Читать книгу Analytical Food Microbiology - Ahmed E. Yousef - Страница 56

After a set of dilutions is prepared from a homogenized food sample or a culture, portions of these dilutions are deposited and spread over the surface of agar plates. Spreading inocula (ideally 0.1 ml) on an agar plate requires the use of a cell spreader. This device can be as simple as a bent‐end glass rod, made in the laboratory by a skilled technician. Alternatively, metal cell‐spreaders are used for their durability. Glass or metal spreaders are decontaminated (sanitized) immediately before use as follows. Dip the spreader into a jar of alcohol, remove the spreader, and pass it quickly through the flame of a Bunsen burner to allow remaining alcohol to catch fire. Notice that alcohol decontaminates the spreader and flaming does not heat the spreader enough to sterilize it; flaming is done only to remove excess alcohol (the spreader should never be held in the flame). Disposable sterile plastic spreaders are also available; these are preferred when the transferred inoculum is expected to contain bacterial spores, as the ethanol dipping (just described) inactivates cells but not spores. Using an alcohol jar, along with glass or metal spreader used to spread spores, is likely to result in contamination of subsequently spread plates.

Calibrated sterile inoculation loops (usually disposable) may also be used to spread a specimen or its dilution on an agar plate. This requires scanning the agar surface with the loop repeatedly in a systematic fashion. This spreading technique is used when a limited number of spread‐plates are needed and the microbial load in the analyzed sample is relatively small. This technique may be used in conjunction with sterility testing.