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There are nearly 4000 species of cockroaches in the world, but only 25 to 30 actually have a pest status.

Cockroaches love to live in filth — but does that make them dirty?

Reporter Dr Andrew Rochford has volunteered to test just how dirty cockroaches are in relation to a human being and other things we have daily contact with.

To get started, Andrew visits Liz Harry, a microbiologist at the University of Technology.

Liz will take two swabs of germs. Firstly, a cockroach will run across a dirty surface, such as the kitchen floor, and will then be left for two hours before the swab is taken. Andrew's fingers will then walk across the same surface and he won't wash his hands for two hours before his swab is taken.

Both samples are then put into separate Petri dishes, then turned upside down and put into an incubator at 37 degrees to be left overnight. If there are any germs, they'll easily be seen in the morning.

Twenty-four hours later it's time to compare the cultures and find out who's the king of clean.

First swab: a control swab was added to the experiment to guarantee the experiment is accurate. There is no bacteria growing on this plate.

Cockroach's swab: There are a few colonies of bacteria there.

Andrew's swab: There's a lot more bacteria on the plate where Andrew's hand was swabbed.

The cockroach is actually cleaner than Andrew. Why? Because cockroaches actually clean themselves fastidiously — all the time.

"Most species of cockroaches are kind of like cats. Cats are considered to be a very clean animal because it's always grooming itself. And cockroaches do that also," says bug collector Darrin Vernier. He lives in the US state of Arizona and is crazy about creepy crawlies.

Darrin's got 10,000 roaches in his personal collection and he claims that his roaches are invaluable in breaking down dead and decaying matter in the eco-system.

So cockroaches could be seen in the insect world as the obsessive compulsive fastidious cleaner?

Darrin says this is a great way to look at it: "Sometimes I walk in from the outside and I track in dirt under my feet accidentally. That's really the only thing the cockroach does that has any relation to filth at all and it's because we've already left it there. If we clean it up it's not a problem."

So if roaches are so clean, what sort of dangers do they really pose?

Dr Noel Tait is an honorary professor in invertebrae zoology at Macquarie University. He says the problem with cockroaches are those nasty little deposits they leave behind.

"The allergens are cockroach allergens themselves. They are in the faeces because they are chemicals from the bodies of the cockroaches. And people who are susceptible to allergic situations can become hyper sensitised to them," says Dr Tait.

There are no available Australian statistics, but in the United States, up to 60 percent of asthma sufferers are affected by cockroach allergens.

If you're one of them, you could get skin rashes, watery eyes, nasal congestion and even asthma attacks.

Therefore, if you don't want cockroaches taking up residence at your house — clean up. Cockroaches in homes are only as dirty as the environment they are living in. If you have a filthy house, they will spread that filth around your kitchen, but if your kitchen is clean and hygienic, you won't be providing them with a food source and they won't bother so much. But if the odd cockroach does show up, at least you know they're not that bad. They're actually quite hygienic.