Both freshwater and saltwater frozen fish are now being tested for Group B Streptococcus (GBS) bacteria, with experts saying the bacteria can survive freezing.

The Agri-Food and Veterinary Authority of Singapore (AVA) started testing last month.

A spokesman said in response to queries: "Following the ban on the use of freshwater fish in ready-to-eat raw fish dishes on 5 Dec, 2015, AVA stepped up the testing on frozen saltwater fish, with a focus on saltwater fish intended for raw consumption."

He elaborated: "We have broadened our tests for established food safety parameters to include the testing for GBS and such tests are performed on both frozen saltwater and freshwater fish."

The tests, so far, have been for various other disease-causing bacteria, parasites and heavy metals.

The spokesman said: "To date, we have not detected Type III GBS (ST283) in our tests on frozen saltwater fish."

Last year, about 150 people became sick after they ate freshwater fish prepared to be eaten raw, said the Ministry of Health (MOH).

It took the total to 355 cases of GBS infections for the year, compared with 150 or so annually from 2011 to 2014.

Last month, MOH confirmed the link between eating such dishes and what appears to be an aggressive strain of GBS bacteria.

Infectious diseases experts say that GBS can survive in frozen fish and revive - and even proliferate - when thawed.

Infectious diseases physician Leong Hoe Nam said: "Freezing doesn't kill the bacteria. It just prevents proliferation."

The bacteria will continue to multiply when thawed.

Dr Leong said: " Fish kept on the table will allow GBS to proliferate. And, with time, a sufficiently large quantity can be found on the fish (and) overcome the body's immune system."

Freezing for a certain duration will kill most of the types of parasites that are present in certain fish, hence the practice for sushi-grade fish. However, bacteria, such as GBS, can survive indefinitely while frozen, and reactivate and multiply again when the fish or meat is thawed.

Infectious diseases specialist Hsu Li Yang said that cooking is the only guaranteed solution to GBS.

Dr Leong suggested flash freezing, in which the whole piece of meat is frozen very quickly, as opposed to slow kitchen freezing, which allows the bacteria to grow inside the meat even as it cools down from the outside.