I have unfortunately just skimmed through the recent open access paper Microbiome analysis and confocal microscopy of used kitchen sponges reveal massive colonization by Acinetobacter, Moraxella and Chryseobacterium species (M. Cardinale et al. Scientific Reports 7, Article number: 5791 (2017) doi:10.1038/s41598-017-06055-9)
It reports an extensive analysis of a group of kitchen sponge specimines. From the abstract:
Two of the ten dominant OTUs, closely related to the RG2-species Chryseobacterium hominis and Moraxella osloensis, showed significantly greater proportions in regularly sanitized sponges, thereby questioning such sanitation methods in a long term perspective. FISH–CLSM showed an ubiquitous distribution of bacteria within the sponge tissue, concentrating in internal cavities and on sponge surfaces, where biofilm–like structures occurred. Image analysis showed local densities of up to 5.4 * 1010 cells per cm3, and confirmed the dominance of Gammaproteobacteria. Our study stresses and visualizes the role of kitchen sponges as microbiological hot spots in the BE, with the capability to collect and spread bacteria with a probable pathogenic potential. (emphasis added)
A quick take-home message from the paper is that typical efforts to try to re-sanitize used sponges may only shift the sponges bacterial population toward a more potentially pathogenic population, and that it is probably a better idea to simply dispose of the sponge perhaps weekly and get a fresh one instead of microwaving it.
There's a nice write-up in the NYTimes science section, Cleaning a Dirty Sponge Only Helps Its Worst Bacteria, Study Says:
Stop. Drop the sponge and step away from the microwave.
That squishy cleaning apparatus is a microscopic universe, teeming with countless bacteria. Some people may think that microwaving a sponge kills its tiny residents, but they are only partly right. It may nuke the weak ones, but the strongest, smelliest and potentially pathogenic bacteria will survive.
Then, they will reproduce and occupy the vacant real estate of the dead. And your sponge will just be stinkier and nastier and you may come to regret having not just tossed it, suggests a study published last month in Scientific Reports.
Personally I've always hated the whole concept of kitchen sponges and now feel vindicated. Isn't something that can trap biomatter (bacteria, food particles) and keep it moist for extended periods of time the antithesis of what you would want to clean your dishes with?
Question: Aren't sponges simply a bad idea for washing dishes in the first place?
I'm not looking for opinion, rather I'm asking about policy and practice. Since what we do in the privacy of our own kitchen sink is our own, I'd like to ask if kitchen sponges are generally approved for use in restaurants or institutional food preparation settings? Do they have sterilization protocols? Does this paper speak to those?
above: "(A) Kitchen sponges, due to their porous nature (evident under the binocular; (B)) and water-soaking capacity, represent ideal incubators for microorganisms. Scale bar (B): 1 mm. (C) Pie charts showing the taxonomic composition of the bacterial kitchen sponge microbiome, as delivered by pyrosequencing of 16S rRNA gene libraries of 28 sponge samples (top and bottom samples of 14 sponges, respectively). For better readability, only the 20 most abundant orders and families are listed." From here.
above: "Neighbor-joining phylogenetic tree of the ten most abundant OTUs in the analyzed kitchen sponges, as retrieved by pyrosequencing of 16S rRNA gene amplicon libraries. The relative abundance (percentage of the total sequence dataset) and the detection frequency (number of sponges where they were detected) are given in parenthesis after the OTU number. The most similar reference sequences retrieved by BLAST and EzTaxon alignment (type strains only) were included in the tree, followed by the corresponding accession numbers. Red circles indicate risk group 2 organisms, according to the German Technical Rule for Biological Agents No. 466 (TRBA 46631). Numbers at the nodes indicate percentage values of 1000 bootstrap re–samplings (only percentages ≥ 50 are shown). Scale bar represents substitution rate per nucleotide position." From here.