA Technical Committee on Microbiological Methods for Foods. Compendium of Methods for the Microbiological Examination of Foods. APHA, Washington, D.C.

7. U.S. Food and Drug Administration. Bacteriological Analytical Manual. Arlington, VA

8. Teramura, Hajime, et al. 2011. Evaluation of the dry sheet medium (Compact Dry LS) for screening of total Listeria count in food samples. African Journal of Microbiology Research 5.30: 5419-5426.

ATCC is a registered trademark of the American Type Culture Collection.
Compact Dry™ is a trademark of Nissui Pharmaceutical Co., Ltd. 


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