PL Agar

PL Agar is recommended for the isolation and cultivation of Plesiomonas shigelloides from food.

Composition

Final pH (at 25°C): 7.4±0.2

Formula adjusted, standardized to suit performance parameters.

Directions

Suspend 43.58 grams in 1000 ml distilled water. Heat to boiling to dissolve the medium completely. Sterilize by autoclaving at 10 lbs pressure (115°C) for 15 minutes. Mix well and pour into sterile Petri plates.

Principle And Interpretation

Plesiomonas shigelloides is an opportunistic pathogen while controversy exists as its role as an enteropathogen (1, 2) and is mainly associated with the consumption of uncooked molluscus or with foreign travel (3). It does not grow on media like Thiosulphate Citrate Bile Salts Sucrose Agar (TCBS, M189) but grows well on PL Agar. PL Agar is formulated as per APHA (4) for isolation and cultivation of P. shigelloides from foods.

Peptic digest of animal tissue and yeast extract supply the nitrogenous compounds, vitamin B complex and trace ingredients. L-lysine is the amino acid source while arabinose, inositol and mannitol are the fermentable carbohydrate sources in the medium. Bile salts inhibit gram-positive bacteria. Phenol red is the pH indicator, which turns yellow at acidic pH.

Food samples are diluted and directly streaked on PL Agar (M1173) and on Inositol Brilliant Green Bile Agar (M574) (5). The samples are also inoculated into 90 ml Tetrathionate Broth (M032) for enrichment. Plates are incubated at 35°C and the broth at 40°C. Following incubation, suspected colonies from plates are inoculated into Triple Sugar Iron Agar slants (M021) and Inositol Gelatin Medium (M1161) (5). The enrichment broth cultures are streaked on M1173 and M574 and incubated at 35°C for 24 hours. Isolates that are alkaline over acid without gas or hydrogen sulphide on TSI, produce acid but no gas from inositol and do not hydrolyze gelatin are tested for oxidase. If the organism is oxidase positive and a gram-negative rod, it is Plesiomonas (4).

Quality Control

Appearance: Light yellow to pink homogeneous free flowing powder

Gelling: Firm, comparable with 1.5% Agar gel

Colour and Clarity of prepared medium: Red coloured clear to slightly opalescent gel forms in Petri plates

Reaction: Reaction of 4.36% w/v aqueous solution at 25°C. pH: 7.4±0.2

pH

7.20-7.60

Cultural Response

M1173: Cultural characteristics observed after an incubation at 35-37°C for 18- 24 hours.

Storage and Shelf Life

Store below 30°C in tightly closed container and prepared medium at 2-8°C. Use before expiry date on the label.

Reference

1. Ingram C. W., Morrison A. J. and Levitz R. E., 1987, J. Clin. Microbiol., 25 : 1791.
2. Holmberg S. D. and Farmer J. J., 1984, Rev. Infect. Dis., 6: 633.
3. Holmberg S. D., Wachsmith K., Hickman, Brenner F. W., Blake P. A., Farmer J. J., 1986, Ann. Intern. Med., 105: 690.
4. Downes F. P. and Ito K., (Eds.), 2001, Compendium of Methods for the Microbiological Examination of Foods, 4th Ed., APHA, Washington, D.C.
5. Miller M. L., and Koburger J. A., 1986, J. Food Prot., 49: 274