A rare bacterial infection has been found in five babies in Adelaide's Flinders Medical Centre neonatal unit, with one in a serious but stable condition.

At least five babies in an Adelaide hospital have tested positive to a rare bacteria, with one in a serious but stable condition.

SA Health says two babies in the neonatal unit at the Flinders Medical Centre are symptomatic with Serratia marcescens, with one now receiving specialised care.

Another three have been found to be carrying the bug but are showing no symptoms or signs of infection, with all 40 babies within the unit tested along with those recently discharged.

The infection can cause urinary and respiratory issues, and lead to pneumonia but Serratia is an organism commonly found in the gut and the environment.

"Once we became aware of the second baby displaying symptoms, we immediately launched a thorough investigation into the origins of the infection by testing the environment where the bacteria can grow," medical services executive director Diana Lawrence said.

"We have exceptionally high levels of infection control within the unit and a very low rate of hospital-acquired infections, so given another three babies returned tests showing they are carrying the bacteria, we have taken immediate action."

Of more than 200 environmental swabs taken within the unit so far, one positive result has been returned from a hand basin which has been removed and will be replaced.

Dr Lawrence said while the internal investigation was under way, extra precautions had been implemented within the neonatal unit to protect babies and their families.

"We are taking all measures possible to prevent further cases and to maintain the safety of the babies within the unit," she said.

"We will continue to test and screen babies and our staff will be wearing gloves and yellow gowns when providing care to babies.

"As an extra precaution, we will temporarily minimise new patient admissions, except for urgent cases."

The first positive case of Serratia marcescens was confirmed on May 18 and was immediately isolated and treated with antibiotics, until the baby made a full recovery.

Under advice from infectious diseases experts, the bed area was deep cleaned and no babies were admitted to the space for more than 24 hours.

A second infant, who used the same bed space following the deep clean, returned a positive result on May 30.

This baby has been transferred for specialised care and is in a serious but stable condition.

The three babies who have been confirmed as carriers without symptoms are receiving prophylactic antibiotics.

The last known case of Serratia Marcescens at Flinders Medical Centre Neonatal Unit was about 20 years ago.