How much does sprayed water remove bacteria?

Question

I was quite skeptical about some company claiming that their device was able to suppress 99.99 % of bacteria. Then I read a report testing the germicidal effect of their device, from a large and independant testing laboratory.

The device was tested against a number of bacteria, including Salmonella, Escherichia coli, Staphylococcus aureus, or Listeria.

They incubated contaminated Petri dishes, then used the device, then incubated the Petri dishes again. It appears that the test samples had a CFU level log-reduced by 4 to 6 (i.e. the number of bacteria is reduced by a factor of 10,000 to 1,000,000) in most cases when compared to control samples, which supports the claim.

However the device implies spraying water, to which the control samples have not been subjected. I am curious about how much this simple spraying may affect the results.

Would rinsing the dishes with sprayed water significantly reduce the amount of bacteria ?

Answer 1

Though conditions are not exactly the same (the water is not sprayed, the duration of water exposition is different, and samples are not Petri dishes ; I am not competent enough in biology to compare methodologies), I found two relevant papers.

Listeria innocua/monocytogenes, pure rinsing

The first paper used Listeria innocua "as a surrogate for L. monocytogenes", while the EMSL report linked in the question tested the latter.

They found the following bacterial log-reduction after rinsing for 15 s under running cold tap water at room temperature and without prior soaking :

• on brocoli and lettuce : 1.41 (96.11 %) ;
• on apples : 2.01 (99.02 %) ;
• on tomatoes : 2.10 (99.21 %).

To be compared to the report from the EMSL which found, after spraying modified water for 10 s on an inoculated Petri dish (which surface is obviously smoother than that of a tomato) :

• at 20 °C : 4.24 (99.994 %) ;
• at 25 °C : 3.01 (99.903 %) ;
• at 30 °C : 3.37 (99.957 %).

Note that although not mentioned, uncertainties on the EMSL numbers must be fairly large since the quantitative counts are below the detection level of the apparatus (but would actually make the numbers conservative…).

Link to the paper : Efficacy of Home Washing Methods in Controlling Surface Microbial Contamination on Fresh Produce (open access PDF), Agnes Kilonzo-Nthenge, Fur-Chi Chen, Sandria Godwin, Journal of Food Protection, 69, 2, 2006.

(NB : it contains further testing which are out of scope for this question but may be of interest, e.g. soaking, rubbing or wiping with paper towel.)

Escherichia coli O157:H7, rinsing while thoroughly rubbing

The second paper tested Escherichia coli 0157:H7, which has also been tested by the EMSL.

The paper reports the following log-reduction after thoroughly rubbing during 15 s under running tap water (no indication of temperature, but flow of 2.0 L/min ; the values in the abstract seem wrong for tomatoes, here are the in-paper values) :

• lettuce leaves (not rubbed ?) : 1.16 (93.08 %) ;
• cabbage leaves (not rubbed ?) : 2.02 (99.05 %) ;
• lemons : 4.10 (99.99 %) ;
• tomatoes : 5.78 (99.9998 %).

Note that the same reduction is reached for tomatoes after only 8 s of washing.

To be compared to the EMSL results (sprayed "modified" water, 10 s rinsing, no rubbing, Petri dish) :

• at 20 °C : 5.67 (99.9998 %) ;
• at 25 °C : 5.14 (99.9993 %) ;
• at 30 °C : 4.88 (99.998 %).

(Same comment about uncertainties applies.)

Interestingly, in this study they also tested the effects of alkaline electrolyzed water, which may not be too far from "ionized water" (I have yet to dig into details), which itself may not be too far from what is supposed to exit the device tested by the EMSL. And… they found no significant difference with regard to plain tap water. (Though they did find significant effects with acidic electrolyzed water and by applying both successively — but that's out of the scope of this question.)

Link to the paper : Reduction of Escherichia coli O157:H7 on Produce by Use of Electrolyzed Water under Simulated Food Service Operation Conditions (open access PDF), Pangloli et al, Journal of Food Protection, 72, 9, 2009.