The antistreptococcal property of milk. II. The effects of anaerobiosis, reducing agents, thiamine, and other chemicals on lactenin action

499. The inhibition of micro-organisms by raw milk: I. The occurrence of inhibitory and stimulatory phenomena. Methods of estimation

1. A diffusion cup technique using Micrococcus lysodeikticus has been found suitable for lysozyme estimation. By this method raw milk gave only small zones of inhibition. The inhibitory substance of milk was shown not to be lysozyme.

2. Dose-response curves (dilution v. pH) were obtained with Streptococcus pyogenes as the test organism. They indicated that the effect of milk on this organism was the result of the interaction of one stimulatory and at least two inhibitory substances.

3. The existence of two inhibitory substances was confirmed by dilution assays, one, L1, occurring mainly in colostrum, the other, L2, occurring mainly in milk. The inhibition is greatest when L1 and L2 act simultaneously.

4. The pH and heat stabilities of the two substances are slightly different. L1 is most stable at pH 6−6·5, 90% being destroyed at 68° C. L2 is most stable at pH 7 and 95% is destroyed at 74°C.

559. The inhibition of micro-organisms by raw milk: III. Distribution and properties of two inhibitory substances, lactenin 1 and lactenin 2

1. The L1 and L2 contents of a number of samples of bulk milk and milk from separate quarters of cows with and without infection have been determined. The milk of cows sensitive to mastitis has about the same content as the milk from resistant ones.

2. The effect of heat on L1 and L2 has been observed for various combinations of time and temperature. Heat treatment corresponding to the legal standards of pasteurization destroyed 50% L1, but did not affect L2.

3. The inhibitory action of L1 and L2 has been observed on several micro-organisms. All the organisms tested were sensitive to lactenin in sterilized milk. Stimulatory substances have been shown to interfere with the inhibition by lactenin.

4. The action of lactenin on the growth of Str. pyogenes has been studied. The effect of hydrogen peroxide on lactenin has been compared with that of cysteine.

The antistreptococcal property of milk. III. The role of lactenin in milk-borne epidemics; the in vivo action of lactenin

The ability of lactenin to prevent the multiplication of group A streptococci when milk becomes contaminated with that organism accounts in part at least, for the infrequency of milk-borne streptococcal epidemics. From epidemiological studies it has been shown that most such epidemics arise from the consumption of raw milk in which streptococci occur as a result of bovine mastitis due to group A streptococcus. Lactenin fails to prevent the establishment of mastitis due to the group A streptococcus because the milk in the cow's udder is at a low oxidation-reduction potential and the lactenin is inactive. Lactenin, being destroyed by temperatures of 80°C. or above, is absent from canned and powdered milk. When the latter have been diluted or reconstituted, they can serve as excellent growth media for group A streptococci, and epidemics have occurred as a result of contamination of milk supplies of those types. The administration of lactenin by mouth or intraperitoneal injection failed to protect mice from peritonitis or subcutaneous infection due to group A streptococcus.

Cystine antagonism of the antibacterial action of lactoperoxidase-thiocyanate-hydrogen peroxide on Streptococcus agalactiae

Cystine reduction in Streptococcus agalactiae, resulting in sulfhydryl formation, may account for antagonism of the antibacterial effect of lactoperoxidase-thiocyanate-hydrogen peroxide when cystine is present in excess of the amount needed for maximum growth. Accumulation of cystine by S. agalactiae and its reduction to form sulfhydryl compounds were demonstrated. The reduction of cystine appeared to occur by a couple reaction between glutathione reductase and glutathione-disulfide transhydrogenase activity, both of which were found in the supernatant fraction from cell homogenates. NADPH-specific glutathione reductase activity was found in the pellet and supernatant fractions from cell homogenates. Two sulfhydryls were formed for each mole of NADPH used during cystine reduction. The information presented offers a plausible explanation of how cystine, when present in excess of growth needs, may be reduced to generate sulfhydryl compounds which neutralize the antibacterial effect of lactoperoxidase-thiocyanate-hydrogen peroxide on S. agalactiae.

Effects of nutritional characteristics of Streptococcus agalactiae on inhibition of growth by lactoperoxidase-thiocyanate-hydrogen peroxide in chemically defined culture medium

Five cultures of Streptococcus agalactiae have an absolute requirement for L-cystine to grow in a chemically defined medium. The L-cystine could be replaced with cysteine, glutathione, or the disulfide form of glutathione. Dithiothreitol could not substitute for the sulfur-containing amino acids of glutathione; hence, the growth requirement appears to be truly nutritional. Growth was maximum with 4 to 5 mug of L-cystine per ml. If the concentration of L-cystine was no greater than 4 to 5 mug/ml, complete growth inhibition could be obtained by the addition of lactoperoxidase, thiocyanate, and H2O2. The growth inhibition, however, was nullified by additions of L-cystine 10-fold or more in excess of the concentration needed for maximum growth. During the aerobic degradation of glucose by cell suspensions, H2O2 accumulation could be shown with cultures 317 and 11-13, the only cultures the growth of which was inhibited without addition of exogenous H2O2. All of the cultures had varying degrees of peroxidase activity. The balance between H2O2 generation and peroxidase activity of the culture evidently determined whether growth could be inhibited with lactoperoxidase and thiocyanate without H2O2 addition. The growth yeilds per 0.5 mol of the disulfide forms (cystine and oxidized glutathione) were 1.5 and 1.9 times greater than that per 1 mol of the sulfhydryl forms (cysteine and glutathione).

Nonspecific bactericidal activity of the lactoperoxidases-thiocyanate-hydrogen peroxide system of milk against Escherichia coli and some gram-negative pathogens

Two strains of Escherichia coli and one strain each of Salmonella typhimurium and Pseudomonas aeruginosa were killed by the bactericidal activity of the lactoperoxidase-thiocyanate-hydrogen peroxide system in milk and in a synthetic medium. H2O2 was supplied exogenously by glucose oxidase, and glucose was produced at a level which was itself noninhibitory. Two phases were distinguished: the first phase was dependent on the oxidation of SCN(-) by lactoperoxidase and H2O2, which was reversed by reducing agent, and the second phase was dependent on the presence of accumulated H2O2, which was reversed by catalase. The latter enzyme could also reverse the first phase, but only when present in excessive and unphysiological levels. The bactericidal activity was greatest at pH 5 and below, and it depended on the SCN(-)concentration and on the number of organisms. Since raw or heated milk neutralizes the acid barrier against infection in the stomach, the bactericidal system discussed may contribute to the prevention of enteric infections in neonates.

The inhibition of streptococci by lactoperoxidase, thiocyanate and hydrogen peroxide. The effect of the inhibitory system on susceptible and resistant strains of group N streptococci

1. The growth of the lactoperoxidase-sensitive Streptococcus cremoris 972 in a synthetic medium was inhibited by lactoperoxidase and thiocyanate. The glycolysis and oxygen uptake of suspensions of Strep. cremoris 972 in glucose or lactose were also inhibited. The lactoperoxidase-resistant Strep. cremoris 803 was not inhibited under these conditions but was inhibited in the absence of a source of energy. 2. Lactoperoxidase (EC 1.11.1.7), thiocyanate and hydrogen peroxide completely inhibited the hexokinases of non-metabolizing suspensions of both strains. The inhibition was reversible, hexokinase and glycolytic activities of Strep. cremoris 972 being restored by washing the cells free from inhibitor. The aldolase and 6-phosphogluconate-dehydrogenase activities of Strep. cremoris 972 were partially inhibited but several other enzymes were unaffected. 3. The resistance of Strep. cremoris 803 to inhibition was not due to the lack of hydrogen peroxide formation, to the destruction of peroxide, to the inactivation of lactoperoxidase or to the operation of alternative pathways of carbohydrate metabolism. 4. A ;reversal factor', which was partially purified from extracts of Strep. cremoris 803, reversed the inhibition of glycolysis of Strep. cremoris 972. The ;reversal factor' also catalysed the oxidation of NADH(2) in the presence of an intermediate oxidation product of thiocyanate and was therefore termed the NADH(2)-oxidizing enzyme. 5. The NADH(2)-oxidizing enzyme was present in lactoperoxidase-resistant streptococci but was absent from lactoperoxidase-sensitive streptococci.