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Hazard Group 3 bacteria in the NCTC collection

In the UK, biological agents are classified into four Hazard Groups (HG1-4) based on their infectious nature, likelihood of spread through the community and availability/requirement of effective prophylaxis or treatment. The list of agents is regularly reviewed and updated by the Advisory Committee on Dangerous Pathogens (ADCP) and the resulting publication The Approved List of biological agents is issued by the Health and Safety Executive (HSE). Similar categorisations of biological agents are also seen in other parts of the world.


Hazard group definitions – The Approved List of biological agents (2013) 

Hazard group definitions

When classifying a biological agent it should be assigned to one of the following groups according to its level of risk of infection to humans.

Group 1

Unlikely to cause human disease.

Group 2

Can cause human disease and may be a hazard to employees; it is unlikely to spread to the community and there is usually effective prophylaxis or treatment available.

Group 3

Can cause severe human disease and may be a serious hazard to employees; it may spread to the community, but there is usually effective prophylaxis or treatment available.

Group 4

Causes severe human disease and is a serious hazard to employees; it is likely to spread to the community and there is usually no effective prophylaxis or treatment available.


Organisms must be handled under appropriate health and safety conditions in the laboratory following risk assessment. In the UK this refers to Containment Levels 1-4 (CL1-4) as defined by the HSE; Biosafety Levels 1-4 are the equivalent in the United States. In other parts of the world national and local policies for categorisation and containment of biological agents should be followed.

The majority of NCTC bacteria fall within the HG2 category and can be handled in a CL2 laboratory on the open bench. The collection also holds HG3 organisms which can cause severe human disease and can spread through the community easily. These organisms routinely require CL3 facilities for handling and may also be subject to transport restrictions.

There are more than 500 strains of HG3 organisms in the NCTC collection including organisms such as Burkholderia and Brucella species, Salmonella enterica subsp. enterica, serotype Typhi, Francisella tularensis, Yersinia pestis and several species of Mycobacterium.


Example: Brucella species

There are 58 isolates of Brucella species within the NCTC collection including the following type strains:

Brucella arbortus      NCTC 10093

Brucella canis          NCTC 10854

Brucella melitensis   NCTC 10094

Brucella neotomae   NCTC 10084

Brucella ovis            NCTC 10512

Brucella suis            NCTC 10316

The majority of isolates are from mammals (such as cows, pigs and rats) and there are several clinical isolates from known cases of human infection. In addition one strain of Brucella abortis, NCTC 7471 was isolated from milk in the UK.

Brucella infection (brucellosis) causes flu like symptoms in humans including an undulant (cyclical) fever in the majority of patients. The infectious dose is low; most clinical cases are due to direct contact with animals or eating unpasteurised milk and milk products.

Research to improve laboratory diagnosis of brucellosis is ongoing. Matrix-assisted laser desorption-ionization time-of-flight mass spectrometry (MALDI-TOF MS) methods are increasingly being used for bacterial identification in clinical laboratories. Compared to most Hazard Group 2 organisms, databases for Hazard Group 3 organisms tend to be less comprehensive and therefore development of high quality databases are required in order to improve clinical diagnosis.


The NCTC collection is dynamic and several new HG3 organisms are in the process of being added to the collection:

Five serotypes of Shiga toxin-producing Escherichia coli

NCTC no.

E. coli serotype


Site of  Isolation




ST1 positive

ST2 negative

eae positive

Human Faeces



ST1 negative

ST2 positive

eae negative




ST1 negative

ST2 positive

eae positive




ST1 positive

ST2 positive

eae negative




ST1 positive

ST2 positive

Human Faeces


Mycobacterium tuberculosis complex

NCTC will be offering more Mycobacterium tuberculosis complex strains in the coming months. The strains form part of a group of 40 Mycobacterium strains (including both HG2 and HG3 strains) from the European Tuberculosis Reference Laboratory Network (ERLTB-Net).

To keep up to date with new additions to the collection visit the new bacterial strains page.

January 2020