3: Hand Washing Procedure
- Rinse hands with plenty of water
- Apply bactericidal soap and lather up
- Clasp hands together and rub fingertips
- Place left palm over right palm interlacing fingertips to wash between fingers
- Place right palm over back of left hand and wash between fingers, and repeat placing left palm over back of right hand
- Wash thumbs by clasping thumb with opposite hand, washing in a rotating motion
- Use sterile nail brush to clean under the nails
- Finally wash both wrists with opposite palm of hand, followed by rinsing the hands with plenty of water
- Using a paper towel or hot air dryer (as specified upon group allocation), dry hands.
4: Plate 2 inoculation
- Again, work in the ‘bubble’ provided by the Bunsen burner flame.
- As for step 2, but with the now ‘clean’ fingertips
- Seal and invert plate
5: Control Plate
5.1 Seal unopened plate as control plate (to check for the sterility of media preparation)
6: Incubation
6.1 Place plates into an incubator set to 35oC ±1oC for 24hrs
7: Data Recording
7.1 Count all colony forming units (CFU), including those of pinpoint size, on selected plate(s). Record dilution(s) used and total number of colonies counted.
7.2 If the number of CFU per plate exceeds 250, record the counts as too numerous to count (TNTC)
Results:
Aerobic Plate Counts Calculated using following equation3:
N = ∑c / [(1*n1) + (0.1*n2)] * d
Where:
N = Number of colonies per ml or g of product
c = Sum of all colonies on all plates counted
n1 = Number of plates in first dilution counted
n2 = Number of plates in second dilution counted
d = Dilution from which the first counts were obtained (in Litres)
i.e.
Plate 1: N = 29 / [(1) + (0)] * 0.0015
N = 29 / 0.0015
N = 1.9 x 104
Plate 1: Dirty
Plate 2(a): Hand Towel
Plate 2(b): Air Dryer
Plate 3: Control
*APC = Aerobic Plate Count
Conclusion:
The first point to make is that the conclusions drawn from this experiment cannot be seen as being conclusive due to the small sample size. In order to state that any deductions are true and accurate, the experiment would have to be repeated many times to give us greater certainty using a higher degree of confidence.
This point aside, the first assumption that can be drawn from the results is that the first plate (i.e. the ‘Dirty’ plate) can be seen as having the greatest APC number, and therefore the highest bacteriological load. This would hold true with what our initial beliefs are, prior to the experiment, in that un-washed hands hold a great number of microbiological organisms, and as such have the greatest potential for cross-contamination and food poisoning.
Reviewing the results for the hand washing, it can be seen that both methods of hand washing are effective at reducing the biological load, to a greater or lesser extent, but it is the apparent best method that is surprising.
It would appear that the best method for the cleaning and drying of hands is with the use of paper towels, over the use of hot air blow-dryers. This would be initially seem counter-intuitive at first, but on further examination, is not as surprising as at once thought. When you consider the media of the two methods in question, this goes part way into explaining the difference.
Firstly, the paper towel; the towel is a piece of wood-pulp paper substrate that is cut into individual sheets. Because the towel is designed to absorb water, it has a very low amount of available water (Aw) that micro-organisms can exploit in its un-used state. By keeping one of the important limiting growth factors low, this may go some way in keeping the microbial load of the hand drying substrate low. This relative sterility may help to explain the lower than expected results of the paper towels.
When considering the second media type; the hot air dryer, this media is increasing one of the limiting factors. By its very nature, the hot air-drying action combination of air movement and heat in order to precipitate in water evaporation from off the hands. This may actually help microbiological growth by providing a warm environment close to the optimum temperature of most mesophiles.
During a blow cycle, the temperature may increase to the point to actually kill some of the organisms, but the metal will cool gradually, and potentially stay warm for longer periods, which would actually promote growth. Couple this with the air movement action of the blower, it could possibly blow organisms that may have been growing on the dryer, onto the freshly cleaned hands, and therefore re-contaminating them.
These ideas are just theories at this stage and would have to be developed further in order to substantiate these.
Consideration should also be given to other sources of experimental error. This might constitute such things as un-pure plate media preparation, contaminated taps/soap dispenser buttons, or accidental touching of surfaces from sink to worktop prior to plate inoculation.
Taking these factors into account, the trends drawn from this limited sample size indicates a positive correlation in favour of the washing of hands in order to reduce the microbiological load naturally found on the human hand.
Discussion:
The next subsequent step to be undertaken would be to collate all of the results obtained from all of the group members that conducted the experiment to see if the trends corroborate the findings from this investigation.
Following that, the next logical progression, in order to verify the validity of the results obtained would be to repeat the experiment, using a much larger sample size.
If the results are synonymous with one other, it may be interesting to try to identify what types of microbiological organisms are actually present on the plates.
The APC or TVC plate helps to provide a general level of hygiene/cleanliness without being selective. Higher TVC/APC counts may result in increased incidences of pathogenic bacteria and as such can be used as a trend indicator for general hygiene levels.
When considering the human skin flora, the most prevalent bacteria on the surface is Staphylococcus epidermis which is one of the best adapted organisms of the human host, but is more of an opportunistic pathogen so direct testing for presence is not required. However, selective testing for the presence and enumeration of pathogenic bacteria is often a prudent, and the next sequential step.
If checking for the usual pathogens then the following selective media could be used4:
S. aureus, E. coli and Salmonella spp. are amongst the few bacteria that actively cause food poisoning, and as such are the targets of hand washing programmes.
By selectively identifying these organisms from the initial inoculate, or future plates, it may be beneficial because certain species of bacterium can be considered as indicator species. For example, S. aureus can sometime indicate that persons have been touching their mouths/nose/skin or hair and not washing their hands after each event. E. coli may indicate that persons are not washing their hands after using the lavatory.
By using these methods it is possible to monitor the effectiveness of the cleaning system that is in place, as well as the hand washing procedures. It also goes some way to help to provide a ‘due-diligence’ defence in relation to any alleged food poisoning incidences.
Appendix:
1: Source:- The Bacterial Flora of Humans, p5/6 ‘The Composition of Normal Flora’; Kenneth Todar, University of Wisconsin (Dept of Biology); 2002
2: Media preparation:-
Beef Extract 3g
Peptone 5g
Agar 15g
Distilled Water 1L
- Heat to boiling to dissolve ingredients
- Dispense into tubes of flasks
-
Autoclave 15min @ 121oC
- pH = 6.8 ± 0.2
-
Pour constant amount of sterile agar @ 60-70oC (e.g. 15ml/100mm plate)
- Allow to solidify on a level surface
-
Place solidified agar plates into poly-bags and seal (store until use @ 0-4.4oC)
- Bring to room temp, prior to inoculation
Source:- M112 Nutrient Agar, Bacterial Analytical Manual (8th Ed, Revision A), FDA/CFSAN, 1998
3: Source:- Aerobic Plate Count, Bacterial Analytical Manual (8th Ed, Revision A), FDA/CFSAN, 1998
4: Source:- Micro Search Laboratories Ltd
Units 3-7 Scotts Trading Complex
Mytholmroyd
Halifax
HX7 5LH
