No abstract

1. The limitations have been explored of a simplified form of Grimes's litmus milk method, as used for a presumptive test for coliform organisms in butter.2. To improve on this test, the merits and demerits of renneted litmus milk and litmus milk agar have been investigated, and the latter found preferable.3. Of 719 samples of export salted butter tested for coliform contamination, only 19·1% gave confirmed positive results using MacConkey broth as the presumptive medium, 21·9% were positive using a litmus milk agar medium, and 30·2% (on 434 samples only) were positive using renneted milk.4. The coliform organisms isolated were classified into the usual types and a seasonal variation in the coli to aerogenes ratio noted.5. An observation is made on what appears to be the inhibitory effect of bile salt on the gas production of several strains of the aerogenes-cloacae group isolated in this investigation.

The roll-tube method for estimating bacterial counts was devised by Esmarch(1), but was soon afterwards superseded by the introduction of Petri plates which have several advantages for cultural and counting purposes. The roll-tube method, however, has been revived by several workers (2,3,4), and where a simple, rapid and economical method is required it is very useful, especially if low counts are being dealt with. For this reason it is particularly suitable for yeast and mould counts in butter and has been so used in this laboratory for many years. In addition to its regular use for this purpose, several series of comparisons have shown that the roll-tube method is capable of giving results which compare very well with Petri plate counts upon milk, using the usual dilution technique in both cases.

1. Using Hammer's litmus milk enrichment technique, a total of ninety-one cultures of Ps. putrefaciens were isolated from 1563 samples of normal salted butter graded for export.2. A correlation was demonstrated between poor hygienic conditions in butter factories and the incidence of Ps. putrefaciens contamination of the butter.