ICMSF Methods Studies. XV. Comparison of Four Media and Methods for Enumerating Staphylococcus aureus in Powdered Milk

A probiotic strain of L. acidophilus reduces DMH-induced large intestinal tumors in male Sprague-Dawley rats

Probiotic bacteria strains were examined for their influence on 1,2-dimethylhydrazine (DMH)-induced intestinal tumors in 100 male Sprague-Dawley rats. Lactobacillus acidophilus (Delvo Pro LA-1), Lactobacillus rhamnosus (GG), Bifidobacterium animalis (CSCC1941), and Streptococcus thermophilus (DD145) strains were examined for their influence when added as freeze-dried bacteria to an experimental diet based on a high-fat semipurified (AIN-93) rodent diet. Four bacterial treatments were compared: L. acidophilus, L. acidophilus + B. animalis, L. rhamnosus, and S. thermophilus, the bacteria being added daily at 1% freeze-dried weight (10(10) colony-forming units/g) to the diet. Trends were observed in the incidence of rats with large intestinal tumors for three treatments: 25% lower than control for L. acidophilus, 20% lower for L. acidophilus + B. animalis and L. rhamnosus treatments, and 10% lower for S. thermophilus. Large intestinal tumor burden was significantly lower for treated rats with L. acidophilus than for the control group (10 and 3 tumors/treatment group, respectively, p = 0.05). Large intestinal tumor mass index was also lower for the L. acidophilus treatment than for control (1.70 and 0.10, respectively, p < 0.05). Other treatments showed no statistically significant change from control for these indexes of tumorigenesis. For rats fed L. acidophilus, no adenocarcinomas were present in the colons. Pulsed-field gel electrophoresis of bacterial chromosomal DNA fragments was used to differentiate introduced (exogenous) bacterial strains from indigenous bacteria of the same genera present in the feces. Survival during gut passage and displacement of indigenous lactobacilli occurred with introduced L. acidophilus and L. rhamnosus GG during the probiotic treatment period. However, introduced strains of B. animalis and S. thermophilus were not able to be isolated from feces. It is concluded that this strain of L. acidophilus supplied as freeze-dried bacteria in the diet was protective, as seen by a small but significant inhibition of tumors within the rat colon.

Media used in the detection and enumeration of Staphylococcus aureus

Over the years a range of selective and diagnostic media have been developed to assist in the detection and enumeration of Staphylococcus aureus in routine food surveillance programmes and food poisoning investigations and these are reviewed here. Baird-Parker agar remains, however, the medium of choice for direct plating and enumeration of S. aureus in both Europe and the US. This paper also reports on a comparison of the productivity and selectivity of trypticase soy broth with 10% NaCl and 1% sodium pyruvate (PTSBS) with trypticase soy broth (TSB) for the isolation of S. aureus. Using three strains of S. aureus and a strain of S. hyicus the productivity ratio of PTSBS to TSBS ranged from -0.17 to 0.57. In the recovery of heat-injured cells of S. aureus, PTSBS offered little or no improvement over TSB with sodium chloride but no pyruvate in a three-tube MPN assessment. Of the seven other bacterial cultures tested, none grew well on the PTSBS. Selectivity ratios of 4.4-7 were found.

Isolation and identification of staphylococci from milk powders produced in Northern Ireland

Milk powders (37 samples) from five different processing centres (A, B, C, D and E) were examined for total viable counts, total staphylococcal counts and staphylococcal enterotoxins. All powders from centres A, B and C contained low numbers of total viable bacteria and staphylococci but five from centres D and E had high total and staphylococcal counts. Nine different staphylococcal species were encountered in low count powders with a wide range of species occurring at each of the five centres. Three species (Staphylococcus capitis, Staph, saprophyticus and Staph. cohnii) were found whose natural hosts are humans. High count powders all contained added fat of various types and had a much more restricted staphylococcal microflora in which Staph. saprophyticus and Staph. cohnii predominated. None of 384 staphylococcal strains isolated were found to be Staph. aureus. In addition, no enterotoxins were detected.