Revealed: How dirty is your supermarket trolley?

Trolley handle surfaces from four types of stores were tested: traditional grocery stores, budget stores, superstores, and upscale stores. Photo / Getty

A supermarket shopping trolley has 361 times more bacteria than a toilet door knob, laboratory test reveals.

And three-quarters of trolleys across a range of stores swabbed by scientists contained the most harmful type of bacteria, which can include deadly escherichia coli (E. coli) and salmonella.

The Daily Mail reports that the discovery was made after testing handle surfaces from four types of stores: traditional grocery stores, budget stores, superstores, and upscale stores.

Budget stores were the next grubbiest, with trolleys found to contain 270 times more germs than a toilet handle.

Those at larger superstores surprisingly were cleaner, but still had three times the level of bacteria than a kitchen counter top.

Upmarket store trolleys fared the best, containing the same bacteria as a computer keyboard. However, despite being the cleanest overall, nearly 29 per cent contained bacillus, which is commonly linked to food poisoning.

Online retailer ReuseThisBag.com commissioned EMLab P&K to carry out swab tests, which based values on an average of five trolleys from each type of store in the US. The full results can be viewed here.

ILLNESS-CAUSING BACTERIA

When it comes to types of bacteria, not all are created equal.

Over 90 per cent of gram-negative bacteria - found on nearly 75 per cent of all trolleys - are considered harmful to humans and most types are resistant to antibiotics.

The report did not break the bacteria down further. But as well as E. coli and salmonella, gram-negative bacteria is known to also cause pneumonia, and sexually transmitted diseases, including gonorrhoea.

A total of 24 percent of surface germs were identified as gram-positive rods. Although mostly harmless, this type causes the majority of skin infections.

Just under one percent of trolleys tested positive for gram-positive cocci, which is associated with skin infections, in addition to pneumonia and blood poisoning.

And although only in trace amounts, bacillus, which is commonly linked to food poisoning was also detected, while yeast, which causes skin infections was also present.

"Like gram-negative rods, gram-positive cocci can carry parasites and pathogens harmful to humans," the report notes.

"In fact, gram-positive cocci are responsible for a third of all bacterial infections that impact humans."

• Gram-negative bacteria

Gram-negative bacteria are usually thought of as the nasty bugs that can make us sick and can be harmful. For example, several species of Escherichia coli (E. coli), are common causes of food-borne disease. Another example is Vibrio cholera, the bacteria responsible for cholera, is a waterborne pathogen. According to the Ministry of Health, cholera is not endemic in New Zealand.

• Gram-positive bacteria

Usually, gram-positive bacteria are the helpful, probiotic bacteria we hear about that live in our gut and help us digests food. But there are different types. Although mostly harmless, they also cause the majority of skin infections.

• Gram-positive cocci

Staphylococcal is the most worrying type. It can cause a wide range of infections, from relatively minor skin infections such as boils, to more serious infections of the blood, lungs and heart.

There are many types of Staphylococci, but most infections are caused by a group called Staphylococcus aureus. This group of bacteria includes meticillin-resistant Staphylococcus aureus (MRSA), which is resistant to certain antibiotics.

• Bacillus

A type of gram-positive bacteria, bacillus is commonly linked to food poisoning. These toxins can cause two types of illness: one type brings on diarrhoea and the other, called emetic toxin, nausea and vomiting.