The Health Protection Agency was fined £25,000 today after up to a billion doses of the deadly bug E.coli were spilt in an accident at its laboratory.
Prosecutors said the incident exposed a "general complacency" about the transfer of infectious waste at the HPA's centre in Colindale, north London.
No-one was infected as a result of the spillage in October 2007 although three employees had been put at risk of contamination, the Old Bailey heard.
The court was told that faulty "bins" used to carry the bug and other infectious waste - including samples of the plague - to be safely disposed of remained in use even though defects had been spotted 17 months earlier.
At the time of the accident, employees taking E.coli to a disposal unit were not wearing protective clothing, said Andrew Marshall, prosecuting.
An initial assessment of the spillage by staff at the centre had shown a "complete lack of understanding of risk", Mr Marshall added.
Judge Martin Stephens said the failings were an "acute embarrassment" for the HPA, an independent body set up by the Government in 2003 to protect the public from threats from infectious diseases and environmental hazards.
In addition to the fine he ordered the agency to pay costs of £20,166.
The court heard that an HPA employee called Neil Perry - who had not been properly trained in the transport of the waste - was handling the bin when one of his hands slipped, it swung down to the floor, and the lid came open.
Glass tubes holding semi-solid samples of the bacteria fell to the ground, with some of the lids breaking and others coming off in the fall, said Mr Marshall.
The spillage of 100-200ml contained approximately 10 billion organisms of the O157 strain E.coli, representing between 100 million and one billion doses of the bug - described in court as a "highly infectious biological agent".
Mr Marshall said there was a "real risk" that "contamination or infection" could have occurred.
He added: "The investigation by the Health and Safety Executive revealed a general complacency around the transfer of infectious waste within the centre.
"There were a number of ongoing failings creating a risk of contamination to employees by infectious biological agents."
The Health Protection Agency pleaded guilty at an earlier hearing to a breach of health and safety legislation.
HSE inspector Jennifer Higham said: "E.coli O157 is a highly infectious and potentially deadly bacterium and there are well established practices for handling this safely.
"But in this case, these practices were not met, exposing several staff and potentially their families to a real risk of infection."
She said the HPA should have developed a "safe system" for dealing with the bug and disposing of it and other such hazardous waste.
"Equipment should have been well maintained and employees should have had appropriate training," she added.
"This was a serious breach of HPA's duty to ensure the health, safety, and welfare at work of all its employees."
A spokesman for the agency said: "The HPA regrets the circumstances that led to today's conviction and accepts the fine imposed by the court.
"We are keen to stress that none of the staff present at the time of this incident suffered any ill health and there was categorically no risk to the public from the spillage.
"Nevertheless we accepted the seriousness of this incident, referred it immediately to the HSE and co-operated fully with their investigation.
"Since the incident, which occurred more than two and a half years ago, the HPA has conducted a thorough review of its health and safety management and has taken the appropriate steps to reduce the likelihood of a similar incident occurring in the future."