The development of chronic and recurrent Staphylococcus aureus infections is associated with the emergence of slow-growing mutants known as small-colony variants (SCVs), which are highly tolerant of antibiotics and can survive inside host cells. However, the host and bacterial factors which underpin SCV emergence during infection are poorly understood. Here, we demonstrate that exposure of S. aureus to sublethal concentrations of H2O2 leads to a specific, dose-dependent increase in the population frequency of gentamicin-resistant SCVs. Time course analyses revealed that H2O2 exposure caused bacteriostasis in wild-type cells during which time SCVs appeared spontaneously within the S. aureus population. This occurred via a mutagenic DNA repair pathway that included DNA double-strand break repair proteins RexAB, recombinase A, and polymerase V. In addition to triggering SCV emergence by increasing the mutation rate, H2O2 also selected for the SCV phenotype, leading to increased phenotypic stability and further enhancing the size of the SCV subpopulation by reducing the rate of SCV reversion to the wild type. Subsequent analyses revealed that SCVs were significantly more resistant to the toxic effects of H2O2 than wild-type bacteria. With the exception of heme auxotrophs, gentamicin-resistant SCVs displayed greater catalase activity than wild-type bacteria, which contributed to their resistance to H2O2. Taken together, these data reveal a mechanism by which S. aureus adapts to oxidative stress via the production of a subpopulation of H2O2-resistant SCVs with enhanced catalase production.

Seven rapid analytical tests (color value, thiobarbituric acid number, extract release volume, pH, “tyrosine” value, pHt, and redox potential) were evaluated as possible indicators of bacterial contamination in intact meat. Color value is a reflectance value related to hedonic acceptance of the meat. Comparisons of results from these seven tests with determination of bacterial load (plate count) and with time of storage were analyzed statistically to determine the relative contributions of bacterial action and of intrinsic reactions. The color values and “tyrosine” values were the most effect monitors of bacterial contamination. Although the thiobarbituric acid test effectively monitored changes in meat quality, it was not specific for those produced by bacteria. The remaining tests were ineffective under conditions employed.

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INTRODUCTION THE COLOR of fresh meat is determined mainly by the relative proportions of three meat pigments: purple reduced myoglobin (Mb), red oxymyoglobin (MbOz) and brown metmyoglobin (MetMb). Under normal marketing conditions reduced myoglobin in the presence of O2 is completely changed to Mb02 within a few hours (Van den Oord and Wesdorp, 1971a). The bright red of the MbOz is the preferred color of fresh beef packaged for retail @es. The change of Mb& to MetMb can be effected by bacterial growth or oxygen tension leyels, resulting in slow oxidation of the heme iron to its ferric state. As this change occurs the meat becomes less acceptable to consumers. The relative proportions of Mb& and MetMb are a prime factor in the acceptability of-fresh beef color to consumers. At approximately 50% conversion to MetMb the meat is unacceptable to most consumers and therefore unsuitable for retail sale (Van den Oord and Wesdorp, 1971a).The preferred method for measuring consumer acceptability is color evaluation by a panel of trained observers This method has several serious disadvantages for continuous evaluation of meat color changes. Panel measurements are time consuming, prone to subjective errors and limited in the number of evaluations which can be made at one time.Reflectance measurement is the instrumental technique of choice because it measures the color on the surface of the meat as observed by the consumer and it is nondestructive. Two types of instruments can be used in reflectance studies: calorimeters and spectrophotometers with reflectance attachments. Previous investigators have employed a variety of techniques for meat studies. Haas and Bratzler (1965) used Munsell disk calorimetry and a Gardner color difference meter to follow oxygenation rates in beef, pork and lamb. Snyder (1965) used the Gardner Rd, a, b, and a/b values to indicate changes taking place in intact beef samples. The "Rd" value is a measure of the total light reflected while "+a" is a measure of redness, " -a" is a measure of greenness, "+b" is a measure of yellowness and "-b" is a measure of blueness in the reflected light. He found that a high a/b value indicated a high concentration on the surface of the meat of either Mb or MbOz, while a low a/b value indicated a high concentration of MetMb. Romans et al. (1965) attempted correlation of Munsell disk calorimetry values with myoglobin concentration but found low correlation coefficients (r = -0.5). Jeremiah et al. (1972) related color difference values to consumer acceptability of beef color. Their study was designed to measure the preferred intensity of color from MbOz. They correlated the values from the Macbeth-Munsell disk calorimeter, the Gardner color difference meter, and the Photovoltreflectance meter. Dean and Ball (1960) used the Gardner aL value as a measure of redness or bloom in beef.Other reflectance methods which could be used for estimating consumer acceptability of beef color measure the concentration of the myoglobin pigments with a reflectance attachment to a s...

Blends were developed to provide 20% protein, 12% fat, 68% carbohydrate and 8% moisture. High protein soy products (full fat flakes, protein isolate and/or concentrate) were formulated with corn meal and soybean oil to provide high protein and fat. The blends were extruded to provide pre-cooked foods that could be reconstituted at 40ЊC to a porridge or gruel, eliminating prolonged cooking or degradation of heat labile nutrients. Two types of soy isolate and concentrate were evaluated under extrusion temperatures from 100 to 130ЊC and feed moistures 8.5 to 18%. The extrusion of lower valued concentrates at 100 to 115ЊC with moisture from 12 to 18% produced a precooked mix that was high in nutrients and contained the most available lysine.

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