362. Studies in the bacteriology of milk: I. The streptococci of milk

Abstract

The bacteria that occur in milk have frequently been studied in the mass by making counts of the organisms or by measuring their total chemical activity. Comparatively little attention has been given, however, to the composition of the bacterial populations in different types of milk. It is generally accepted that streptococci are frequently the dominant organisms, and much information is available concerning the coliform group; but for other bacteria and the individual species of streptococci there are few records of the frequency with which they occur. The question whether market milk possesses a special and characteristic microflora, or one that is too variable and heterogeneous to define, cannot be answered by the information at present available. The solution of practical problems in the dairy industry is often impeded by this lack of knowledge; too frequently the only approach must be an empirical one.The object of the present investigation was to provide a clearer picture of the microbic populations in raw and pasteurized milks of varying purity. The general plan of the work was to make quantitative platings and to investigate the organisms that formed colonies. The pure cultures were studied for the purposes of identifying them and of discovering their properties, especially those properties that appeared to be of significance in relation to dairy products. Problems arising during the course of the diagnostic work were investigated with the object of improving methods of recognizing the more frequently occurring organisms. This, the first paper of a series, describes work on the streptococci of milk. Subsequent papers will be concerned with the other major groups of bacteria and with the quantitative relationships of the organisms in different classes of milk. METHOD OF EXAMINING MILKIn selecting milk for examination the intention was to secure specimens varying widely in bacteriological condition. Thus, three main types of raw milk were used: (1) samples obtained by aseptic milking, (2) milk produced under good or fairly good conditions and bottled on the farm, and (3) bulked milk in which heavily contaminated supplies were invariably represented.Samples were examined in the raw state and also after pasteurization in the laboratory at 63° C. for 30 min. They were plated in duplicate on meat infusion agar containing 1 % tryptone peptone and 0-25 % glucose. Each plate inoculated with less than 1 ml. of the sample received an addition of 0-5 ml. of sterile skim milk. Plates were incubated for 5-6 days at 30° C. This combination of medium and incubation was expected to permit the growth of most types of the bacteria occurring in the samples. If this expectation was in fact realized, only the predominant organisms would be obtained from samples giving

Heat Resistance of Some Selected Toxigenic Enterococci in Milk and Other Suspending Media

“…liquefuciens NP was the least heat resistant as indicated by the lowest D-values (3.5 min each) at 70°C in the menstrua. These findings corroborate the earlier reports by other workers on the heat resistance characteristics among enterococci in milk and milk products (Abd-el-Malik and Gibson, 1948;Iyengar et al, 1957;White, 1963;Batish et al, 1985b). Shannon et al (1970) found that S. &runs was the most heat resistant among enterococci since the above organism survived heat treatment at 71.7"C for 1.5 sec.…”

Identification and first characterization of lactic acid bacteria isolated from the artisanal ovine cheese Pecorino del Poro

“…Figure 2 shows the behaviour of a typical Lactobacillus (L356) and a typical Enterococcus (C310) strain. The observed inhibitory activity against E. coli is interesting, because it could be possible that these strains inhibit not only E. coli but also different food‐borne pathogens, as do other bacteriocin‐producing lactobacilli 11–26 …”

“…Working cultures were grown in appropriate (MRS or M17) broth media. All isolates were tested for their Gram reaction, catalase using 3 g H 2 O 2 /L, shape by observation of overnight cultures using a phase contrast microscope Standard 20 (Carl Zeiss, Göttingen, Germany), and for heterofermentation or homofermentation of sugars 18 . Growth of the coccal‐shaped bacteria was examined in M17 broth after incubation at 10°C for 7 days, at 45°C for 2 days, and in M17 broth containing 20, 40 and 65 g NaCl/L after incubation at 30°C for 4 days.…”

Some remarks on the classification of micrococci

“…T h e differentiation of t h e s e o r g a n i s m s into "groups" and "subgroups" w a s first u s e d by Abd-El-Malek and Gibson (1948), and l a t e r by Hill (1959), P o h j a (1960), P i k e (1962), and B a i r d -P a r k e r (1963). This way of classifying h a s at l e a s t two advantages: 1) methodically, a new and rapid way of differentiating a g r e a t amount of m a t e r i a l into s e p a r a t e g r o u p s i s introduced, 2) not being burdened by t h e s o -c a l l e d s p e c i e s concept, t h i s classification a s c e r t a i n s which n a t u r a l g r o u p s (biotypes) r e a l l y e x i s t within t h e genus Micrococcus.…”