Determination of Untreated Whole-Milk Effects on In Vitro Antibacterial Activity

Abstract

The effect of fresh whole milk without pasteurization or other pretreatment on in vitro antibacterial activity of selected compounds was determined in broth dilution. The milk was collected by hand directly from dairy goats, or by syringe or cannula from bovine quarters showing low bacterial counts. Antibacterial activity was determined in 50% (v/v) milk-broth medium against sensitive mastitis-etiologic strains of Streptococcus agalactiae and Staphylococcus aureus. The indicator saJt 2,3, 5-triphenyltetrazolium chloride was incorporated in the milk broth medium to determine inoculum growth. Contaminant interference was circumvented through early as well as late readings and comparisons with uninoculated control tubes, with and without the test compounds. Application of the method with more than 75 compounds, including nitrofurans, antibiotics, and other chemicals uncovered marked degrees of milk interference. The method warrants routine use among preliminary screens to relate in vitro with in vivo observations of antimicrobial activity. Similar procedures may be used with serum, skim milk, or mastitis-milk media for separating effects due to protein, lipid, or other elements in product evaluation. Few studies have been conducted to demonstrate the effect of milk on the antibacterial

The in vitro Effects of pH and Type of milk media on the Growth of Mastitis Organisms as related to Antibiotic Sensitivity Tests

“…Most data were obtained from studies conducted under accepted standard bacteriological procedures. It was, however, claimed (5,7,14,19,20) that the standard test conditions do not resemble the environment which characterizes the acutely or chronically inflamed udder and that the in vitro test results were not wing a true measure of the sensitivity of the mastitis results from different laboratories which tried in various ways to adopt changes in the standard in vitro test procedures.To determine whether the medium used in sensitivity tests has any influence on the minimum bactericidal concentrations (MBC) of antibiotis for mastitis organisms, several workers conducted comparative tests employing whole milk, skim milk, mastitic milk and nutrient broth ( 5 , 7, 11, 14, 19).Their results could be summarized by stating that the MBC for almost all antibiotics studied, using several types of udder pathogens, were considerably higher in skim milk than in broth, much higher in whole milk and highest in mastitic milk. Partial loss of antibiotic activity in milk due to protein binding, mainly to the serum albumin fraction, is probably the principal cause for the reported higher MBC as it is well known that protein-bound antibiotics have little or no bactericidal activity.…”

“…Most data were obtained from studies conducted under accepted standard bacteriological procedures. It was, however, claimed (5,7,14,19,20) that the standard test conditions do not resemble the environment which characterizes the acutely or chronically inflamed udder and that the in vitro test results were not wing a true measure of the sensitivity of the mastitis results from different laboratories which tried in various ways to adopt changes in the standard in vitro test procedures.…”