Facilitation of Dilutions for Enumeration of Bacteria

Facilitation of Dilutions for Enumeration of Bacteria

Research Note Facilitation of Dilutions for Enumeration of Bacteria Colin Thacker, L. Kelly, and Hilliard Pivnick Microbiology Division, Food and Dru...

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Research Note

Facilitation of Dilutions for Enumeration of Bacteria Colin Thacker, L. Kelly, and Hilliard Pivnick Microbiology Division, Food and Drug Directorate Tunney's Pasture, Ottawa

Dilution of foods or other materials for agar plate counts or MPN determinations is usually done in 90 or 99 ml dilution blanks. The diluent is contained in rubber-stoppered or screw-capped bottles. Screw-capped bottles frequently leak because of imperfect cap liners or chipped bottle rims; and they require more manipulation in removing and replacing the caps than do rubber-stoppered bottles. Bottles (e.q., Fisher Scientific Co. No. 2-943) and Escher rubber stoppers (e ..1., No. 2-945) have two advantages: they rarely leak during shaking, and the stoppers are readily manipulated; but they have the disadvantage of microorganisms and dust settling on the unprotected bottle lip during storage. We have developed a system for handling Escher stoppered bottles which obviates the disadvantage mentioned, offers considerable saving of labour, and reduces the probability of making errors during dilution.

ing and it reduces the amount of dust settling on the bottle lips during storage. Fig. 1 shows a tray

Fig, 2

hg. 1


ot bott,es with iid partially removed.

Bottles are handled in groups of 35 in stainless steel storage trays 13.0" x 9.75" x 3" high with a few holes in the bottom, and the filled, stoppered bottles are covered with a 2" high tray of the same length and width which has no holes in the bottom. The cover prevents stoppers from blowing out during autoclavJ. Inst. Can. Techno!. Aliment.

Vo!. 3, No 3, 1970

Compartment tray for holding bottles while making dilutions.

containing bottles; the lid is partially removed. A third type of tray is used for holding bottles during dilution of samples. This tray is 11" x 15" x 2" and has 4 parallel spacers running lengthwise each I" high x 0.1" thick and separated from each other by 2.1". The 5 longitudinal compartments each ac· commodates up to 8 dilution bottles, and the spacers reduce accidental overturning of the bottles (Fig. 2). To prepare for diluting samples, bottles from the storage trays are marked and placed in the compartment tray. Just before beginning the dilution, the tray and bottles are placed in an ultra-violet cabinet 1-2 min to reduce microbial contamination on the bottle lip; this eliminates the necessity of flaming. The ultra-violet cabinet is a simple box about 15" x 15" x 20" with a single 15 watt germicidal bulb attached to the top. The cabinet has a door. After the diluted samples have been used to inoculate culture medium, the compartment tray containing contaminated bottles may be taken directly to the autoclave for sterilizing and then to the washing facilities. After washing, bottles are returned to the storage trays and refilled, and the compartment trays are returned to the laboratory. Received Mar. 11, 1970.