those used for work with common pathogens* corresponding to the conditions 
of our level I. The four levels are described in detail in the code of practice 
(Appendix II) but may be briefly summarised as follows: 
I. conditions conforming to the standard necessary for the maintenance 
of good microbiological practice, as required for work with common 
pathogens m a medical microbiology laboratory; 
II. conditions as at I above in a laboratory which is additionally sited 
away from areas used by the general public and has controlled air 
flow and an exhaust-protective cabinet for aerosol-producing 
operations; 
III. conditions as at II above but with access to the laboratory only 
through an airlock containing washing facilities and with an autoclave 
and provision for decontamination of all effluent from the laboratory; 
IV. conditions provided by a laboratory equivalent to the category A 
toxic laboratory referred to in Appendix III to the Report of the 
Working Party on the Laboratory Use of Dangerous Pathogens, ie 
those at III above with, additionally, provision for a full change of 
clothing and showering on leaving the laboratory and a double-ended 
autoclave. 
2.9 Categorisation 
There is a continuous spectrum of hazard but any categorisation must be done 
in steps. We assume that there are conditions of biological containment and 
nucleic acid purity that will allow an experiment to be moved from one category 
to another but these cannot be absolutely defined without reference to the 
individual experiment. We therefore give some examples to illustrate the 
principles we have adopted in the following table. It will be seen that, experi- 
ments involving recombination between the nucleic acids of non-pathogenic 
bacteria are considered to require only category I containment. Depending 
on the degree of biological containment achieved, and on the nature of the 
nucleic acid sequence inserted, certain other classes of experiment can be 
acceptably undertaken with Category I or II containment and some examples 
of such experiments are given in the following table. We stress that the table 
gives illustrative examples of some typical experiments and is intended to serve 
only as a guide and not as a definition of the whole area of possible work in 
each category. It will of course be for the GMAG to advise on specific cases 
and to establish a more complete categorisation as part of the “case law” as 
the work proceeds. 
• For example, streptococci, staphylococci, salmonellae and other pathogens commonly 
present in the community and not especially liable to cause laboratory-acquired infection. 
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