746 A MANUAL OF BACTERIOLOGY 



pipette (which is afterwards used for filling the counting chamber). 

 After fifteen minutes, water is added to the 1-c.c. mark, and 

 counting done in the usual way with a Thoma-Zeiss blood counter. 

 Counting should not be restricted to the ruled spaces, but the 

 field should be so arranged that a definite number of squares is 

 included, and fields are counted all over the chamber. At least 

 two different preparations should be made of the same deposit for 

 counting. 



Food Poisoning l 



Apart from being naturally poisonous or from the 

 presence of poisonous substances, food which is normally 

 wholesome may become poisonous as a result of the 

 activity of micro-organisms. This action may be exerted in 

 two ways : (1) the foodstuff may be attacked by bacterial 

 or fungoid organisms and poisonous products formed, 

 resulting in an intoxication, or (2) the foodstuff may be 

 infected with living micro-organisms which give rise to 

 an infection. 



1. In the first class of food-poisoning the well-known 

 "ptomaine poisoning" must be placed. By bacterial 

 action on proteins, nitrogenous compounds or ptomaines, 

 chemically allied to the vegetable alkaloids, may be 

 formed, and many of them being intensely poisonous, 

 the consumption of food so attacked may give rise to 

 poisoning. It is supposed to occur particularly in connec- 

 tion with tinned foods. In tinning and canning the tins 

 containing the food are heated in steam to sterilise them 

 and the tins are closed while hot. In some cases the 

 sterilisation is imperfect, certain bacterial spores retain 

 their vitality and subsequently grow and multiply with 

 the formation of toxic products, and the poisoning 

 becomes manifest within an hour or so of consumption as 

 a severe gastro -enteritis. Tinned foods are rarely quite 



1 See Savage, Food Poisoning and Food Infections, 1920, and Hewlett 

 and Nankivell, Principles of Preventive Medicine, 1921. 



