STAPHYLOCOCCI IN COMPETITION. V. EFFECT OF EGGS, EGGS PLUS CARBOHYDRATES, AND LIPIDS ON STAPHYLOCOCCAL GROWTH

As is generally known, custards have been frequently involved in staphylococcal food poisonings and are regarded by some as an ideal culture medium. Previous studies showed that high carbohydrate concentrations repressed the growth of competing saprophytic species, allowing the growth of staphylococci in a mixture rather than stimulating the growth of the staphylococci. In an extension of those studies, the influence of the egg constituent of custard was investigated to determine its role in affecting the growth of pathogenic staphylococci in a competing mixture of psychrotrophic saprophytes. The normal competitive effect of the saprophytes on the growth of staphylococci was very slightly affected by the addition of 25% whole egg to the growth media. Approximately 9% egg yolk alone added to the medium resulted in a slight increase in the length of the lag period of the psychrotrophs and a slight increase in the number of staphylococci which grew. Addition of 25% whole egg plus 14.5% sucrose resulted in repression of saprophyte growth similar to that seen in high sucrose concentrations. Staphylococcal growth was more extensive in the presence of both whole egg and sucrose than in the presence of either ingredient alone. Incorporation of 4% corn oil in media was effective in repressing growth of the saprophytes at 37 C only. This allowed the staphylococci to dominate the population. At lower temperatures, staphylococci were unable to compete effectively. Buffering media of high carbohydrate content resulted in lengthened lag periods for the psychrotrophs and the appearance of very large staphylococcal populations.