"Study finds just a sugar-cube sized piece of kitchen sponge can contain 54 BILLION bacterial cells," the Mail Online reports. A German study sampled 14 different kitchen sponges and found they contained far more bacteria than expected.
Genetic analysis revealed that the used sponges contained billions of bacteria, from 362 species-like groups called "operational taxonomic units" (OTUs).
However, it's not clear that any would be harmful in the context of someone's typical exposure to a kitchen sponge, despite 5 of the 10 most common OTUs being bacteria from "risk group 2" (RG2) – a classification including bacteria that may cause disease in certain circumstances.
For example, researchers found high levels of the Acinetobacter strain of bacteria. This can cause potentially serious infections – but only if it penetrates deep inside the body, or infects traumatic wounds or burns.
People associate bacteria with germs. But we are all covered in bacteria, inside and out, and so are our homes. Most are either harmless or actually play a useful role in biological processes, such as digestion. Only a few cause diseases, so the fact kitchen sponges harbour bacteria is not as alarming as it sounds.
The researchers found that methods to clean sponges, such as heating them in microwaves to kill bacteria, don't work particularly well. They suggest replacing sponges weekly, rather than cleaning and re-using them.
The study was carried out by researchers from Justus-Liebig University Giessen, Furtwangen University and the German Research Centre for Environmental Health, all in Germany. It was funded by the Institute of Applied Research (IAF) of Furtwangen University and published in the peer-reviewed journal Nature Scientific Reports on an open-access basis, so it can be read free of charge online.
The Mail Online carried a reasonably accurate report of the research. However, it made much of the fact that some of the bacteria identified came from RG2, a class that includes "bacteria that cause typhoid fever, the plague, cholera and food poisoning". While this is correct, the researchers did not find any of the actual bacteria that cause typhoid, cholera, the plague or food poisoning in the sponges.
This was a genetic analysis of a small sample of kitchen sponges to assess the number, variety and density of bacteria living in and on them. This type of study can investigate the amount and type of bacteria present in the sponges. However, it can't tell us where the bacteria came from or how they may have affected the health of the people using the sponges.
Researchers collected 14 used kitchen sponges from houses in a German town, along with information about how regularly sponges were changed and whether they were specially cleaned to remove bacteria. The type, number and density of bacteria within the sponges were assessed using the latest genome sequencing techniques and a microscopy visualisation technique.
Most previous studies of bacteria in kitchens and kitchen accessories – such as dishcloths and sponges – used bacterial culturing, which can only detect species that can be grown on culture plates in the laboratory. This study used a genetic sequencing technique, called 454-pyrosequencing, of 16S RNA genes to find a much larger range of bacteria, including those that are difficult or impossible to culture in the laboratory.
Laser scanning microscopy was used on fixed samples of sponge to visualise the numbers and density of bacteria.
The researchers grouped the bacteria into OTUs, which was a way of classifying closely related bacteria. They could then divide the bacteria into types that might cause infection and they also checked to see if special cleaning processes affected the number or types of bacteria.
They also checked to see if special cleaning processes affected the number or types of bacteria.
The researchers found billions of bacteria on the surfaces of and walls of the interior spaces within the sponges. Among these, gene sequencing identified 362 OTUs, the majority of which were related to the gammaproteobacteria phylum (a group of classes that share distinctive characteristics).
The 10 most frequently found OTUs were responsible for almost 70% of all the sequences found, and 5 of these 10 fell into the "German Technical Rule for Biological Agents Risk Group 2", suggesting they may have the potential to cause disease in humans.
The researchers didn't find any signs of salmonella, proteus or campylobacter, which are known to cause food poisoning and obviously would be a concern in a kitchen or similar environment.
Imaging showed that most of the bacteria were still growing at the time of analysis. The highest density of bacteria recorded was 54 billion bacterial cells in a 1cm cube of sponge.
The researchers concluded that "kitchen sponges harbour a higher bacterial diversity than previously thought" and "human pathogens might represent just a minority" of the bacteria found.
They added: "Sponge sanitation methods appear not sufficient to effectively reduce the bacterial load and might even increase the shares of RG2-related bacteria." Instead of attempting to clean sponges, they suggest "a regular (and easily affordable) replacement of kitchen sponges, for example on a weekly basis".
There's no need to panic about the results of this study. Bacteria are everywhere, so it's not a surprise to find them growing in kitchens. The researchers say sponges, being porous and usually damp, represent ideal conditions for bacteria to grow.
The study found that one of the most dominant types of bacteria came from the Moraxella family. These bacteria are often found on human skin, so it's likely they got onto the sponges from people's hands. Moraxella are also linked to the unpleasant smell sometimes found after laundry has taken longer to dry, so they seem to be common in the household environment.
The study has a few limitations. As only 14 sponges from one area of Germany were tested, we don't know if the results would apply to households in other parts of the world.
The researchers say the relation of the ONU gene sequences to RG2 species provides "only a weak indicator for the pathogenic [disease-causing] potential of the identified bacteria" and that they "are not aware of any case in which an infection from these bacteria was explicitly reported from a domestic environment". The technology is not yet precise enough to show that any specific bacteria found growing in sponges causes disease.
However, poor kitchen hygiene can cause disease, especially when preparing uncooked food such as raw chicken or salad leaves. Bacteria-laden sponges, if used to wipe down surfaces, could spread pathogenic bacteria around and make infection more likely. You might want to consider simply replacing your sponge regularly instead of rinsing it in hot water or zapping it in the microwave.