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foundations of our knowledge concerning the bacterial flora of the air must be thus laid, and they are
only beginning to be laid. Had pathogenic bacteria been present in any numbers in the 240 litres of
air which altogether was submitted to analysis, they would almost certainly have been detected.
But from what is known as to the distribution of pathogenic organisms in other media, water
for example, it may safely be inferred that their distribution in air must be extremely sparse. So long
as our knowledge of the distribution of bacillus coli communis in water was confined to the examination
of plate cultivations made from a single cubic centimetre of the water sample, we remained in
comparative ignorance. But improved methods, directed to the recognition of this particular organism,
have shown that it is far more widely distributed than was at first imagined. We now take 1000 cc.
of the water, concentrate the bacteria in it into small compass, and make cultures in a medium which
favours the growth of this special bacillus while hindering the growth of other organisms. Methods
of this nature will have to be evolved and applied to the examination of air if we are to learn anything
of the distribution of pathogenic organisms in it. And special methods will have to be evolved
for the recognition of each particular pathogenic organism. The method which I employed in the
foregoing investigation was, I believe, excellently adapted to the study of the general bacterial flora of
the air. But it was probably very ill adapted to the recognition of given pathogenic micro-organisms
on account of the small volumes of air to which I was necessarily confined. It will be needful in the
future, when once the general nature of the bacteria present in the air is well established, to devise
special methods for the recognition of the more important species pathogenic to man, and to employ
these methods upon very large volumes of air. The bacteriological examination of air is, in fact, in its
infancy, but those principles which have advanced our knowledge as to the bacteriology of water may
be expected to prove equally fruitful in this field also. I have felt compelled to pass this criticism
upon my own results, lest any undue stress should be laid upon the absence of important pathogenic
bacteria from the list of species which I succeeded in isolating from the air of London generally, and
that of the Central London Railway in particular. Had I not done so others would doubtless have
done it for me.
The general results of the investigation may be summarised in the following propositions—
(1.) Micro-organisms were found present in the air of the Central London Railway in a
somewhat greater proportion than in the fresh air outside. Taking all the results together,
the exact proportion was as 13 to 10.
(2.) The number of micro-organisms found was high in proportion to the concentration
of human traffic ; highest in the railway carriages, platforms and lifts. Elsewhere I found
it low.
(3.) The air of the railway does not, in it bacterial content, compare unfavourably with
that known to exist in inhabited rooms generally.
(4.) No pathogenic germs, other than those commonly present as saprophytes upon the
normal body, were detected in such small volumes of the air as could be submitted to
analysis under the conditions of the investigation.
(5.) The number of organisms capable of growing at the temperature of the human
body was considerably greater in the air of the Central London Railway than in the fresh
air, but the excess was due to non-pathogenic sarcinae and allied species.
(6.) The number of micro-organisms in the air of the railway was found generally
proportional to the degree of chemical contamination, but this rule was subject to striking
exceptions.
(7.) The species of micro-organisms found were in the main identical with those
occurring in the fresh air.
Frederick W. Andrewes, M.D., F.R.C.P., D.P.H.
St. Bartholomew's Hospital,
July 31st, 1902.