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The Laypersons guide to Bacteria Microbiology in the Food Industry
1] Interpretation of results. We take two types of swab, one known as a plain swab (looks like a cotton-wool bud on a long stick); the other is a sponge swab, specifically for Listeria species, looking like a piece of cheap dish washing sponge pad. The plain swab is used for collecting any contaminant organisms on any surface. It is a general-purpose swab, with a low level of reproducibility, but a useful indicator to the general state of hygiene on the test surface. Typically, the swab would be dissolved in 10ml of dilutent to give a nominal 1 in 10 dilution; this would be then further diluted in 1 in 10 stages i.e. 1/10 1/100 1/1000 1/10000 1/100000 1/1000000, which corresponds to: - 1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 One ml of each dilution is spread evenly over an agar plate containing nutrients to support the growth of all bacterial organisms except yeasts and molds, these are incubated overnight @ 30 degrees C. Then the number of bacteria “colonies” as indicated by small colored spots of growth on the agar is counted and multiplied by the dilution. Thus typically, the agar plates may all have growing bacteria colonies up to the 1.0E+05 dilution, but none at the 1.0E+06 dilution. This is known as the “end point”. If there were say fifteen bacteria colonies on the end point, then the count would be:- 15 x 100,000 which = 1.5 million, or 1.5E+06.
This number is then divided by the area originally swabbed to give the total viable count (tvc) per square centimetre, in our case 10 sq. cm therefore; TVC = 1.5E+06 / 10 = 1.5E+05 cfu (colony-forming units) cm2. The fact that there was no growth at the higher dilution reflects the statistical inaccuracy of the test. To get the same count at the higher dilution, one would have to recognise 1.5 bacteria colonies which is nonsensical. In this type of test, a single bacteria colony would be ignored at any dilution as being statistically insignificant. As a general rule, most people do not even have a standardised area to swab, so some swabs may reflect a light touch on a clean surface, others may reflect a damn good scrub somewhere filthy. Comparisons to someone else’s results are therefore invalid. At best, use a completely uniform technique, which will give comparable results across a range of sites. This uniformity of approach must be highlighted in the resulting report. The other swab, specifically targeted at the bacterium Listeria spp is taken over 500-sq. cm, and only processed at a 1 in 10 dilution, but in a way to encourage the growth of any Listeria, while discouraging the growth of all other organisms. Here we are looking for Presence/Absence of Listeria . Because the support medium encourages the Listeria to replicate, counts are meaningless, but the test will typically detect single numbers of Listeria on a surface. The bad-guy of the bunch is L. monocytogenes; this is a pathogenic bacterium and of great concern to the food industry. However, the presence of other Listeria such as L. innocua, indicates that L.monocytogenes might be present at numbers below the limit of detection by the swab, but still a significant risk of contamination of product. |
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May, 2012
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